.
‘17. ' ‘
2
2x

 

SYNOPTIC ORIGIN OF PRECIPITATION IN IRAN

By
Bohloul Alijani

A DISSERTATION

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of
DOCTOR OF PHILOSOPHY

Department of Geography

1981

\uxbng

ABSTRACT

SYNOPTIC ORIGIN OF PRECIPITATION IN IRAN
By Bohloul Alijani

This research was undertaken to investigate the
spatial and seasonal variations of precipitation mechanisms
in Iran. For the period 1965 to 1969, uplift mechanisms
associated with precipitation events at 40 different stations
across Iran were determined from lZGMT surface and 500mb syn-
optic maps of the Northern Hemisphere. Importance of the up-
lift mechanisms was judged on the basis of their contribution
to the number of each station's annual or seasonal precipita-
tion days. Westerly disturbances, including both upper level
and surface disturbances, were the most important uplift mech-
anism.over the entire country throughout the year; their con-
tribution to annual precipitation days was higher in the South
and the Northeast but decreased to the north and northwest.
Sea-effect was observed only on the southwest coast of the
Caspian Sea in fall and early winter. The contribution by
surface heating was low. Based on the patterns of contribu-
tion by westerly disturbances to annual precipitation days,
the country was divided into six different regions. Represent-

ative stations of these regions were then used to analyze the

_Bohloul Alijani
seasonal importance of the identified mechanisms as well as
the moisture sources of the precipitation. Seasonally,
upper level disturbances were the most frequent and import-
ant uplift mechanism during the transition seasons but were
co-dominant with surface disturbances in winter, whereas
summer was the season of greatest contribution by surface
heating.

Important moisture sources of the country were deter-
mined for days with 10mm or more precipitation at the repre-
sentative stations through the use of both surface and 700mb
maps of the period July 1967 through December 1969. Although
moisture frmm the Caspian Sea appeared to contribute the
highest percentage to precipitation totals, moisture from
the Mediterranean Sea affected a larger portion of the country.
Persian Gulf moisture contributed the highest percentage of
warm period precipitation days in the relatively dry South;
the southeastern part of the country received most of its

summer moisture from the Bay of Bengal.

To my wife Robab, who shared with me the difficulties
and gave me encouragement, and

to my children, Vajiheh and Taha.

ACKNOWLEDGEMENT

The author expresses his thanks to the many indi-
viduals who assisted in the successful completion of this
research. Above all, Dr. J.R. Harman, my major advisor,
deserves special recognition. I extend my sincere appre-
ciation to hflm for his invaluable and constructive advice,
patience, and words of encouragement throughout the
research. Thanks is also extended to the other members of
my dissertation committee, Dr. Harold A. Winters, whose
encouragement throughout my career was invaluable and very
constructive, Dr. Richard E. Groop, for his statistical
and cartographical advice, and Dr. Ian M. Matley.

Mr. Moslem Saraj of Arak College of Science, Iran,
deserves special appreciation for his fundamental and vital
assistance in obtaining the basic data for this research.

An expression of appreciation is given to my parents
who bore the difficulties of life and gave encouragement
throughout my academic life. I thank also my other rela-
tives, especially my cousin, Mr. Ali Akbar Alijani, for

their spiritual and material encouragement.

ii

PREFACE

This research was undertaken in partial response
to personal feelings I had toward my country, Iran. As a
student of physical geography with particular interest in
climatology, I had long wished to study aspects of the
Iranian physical environment which had not been considered
by others. Indeed, this goal was a major reason that led
me to study abroad. After having been exposed to some
physical geography courses at Michigan State University and
becoming familiar with the literature about Iran, I realized
that climatological studies of the country were mostly des-
criptive rather than explanatory in nature. As a geographer,
I organized my program to permit me to better understand and
explain the spatial pattern of weather elements in Iran, and
elsewhere. Since the explanation of surface weather condi-
tions is not possible without first understanding upper-level
flow processes, I was particularly interested in the relation-
ship between the surface weather patterns and upper level flow
conditions, the field of "synoptic climatology".

In preparing this thesis I have tried my best to
review the relevant literature and gather appropriate data,
and I hope the product represents a contribution to the precip-

itation climatology of Iran. Its preparation by a writer

iii

whose primary language is not English may result in occa-
sional ambiguity, and the author apologizes for any awkward-
ness or error in the style and structure of the text.

This thesis should be considered as only a prelimin-
ary work about the precipitation climatology of Iran which

may be used as starting point for future research.

Bohloul Alijani
June 1981

iv

 

TABLE OF CONTENTS

LIST OF TABLES
LIST OF FIGURES
CHAPTER
I. INTRODUCTION .
LIST OF REFERENCES - CHAPTER I .
II LITERATURE REVIEW

Introduction .

Causes of Precipitation
Precipitation Studies in Iran .
LIST OF REFERENCES - CHAPTER II

III PHYSICAL SETTING .

Topography .
Climate . .

Winter .
Summer .

LIST OF REFERENCES - CHAPTER III .
IV METHODS .

Data Sources

Precipitation Mechanisms

Procedure of Map Analysis

Regionalization . .

Importance of the Precipitation Mechanisms

Moisture Sources . .
Determining the Moisture Sources and
Trajectories . . . . . . . . .

Heavy Storms . .

LIST OF REFERENCES - CHAPTER IV

V RESULTS

TABLE OF CONTENTS (Continued. . .)

Introduction . .
Annual Precipitation Days
Precipitation Mechanisms

Introduction . .

Upper Level Disturbances

Surface Disturbances

Surface Heating

Sea Effect .
Regions of Precipitation Mechanisms
Seasonal Variation of Precipitation Mechanisms

Winter .
Spring
Summer .
Fall .
Summary

Importance of Precipitation Mechanisms

Introduction . .

Annual Precipitation . . .

Annual Importance of Precipitation Mechanisms.

Seasonal Importance of Precipitation
Mechanisms . . . . . . . . .

Winter
Spring
Summer .
Fall .
Summary

Regional and Seasonal Summary

Caspian Region .
Northwestern Region
Northeastern Region
Southwestern Region
Southern Region
Southeastern Region

Moisture Sources
Seasonal Distribution of Moisture Sources
Winter

Spring .
Summer .

vi

112
112
113

TABLE OF CONTENTS (Continued . . .)

CHAPTER

VI DISCUSSION .
Sea Effect .
Surface Heating .
Surface Disturbances .
Upper Level Disturbances . . .
Mbst Frequent Uplift Mechanism . .
Regions of Precipitation Mechanisms
MOisture Sources . .
The Climatology of Heavy Storms
Original Contributions . . .
Research Design
Limitations of the Study .
LIST OF REFERENCES - CHAPTER VI
VII SUMMARY AND CONCLUSION .
APPENDIX
I
II
III
BIBLIOGRAPHY .

Fall .
Importance of Moisture Sources

Heavy Storms .

vii

. 150
. 154
. 155

10.

ll.
12.
13.
14.

LIST OF TABLES

Page
Percentage Contribution by Mechanisms to Mean
Annual Precipitation Days, averaged for all
selected Stations (1965-69) . . . . . . . . . . . . . 67
Total Number of Precipitation Days caused by
different types of Upper Level Disturbances at
selected Stations (1965-69) . . . . . . . . . . . . . 66
Total Number of Precipitation Days caused by
different types of Surface IBsturbances. . . . . . .69

Total Number of Precipitation days caused by different
types of Surface Frontal Disturbances at Selected
Stations (1965-69) . . . . . . . . . . . . . . . . .71

Mean Regional Number of Annual Precipitation days
and the percentage contribution by each mechanism
(1965-69) . . . . . . . . . . . . . . . . . . . . . .85
Representative Stations of the Regions . . . . . . .87

Predominant (most frequent) Precipitation Mechanisms
of the Regions, by Seasons . . . . . . . . . . . . .93

Mbst important Precipitation Producing Mechanisms
by Seasons . . . . . . . . . . . . . . . . . . . . .103

Regional and Seasonal Summary of Precipitation
Mechanisms in Iran . . . . . . . . . . . . . . . . .105

Annual Percentage Contribution by Moisture
Trajectories to Regional Precipitation Days

(10mm or more), (July 1967- December 1969) . . . . .108
Same as Table 10 except for Winter . . . . . . . . . 114
Same as Table 10 except for Spring . . . . . . . . .115
Same as Table 10 except for Summer . . . . . . . . .117
Same as Table 10 except for Fall . . . . . . . . . .118

viii

LIST OF TABLES (Continued . . .)

16.

Page
15. Mean Regional Rainfall and percentage contribution
to the total Mean Precipitation of all Regions . . . . 120
Primary Mechanisms and Moisture Trajectories of
Heavy Storms (heaviest 10% of precipitation days
with 10mm or more) and their percentage contribution
to Regional and Annual Precipitation. . . . . . . . . 122

ix

10.

ll.

12.

13.

14.

 

LIST OF FIGURES

Locational Map of Iran With Selected Stations.

Mean Monthly Precipitation (in millimeters) and
Temperature (in centigrade) at Selected Stations

Mean Annual Precipitation Days (1965-69)

Percentage of Mean Annual Precipitation Days
Caused by Upper Level Disturbances With or With-
out Surface Component . . . . . . . .

Percentage of Mean Annual Precipitation Days
Caused by Surface Disturbances . . . .

Percentage of Mean Annual Precipitation Days
Caused by Surface Heating . . . . .

Percentage of Mean Annual Precipitation Days
Caused by Sea Effect . . . . . . . . . . .

Regions of Precipitation Mechanisms Based on
Cluster Analysis and Using A11 Uplift Mechanisms

Mean Z- scores of the Percentage Contribution by
Uplift Mechanisms to Annual Precipitation Days
for each Cluster . . . . .

Z-score Values of Percentage Contribution by
Westerly Disturbances to Mean Annual Precipita—
tion Days . . . . . . . . . . . . . .
Regionalization Based on the Percentage Contri-

bution of the most frequent Uplift Mechanism to
Mean Annual Precipitation Days . . . . .

Percentage Contribution by Seasons to Mean
Annual Precipitation Days at Selected Stations

Percentage Contribution by Mechanisms to
Seasonal Precipitation Days

Mean Annual Precipitation in millimeters
(1965-1969) . . . . . . . . . . . .

Page

. 32

. 34

62

. 67

. 72

. 74

. 76

78

. 79

. 81

. 83

88

90

95

LIST OF FIGURES(CONTINUED. . .)

15.

16.

17.

18.
19.

Percentage Contribution by Mechanisms to Mean
Annual Precipitation at Selected Stations .

Percentage Contribution by Season to Mean
Annual Precipitation at Selected Stations

Percentage Contribution by Mechanisms to
Seasonal Total Precipitation . .

Annual Moisture Trajectories

A

Relative Importance of Seasonal Moisture
Trajectories .

xi

. 97

98

.100

109

113

CHAPTER I

INTRODUCTION

The location of Iran between 25°N, and 39°N. lati-
tudes places it under the climatic controls of both tropical
and extratropical latitudes. During winter the cold Siberian
anticyclone expands over the country while during the summer
season the shallow heat low centered over west Pakistan and
southern Iran is a dominant surface circulation feature.
Another control of the climate of Iran is the shift of the
'mean position of the subtroPical jet stream in the upper
troposphere. In summer the region of its speed maximum shifts
northward over Turkey (1). Since the subtropical jet stream
is associated with subsidence to its south, Iran is affected
by midtropospheric subsidence in summer (about 700mb above
the surface heat low (2)). On the other hand, the westerly
polar vortex and its accompanying cyclonic activity expand
during winter, bringing the southern branch of the polar front
jet stream.over Iran. This occurs in association with height-
ened baroclinity. The regional climate, furthermore, is comr
plicated by the configuration of the surface terrain. In the
western and northern portions of the country the higher
mountains add to the severity of winters and block the mois-

ture flux to the interior parts of the country.

1

 

2

All of these variations in the circulation, as modi-
fied by the configuration of the terrain, bring different
kinds of air masses to the country. For example, in winter
continental polar air masses invade Iran from the north
while modified Mediterranean air masses bring moisture-laden
air from the west. In summer, continental tropical air orig-
inates over the country itself. As a result of these diverse
climatic controls, Iran harbors a surprisingly complex climate.'

According to the Koppen classification, Mediterranean
climate is found in the north, northwest, and western high-
lands. Hot desert climates are found in the central areas
east of the Zagros Mountains and in the coastal strip of the
southern water bodies; the remaining areas are steppe climates.
Based on the aridity index, Dehsara (3) classified the country
into three broad regions of forest, steppe, and desert.

The forest region covers the coastal areas of the Caspian
Sea and northern parts of the Zagros Mountains. The steppe
regions cover all the mountainous areas, while deserts cover
the lowlands. Martonne (4) considers the southeasternmost 3
part of the country to have a monsoon climate.

Regardless of the scheme, classifications convey the
diversity of the climate of Iran. For example, mean annual
precipitation ranges from very high values in Anzali on the
southwest coast of the Caspian Sea to values below 50mm in
the central deserts. In general, precipitation decreases from
north to south and from west to east. Most of the country

experiences a winter maximum while the Caspian coast is

 

3
characterized by a fall maximum. A spring maximum exists
on the sunny slopes of the northwestern highlands. Summer
is the driest period over most of the country except in the
coastal regions of the Caspian Sea. Ganji (5) was one of
the first to recognize this diversity and divided the country
into six precipitation provinces based mostly on mean annual
precipitation and its degree of seasonality. Using the same
subjective methods, Adl (6) organized the precipitation
patterns into several subdivisions based on only the range of
annual precipitation.

Why is there so much diversity in the Iranian precipi-
tation pattern? What factors other than those discussed above
generate this diversity? Since the occurrence of precipita-
tion depends upon the coexistence of a lifting mechanism and
moist air (7, 8, 9, 10), the explanation for this diversity
lies in variations of both these components.

Uplift can be generated by several mechanisms (10, 11),
yet all of them can be classified under two categories--
dynamic and thermodynamic. Dynamic mechanisms include upper
level divergence, surface cyclones, and surface convergence
induced by terrain irregularities such as rough surfaces and
mountain ranges. Among these mechanisms, upper level diver-
gence produced by vorticity advection in the westerlies is by
far the most important mechanism (12); indeed, surface cyclones
are generated by this vorticity principle (13). Regions of
positive vorticity advection exist to the east of upper level

short-wave westerly troughs. Because of compensation, these

4

regions generate surface convergence and hence, ascending

air (7). Within surface cyclones the areas of most import-
ant uplift are located along the frontal zones. At the warm
front, the warm air glides over the cold air because of sur-
face wind convergence and contrasting air mass densities.

At the cold front the cold air subsides and undercuts the
warm air forcing it to rise (7). Accordingly, frontal zones
are often associated with the heaviest precipitation. In
'addition, another dynamic factor causing uplift is surface
convergence created by such terrain irregularities as mountain
ranges. For example, when an air stream approaches the Zagros
MOuntains from the west it is forced to ascend (14), resulting
in condensation and possibly in precipitation at some point.
Browning and Harroid (15) observed that such orographic uplift
is an enhancing factor for already-existing precipitation areas;
this effect usually occurs at more than 1000m above the mount-
ain base, where uplift begins (16, 17, 18, 19).

Thermodynamic processes are induced directly or in-
directly by vertical temperature differentiation in the atmos-
phere. Such processes as the warming of cold air by wanm
water surfaces, cold air advection, and extensive surface heat-
ing are included because they all lead to instability (20).
Heating by an underlying water surface occurs when very cold
air passes over a warm water body. In this process the air
gains heat and moisture from the water and it may become de-

stabilized. Cold advection occurs when cold air behind a

surface cold front advances more rapidly in the lower

 

5

troposphere than at the surface, leading to an increased
lapse rate that maximizes beneath upper cold lows and short
wave trough axes. Extensive surface heating, though not un-
common in the mid-latitudes, is most important as a de-
stabilizing factor in equatorial lands (21). It is also a
common uplift mechanism in hot coastal regions where moist
air is brought in by sea breezes (21, 22, 23). In the mid-
latitudes, intense surface heating may be a rain-inducing
mechanism on the sunny slopes of the high mountains (24).

It is obvious that some, if not all, of these uplift
mechanisms operate in Iran. Unfortunately, few studies have
been published regarding the origin of precipitation in the
country. Some references are available in general textbooks
which relate most of the Iranian cold season precipitation to
surface and upper level disturbances originating in the
Mediterranean Sea region (25, 26). The fall precipitation
maximum of the Caspian Sea (27), the spring maximum of the
western highlands, and the summer precipitation over the entire
country have been regarded (probably erroneously) as convec-
tional in origin (5).

. As mentioned earlier, availability of moist air is
another important variable in the generation of precipitation
(20, 24). Like the uplift mechanisms, moisture advectiOn over
Iran has been studied only in the context of regional or global
patterns. According to Tuller (28), the amount of precipit-
able water is very high in the south and decreases northward.

Bannon (29), in studying the moisture flux of the Northern

6
Hemisphere, found that the moisture flux is from the west
to the east in the Middle East. Except for these general
studies, no detailed research regarding the moisture pat-
terns of the country is known to the author.

According to the literature presented here, it
appears that no comprehensive study has been done regarding
either uplift mechanisms or moist air as they contribute to
Iranian precipitation patterns, in spite of their import-
ance. Therefore, this study is undertaken to determine the
spatial distribution of uplift mechanisms over Iran and to
identify more precisely their contribution to overall pre-
cipitation. Also, an attempt is made to determine the major
contributing moisture sources of the country.

The specific aims of the study are:

1) to determine the uplifting mechanisms operating

over Iran,

2) to study the seasonal and spatial variations of
these mechanisms,

3) to regionalize the country on the basis of
dominant uplifting mechanisms,

4) to determine the most frequent and significant
mechanism for the country and for each region on
an annual and seasonal basis,

5) to study the moisture sources and trajectories

contributing to regional precipitation, and

7
6) to study the triggering mechanisms, moisture
sources, and relative contribution of heavy

storms to regional precipitation.

10.

ll.

12.

 

LIST OF REFERENCES - CHAPTER I

Meteorological Office, 1962. Weather in the Medi-
terranean, Vol. I General MeteoroIogy, HMSO, London.

Ramage, C.S. 1966. The summer atmospheric circulation
over the Arabian Sea. J. Atm. Sci., 23:144-50.

 

Dehsara, M. 1972. An Agroclimatological Map of Iran.
Arch. Met. Geoph. Biok1., Ser. B. 21:393-402.

De Martonne E. 1925. Traite de Geographie physique,
Vol. 1, Paris, in Ganji, M.H. 1954. A contribution
to the climatology of Iran. Ph.D. Thesis, Clark
Univ., Worcester, Massachusetts.

Ganji, MQH. 1954. A Contribution to the Climatology
of Iran. Ph.D. Thesis, Clark Univ., Worcester,

Massachusetts.

Hosseini, A.A. 1960. Climate of Iran. Tehran Univer-
sity Public No. 586.

Palmen, E. and C.W. Newton. 1968. Atmospheric Circula-
tion Systems; their structure and physicaIiinterpre-
tatibn. Academic Press, New York.

Kidson, J.W. 1977. African Rainfall and its relation

to the Upper Air Circulation. Quart. J.R~ Met. Soc.
1032441-56. ,

wasserman, S.E. and H. Rosenblumo 1972. Use of Primi-
tive-Equation Models Output to Forecast Winter Pre-
cipitation in the Northeast Coastal Section of the
United States. J. Appl. Met. 11:16-22.

Starret, L.G. 1949. The relation of Precipitation
Patterns to certain types of Jet Stream at the 300mb
Level. J. of Met. 6:347-52.

 

Schermerhorn, V.P. 1967. Relations Between Topography
and Annual Precipitation in Western Oregon and
Washington. Water Resource Research, 3:707-711.

Jenrette, J.P. 1960. An Objective Application of Vor-
ticity Principles to Precipitation Forecasting.
Bull. Amer. Met. Soc., 41:317-23.

8

13.

14.

15.

16.

17.

18.

19.

20.

21.

22.

23.

24.

25.

26.

27.

9

Harman, J.R. 1971. Tropospheric waves, Jet Streams,
and United States Weather Patterns. Assoc. Amer.
Geogr. Res. Pap.No. 11, washington, D.C.

Feteris, P.J. 1973. The Role of Deep Convectional
and Strong Winds Aloft in Triggering Gales Over
the Persian Gulf: Comparative Case Studies.

Mon. Wea. Rev., 101:455-60.

Browning, K.A. and T.W. Harrold. 1969. Air Motion
and Precipitation Growth in a wave Depression.
Quart. J.R. Met. Soc., 95: 288-309.

Elliot, R.D. and E.L. Hovind. 1964. The Water
Balance of Orographic Clouds. J.App1. Met., 3:235-39.

Duckstein, L., M.N. Fogel, and J.L. Thames. 1973.
Elevation Effects on Rainfall: A Stochastic Model.
J. Hydrol., 18:21-35.

Lamb, D. et al., 1976. Measurement of Liquid Water
Content in Winter Cloud Systems Over the Sierra
Nevada. J. Appl. Met., 15:763-75.

 

Chuan, G.K. and J.Q. Lockwood. 1974. An Assessment of
Topographical Controls on the Distribution of Rain-
fall in the Central Pennines. Met. Magg, 103: 275-87.

Strommen, D.N. 1975. Seasonal Change in the Axis of
‘Maximum.Lake Snow in Western Lower Michigan. Ph.D.
Thesis,IMichigan State Univ., E.Lansing, MiChigan.

Trewartha and Horn. 1980. An Introduction to Climate
(Fifth Edition), McGraw-Hill Book Company{INew York.

Byers, H.R. and R.R. Harriet. 1948. Causes of the
Thunderstorms of the Florida Peninsula. J. Met.,5:
275-80. '

Eddy, G.A. 1966. The Summer Atmospheric Circulation
Over the Arabian Sea. J. Atm. Sci., 23:144-150.

Scorer, R.S. 1955. The Growth of Cumulus Over Mount-
ains. Arch. Met. Gedph. Biok1., Ser.A. 8:25-34.

Trewartha. 1961. Earth's Problem Climates. University
of Wisconsin Press, Madison.

Beaumont et a1. 1974. The Middle East: A Geographical
Study, John Wiley & Sons, New YOfk.

Khalili, A. 1971. On the origin of Southern Caspian
Sea Rainfall. Publ. of Iranian Met. Dept., March 1971.

10

28. Tuller, S.E. 1968. WOrld Distribution of Mean Monthly
and Annual Precipitable Water. Mon. Wea. Rev., 96:
785-97.

29. Bannon, J.K. et al. 1961. The flux of water vapor
due to the mean winds and the convergence of this
flux over the Northern Hemisphere in January and
July. Quart. J. Ro‘Met. Soc., 87: 502-11.

CHAPTER II

LITERATURE REVIEW

Introduction

Precipitation is produced when moist air and a lift-
ing mechanism co-occur. The process of uplifting and the
availability of moist air are governed by circulation both
at the surface and aloft. Many surface pressure systems
are generated and guided by atmospheric disturbances im-
bedded in the upper level flow; however, thermally-induced
convectional lifting may develop independent from upper
circulation. Day-to-day changes of the upper level flow
pattern cause variations in the surface pressure pattern
which, in turn, influence the distribution of such weather
elements as precipitation.

Considerable spatial and temporal variation of preci-
pitation exists in Iran. For example, the coastal areas of
the Caspian Sea receive the most rain, with heaviest precipi-
tation occurring in the fall. The southern parts of Iran,
however, are almost rainless except for scattered and sporad-
ic winter precipitation. In an area so large and topographic-
ally diverse, several mechanisms may operate on moist air
derived from.several possible sources. In the following

sections 1 will summarize the factors involved in the

11

12
generation of precipitation generally and then will review
literature specifically concerned with the synoptic preci-

pitation climatology of Iran.

Causes of Precipitation

The question of the origin of precipitation occupied
meteorologists and climatologists beginning in earliest
times and explanations varied widely. In his theory of rain
formation presented in 1788, Hutton (1) stated that when
moist air masses with different temperatures mix, super-
saturation will occur resulting in condensation or rain.
This theory stood as the primary explanation for the origin of
rain for nearly a century and was supported by other studies (2).
At the same time, Velschow (3) proposed that rain is produced
by the descent of moist air, which removes the moisture
through compression. This idea was rejected by Hazen because
areas of atmospheric subsidence usually experience fair weather
and clear skies. By the end of the nineteenth century Hutton's
theory was abandoned when it became obvious through advances
in the field that the amount of water which could be condensed
in this way was too small. The process of air mass mixture
may bring the temperature of warm air to the saturation point
and produce fog, but the process will not produce significant
amounts of rain. It was later understood that ascent of air
would lead to adiabatic cooling which would then produce con-
densation in amounts adequate for precipitation. The necess-

ary rate of ascent was determined to be 30 feet per minute

13
for steady rain at fronts and up to one or two thousand
feet per minute for heavy rains or thunderstorms (1).

Shaw (4) described a stable atmosphere as having
successive layers of air which preserve their distinctive
characteristics. He said that three physical processes
could disturb this stability and could cause displacement:
eddy motion caused from frictional or viscous forces, well
organized air flow across the isobars resultingin upward
motion in cyclones and descending motion in anticyclones,
and thermal convection caused from surface heating.

A most significant contribution to the subject came
from the Norwegian school through the development of frontal
theory. The central theme of this theory was that the atmos-
phere consists of extensive bodies of fairly homogeneous air
masses, separated by gently sloping frontal surfaces (5).
According to this theory, wave cyclones develop when two con-
trasting air masses approach each other and wavey movements
begin along the zone of contrast due to frictional forces._
Within these cyclones the warm air glides over the cold air at
the warm front, while at the cold front the cold air undercuts
the warm air. At both regions upward motion occurs. In
acknowledging the role of frontal uplift, V. Bjerknes (6)
classified rainfall types as being orographic, frontal, and
convectional. Following the same idea, Bjerknes and Solberg
(7) classified the origin of rainfall over Norway as being
caused by orographic uplift, instability, cyclonic uplift, warm

front and cold front, or low level cooling. Douglas (8),on the

14
other hand, classified rain formation into two broad cate-
gories of dynamic ascent caused by wind shear of frontal
zones, and thermodynamic ascent produced either by surface
heating or by the passage of cold air over a warm surface.

Bjerknes and Solberg (7) defined orographic precipi-
tation as that produced by air ascending a mountain slope
when there is no cyclonic or instability precipitation in the
vicinity. At other times such uplift may enhance the already
existing precipitation (9, 10, ll). Druyan (11), Eliot and
Shaffer (12) all concluded that orographic rain will be
greater with the ascent of unstable air than with the ascent
of stable air. Stable air tends to pass around the mountain (7)
and, if forced to rise, will yield less precipitation or none
at all.

Several studies have correlated precipitation with
elevation. For example, Chuan and Luckwood (13) found a very
high correlation between mean altitude and rainfall in the
east Pennine region. Similar results were obtained by
Schermerhorn (14). Lamb, et al. (15), in a study of the
liquid content of orographic clouds for six storms over the
Sierra Nevada, found that the highest values of precipitable
water existed on the windward slope of the mountain. The
strong buoyant region started from the height of 1000 meters
(see also 7, 10). The high instability on the west side was
thought to be caused by differential advection. Duckstein,et
a1. (16) attributed greater mean rainfall with greater eleva-

tion to an increase in the number of mountain slope showers

15
and to the amount of precipitation per storm. In a study
of orographic water balance for the Santa Ynez and Gabril
ranges, California, Elliot and Hovind (17) found that oro-
graphic ascent could account for about 25 percent of the
condensed water, an estimate increased for larger barriers.
However, since atmospheric instability is very low in winter,
small scale uplift (areas with lOOkm. in dimension or smaller)
occurs only when the air is forced to ascend the mountain
slope. Thus the mountains are more important when stability
is greater, such as during the winter or at night, and when
other lifting triggers operate (13)”

Types of precipitation other than orographic are pro-
duced in the free atmosphere by surface convergence and
resulting uplift. Bjerknes and Solberg (7) showed that these
convergence regions are usually linear. In cyclones these
regions are associated along lines of wind and temperature
discontinuity, i.e” frontal surfaces (see also 18). Numerous
studies have been done concerning the structure, mechanism,
and efficiency of frontal or cyclonic precipitation (7, 9, 19,
20, 21, 22, 23). Bjerknes and Solberg (7) concluded that
warm frontal precipitation results from the gliding of warm
air over a gentle slope of cold air in a broad zone of 150 to
400 miles ahead of the surface front. Cold-front rain is
produced when the cold air undercuts the warm air and uplifts
it. Atkinson (22) found several types of large mesoscale pre-

cipitation areas within the warm frontal rain area, including

16

linear cells, which originated in synoptic scale uplift.
According to Browning and Harrold (21), cold front precipi-
tation occupies a narrower area and results from a two phase
convectional ascent. This includes near-vertical ascent
up to 3km. over the front and then further ascent through a
shallow slope up to 6km. behind the surface front. In study-
ing the efficiency of the dynamics of cyclones in releasing
atmospheric moisture, Robinson and Lutz (23) found that at the
cold front the areal distribution of precipitation is narrow
but its efficiency is very high. At the warm front, however,
the areal distribution is broad and its efficiency is lower.
Browning and Harrold (9) found that the warm sector
precipitation of cyclones was mostly affected by the instabil-
ity of warm air and was organized as linear bands parallel to
the warm sector wind flow. The instability showers grew more
vigorous if the air being lifted contained abundant moisture.
Curtis and Panofsky (24) found that in the presence of suffi-
cient moisture no type of large scale vertical motion was re-
lated particularly well to daytime precipitation, since showers
and thunderstorms can be produced by surface heating alone.
Nocturnal showers, on the other hand, showed signifi-
cant correlation with large scale vertical motion. The noc-
turnal atmosphere is often more stable, and organized uplift
is more necessary to promote precipitation. Oliver and Shaw
(25) concluded that warm sector precipitation is highly related

to the moisture content and instability of the air.

17

Instability showers are also common features of
coastal regions as well as sunny mountain slopes. According
to Bjerknes and Solberg (7), moist sea air invades coastal
regions on hot summer days and with low level heating it
rises and produces instability showers. Wexler (26) stated
that sea breezes in mid-latitudes extend inland 30 to 40km.

He also described the best condition for sea-breeZe develop-
ment as being hot, calm sunny days preferably with an off-
shore flow. Flower (27) studied the sea breezes at Ismailia,
Egypt, and concluded that they developed from a diurnal change
of pressure and temperature and then moved overland as true
cold fronts. In a study of the structure of these breezes,
Simpson, et al.,(28) showed that the humidity of the air is
higher at lower levels near the front and decreases toward the
higher levels over the front due to high mixing rate at this
level. The vertical velocity is highest near the leading edge
of the sea breeze. Byers and Harriet (29) found that when
there is no significant large scale vertical motion the conver-
gence of two sea breeze fronts from the opposite sides of a
peninsula stimulates surface convergence and hence thunder-
storms. The same result was found by Harman and Hehr (30) for
the eastern Upper Peninsula of Michigan.

Low-level heating may also lead to precipitation during
the advection of cold air over a warm surface. For example,
Bjerknes and Solberg (7) found that behind surface cold fronts

heating from below occurs when polar air invades the relatively

18

warmer southern latitudes. Furthermore, if moisture is
adequate in the cold air, instability showers develop. They
found that the intensity of these cold advection showers
depends on the moisture of the cold air. According to
Stromman (31), when cold polar air passes over the warm sur-
face of Lake Michigan it gains heat and moisture. By the time
it reaches the lee shore it becomes unstable and may yield
rain or snow showers. Druyan (11) found that when this air_
is not sufficiently unstable to produce showers on the coastal
regions it may yield precipitation when it ascends mountain
slopes near the coast. Douglas and Glasspool (10) concluded
that over England the instability showers caused by surface
heating (air mass showers) are important in summer and those
of cold air advection are dominant in fall and winter. Accord-
ing to Harrold (20), squall-line precipitation is a good ex-
ample of instability showers resulting from.a cooling of the
air column from above. However, in many instability showers
the prime triggering factor of vertical motion is the large-
scale uplift caused by the vorticity principle (22, 32, 33),
and the vertical instability of air enhances the vertical
motion or causes small scale variations (34).

The vorticity principle was not well understood until
just prior to World War II; important uplifting mechanisms
previously were thought to be orographic, low-level convergence
or upslope motions at frontal boundaries, and instability due

to surface heating. After WOrld war II, research at the

19

University of Chicago opened a new era in the field of syn-
optic meteorology. In one paper, surface weather patterns
were related to the wind flow pattern in the mid to upper
troposphere (35). In interpreting these principles, Harmon
qualitatively described the connection between vorticity
and associated vertical motion. Positive vorticity advection
(upper level divergence) in the westerlies occurs where the
upper level air streams undergo a vorticity decrease east of
a trough linecn: the lee side of upper cold lows and cut-off
lows. Less often this advection occurs under trough axes
themselves and in the left exit of a jet stream maximum. The
inflection point, the location where curvature vorticity
changes sign, often lies near the center of the uplift region.

Several studies have related precipitation patterns
to the upper flow pattern. Klein (37) presented a quantitative
relationship between 700mb circulation patterns and surface
precipitation at Knoxville, Tennessee. He studied the inten-
sity of 5 day mean rainfall for each 5 degree longitude behind
and ahead of trough and ridge lines. His computations showed
that the heaviest rainfall occurred under southwesterly flow,
i.e.,where positive vorticity advection and resulting vertical
motion were the highest. This same relationship was described
by O'Conner (38). Jenrette (32) showed that positive vorticity
advection is associated with surface convergence and with
decreasing atmospheric stability, resulting in high vertical

motion.

20

As mentioned before, according to Harman (36),
ascending motion is concentrated beneath the left exit
region of the jet maxima. Starrett (39) studied the pre-
cipitation patterns in relation to jet maxima within 10
degrees latitude to the north and south of the quasi-linear
jet axis. He found that the area of maximum.precipitation
occurs within a few degrees north of the jet axis. Others
such as Johnson and Daniels (40) and Richter and Dahl (41),
have done similar studies. In addition, Smith and Younkin
(42) developed a forecasting model for jet streams associated
with upper troughs. They found the precipitation maximum
located in the area about 2.5 degrees longitude to the right
side of the jet axis having a mean width of 3 degrees latitude
and a length of 12 degrees longitude.

As a result of these and other studies, it has been
established that by far the most important uplifting process
in the baroclinic westerlies is associated with the vorticity
principle. This relationship has become a tool for forecasting
precipitation (43). However, this does not overshadow either
the importance of the moisture content of the air mass or the
degree of its stability in precipitation prediction. In addi-
tion, Oliver and Shaw (25) found that these variables played
an important role in determining surface rainfall patterns.

With the development of a more thorough theoretical
background in synoptic climatology (44), meteorologists and

climatologists have attempted to classify precipitation

21
according to the causative synoptic weather types (44, 45,
46, 47, 48, 49, 50, 66). In England and Wales, for instance,
Lawrence (47) studied the rainfall averages for 27 surface
flow categories such as cyclonic and anticyclonic. In a
study of areal correlation of surface precipitation with
upper level cold lows, Klein, et al., (51) found that the Opti-
mum.precipitation occurs 2 degrees east of 850mb lows in the
region of southwesterly flow, 3 degrees southeast of 700mb low,
and 5 to 6 degrees east of 500mb and 300mb lows. The least
amount of precipitation was observed to the northeast and west
of the low centers. As a whole, the 850mb lows accounted for
much of the precipitation. Williams,et al.,(52) found that
precipitation events associated with upper level low centers,
those with at least one closed contourline, yielded heavier
precipitation than those associated only with surface features
such as frontal systems. They classified those associated
with surface features according to whether the trigger was
warm frontal, cold frontal, or attributed to other factors
such as air mass showers, or occlusion. They observed that
the cold frontal storms yielded the highest amount of precipi-
tation among the classes. Others have studied precipitation
types in relation to very specific synoptic situations (19, 53,

54, 55, 56, 57, 58, 59).

Precipitation Studies In Iran
Very little has been written about the origin of pre-

cipitation in Iran. Yet some references are available in

22

general climatic textbooks. Trewartha (67) attributes the
precipitation of the country to three synoptic origins.
According to him, most of the cold-period rainfall over the
country is caused by Mediterranean cyclones. He also noted
that Siberian cold air becmmes moist and unstable through
passage over the Caspian Sea, resulting in "sea effect" pre-
cipitation on the lee shore. In the southeastern part of the
country moist monsoon air provides moisture for summer season
convectional rainfall.

Few synoptic studies have been done in Iran regarding
the origin of precipitation. Gangi (65) classified the rain
events over Iran according to whether the primary mechanism
was cyclonic, convectional, or orographic. According to him,
almost all of the cold period rain is of cyclonic origin where-
as the summer rain is of convectional origin. The western
highlands receive their maximum rainfall in spring because of
convectional processes, while in the Caspian area northeast-
erly winds from.central Asia cross the Caspian Sea to yield
the fall rainfall maximum during orographic ascent on the lee
shoreline. Recently, Khalili (64) related the origin of pre-
cipitation in the Caspian area to instability resulting from
thermodynamic processes. These thermodynamic processes are
produced by the passage of Siberian Polar air over the warm
sea surface during the cold season. In describing the climate
of Kerman, Becket and Gordon (60) stated that, apart from very

rare monsoon rain, most of the rain in the area is caused by

 

23

Mediterranean disturbances. In summer southwesterly or
southerly winds bring moist air from the Persian Gulf and
permit convectional precipitation to develop over the area.
To the author's knowledge, the first general explanatory
study of rainfall dynamics in Iran was completed by Weickman
(61).. He concluded that the precipitation mechanisms affect-
ing Tehran are similar to those over much of the rest of the
country. The main control of precipitation over Tehran was
found to be divergence associated with different wave types
in the middle and upper atmosphere.

When studying an abnormal wet spell with flooding in
Iran during July 1956, Ramaswamy (62) investigated the dynamic
and physical interactions between middle and low latitude
weather systems. During the wet spell, the monsoon air was
deflected over Iran as far as the Caspian Sea coastal area
because of a westward displacement of the Tibetan high. At
the same time an upper level trough developed in the mid latitude
westerlies over the Caspian Sea. As a result, the moist monsoon
air was uplifted by upper level divergence yielding torrential
rains over the area. Further to the west the physical inter-
action of warm, wet monsoon air with cool, continental air
from the northwest caused frontal uplift aiding the intensity
of the interaction of mid and low latitude atmospheric features.
Except for this abnormal occurrence the author concluded that
the moisture source for precipitation in Iran is normally
provided by southwesterly and westerly winds from either the

iMediterranean Sea or the Persian Gulf.

24

The effect of cyclonic uplift in Iran has been
observed in areas as far south as the Persian Gulf. For
example, Murray and Coulthard (63) attributed heavy thunder-
storms in Sharjah to the instability associated with cyclonic
circulation on the surface which was the result of an upper
level trough over the area.

Most of these studies, however, have been concerned
with restricted problems or areas, and no comprehensive study
regarding the dynamics and moisture source of precipitation
over Iran has been done. A.study of the genetic climatology
of precipitation in a comprehensive approach for the whole

country is needed.

10.

11.

12.

LIST OF REFERENCES - CHAPTER II

Brewer, A.W. 1952. Why Does It Rain? Weather, 7:195-98.

Hazen, H.A. 1891. Rain Formation. Amer. Met. J., 8:
2

Velschow, F.A. 1891. On Rain Formation. Amer. Met. J.,
8:177-78.

Shaw, S.N. 1922. Convection in the Atmosphere (Lecture).
Met. Mag., 57:238.

Taylor, G. 1951. Geography in the Twentieth Centur .
Philosophical Library Inc., New YOrk, pp. 1 8-96.

Bjerknes, V. 1919. The Structure of the Atmosphere when
Rain is Falling. Met. Mag. 54:116.

Bjerknes, J. and H. Solberg. 1921. Meteorological Con-
ditions for the Formation of Rain. Geofys. Pub1.,
Vol. 2, No. 3, 60pp.

Douglas, C.K.M. 1934. Some Problems of Modern Meteor-
ology, No. 14, The Problem of Rainfall. anrt. J.R.
Met. Soc., 60:143-152.

Browning, K.A. and T.W. Harrold. 1969. Air Motion and
Precipitation Growth in a Wave Depression. anrt. J.
R. Met. Soc., 95:288-309. -

Douglas, C.K.M. and J. Glasspool. 1947. Meteorological
conditions in Heavy Orographic Rainfall in the
British Isles. anrt. J.R. Mgt. Soc., 73:11-43.

 

Druyan, L.M. 1979. The Role of Static Stability in
Orographically Influenced Precipitation in Israel.
Israel J. Earth Sci. 28:100-102.

 

Elliot, R. and R. Shaffer. 1962. The Development of
Quantitative Relationship Between Orographic Precipi-
tation and Air-Mass Parameters for use in Forecasting
and Cloud Seeding Evaluation. J. Appl. Met. 1:218-28.

25

13.

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26

Chuan, G.K. and J.G. Lockwood. 1974. An Assessment of
Topographical Controls on the Distribution of Rain-
fall in the Central Pennines. Met. Mag., 103:275—87.

Schermerhorn, V.P. 1967. Relation between Topography
and Annual Precipitation in Western Oregon and
Washington. Water Resource Research 32707-11.

Lamb, D. et al., 1976. Measurement of Liquid Water
Content in Winter Cloud Systems Over the Sierra
Nevada. J. Appl. Met. 15:763-75.

Duckstein, L. et a1. 1973. Elevation Effects on Rain-
fall: A Stochastic Model. J. Hydr., 18:21-35.

Elliot, R.D. and E.L. Hovind. 1964. The Water Balance
of Orographic Clouds. J. Appl. Met., 3:235-39.

Haurwitz, B. 1933. The Recent Theory of Giao Concern—
ing the Formation of Precipitation in Relation to

the Polar Front Theory. Trans. Amer. Geoph. Union.
14:89-91.

Soliman, K.H. 1958. Rainfall Over Egypt. Quart. J.R.
Met. Soc., 79:

Harrold, T.W. 1973. Mechanisms Influencing the Distri-
bution of Precipitation Within Baroclinic Disturb-
ances. Quart. J.R. Mgt. Soc., 99:232-51.

Browning, K.A. and T.W. Harrold. 1970. Air Motion and
Precipitation Growth at a Cold Front. anrt. J.R.
Met. Soc., 96:369-89.

Atkinson, B.W. and P.A. Simpson. 1978. Mesoscale Preci-
pitation Areas in a Warm Frontal Wave. Mon. Wea. Rev.
106:211-22.

Robinson, P.J. and J.T. Lutz. 1978. Precipitation
Efficiency of Cyclonic Storms. Ann. Amer. Geogr. 68:
81-88.

Curtis, R.C. and H.A. Panofsky. 1958. The Relation
Between Large Scale Vertical Motion and Weather in
Summer. Bull. Amer. Met. Soc., 39:521-31.

Oliver, V.J. and R.F. Shaw. 1956. Heavy Warm Section
Rains from Illinois to Middle Atlantic Coast. Mon.
Wea. Rev., 84:198-204.

Wexler, R. 1946. Theory and Observations of Land and
Sea Breezes. Bull. Amer. Met. Soc., 27:272-87.

 

27.

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29.

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31.

32.

33.

34.

35.

36.

37.

38.

39.

40.

41.

27

Flower, W.D. 1936. Minor ”Haboob" at Ismailia, Egypt.
Met. Mag., 71: 111-113.

Simson, J.E. et a1. 1977. Inland Penetration of
Sea-breeze Fronts. Quart. J.R. Met. Soc. 103:47-76.

Byers, H.R. and R.R. Harriet. 1948. Causes of
Thunderstorms of the Florida Peninsula. J. Met.,
5:275-80.

Harman, J.R. and J.G. Hehr. 1972. Lake Breezes and
Summer Rainfall. Ann. Amer. Geogr., 62:375-87.

Strommen, N.D. 1975: Seasonal Changes in the Axis of
Maximum Lake Snow in_WEstern Lower Michi an. . .
thes1s, M1cH1gan State Un1ver31ty, East Eansing,
Michigan.

Jenrette, J.P. 1960. An Objective Application of
Voricity Principles to Precipitation Forecasting.
Bull. Amer. Met. Soc., 41:317-23.

Palmen, E. and C.W. Newton. 1969. Atmos heric Circula-
tion Systems. Academic Press, New York.

Miller, J.E. 1955. Intensification of Precipitation by
Differential Advection. J. Met., 12:472-77.

University of Chicago Staff Members. 1947. On the
General Circulation of the Atmosphere in Middle
Latitudes. Bull. Amer. Met. Soc., 28:255-80.

Harman, J.R. 1971. Topospheric Waves, Jet Streams,
and United States Weather Patterns, Assoc. Amer.

Geogr. Res. Pap. No. 11, Washington, D.C.

Klein, W.H. 1948. Winter Precipitation as Related to
700mb Circulation, Bull. Amer. Met. Soc., 29:439-53.

 

O'Connor, J.F. 1963. Extended and Long Range Fore-
casting. J. Amer. Waterworks Assoc., 55:1006-1018.

 

Starret, L.G. 1949. The Relation of Precipitation
Patterns in North America to Certain Types of Jet
Streams at the 300 millibar level. J. Met., 6 347-
52.

Johnson, D.H. and S.M. Daniels. 1954. Rainfall in
Relation to Jet Streams. Quart. J.R. Met. Soc., 80:
212-17. ——

Richter, D.A. and R.A. Dahl. 1958. Relationship of
heavy Precipitation to the Jet Maximum in the Eastern
ggigzg Sgates, September 19-21, 1958. Mon. Wea. Rev.

42.

43.

44.

45.

46.

47.

48.

49.

50.

51.

52.

53.

54.

55.

28

Smith, W. and R.J. Younkin. 1972. An Operational
Useful Relationship Between the Polar Jet Stream
and Heavy Precipitation. Mon. Wea. Rev., 100:
434-40.

Riehl, H. et al., 1952. A Quantitative Method for the
Prediction of Rainfall Patterns. J. Met., 9:291-98.

.Knowles, H.T. and K.H. Jehn. 1975. A Central Texas

Synoptic Climatology and its Use as a Precipitation
Forecast Tool. Mon. Wea. Rev., 103:730-36.

Paegle, J.N. 1974. Prediction of Precipitation Prob-
ability Based on 500mb Flow Type. J. Appl. Met.,
13:213-20.

Rahmatullah, M. 1952. Synoptic Aspects of the Monsoon
Circulation and Rainfall over Indo-Pakistan. J. Met.
9:176-79.

Lawrence, E.N. 1971. Synoptic Type Rainfall Averages
Over England and Wales. Met. Mag., 100:333-39.

Howell, W.E. 1953. Study of Rainfall of Central Cuba.
J. Met., 10:270-78.

Lawrence, E.N. 1973. High Values of Daily Areal Rain-
fall Over England and Wales and Synoptic Patterns.
Met. Mag., 102:361-66.

Houghton, J.G. et al., 1976. Synoptic Origin of Irish
Precipitation 1979-81. Weather 31:11-25.

Klein, W.H. et al., 1968. Relation Between Upper Air
Lows and Winter Precipitation in the Western Plateau
States. Mon. Wea. Rev., 96:162-168.

Williams, P. Jr., and E.L. Peck. 1962. Terrain Influ-
ence on Precipitation in the Itermontane West as
Related to Synoptic Situations. J. Appl. Met. 1:
343-47.

Lumb, F.E. 1966. Synoptic Disturbances Causing Rainy
Periods Along the East African Coast. Met. Mag., 95:
150-59.

Hay, R.F.M. 1950. Rainfall in East Scotland in Rela-
tignlto the Synoptic Situation. Met. Mag., 79:
1 — 9.

Changnon, S.A. 1970. Hailstreaks. J. Appl. Met., 27:
109-125.

56.

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64.

 

29

Hiser, H.W. 1956. Type Distribution of Precipitation
at Selected Stations in Illinois. Trans. Amer.

Geoph. Union, 37:421-24.

Shaw, 1962a. An Analysis of the Origin of Precipita-
tion in Northern England, 1956-60. Quart. J.R. Met.
Soc., 88:539-47.

Matthews, R.P. 1972. Variation of Precipitation
Intensity with Synoptic Type Over the Midlands.
Weather, 25:63-72.

Smithson, P.A. 1969. Regional Variations in the
-Synoptic Origin of Rainfall Across Scotland.

Scottish Geographical Mag., 85:182-95.

Beckett, P.H.T. and E.D. Gordon. 1956. The Climate of
Kerman, South Persia. Quart. J.R. Met. Soc., 82:
503—14.

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and Physical Interaction Between Middle and Low
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88:176-78.

Khalili, A. 1971. On the Origin of Southgrn Caspian
Sea Rainfall. PubI. of Iranian Met. Dept., March 1971.

CHAPTER III

PHYSICAL SETTING

Iran's physical geography has made it a region of
diverse and complex climate (1). At least three factors
cause this diversity. First, its latitudinal position
brings it under the influence of both tropical and extra-
tropical controls during the course of the year. Second,
its location between Siberia and central Asia to the north,
the Sahara and Arabia to the southwest, the Mediterranean Sea
to the west, and the Indian region to the east makes it poss-
ible for weather from a variety of source areas to affect the
country. The proximity of the country to these source areas
results in little modification of the air masses before they
reach it. Third, the location, configuration, and altitude
of topographic features affect both the surface circulation
and local climate elements such as temperature and precipita-
tion. In this section I will describe some of the important
topographic features of the country and then summarize other

general controls of the climate of Iran.

Topography
Much of Iran is a plateau with high mountain belts

forming its borders. These mountain belts consist of high

30

31

continuous ranges on the North and West, but are made of
broken, isolated ranges in the Northeast, East, and Southeast.
The area enclosed by these belts is largely salty desert.

Both the northern and western ranges converge in the northwest
portion of the country and join in the extensive highlands of
Turkey (Fig. l).

The northern belt includes the west-east trending
highlands of Azarbaijan, the north-south range near the west
coast of the Caspian Sea, and the main Elburz Mountains. The
mean elevation of the northern mountain belt is more than 2500m
in Azarbaijan but increases to above 3000m in Elburz. To the
southeast of the Caspian Sea the elevation drops to 900-1500m
in the lowlands between the Elburz and the northeastern high-
lands. The northeastern highlands consist of parallel north-
east-southwest ranges with lowlands between them.

The western belt runs southward parallel to the border
between Iran and Turkey where it attains elevation above
2750m. To the south its elevation decreases to about 1500m
in the lowlands between the northwestern highlands to the
north and the main Zagros Mountains to the south. The Zagros
Mountains run southeastward to a point north of Bandar Abbas
and consist of several parallel ranges together more than
200km wide and 1000km long, attaining heights above 4500m (2).
The Zagros Mountains separate the interior deserts from the
Mesopotamian lowlands in the north and from the coastal areas

of the Persian Gulf in the south. To the east of Bandar

32

    
    
  
  

    
  
  
     

4 0 °
N
’1’ ’/ I
oemga" MashTIad
iflP/d! , / 3'"th .Sabzevar
0/ / rbat He darieh ,
///.T9hf3n4mnan ', /"T9’., y 35
Eda"
a 0 °

30
I / ’/
Bushehr //, ',

Iranshahr
.

      
   
   

Bandar Abbas

BgndEr Lengeh

25“

'I/j,1o.ooo - 14,000 feet . ,

FIGURE 1. Locational Map Of Iran With Selected Stations

33

Abbas, the mountain ranges attain relatively lower elevations
and more closely approach the coast. Just to the east of and
parallel with the Zagros lie a series of broken mountains
with elevations of more than 2750m separating such fertile
basins as the Esfahan from the deserts of central Iran. In
contrast to the North and West where the mountains are very
high, the eastern portion of the country consists of isolated
upland massifs with elevations not exceeding 3000m.

. The mountain ranges are very important to the climate
of Iran. The Elburz Mountains block the influx of moisture
from the Caspian Sea to the interior of the country just as
the Zagros Mountains block the eastward movement of moist air
from the Mediterranean Sea. As a result, regions to the north
of the Elburz Mountains and the western slopes of the Zagros
Mountains have a moist mild climate, while the interior is
desert. On the other hand, the northeastern lowlands allow
cold, dry Siberian air to enter the country, producing local
temperatures below 0°C. The mountains exercise their influ—
ence very significantly during early spring when the relatively
cold atmosphere warms in the lower layers because of increas-
ing isolation on the mountain slopes. Therefore, in the pre-
sence of moist air, convectional mountain showers develop (3).

This is clearly seen in the April maximum of Sanandaj (Fig.2).

Climate
Figure 2 shows the annual march of temperature and

precipitation for selected stations. The country experiences

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,.
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JFMA1M>JJAS-OIND JF-MAMQJAS‘IOND JFMAM'JJASOND
IRANSHAHR ' m'"
- 2w
1““.
I \
I \,
I’ ‘\. -'200
." \\ Source: mm of tho Mineralogical
I; '\ . 150 ngnniuflon oHrnn.
(I. ‘0
- 100
- 50
J FoMzA Mu-J‘AsooN-D J FDM A.MJ'J ASOINID

FIGURE 2. Mean Monthly Precipitation (in millimeter) and
Temperature (in centigrad) at Selected Stations.

35
cold winters in the highlands and warm summers in the low-

lands, particularly in the South. The highest mean monthly
temperature ranges from 25°C. in such northern stations as
Tabriz to as high as 39°C. at Iranshahr. In winter most of
the highland stations record sub zero temperatures, as at
Tabriz, while those with low elevation experience milder
weather. Except in the narrow coastal regions, January is
the coldest and July is the warmest month; in the coastal
regions the influence of the water bodies postpones the cold-
est and warmest months to February and August, respectively.
In general, almost all of the country experiences a
winter precipitation maximum, with little or no rain in summer.
More than two thirds of the country receives more than 50% of
its annual precipitation during winter (2). This percentage
decreases with an increase in latitude and altitude, and the
amount of precipitation decreases from northwest to southeast
(Fig. 2). Most stations show only one winter precipitation
maximum, whereas Ramsar on the Caspian Sea shows both a fall
maximum and a secondary one in winter; Iranshahr, in the south-
east, receives significant rain in July. The wettest part of
the country throughout the year is the coastal region of the
Caspian Sea. Even in winter, the wettest season, half of the
country receives less than 10cm; two thirds of it receives
less than 20cm annual precipitation. Therefore, most of the
country falls under the desert or steppe climates and only the

Caspian Sea region and western slopes of the Zagros Mountains

36
experience a kind of modified Mediterranean climate.

Within Iran the transitional seasons of spring and
fall are short and winter and summer dominate. In most of
the stations the transition period from summer to winter
is sharper than is the transition from winter to summer
(Fig. 2). Later in this section it will be shown that,
even in terms of circulation patterns, spring and fall are
not distinct seasons. Thus, I will review the circulation
patterns for only winter and summer.

Winter

During winter the westerly vortex of the Northern
Hemisphere expands, pushing the subtropical jet stream (STJ)
southward over the Persian Gulf to about 290 to 30°N, at the
200mb level (13). At this time the maximum winds are over
the Arabian peninsula, frequently placing Iran in the left
exit region where uplifting may be common (5). At the same
time, in the baroclinic westerlies, a deep mean trough
develops over the eastern Mediterranean Sea, producing a mid-
tropospheric southwesterly flow over Iran. As a result,
the speed'maxima and short wave troughs characteristic of
the polar front jet streams pass over Iran to bring Medi-
terranean cyclones to the country (4).

On the surface, the very intense thermal anticyclone
of central Asia (hereafter referred to as the Siberian
anticyclone) develops from the extreme cooling of that

large continent (6). This anticyclone extends over Iran and is

37

indeed the dominant weather control over the country.

Because it is a cold system, it is confined mostly
to the lower troposphere and drives very dry and cold polar
air from Siberia into Iran, resulting in clear skies and light
winds with very cold nights; daytime temperatures are relativ-
ely compensated by high solar radiation receipts. As this
air moves south and-southwestward it is modified by the
terrain and gains sensible heat. When it descends the south-
western slopes of the Zagros Mountains, particularly, it is
heated adiabatically and produces moderate temperatures over
the region. Its main access to the interior of Iran is through
the lowlands of the Northeast and it is accompanied by norther-
ly or northeasterly winds over the country.

As was mentioned before, this air mass is very dry
and produces very little, if any, precipitation except when
it passes over a considerably warmer water body, gaining both
heat and moisture in the lower layers. This modification
occurs over the Caspian Sea in late fall or early winter when
the water is much warmer than the overlying polar air, which
crosses the Caspian Sea and is destabilized by the acquisition
of heat and moisture. By the time it reaches the southwest
coast of the Caspian Sea it is relatively moist and highly un-
stable, causing the fall rainfall maximum of this region.

Even though the Siberian anticyclone is a thermally-
generated low level feature, its intensity, strength, and
exact location are affected by upper level circulation patterns.

For example, the superposition of upper level divergence over

38
the anticyclone reduces it to several weak cells, and the
westward movement of the Mediterranean cyclones forces
it to retreat (7).

The Mediterranean cyclones begin invading Iran in
late fall and continue until the end of spring, travelling
toward Iran in two main tracks (4). The northern track
crosses both the very northwestern part of the country and
adjacent coastal areas of the Caspian Sea. Cyclones
following this track appear in late fall and may be another
reason for the fall rainfall maximum of the Caspian Sea
region. The other track passes through the Kermanshah low-
lands and north of the Persian Gulf. By January, cyclones
affect all of the country. They yield maximum precipitation
west of the Zagros Mountains and carry little rain eastward
to the interior of the country (Fig. 2, Esfahan). Besides
the Mediterranean cyclones, others develop inside Iran to
the lee of the southern Zagros Mountains (4). These cyclones
are shallow and develop in late winter when the polar front
jet stream crosses over the Persian Gulf; the moisture source
accessible to these cyclones is the Persian Gulf and adjoin-
ing parts of the Indian Ocean.

The Mediterranean cyclones bring modified air masses
of both tropical and continental origin to Iran. The tropi-
cal air mass originates over north Africa and gains moisture
when passing over the Mediterranean Sea or the Red Sea. Still,

its moisture content is low and it may be associated with dust

39
storms over Arabia and occasionally over Iran. This air
mass usually lacks the leading warm front at the surface.
Often while passing over the Persian Gulf it gains mois-
ture and thus brings hot and moist air to southern Iran (8).
The most widespread air mass over Iran is the continental
air mass which follows the cold fronts associated with
Mediterranean cyclones (9). Many of these air masses
originate over the Atlantic Ocean and release their mois-
ture while crossing west Europe, but may acquire some
additional moisture when passing over the Mediterranean Sea
or Black Sea. These air masses enter Iran from the north-
west and produce almost all of the cold-period precipitation.
At first they have a distinct cold front, but as they move
east or southeastward they become warmer and lose their in-
itial characteristics so that when they reach southeastern
Iran they are hardly distinguishable, and the associated
cold front is seldom recognized (9).

The transition from the winter to summer circulation
pattern takes place so sharply (13) that spring is very V
short and only April could be designated as a spring month,
based on temperature (2). Spring is conspicuous, mainly in
highland regions of the Zagros Mountains where a spring
maximum in rainfall (Figure 2) can be recognized (Sanandaj).
By May the summer pattern is established all over the country.

Summer

During summer the subtropical jet stream has moved

northward over the Caspian Sea, placing Iran under the

40

anticyclonic region to the south of the jet stream. Because
of subsidence, the subtropical high pressure center develops,
prohibiting major uplifting in the warm period of the year
(10). This subsidence creates an inversion layer over Iran
which in some places (such as the south) may be as low as

1000 feet above mean sea level (9). The lower troposphere
warms because of high solar radiation and a shallow surface
low develops over west Pakistan and north of the Persian Gulf.
At the same time a high pressure center develops over the
Caspian Sea due to its colder temperatures. As a result,
surface winds blow from the north or northwest toward the low
center. One of these famous winds is the ”120 Days" wind of
Sistan which starts around May 21 and lasts for about four
months. This particular wind is best developed in the east-
ern and southeastern parts of the country. To the west of the
Zagros Mountains the winds flow toward the low center over the
Persian Gulf and are channeled by the Iraq lowlands, locally
they are called "Shamal" (northerly) winds (11). Sometimes
these winds are interrupted by the southwesterly winds blowing
toward the low center from Arabia. Through passage over the
Persian Gulf, these winds from Arabia gain moisture and bring
hot moist air to the Iranian coastal area, producing uncom-
fortable summer weather. These winds are thus called ”Simum"
(poison) (19). Although the moisture content of the air is
high and surface temperatures are adequate to support some
convection, instability is shallow because of the low level

inversion and seldom results in significant rain.

41

In general, during summer Iran is under the dry,
hot continental tropical air mass which originates over the
country itself. The aridity is widespread at this time ex-
cept in the coastal areas of the Caspian Sea. The coastal
areas are not completely under the subsidence, and conse-
quently, occasional mid-latitude disturbances invade the
area to produce precipitation. Thus, even in summer, this
area is the wettest part of the country.

Besides the continental tropical air mass originating
over the country itself, remnant monsoon air masses may affect
portions of eastern and southeastern Iran. Through its cir-
culation, surface low pressure located over southern Iran
deflects the Southwest monsoon away from the Iranian coasts
toward the west coast of India. This shallow, moist air
circles the low center through the Indian peninsula, and by
the time it reaches Iran from the east it has released its
moisture over India and is a relatively dry air mass resulting
in little rain. Even when this air is exceptionally moist, it
will seldom produce rain because of the inversion layer aloft;
only if a disturbance destroys the inversion will convectional
systems develop and produce significant precipitation. The
July precipitation of Iranshahr (Fig. 2) may be attributed to

moisture brought by the monsoon air mass.

 

 

10.

11.

12.

13.

LIST OF REFERENCES - CHAPTER 111

Fisher, W.B. 1971. The Middle East: A Physical, Social,
and Regional Geography. Methuen & Co. Ltdi, London.

Ganji, M.H. 1954. A Contribution to the Climatology of
Iran. Ph.D. Thesis, Clark University, Worcester,
Massachusetts.

 

. .Scorer, R.S. 1955. The Growth of Cumulus Over Mountains.

Arch. Met. Geoph. Biokl. Ser. A, 8:25-34

 

Alijani, B. 1979. Cyclone Tracks in Relation to the
Upper Flow Pattern in thg Middle East, December-
March 1964-67. M.A. Thesis, Michigan State Univer-
sity, East Lansing, Michigan.

Walker, J.M. 1967. Some Ideas on Winter Atmospheric
Processes Over Southwest Asia. Met. Mag., 96:161-67.

Gupta, B.R.D. 1972. Periodicity in the Average Daily
Strength of Siberian Anticyclone from November
67-April 68.

Pedgley, D.E. 1974. Winter and Spring Weather at Riyadh,
Saudi Arabia, Met. Mag., 103:225-36.

Khalili, A. 1971. On the origin of Southern Caspian
Sea Rainfall. Publ. of Iranian’Met.‘Dept., March
1971.

Snead, R.E. 1968. Weather Patterns in Southern West
Pakistan. Arch. Met. Geogh. Biokl., Ser. B. 16:
316-46.

Pedgley, D.E. 1970. A Heavy Rainstorm over North—
Western Arabia. Proced. S o. Tro 1. Met.,
Amer. Met. Org., June 2-11, I970.

Brice, W.J. 1966. South West Asia. University of
London Press, Lon on.

Ganji, M.H. 1968. Climate, in Fisher, W.B. (ed.), 1968,

The Cambridge History of Iran, Volume 1, The Land
3f‘lran, Cambridge’University Press, England.

 

Weickmann, L. 1961. Some Characteristics of the Sub-
tropical Jet Stream in thngiddle East and’Adjacent
Regions. Iran, Met. Dept., Tehran, Iran.

42

CHAPTER 4

METHODS

Data Sources

As discussed in the preceding chapter, precipitation
in Iran is variable both spatially and temporally. The objec-
tive of this study is to determine the origin of this precip-
itation in terms of both uplift mechanisms and moisture
sources. To accomplish this objective, daily total precipita-
tion records were obtained for forty stations for the period
1965-1969 from the Meteorological Organization of Iran; 1969
was the latest year used because it is the most recent year
for which the synoptic weather charts were available. Since
similar studies (1,2) have been carried out for five-year
periods in other regions, this duration of record was thought
to be adequate for this study as well. Also, the number of
selected stations was limited by their availability from the
Meteorological Organization of Iran. For the stations of Khoy,
Kashan, and Bandar Lengeh, only four years of precipitation
data were available, but since each day in this study is an
observation, the one-year difference between samples is accept-
able. All the selected stations are listed in Appendix 1 with
their geographic coordinates, elevation, and mean annual sur-
face pressure. The location of these stations is depicted in

Figure l.
43

44
Daily 12GMT (15:30 in Tehran) surface and 500mb

synoptic maps of the Northern Hemisphere were obtained on
microfilms for analysis from the National Climatic Center (3).
Missing 500mb-level microfilm data for the period November
l967-December 1969 were replaced with data obtained from 12GMT
500mb synoptic charts available on microfilm from the Michigan
Weather Service; the 700mb synoptic charts for moisture study
were also obtained there. Both of these synoptic charts were

published by the National Climatic Center.

Precipitation Mgchanisms

For the purpose of this study any day with total pre-
cipitation of 0.1 millimeter or more was considered one pre-
cipitation day and included in the analysis. This low thres-
hold was chosen because the initial objective was to determine
the uplift mechanism of any precipitation regardless of the
strength of that mechanism. Although more than one mechanism
might combine to produce a precipitation day, only the mechan-
ism detectable on the 12GMT synoptic maps was regarded as res-
ponsible for the day's total precipitation.

The mechanisms were classified according to the follow-
ing scheme:
1) Upper level disturbances. This type included upper level
short wave troughs embedded in the westerlies. Klein (4)
demonstrated the relationship between these short wave troughs

and the distribution of surface precipitation very well.

45

Precipitation days included in this type were those on which
the station under consideration was located in one of the
following situations:

a. Beneath the region of positive vorticity advection
(upper level divergence). This region was defined in this
study as the area between the trough axis and the next down-
stream ridge axis (5). The region of upper level divergence
produces uplifting and surface convergence through atmospheric
continuity (6).

b. Under an upper topospheric trough axis (4). In
this case the uplifting is triggered mostly by the advection
of cold air aloft which destabilizes the lower atmosphere.

c. Beneath upper cold lows, which most of the time
combine the effects of both a and b above. Upper cold lows
were defined as those having at least one closed contour line
at the 500mb level. A precipitation day was considered in
this category if the station under consideration was under a
small, cold low and was difficult to classify into either
category a or b.

2) Surface disturbances. This type included all eastward-
moving surface cyclones regardless of their surface fronts,
because most of the time these fronts are drawn subjectively.
Any precipitation day was classified into this type if the
station under consideration was located in one of the follow-
ing situations:

a. Within an estimated 100 miles on either side of

the surface cold front. At the cold front the cold air lifts

46
the warm air, resulting in precipitation along a narrow
strip (7, 8).

b. At or within about 400 miles ahead of the sur-
face warm front. Here the warm air overruns the cold air,
resulting in extensive precipitation over a broad area ahead
of the surface front (7, 9, 10).

c. Days in which it was difficult to determine the
precise frontal trigger were classified in a general cyclonic
group.

d. Frontless cyclones. These were the cyclones
surrounded by rather homogeneous air in which no fronts were
recognized but were still associated with precipitation.
Since surface disturbances are generated and guided by
features of upper level circulation (5), this category could
be combined with the first into one type. Nonetheless, the
separation was maintained because, first, upper level disturb-
ances may or may not stimulate surface cyclones, and second,
I was interested in determining the contribution of surface
disturbances regardless of their upper level support.

3) Sea effect. This mechanism occurs when very cold air
crosses a warm water body. In this process, the air gains
heat and moisture, and by the time it reaches the lee coast
it is destabilized, thus leading to uplift. In this study,
the conditions necessary for a precipitation event to be

placed in this category were as follows:

47

a. The surface wind flow must cross over the water
before reaching the station (the direction of the surface
wind was estimated to be about 300 to the right of the sur-
face isobars (11).

b. Air temperature on the lee shore should be at
least 15°F. higher than the windward shore, evidence of
modification from an overwater trajectory (sea temperatures
were not available).

c. No other uplift mechanism should be apparent.

Because sea effect does co-occur with other triggers on

 

occasion, this last criterion probably led to an underestima-
tion of its contribution near the Caspian Sea coast.

4) Surface heating. This type of uplift occurs when the air
in the lower layers of the atmosphere has been heated from
below to such a degree that autoconvection* develops. Those
precipitation days were considered in this category when no
other kind of uplift was evident and the temperature of the
station or its surrounding area was either equal to or more
than 80°F., an arbitrary threshold. Only those days on which
no other mechanism was apparent were placed in this category
and, as in the case of sea effect, the days in this group are
probably underrepresented, for this reason,since instability
from air mass heating often assists other uplife mechanisms.Be-
cause the temperatures above 80°F most often ocmu'mnjngxmrytmt
*When the lapse rate is more than 3.43C. per one hundred

meters, the air parcel rises without any upper level trigger;
this is called "autoconvection" (12).

48
summer days, all summer precipitation days with no other
lifting trigger were included in this type.
5) Indeterminable. All precipitation days not falling into
one of the above-mentioned types were considered indetermin-
able.

Orographic effects were not regarded as a separate
uplift mechanism here because the micro-environments of the
stations were not clearly known; most of the highland
stations were located in valley bottoms or flat areas surround-
ed by mountain ranges; and above all, orographic effects are
often enhancing and are seldom an independent lifting mech-

'anism (10).

Procedure of Map Analysis

Pre-constructed tables were used to record the mechan-
ism responsible for each precipitation day. Also, the date
and the amount of precipitation of each day were recorded in
these tables (Appendix 2). To determine the mechanism of
each day on the maps, the following steps were taken in this
order:
1) The 500mb maps were inspected to see whether the station
under consideration was affected by any category of upper level
disturbances. If so, the day was placed in this category.
2) The surface maps were then examined for the existence of
surface disturbances, regardless of the existence of upper
level disturbances. To determine whether the station fell

within the pre-set range of frontal boundaries, the distance

49
between the front and station under consideration was esti-
mated on the map by using the scale of the map.
3) If the event was still unclassified (in the absence of
any kind of disturbance) and the station was coastal, the
station was examined for sea effect.
4) Days in which none of the above mechanisms were evident and
days with temperature above 80°F were classified into the sur-
face heating type.
5) In the absence of all the above mechanisms the precipitation
day of the station was considered indeterminable.

For each type the mean annual number of precipitation
days falling into that type was computed and divided by the
mean number of all precipitation days of the respective
station. This ratio was then multiplied by 100 to get the
percentage of annual contribution of a specific mechanism to
the station's annual mean total of precipitation days. Because
of the sequence of steps followed in the analysis, days in
which a number of mechanisms co-occurred were automatically
placed in the first appropriate category. Thus, days of
multiple causation were placed in the upper level or surface
disturbance category, a procedure that introduced some bias
into the results but which was justified nevertheless because
it was impossible to determine the relative influence of each
mechanism during such situations from the data available.
Also, this procedure was useful because days ultimately placed

in the surface heating category included only those in which

 

 

 

50
other mechanisms had been discounted, providing an accurate

assessment of the role of unassisted convection.

Regionalization

In order to regionalize the country on the basis of
uplift mechanisms, Ward's method of hierarchical clustering
analysis was adopted (13,14). In this method, clustering
was based on the total sum of squared deviations of percent-
age contribution of each mechanism from the mean of the
clustered stations. In other words, at each step only those
individuals or clusters would be combined if their fusion
resulted in the least increase of the total sum of squared
deviations of percentage contribution. Ward named this
term the ”Error of Sum of Square” (ESS) which is computed

from the equation:

n

ESS 1:1 (Dix) 2 (l)

n

where D is the deviation of the value of an individual in
cluster 1 from the cluster mean (X), and n is the number of
members of cluster 1. For cases with more than one variable
this formula is repeated for each variable and the results
are summed together. Since this study included five vari-
ables (mechanisms), standardized scores. (Z-scores) were

computed from:

_ X-X
z - _Sd__ (2)

51
where X is the value of each observation for each variable,
X is the mean of observations for the same variable, and Sd
is the standard deviation.

By applying Ward's method the maximum similarity
between climatic stations was computed through the use of
the computer software program CLUSTAN, written for use on
Michigan State University's Cuber 750 computer. The input
data were the Z-score values of the five variables for all
stations. The variables were the percentage contribution to

the mean annual precipitation days by upper level disturb-

 

ances only; surface disturbances, with or without upper level
support; surface heating; upper level and surface disturb-
ances combined (westerly disturbances); and sea effect.

The output from program CLUSTAN included the within-
cluster dissimilarity coefficients; the lower the coefficient
the lower the total within-cluster ESS. The final clusters
were determined on the basis of these coefficients.

Because of reasons described in the next chapter,
the region-groups resulting from the multi-variable analysis
were rejected and the classification procedure proceeded on
the basis of a single variable, the predominant (most frequent)
uplift mechanism which was determined according to the percent-
age contribution of each mechanism to the station's annual pre-
cipitation days. In this procedure regionalization was

attempted through the use of the Z-scores of this one variable

 

52
far the stations. First, the spatial distribution and
clustering of the single-variable Z-scores were observed
on the map. Regional boundaries were then constructed
using interpolated isolines selected to provide the greatest
contrast between regions. The greatest contrast was defined
as maximum slope on the surface. The resultant regions
were tested by an Analysis of Variance test for their dis-
tinctiveness from each other (15) by using the following
equation:

F= Betwee_n-Group Mean Square of the Predotninant Mechanism' s Contribution
MGrouplifin18quare of fixaPr edafifiiiit WEEEEIEE's(kxur15ution

To study the regional and seasonal distribution of
the precipitation mechanisms, one representative station was
selected from each region using the following criteria. First,
the percentage contribution of the most frequent mechanism to
the annual precipitation days at the station had to be the
highest in the region; and second, the station was located, as
much as possible, in the geographic center of the region.
Finally, based on the percentage contribution of the mechanisms,
the regional and seasonal predominant mechanisms were deter-
mined. In addition, each year was divided into four seasons:
winter (January-March), spring (April-June), summer (July-

September), fall (October-December).

Importance of the Precipitation Mechanisms
The importance of the mechanisms was defined as their

percentage contribution to the annual or seasonal precipitation

53

total of the station. The higher the contribution, the more
important the mechanism was assumed to be. The importance of
the mechanisms was determined through the selection and
analysis of representative stations within the regions. At
each such station, the mean annual or seasonal precipitation
which occurred during the days of each mechanism was totaled
and then divided by the station's annual precipitation. By
multiplying this ratio by 100, the percentage contribution of

each mechanism was computed.

Moisture Sources

In the presence of atmospheric uplift mechanisms, the
intensity of precipitation depends on the amount of available
atmospheric moisture (16). The role of moisture in determining
precipitation yield from various triggers is particularly
important in an area such as Iran which is located far from
larger bodies of water. The second objective of this study,
after a determination of the precipitation mechanisms, was to
identify the moisture sources and advective trajectories and
their contribution to the country's precipitation. This objec-
tive involved analyzing all precipitation days with ten or more
millimeters of precipitation at representative stations on
12 GMT surface and 700mb weather charts for the period July
1967-December 1969.

More than half of the atmospheric moisture exists be-
low 850mb (17, 18), but since Iran is a plateau country, the

mean surface pressure of most of the stations is about 850mb

54

(Appendix 1). Thus, the surface and 850mb level can be
assumed to be roughly the same except in the narrow coastal
areas. Therefore, most of the moisture flux must occur at
levels about 850mb. For this reason the 700mb level, the
next lowest available level, was selected in this study in
agreement with other studies (19). In some situations, pre-
cipitation can be produced when a trigger acts on moist air
left by a previous weather event. Because such precipitation
is usually light, in this study only days with 10 millimeters
or more were chosen to increase the likelihood that the pre-
cipitation was associated with a distinct influx of moisture.
Since the 700mb weather charts were available for July 1967-
December 1969, the moisture analysis was limited to this
period at both the surface and the 700mb level.
Determining the Moisture Sources and Trajectories

On the surface maps the moisture trajectories and
sources were determined according to the surface wind flow.
In estimating trajectories, the mean surface wind direction
was assumed to be about 350 from the mean sea-level isobars
(ll).' To further support the final decision, the surface dew
point temperature gradient was checked to see whether the area
of higher dew point temperatures was located upwind from the
station. The moisture source was assumed to be the closest
body of water in the upwind direction. In this study the term
"water body" is applied only to water bodies not smaller than

the Persian Gulf.

55

On the 700mb maps the moisture trajectory was
assumed to be parallel to the height contours because at this
level the wind flows generally parallel to the contour lines.
The moisture trajectory of the station under consideration
was determined as being parallel to the contour lines passing
through station, and its source, as on the surface, was
assumed to be the closest body of water in the upwind direction
over which the contour lines pass.

The moisture source of the station for each study day
was determined by combining the surface and 700mb level

results. If the moisture sources indicated on the surface and s

 

700mb on a given day were different, a combined category was
recognized. If the source of one level was indeterminable,
however, the source indicated at the other level was assumed to
be the most important. Based on these computations, the
moisture trajectories were constructed and shown on the maps.
The moisture sources were classified on the basis of the pre-
cipitation days with identical sources both at the surface and
700mb level, and the days with combined sources were used as a
supplementary aid. The sources with highest percentage of con-
tribution were designated as the primary source and the sources
with lower contribution were secondary. For example, a station
having 20% of days with moisture influx from the Mediterranean
Sea, 25% of days from the Persian Gulf, and 30% of days from
both the Red Sea and the Persian Gulf was determined as having
a primary source from the Persian Gulf and a secondary source

from the Mediterranean Sea.

56

Because of a lack of adequate data, the magnitude
of moisture influx was not measured, so the significance
of the moisture sources was determined according to the areal
average of the mean annual precipitation of the regions; the
areal average was calculated by summing the annual mean pre-
cipitation of all stations and dividing by the number of
stations included in the region. The areal averages of all
regions were then added to yield the total of all the regional
means, which was then used to calculate the percentage contri-
bution of each region to the total of all the regions by the
formula:

Mean of each re ion

m “00
The grand total of all regional means was assumed to be a measure
of the total national precipitation provided by all regions.
Therefore, a region with higher percentage contribution to the
total provides a higher percentage of the country's total pre-
cipitation. Therefore, it can be assumed that the primary
source of a region with higher contribution to the national
total precipitation is more significant than the moisture source
of the region with lower contribution to the national total pre-
cipitation. Based on this conclusion the relative importance
of the moisture sources to the country's precipitation was

qualititatively evaluated.

Heavy Storms

The heaviest ten per cent of precipitation days studied

for moisture analysis were arbitrarily regarded as days with

57
heavy storms. For these days the uplift mechanisms and
moisture sources were analyzed to indicate the dominant syn-

optic features associated with these heavy storms over Iran.

LIST OF REFERENCES - CHAPTER IV

1. Shaw, 1962a. An Analysis of the Origin of Precipitation
in Northern England, 1956-60. Quart. J.R. Met. Soc.
88: 539-47.

2. Smithson, P.A. 1969. Regional Variations in the Synoptic
Origin of Rainfall Across Scotland. Scottish

Geographical Mag., 85: 182-95.

3. National Climatic Center. Daily Series Synoptic Weather
Maps: Part 1z Northern Hemisphere, Supface an
Millibar Charts. U. . Dept. Comer., NOAA,
AsheviIle, North Carolina.

4. Klein, W.H. 1948. Winter Precipitation as Related to
700mb Circulation, Bull. Amer. Met. Soc., 29:439-53.

 

5. Harman, J.R. 1971. Topospheric Wavgs, Jet Streams, and
United States Weather Patterns, Assoc. Amer. Geogr.
Res. Pap. No. 11, Washington, D.C

6. Palmen, E. and C.W. Newton. 1968. Atmos heric Circulation
Systems. Academic Press, New York.

7. Douglas, C.K.M. and J. Glasspool. 1947. Meteorological
conditions in Heavy Orographic Rainfall in the
British Isles. Quart. J.R.;Mgt. Soc. 73:11-43.

 

8. Browning, K.A. and T.W. Harrold. 1970. Air Motion and
Precipitation Growth at a Cold Front. Quart. J.R.
Met. Soc., 96:369-89.

9. Bjerknes, J. and H. Solberg. 1921. Meteorological Condi-
tions for the formation of rain. Geofys. Publ.,
Vol. 2, No. 3, 60pp.

10.Browning, K.A. and T.W. Harrold. 1969. Air Motion and
Precipitation growth in a Wave Depression. Qpar.
J.R. Met. Soc., 95:288-309.

11.Bannon, J.K. et al., 1961. The Flux of Water Vapor due to
the mean Winds and the Convergence of this Flux over
the Northern Hemisphere in January and July. Qpa .
J.R. Met. Soc., 87:502-11.

58

12.

13.

14.

15.

l6.

17.

18.

19.

59

Patton, P.C. et a1. 1974. Physical Geography. Duxbury
Press, Belmont, California.

Ward, J.H. 1963. Hierarchical Grouping to Optimize an
Objective Function. Ampr. Statis. Assoc . J., -
236-44.

Johnston, R.J. 1976. Classification in Geography,
CATMOG, Series No; 6

Till, R. 1974. Statistical Methods for the Earth Science.
Macmillan.

Miller, J.E. 1955. Intensification of Precipitation by
Differential Advection. J. Met., 12: 472-77.

Newell, R.E., J.W. Kidson, D.G. Vincent, and G.J. Boer,
1972. The General Circulation of the tropical
Atmos here and Interactions wifh Extratropical
Latitudes, Volume 1. The Massachusetts Institute of

Technolog , U.S.A.

Changon, S.A. Jr. and F.A. Huff. 1980. Review of Illinois
Summer Precipitation Conditions. I1 1. tate ater
Survey, Urbana, Bull. 64

 

 

Ramaswamy, C. 1965. On a Remarkable Case of Dynamical
and Physical Interaction Between Middle and Low
Latitude, Weather Systems in Iran. Ind. J. Met.
Geoph. 16: 177-200.

CHAPTER V

RESULTS

Introduction

As was mentioned previously, the occurrence of pre—
cipitation depends upon the presence of an uplift mechanism
and the availability of moist air. The uplift mechanisms
affecting Iran recognized in this study included upper level
disturbances,* surface disturbances, surface heating, and sea
effect. Since daily 12 GMT weather maps were available for
data analysis, each precipitation day at each station was
attributed to at least one precipitation-forming mechanism.
The first section of this chapter is devoted to a description
of the spatial distribution of precipitation days over Iran.
This is done to summarize the distribution of the precipita-
tion days, and because the contribution of each mechanism was

computed from these data.

 

*

The category "upper level disturbances" always designates
upper level disturbances without any surface disturbance, un-
less otherwise specified.

60

61

However, since the main purpose of this study was
to analyze the synoptic origin of the precipitation over
the country and to determine its moisture sources, the other
sections of this chapter presents l) the percentage contribu-
tion of each type of uplift mechanism, 2) the regions derived
from these frequency contributions, 3) seasonal distribution
of uplift mechanisms within each region, 4) the efficiency of
the uplift mechanisms in terms of their percentage contribu-
tion to the regions precipitation amounts, 5) the moisture
source of each region, and 6) the synoptic conditions associ-

ated with the very heavy precipitation days within each region.

 

Annual Precipitation Days

The number of annual precipitation days for each
station was averaged for the study period (Figure 3). This map
provides a picture of the spatial distribution of all precipi-
tation forming mechanisms over the country. The number was
the highest on the coastal regions of the Caspian Sea, where
the mean annual number was 133 days in Anzali but decreased
to 101 days at Gorgan on the east coast. Outside the Caspian
region the only comparable area was the Northwest, which is
represented by Khoy with 101 days per annum. To the south,
the number of precipitation days diminished but still remained
above 80 days per year in the northwest and in the western
slope of the northern Zagros Mountains.

The Zagros Mountains and the northeastern sections of

the country, including the intervening foothills of the

62

§§ «a
Q)
‘5
«‘3
3° '0
- ‘9
'9

.9

   

C}

FIGURE 3. Mean annual precipitation days (1965-69)

SOURCE: Iranian Meteorological Organization

63

Elburz Mountains, all receive about the same range of annual
precipitation days, varying from 38 days in Birjand to 65 days
in Tehran. This portion of the country separates the highland
regions to the north and west from the lowlying interior
deserts. The deserts experienced fewer than 20 precipitation
days annually; however, precipitation days increased again to
the south (Figure 3).

In general, as is seen in this map, the annual number
of precipitation days (precipitation mechanisms) decreased
from north to south. A decreasing trend existed also in a
west-to-east direction but with a lesser gradient, and it is
marked by two outstanding patterns: the Caspian Sea showed an
area of very high values, whereas,in the South, the central

deserts developed a trough of relatively lower values.

Precipitation Mechanisms

 

Introduction

As was mentioned in the previous section, each precipi-
tation day was regarded as the result of one precipitation
mechanism depicted on the 12GMT surface and 500mb weather maps.
The percentage of the mean annual precipitation days at each
station, caused by each type, is shown in Figures 4, 5, 6 and 7.
Upper level disturbances caused the highest percentage of
annual precipitation days, followed by surface disturbances
(Table l). The sea effect was observed only in the coastal
stations of the Caspian Sea (Figure 7). The percentage contri-

bution by upper level disturbances showed the least degree of

64

 

Mechanisms Contribution Coefficient
of variation
% %
Upper level disturb- 55 15
ances
Surface disturbances 18.5 28
Surface heating 7.5 68
Sea effect* 13
Indeterminable 14

 

*
Only for Anzali and Ramsar

TABLE 1. Percentage contribution by mechanisms to mean
annual precipitation days, averaged for all

selected stations (1965-69).

65
spatial variation, while contribution by surface heating dis-
played the highest degree of spatial fluctuation (Table 1).
Upper Level Disturbances

This category included all upper level disturbances
without any surface features. Eighty-one percent of upper
level disturbances consisted of cases with upper level diver-
gence (positive vorticity advection), whereas troughs aloft
contributed 17% of the disturbances (Table 2). Uplift caused
by upper cold lows was numerically least important. All three
components of the upper level disturbance category decreased
from north to south and from winter to summer. An important
result is the complete absence of upper c01d lows at Bandar
Abbas and Iranshahr and their absence at all other stations in
the second half of the year. Upper level divergence and
trough axes aloft were most common in winter, whereas spring
produced more upper cold lows, especially at Ramsar, which had
the highest annual number of cold lows and total incidents of
upper level divergence among all stations.

The annual contribution of upper level disturbances at
each station was averaged for the study period. According to
Figure 4, the percentage contribution of upper level disturb-
ances decreased from south to north. For instance, in the
south-central portions of the county, more than 80% of the
precipitation days were caused by upper level disturbances,
whereas in the southwest corner of the Caspian Sea this value

fell below 40%. The maximum contribution was located in the

66

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67

 

FIGURE 4. Percentage of mean annual precipitation days caused
by upper level disturbances with no surface component.

68
South along the Persian Gulf coast with a secondary maximum
in the northeastern part of the country. In the Northwest,
the center of the lowest contribution was over the south-
western coast of the Caspian Sea and extended southward over
the northern portions of the Zagros Mountains and eastward
over the Elburz Mountains. Generally, along parallels the
values decreased from east to west.
Surface Disturbances

This type included all mid-latitude cyclones with or
without frontal systems. Table 3 shows the different con-
stituents of the surface disturbances at selected stations.
0n the annual basis, cyclones with fronts outnumbered front-
less lows over the entire country. On the whole, 64% of the days
with surface disturbances during the study period experienced
cyclonic activity with surface fronts, whereas in 35% of the
days the disturbances had no surface front.

On a seasonal basis, summer lacked any cyclonic activ-
ity over the country and, during the transitional seasons of
spring and fall, no surface disturbance migrated south of 32°N.
Cyclonic activity maximized in winter but was high during
fall also over the entire country. Although spring experi-
enced less cyclonic activity than winter, it was the only
season during which frontless lows outnumbered cyclones with
fronts. These data reveal that surface disturbances were most

frequent during the cold season over northern Iran while their
occurrence in the southern region was restricted to only the

winter.

69

 

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70

Further breakdown of the cyclones with fronts (Table 4) reveal—
ed that 73% were cold fronts, whereas only 24% of them were
cyclones with both frontal systems. Most of the cyclones
affected the northern parts of the country, whereas to the
south of 32°N only cold fronts were dominant, during winter
only (Table 3).

The percentage contribution of surface disturbances of
all kinds was averaged for the study period on an annual
basis for each station (Figure 5). In contrast to upper level
disturbances, the maximum contribution by surface disturbances
occurred along the 32°N parallel. The highest contribution
(30%) was observed at Shahr Kord. On Figure 5, three separate
zones of more than 20% contribution are found to the north of
30°N latitude. These zones include the western highlands of
Azarbaijan, the western slopes of the Zagros Mountains in the
Southwest, and northeastern sections to the north of the cen-
tral deserts. Both the northern and the southern parts of the
country experienced relatively lower values, particularly in
the Southeast, where the values fell below 10% (1% at Jask).
The central trough of the isolines with values below 20% ex-
tended generally in a northwest-to-southeast direction. Com-
parison of this map with Figure 4 reveals that, in the South,
the primary maximum of upper level disturbances coincided
nearly with the minimum of surface disturbances, but in the

northern parts both mechanisms showed low contributions.

71

 

Station Cyclones Cold Warm Occluded Stationary
with front front front front
fronts

Iranshahr 5

Bandar

Abbas 1 7

Shahr

Kord 13 26 1

Torbat

Heydarieh 13 25 2

Zanjan 4 39 1 2 l

Ramsar 12 28

Mean 7 27 l

% of all

frontal 24 73 2

disturbances

 

TABLE 4. Total number of precipitation days caused by differ-
ent types of surface frontal disturbances at
selected stations (1965-69).

72

 

FIGURE 5. Percentage of mean annual precipitation days
caused by surface disturbances.

73

Surface Heating
This type included those days when no other uplift

mechanism was observed and the surface temperature remained
over 80°F at or near the station. Since in some cases sur-
face heating contributed to precipitation concurrently with
upper level or surface disturbances and was therefore in-
advertently included in those types, the surface heating
category may underrepresent the number of qualified days.

In this study this category includes the days when only sur-
face heating appears to have been the primary cause of pre-
cipitation. This process occurs only when the air aloft is
unstable, without interference from subsidence or temperature
inversions. Therefore, occurrence of this type indirectly
indicated the absence of atmospheric subsidence or an inver-
sion which, otherwise, would not permit convection to proceed
to the point of producing precipitation.

Figure 6 shows the spatial distribution of the per-
centage contribution of the surface heating process over the
country. It is apparent from this map that the highest values
were aligned in a northwest-southeast direction, with the
primary maximum over the southeasternmost part of the country.
In the North, the southeastern coast of the Caspian Sea com-
prised the secondary maximum. Areas of least contribution
were located in the northeast and southwest extremes of the
country where the contribution of upper level disturbances was
at a maximum (Figure 4). For example, no convective precipi-

tation was detected during the study period at Bushehr on the

74

 

FIGURE 6. Percentage of mean annual precipitation days
caused by surface heating.

75

Persian Gulf coast.
Sea Effect

This type was defined as those uplift processes which
resulted from the passage of cold air over the warm water
surfaces and, in this study, was found to be confined to the
southern coast of the Caspian Sea (Figure 7). In this
region, cold Siberian air gained heat and moisture through
its passage over the Caspian Sea and by the time it reached
the Iranian coast became unstable and caused precipitation.
The maximum concentration of this activity occurred specific-
ally on the southwestern coast of the Caspian Sea and
decreased eastward. The weather charts of the days with sea
effect type all showed an anticyclone centered to the north
of the Caspian Sea, which caused the northerly, northeasterly,
or easterly winds over the sea and carried moist, unstable
air to the southwestern coast.
Eggions of Precipitation Mechanisms

One of the objectives of this study was to regional-
ize the country on the basis of the dominant uplift mechanisms.
A hierarchical clustering method was used with "ward's"
least-squared distance algorithm as the measure of dissimilar-
ity between observations. Five clusters were subjectively
chosen to regionalize the country. This scheme ultimately
was not found suitable for this study because

the resulting regions were often fragmented and conveyed

76

 

FIGURE 7. Percentage of mean annual precipitation days
caused by sea effect.

77

confusing information. For example, cluster 5 (dominated by
upper level disturbances) is interrupted by cluster 1 (dominated
by surface disturbances); such a pattern is climatologically
incoherent. Nonetheless, the clusterings are shown in Figure 8
and are presented statistically in Figure 9:

l) The stations on the northeastern and southwestern
parts of the country were clustered together. This cluster
is unique because of the-relatively large contribution by
surface disturbances; surface heating was the least important
uplift mechanism to be recognized in this cluster.

2) The extreme northeastern and northwestern areas to-
gether with the south central parts of the country made up the
second cluster; no single uplift mechanism accounted for most
of the precipitation days.

3) The southeasternmost part of the country and the
southeast Caspian coast, along with some other scattered areas,
comprise the third cluster. This cluster is well distinguished
because of a contribution by surface heating; westerly dis-
turbances were least important in this cluster.

4) The fourth cluster is located mainly over the lands
to the southwest of the Caspian Sea with a few outliers in the
other areas. This cluster is characterized by the lowest
contribution by upper level disturbances.

5) Only two stations in the South are included in the
fifth cluster, which is distinguished by a strong contribu-

tion by upper level disturbances.

78

 

 

 

 

 

 

 

 

 

 

 

 

  

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.,

hanisms based on

cluster analysis and using all uplift mechanisms.

10D. mec

Regions of precipitat

FIGURE 8.

‘to.

Cluster 1

 

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Cluster 3

 

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FIGURE 9.

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Mean Z-scores of the percentage contribution by
uplift mechanisms to annual precipitation days

for each cluster.

80

In order to produce contiguous and more understand-
able regions, further regionalization was based on analysis
of the predominant uplift mechanisms. According to Table 1,
the highest percentage of annual precipitation days was
caused by upper level disturbances (55%), and surface dis-
turbances (18%) were second in importance. Since any travel-
ing surface disturbance is associated with an upper level
disturbance, both the surface and upper level disturbances
‘were combined into one group. As a result, this combined
type (westerly disturbances) was by far the most predominant
uplift mechanism, reflecting the dominance of westerly dis-
turbances, and contributing 73% to the country's annual pre-
cipitation days (Table 1). For the purpose of regionaliza-
tion the Z-scores of the percentage contribution of this com-
bined category for the stations were computed and are shown
in Figure 10 (their numerical percentages are listed in
Appendix 1); the Z-scores best demonstrated the spatial
variation of percentage contribution of this combined cate-
gory among the stations. Spatial clustering can be interpreted
from the isoline patterns, with the highest positive deviation
from.the mean percentage contribution by the predominant mech-
anism.on the coastal areas of the Persian Gulf and the highest
negative deviation on the southwest corner of the Caspian Sea.
The isoline +1 in the South and the isoline -l.5 in the North
were arbitrarily chosen to distinguish these extremes:firmndufir
smomtdings. In the northern parts of the country all the north-

wesmamihhaflamdsankltheb:contuuuufioniemnmmrd,:uxfludfiugtheemmtennlmflf

81

 

FIGURE 10. Z-score values of percentage contribution made by
upper level disturbances, with or without surface
disturbances, to the mean annual precipitation days.

82

of the Caspian Sea coastal region, are separated by the iso-
line 0.00 from those areas to the south. In the central
portions of the country there are two secondary maxima in

the Northeast and Southwest. Values decrease toward

central Iran from these maxima. A zone of lower values pass-
ing through Kashan, Yazd, and Bam separates these two maxima
from each other. The final regions are shown in Figure 11
and are as follows:

1) Caspian region. This region encompasses the coastal
areas of the western Caspian Sea. It led all other regions
in terms of the annual precipitation days (Table 5), with
precipitation occurring on approximately a third of the days.
Upper level disturbances were the dominant mechanism in the
region with 45% contribution (Table 5). Nevertheless, this
value was the lowest among the regions. Sea effect was
second to upper level disturbances in causing the precipita-
tion days (13%), whereas surface heating (9%) was the least
important mechanism.

2) Northwestern region. This region covers all the
northwestern highlands, the Elburz Mountains with their south-
ern foothills, and the eastern half of the Caspian Sea coast.
It was second to the Caspian region in terms of annual pre-
cipitation days (Table 5) and 49% of its precipitation days
were attributed to upper level disturbances, whereas surface
disturbances accounted for 20%. Surface disturbances
affected both northern and southern parts of the region more

than the central part (Figure 5), whereas the central areas

83

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FIGURE 11. Regions of precipitation mechanism based on the
percentage contribution of the predominant uplift
‘mechanimm to the mean annual precipitation days.

84

had a greater contribution from surface heating.

3) Southwestern region. This region consists of al-
most all the Zagros Mountains and the lowlands to their west
except the coastal areas of the Persian Gulf. Mean annual
precipitation days were less than in either the Caspian
region or the northwestern region (Table 5). Upper level
disturbances were associated with the highest percentage of
the precipitation days (57%), and surface disturbances were
more important here than elsewhere, too. Contribution by
surface heating was very low (4%).

4) Northeastern region. This region includes the
lands to the south of the Elburz Mountains and east of the
Zagros Mountains up to the border of Afghanistan. The region
includes all the central deserts of Iran. The highest per-
centage of the precipitation days was caused by upper level
disturbances (Table 5). The 20% contribution by surface dis-
turbances was second only to the southwestern region. Like
the latter region, surface heating contributed a very low
percentage of days.

5) Southern region. The coastal lowlands of the
Persian Gulf and the western half of the Gulf of Oman make up
this region. Upper level disturbances accounted for a higher
percentage (70%) of days here than in any other region, but
the contribution by surface disturbances was very low (2%).
Despite its very southerly location, surface disturbances

contributed 16% to the region's annual precipitation days.

85

Regional Mean

 

% of precipitation days caused by

 

 

Region Number Upper Surface Surface Sea
of level disturb- heating effect
annual disturb- ances
precip- ances
itation
days
Southeastern 16 60 10 11
Southern ' 18 72 16 2
Southwestern 48 57 27 4
Northeastern 34 59 20 4.5
Northwestern 78 49 20 9.5
Caspian 126 45 12.5' 9 l3

 

TABLE 5. Mean regional number of annual precipitation days
and the percentage contribution by each mechanism
1965-69 .

86

6) Southeastern region. The southeasternmost part
of the country, with the lowest number of precipitation
days (15 days), is included in this region. It was the
leading region in the percentage contribution made by sur-
face heating (14%), but, at the same time, surface disturb-
ances contributed the least to its annual precipitation
days compared with all other regions (Table 5). Still, like
other regions, upper level disturbances were the most fre-
quent trigger among the various mechanisms.
Seasonal Variation of Precipitation Mechanisms

To study the seasonal variation of precipitation
mechanisms, one representative station was selected for each
region (see Table 6). Among all seasons, winter had the
highest number of precipitation days (Figure 12). The per-
centage contributionof this season to annual totals was
highest in the southern and southeastern regions but decreased
northward until it fell to 29% in the Caspian region. Summer
was almost rainless in the southern, southwestern, and north-
eastern regions. The highest number of summer precipitation
days was observed in the Caspian region where the seasonal
range in the number of such days was lowest. Autumn contri-
buted the lowest percentage to annual totals in the south-
eastern and southern regions, where the seasonal value fell
as low as 7%. In other regions, fall contributed about one-
third of the annual precipitation days. Greatest contribu-
tion was in the western highlands (the northwestern and

southwestern regions). Like fall, spring contributed lower

 

87

Region Representative

Station
Southeastern Iranshahr
Southern Bandar Abbas
Southwestern Shahr Kord
Northeastern Torbat Heydarieh
Northwestern Zanjan
Caspian Ramsar

 

TABLE 6. Representative stations of
the regions.

 

88

 

 

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Percentage contribution by season to mean annual
precipitation days at selected stations.

FIGURE 12.

89

percentages to the South and the North.’ The spring contri-
bution to the annual precipitation days was greatest in the
northwestern region. Figure 13 depicts the seasonal import-
ance of the mechanisms by region.
Winter

The predominant cause of precipitation throughout
the country, except in the northwestern and southwestern
regions, was upper level disturbances, although this pre-
dominance decreased from south to north. For instance, this
‘mechanism accounted for more than 70%.at Iranshahr and Bandar
Abbas but only 46% at Ramsar. In these regions, surface dis-
turbances were of second greatest importance; indeed, their
role in the southeastern region was substantial (69%). In
the northwestern and southwestern regions the contribution
by surface disturbances was equal to, or higher than, that by
upper level disturbances. No contribution by surface heating
was observed in this season except in the southeastern region
where it accounted for 14% of the precipitation days at
Iranshahr. In general, during the winter season upper level
disturbances were predominant in the Caspian and southeastern
regions, but in the remaining parts of the country surface
disturbances were the leading mechanism.
Spring

In contrast to winter, the most frequent uplift mech-
anism in spring in all regions was upper level disturbances

(Figure 13); their dominance ranged from 89% at Bandar Abbas

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91

to 51% at Zanjan in the northwest. Surface disturbances
were absent in the southern and southeastern regions and
played a secondary role in the southeastern and north-
eastern regions , with 18% contribution at Shahr Kord and
13% at Torbat Heydarieh. They made even less contribution
in the northwestern and Caspian regions, with 8% at Zanjan
and 7% at Ramsar. The northwestern region was unique in
that surface heating accounted for up to 33% of precipita-
tion days. This region was followed by the southeastern
region (22% at Iranshahr). In other regions, the contribu-
tion by surface heating was generally low and, in some
places such as Shahr Kord, was negligible.
Summer

Surface disturbances were unimportant over much of
the country, and upper level disturbances were the most fre-
quent only in the northeastern region. Over the rest of the
country surface heating was the most frequent mechanism
(Figure 13); however, the difference between the contributions
of upper level disturbances and surface heating was not sig-
nificant in the Caspian and northwestern regions.
Fall

Upper level disturbances were the predominant mech-
anism over the entire country, as they were in spring. Their
contribution was as high as 89% at Bandar Abbas in the
southern region but fell to 43% at Ramsar on the Caspian

coast. Except for the southern region, all the country was

92

affected by some surface disturbances. The highest contri-
bution was 25% at Iranshahr in the southeastern region
followed by 21% at Torbat Heydarieh in the Northeast.

In this season, the Caspian region benefitted
also from sea effect. This process accounted for 20% of the
region's fall precipitation days (Ramsar, Figure 13). Sur-

face heating was not important anywhere in the country.

Summary

The predominant seasonal precipitation mechanisms
of the regions, based on their contribution to the annual
number of precipitation days, are shown in Table 7. The
most frequent uplift mechanism in the transitional seasons of
spring and fall over the entire country was upper level dis-
turbances. During the summer, surface heating was important
except in the northeastern region where upper level disturb-
ances were significant. In winter, upper level disturbances
predominated in the southern, northeastern, southeastern,
and Caspian regions, but surface disturbances were dominant
in the southwestern region. The northwestern region was

equally affected by both surface and upper level disturbances.

Importance Of PrecipitationgMgchanismp
Introduction
The importance of a precipitation mechanism is
assessed according to its contribution to a station's annual

or seasonal precipitation total, rather than frequency, and

93

 

 

 

Mechanisms
Region Winter Spring Summer Fall
Southeastern upper upper surface upper
level level heat1ng level
disturb- disturb- disturb-
ances ances ances
southern II II ' H H II II I! ll
Southwestern surface " " " " " "
disturb-
ances
Northeastern upper " " upper " "
level level
disturb— disturb-
ances ances
Northwestern surface " " surface " "
disturb- heating
ances
Caspian upper H H II H H II
level
disturb-
ances
TABLE 7. Predominant (most frequent) precipitation mechanisms

of the regions, by seasons.

94

these results will be described in detail later. However,
before moving into this discussion I will describe the
spatial variation of the annual precipitation amounts over
the country to give the reader a general understanding of
the precipitation pattern.
Annual Precipitation

Mean annual precipitation for the study period is
shown in Figure 14, which depicts a highly diverse pattern
across the country. For instance, the southwestern coast
of the Caspian Sea, the region of highest precipitation,
received more than 1700mm annual precipitation, whereas the
central deserts of the country were driest with annual pre-
cipitation of less than 100mm. The northern slopes of the
Elburz Mountains received annual mean precipitation as much
as four times that of the southern slopes. The northwestern
highlands of the country received mean totals between 300mm
and 600mm, whereas the highlands of the northeastern portion
of the country had values below 300mm. The overall precipi-
tation pattern of the country can be portrayed as follows:
Caspian Sea area, 600mm to 1800mm; northwestern parts, 300mm
to 600mm; northeastern section, 200mm to 300mm; southern
coastal strip, 100mm to 200mm; and central deserts, below

100mm.

Annual Importance of Precipitation Mechanisms
The annual contribution of each mechanism to the total

prafipimnimn of the representative stations was determined

95

 

FIGURE 14. Mean Annual precipitation in millimeters (1965-69)
SOURCE: Iranian Meteorological Organization.

96
(Figure 15). Upper level disturbances contributed the high-
est percentage to the annual precipitation (Figure 15),
with values ranging from 46% at Shahr Kord in the southwestern
region to 67% at Bandar Abbas in the southern region. Surface
disturbances were the second most important mechanism overall
except in the southern region where surface heating was second.
The highest contribution by surface disturbances was at
Shahr Kard (40%), whereas Ramsar received the lowest contribu-
tion (11%). Surface heating was almost inconsequential in the
southern, southwestern, and northeastern regions, but contri-
buted up to 18% in the southeastern region. Contribution by
sea effect was observed only at Ramsar (13%). The mean contri-
bution to the country's annual precipitation by each mechanism
was computed from the representative stations as follows:
upper level disturbances 57%, surface disturbances 26%, and

surface heating 7%.

Seasonal Importance of Precipitation Mgghanisms

To assess the seasonal importance of the precipitation
mechanisms, the percentage of annual precipitation contributed
by each season was computed for the representative stations
(Figure 16). Winter provided more than half of the country's
annual precipitation, except in the northwestern and Caspian
regions. Winter precipitation constituted 84% of the annual
precipitation at Bandar Abbas in the southern region but only

24% at Ramsar in the Caspian region.

97

£14044!
”“4845

 

 

1234 1234 1234 234 1234 12"3‘

FIGURE 15. Percentage contribution by mechanisms to mean
annual precipitation at selected stations.
Columns are as follows: 1. Upper level dis-
turbance, 2. Surface disturbances, 3. Surface
heating, 4. Sea effect.

98

 

 

 

 

1%-
go-
1'
so- In
3 S
0
° 57
Q Q 1‘
70- v 3 E a Q E
5 g T J g
«- 2 g
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”'-
96 5°-
”'1
so— '
20—
10- l
o-WSPSF WSPSF -SPSF SPSF WSPSF WSPSF WSPSF

 

 

FIGURE 16. Percentage contribution by season to mean
annual precipitation at selected stations.

99

Spring was the driest season in the southeastern
and Caspian regions (5% of the annual in the former and 15%
in the latter). Spring contributed less than did winter in
the northeastern region but equalled winter in the north-
western region (31% for both at Zanjan).

Summer saw almost no rain in the southern, south-
western, and northeastern regions. Greatest summer precipi-
tation was observed in the Caspian region where about 20% of
the region's annual precipitation occurred. This region was
followed by the southeastern region where summer accounted for
19% of the annual precipitation, but only 6% in the north-
western region.

Autumn accounted for much of the annual precipitation
in the Caspian region (43% at Ramsar), but only 8% at Iranshahr
in the southeastern region.

For the country as a whole, winter accounted for more
than half of the annual precipitation. This season was
followed by fall, with summer providing the least precipita-

tion over the entire country.

Winter:

Surface disturbances provided the highest proportion
of the season's precipitation in the northwestern and south-
western regions (55% at Shahr Kord and 47% at Zanjan). Yet,
in the other parts of the country, upper level disturbances
were the most important mechanisms (Figure 17). These disturb-

ances contributed 59% of the seasonal precipitation at

 

100

     
 

 

 

 

 

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101
Iranshahr. The contribution by surface heating was not

important over any part of the country.

Spring:

The most important uplift mechanism was upper level
disturbances (Figure 17). They contributed the highest per-
centage in the southeastern region (87%), with 56% in the
southwestern region. The contribution by surface disturbances
was highest at Shahr Kord (36%), but they were absent in the
southern and southeastern regions. In other regions their
contribution was not significant. Surface heating produced
relatively high amounts of seasonal precipitation in the
southern regions (36% at Bandar Abbas), and also in the north-
western region (17% at Zanjan), This contribution was very

low in the remaining regions.

Summer:

During this season, the southern half of Iran received
all of its seasonal precipitation, if any, from surface heat-
ing (see Bandar Abbas, Iranshahr, and Shahr Kord in Figure 17).
In the northern half of the country, with the exception of the
Caspian region, upper level disturbances produced more than
half of the season's precipitation (65% at Torbat Heydarieh and
55% at Zanjan); the remainder of the precipitation in the North
was the result of either surface heating or other mechanisms
undetermined by this study. Both upper level disturbances
and surface heating produced almost equal amounts of precipita-

tion in the Caspian region, 45% and 49% respectively. Only

102

this region received a measurable percentage (3%) of its
precipitation from surface disturbances, which were completely

absent from the rest of the country.

Fall:

Fall was the season of upper level disturbances
over all the country. .Their contribution to the season's
precipitation varied from 100% at Iranshahr to 41% at Ramsar
(Figure 17). Surface disturbances ranked second to upper
level disturbances outside the Caspian region and achieved
their highest percentage in the southeastern region. In the
Caspian region, the mechanism second in importance to upper
level disturbances was sea effect, which accounted for 29% of
the region‘s autumn precipitation. The contribution by surface

heating was slight over the entire country.

Summary

Seasonally important uplift mechanisms of the regions
are listed in Table 8. Upper level disturbances were the most
important mechanism during the transitional seasons over all
the country. In winter, except in the northwestern and south-
western regions where surface disturbances were the most
important, upper level disturbances accounted for over half of
the seasonal precipitation. Summer was the only season when
surface heating was the most important mechanism, except in the
northeastern region where upper level disturbances continued to

be the most important.

 

103

 

 

 

Mechanisms
Region Winter Spring Summer Fall
Southeastern upper upper surface upper
level level heat1ng level
disturb- disturb- disturb-
ances ances ances
southern II H II II I! I! H H
Southwestern surface " " " ” " ”
disturb-
ances
Northeastern upper " " upper ” "
level level
disturb- disturb-
ances ances
Northwestern surface " " " " " "
disturb-
ances
Caspian upper ” " surface " "
level heating
disturb-
ances
TABLE 8. Most important precipitation producing mechanisms,

by season.

104
Regional and Seasonal Summary

I have presented the findings of this study regard-
ing the contribution of the mechanisms to the annual and
seasonal precipitation over Iran. In this section, I will
summarize the importance of these mechanisms in each region

by seasons (Table 9).

Caspian Region:

This region received maximum precipitation during
fall, while the remainder of the precipitation is distributed
rather equally among other seasons. During the colder half
of the year, upper level disturbances were the most important
and frequent mechanism, but, in the warmer period of the year,

surface disturbances were most frequent.

Northwestern Region:

Most of the annual precipitation occurred during the
winter and spring seasons. Summer was the driest season of the
year. Surface disturbances were the most frequent and import-
ant uplift mechanism in winter, but upper level disturbances

prevailed through the other seasons.

Northeastern Region:

In this region summer precipitation was negligible
and winter was the rainiest season. The most frequent and
important mechanism was upper level disturbances throughout the

year.

105

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106

Smmhmmtaleqfion:
andhce<fistuflxumesvmxe fluamom:frammmtanm.flmannm
maflmmisn in winter. The transitional seasons were character-

ized by upper level disturbances. Summer was almost rainless.

Southern Region:

Winter was the rainiest season of the year and summer
was the driest and nearly rainless season. Precipitation in
the summer, if any, was caused by surface heating. In other
seasons, upper level disturbances were the most frequent and

important mechanisms.

Southeastern Region:

The rainiest season of the year was winter, whereas
autumn was the driest season. The most frequent and important
precipitation mechanism was upper level disturbances through
the cold period of the year and surface heating through the

summer .

Moisture Sources

As mentioned earlier, the two important factors res-
ponsible for precipitation are an uplift mechanism and an
adequate amount of moist air. In previous sections of this
chapter, mechanisms contributing to Iran's precipitation were
described. In this section, I identify the principal moisture
trajectories and sources of moist air involved in this preci-
pication. For this purpose, precipitation days with amounts of

10mm or more at the representative stations were analyzed using

 

107

data from both the surface and 700mb level for the period July
1967 to December 1969. The relative annual contribution of
the moisture sources is listed in Table 10 and illustrated in
Figure 18. These results show that the country received
moisture from nearby water bodies such as the Persian Gulf,
the Black Sea, the Red Sea, the Gulf of Oman, the Caspian Sea,
the Mediterranean Sea, and as far as the Bay of Bengal. Among
these sources, the most important were the Persian Gulf, the

Caspian Sea, and the Mediterranean Sea (Table 10, bottom row).

 

The Caspian Sea\moisture reached only the nearby coastal
areas and was the predominant moisture source of this region
(Figure 18). This moisture invaded the area in association
with different synoptic patterns. The major pattern involved
anticyclones traveling north of the Caspian Sea under the
northern branch of the European Polar Front jet stream; these
anticyclones were common throughout the year. When these
anticyclones were to the west of the Caspian Sea, northerly
winds on their east side crossed the sea and carried moisture
to nearly all the adjacent southern coastal areas. When they
were to the north or northeast of the Caspian Sea, however,
the northeasterly or easterly winds affected mostly the south-
western coast. A variant of this pattern developed when the
thermally-reinforced Siberian anticyclone was located to the
east or northeast of the Caspian Sea. Moisture acquired by
the resulting wind flow from this very cold anticyclone

affected the southwestern coast. During the warm season, moist

108

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m H H H o o. m m 2 H m m m m 3 982
H o H N 2 mm H a Shams
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109

 

— Primary ___" 5°°°ndary

FIGURE 18. Annual moisture trajectories.

110

air fran the Caspian Sea affected the coastal strip through
the establishment of sea breezes.

Moisture from the Mediterranean Sea reached all the
northern half of the country (Table 10) but it was the primary
source only in the northwestern region (Figure.l8). This
moist air often invaded the country with anticyclones steered
eastward by the southern branch of the Polar Front jet stream
behind surface cold fronts. These anticyclones originated
over southern or western Europe and gained moisture from the
Black and Mediterranean Seas, yielding precipitation over Iran.
Moisture from these sources was also brought by the upper
level flow pattern, as when a trough at 700mb developed over
the Mediterranean Sea, resulting a moist southwesterly flow
into the country. This mild Mediterranean air mass was uplifted
over Iran by one or more of four different synoptic processes;

1) Cold front uplift.

2) Warm occlusion. In some cases, especially in
winter, the western extremity of a very cold Siberian anti-
cyclone occupied Iran. This cold air forced the mild Medi-
terranean air mass over it, which yielded precipitation.

3) Uplift by upper level disturbances. Following the
passage of cold fronts the Mediterranean air mass remained
over the country and was uplifted by passing upper level dis—
turbances.

4) Slope instability. Air over the slopes of the
western mountains became warmer than the ambient air because

of differential heating. This lapse rate increase resulted in

 

 

lll

convectional uplifting of the Mediterranean air mass; however,
this process was affected by orographic influences as well.
Persian Gulf moisture together with that from the Gulf
of Oman affected the southern half of the country (Figure 18).
Moist air from these water bodies invaded the country with
several synoptic patterns. First, as Mediterranean cyclones
invaded Iran, the wind flow ahead of these systems drew
moisture northward from these water bodies and yielded precipi-
tation over the country, especially in the northeastern section.
Second, especially in the transitional seasons, either the
Mediterranean cyclones became stationary or inverted troughs
developed over Iraq. These cyclones or troughs drew in moisture
from the Persian Gulf on their eastern flank to the western
slopes of the Zagros Mountains. Third, during the summer,
moisture was drawn in from these sources by the establishment
of sea breezes in the lower atmosphere. Fourth, development of
an upper level (700mb) trough over Arabia generated southwester-
ly flow from the Persian Gulf. This circulation was the only
one responsible for transporting moisture from the Red Sea and
it arose when the trough was located over northwestern Africa.
The Bay of Bengal was the farthest water body to affect
the southeastern region and was important only in summer
(Figures l8, 19). During this season, the establishment of the
Indian low in the lower atmosphere drew moist air from the Bay
of Bengal across the Indian subcontinent into Iran around the

northern flank of this low.

 

112

Seasonal Distribution of Moisture Sources
Winter

This season was most similar to the annual pattern
of moisture flux (Figure 19). In the Caspian region, the
primary source was the Caspian Sea, accounting for 47% of
total seasonal frequencies of moisture trajectories (Table 11).
The Mediterranean Sea*, accounting for 35% of the total, was
the second most important source of moist air. The primary
source in the northwestern region was the Mediterranean Sea
(12% of the total), whereas the Red Sea was the secondary
source. Moisture from the Persian Gulf often combined with
these sources, but in the eastern and southeastern regions
the Persian Gulf was the primary source, and the Mediterranean
Sea was the secondary source. Both southern and southeastern
regions received all their moisture from the southern water

bodies.

Spring

The moisture influx in the Caspian, northeastern, and
southwestern regions was similar to the winter pattern (Figure
19). No moisture source was identified in the southern and
southeastern regions because these areas did not receive any
precipitation values of 10mm or more per day (Table 12). The
primary sources for the northwestern region were the Medi-

terranean and the Red Seas.

* l
Days with moisture trajectories from both Mediterranean Sea
and Red Sea concurrently were classified into a combined group
and included in the category of the Mediterranean Sea.

113

 

WINTER Li.” SPRING ' °._._L°°

   

SUMMER LL” FALL La—‘am
Milo. Mil.
‘ Primary ———-> Secondary

FIGURE 19. Relative importance of seasonal moisture trajectories.

114

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115

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116

Summer

During this season, the southwestern, northeastern,
and southern regions did not receive precipitation values
of 10mm or more per day. The northwestern region received
all of its precipitation from the Mediterranean Sea and the
southeastern region from the Bay of Bengal (Table 13). The
Caspian region had the same pattern as the previous seasons
but with an increase in contribution by the Caspian Sea

(Table 13).

Fall

Fall was the driest season in the southeastern region
(Figure 19). The southern, southwestern, and northeastern
regions received most of their moisture from the Persian Gulf.
In the northwestern region, in contrast to spring, the Red
Sea was the second most important source after the Medi-
terranean Sea. The Caspian region experienced the same pat-
tern as in the other seasons but with the addition of a very

low contribution from the Mediterranean Sea (Table 14).

Importance of Moisture Sources

According to Table 10, the moisture sources contribut-
ing the greatest amount of precipitation over all the country
‘were the southern water bodies, with the Mediterranean Sea
ranking third after the Caspian Sea. To determine their
regional importance, the percentage contribution of each

iregional mean to the national mean (computed from regional

 

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117

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mo 8 mm mm 8 mu m: mm m: OO
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119
means) was determined (Table 15). The Caspian region
received more than half of the country's precipitation
(51%), whereas the southeastern region received only 4% of the
precipitation. Because the primary source of moisture in the
southern, southwestern, northeastern, and southeastern regions
was from southern water bodies (Figure 18), I conclude that
these water bodies were of limited total importance. The
Mediterranean Sea was the_primary source in the northwestern
region and the secondary source in the northeastern region.
Consequently, the most important moisture sources for the
country as a whole were the Caspian Sea and the Mediterranean
Sea (Table 15). The influence of the Caspian Sea was limited
to a very small area on its coast, whereas the Mediterranean

Sea moisture flux affected most of the country.

Heavy Storms

The heaviest ten percent of precipitation days (those
used in moisture analysis) in which 10mm or more occurred per
day was defined as days with heavy storms. The mean intensity
of these heavy storms was computed for the representative
stations and ranged from 23mm in the northeastern region to
74mm in the Caspian region (Table 16). Their contribution to
the regions' precipitation was very high in the southern region
(58%),but decreased to 14% in the northwestern and northeastern
regions. Thus, most of the annual precipitation in the southern

'region came in the form of heavy storms, but heavy rainfalls

120

 

Region Mean Contribution
annual to total
rainfall mean

(min) (7.)

Southeastern 86 4

Southern 133 6

Southwestern 312 13

Northeastern 252.6 11

Northwestern 358 15

Caspian 1204 51

 

TABLE 15. Mean regional rainfall and percentage
contribution to the total mean precip-
itation of all regions.

121
were less common in the northwestern and northeastern regions.
Positive vorticity advection (upper level divergence) was
the main uplift mechanism associated with these events in all
regions, except in the Caspian region where sea effect often
caused the heavy storms. The main moisture source was the
Caspian Sea in the Caspian region, the Mediterranean Sea in
the northwestern and northeastern regions, and the southern

water bodies in the remaining regions (Table 16).

122

 

 

Mean Primary Contribution
Region intensity Predominant moisture to the region's
(mm) mechanism trajectory annual precipi-
tation
(7°)
Southeastern 25 Positive Persian 37
vorticity Gulf
advection
Southern 50 " " " " 58
Southwestern 44.3 " ” " " 25
Northeastern 23.24 " " Mediterranean 14
Sea
Northwestern 27 " " " " 14
Caspian 74 Sea effect Caspian Sea 35
TABLE 16. Primary mechanisms and moisture trajectories of

heavy storms (heaviest 10% of precipitation days
with 10 millimeter or more) and their percentage
contribution to regional mean annual precipitation.

 

CHAPTER VI

DISCUSSION

The results of this study have shown the following
patterns. First, despite their higher absolute frequency
in the North, upper level disturbances were responsible for a
higher percentage of the annual precipitation in the South,
and they were the most frequent and significant mechanism
across the country throughout the year; their spatial distri-
bution showed a form of regionality. Second, surface disturb-
ances were most frequent and made the greatest contribution to
annual precipitation totals in the central parts of the coun-
try, although their regional importance was similar to upper
level disturbances in winter. Third, despite the higher aver-
age dew points on the southern coastal areas, surface heating
contributed a higher percentage of summertime precipitation
days in the non-coastal areas of the Southeast and Northwest.
Fourth, over the entire country, only the southwestern coast
of the Caspian Sea received sea effect precipitation. Fifth,
although the Caspian Sea provided the highest percentage of
the national mean precipitation, moisture from the Mediter-
ranean Sea was more widespread over the country. Sixth, the

occurrence of precipitation was sporadic and was in the form

123

 

124

of heavy single events more in the South than in the North.
In the following sections these patterns will be
discussed in more detail. In addition, the final sections
are devoted to a discussion of the validity of the findings
and the methods of study, including their application and

limitations.

Sea Effect

 

Although the country is bordered by bodies of water
on the north and south, sea effect was significant only along
the coastal regions bordering the Caspian Sea (Figure 7).

The required conditions for this process occurred during fall
and winter, and the contribution by this mechanism was high-
est along the southwest portion of the coast. The establish-
ment of an anticyclone to the northwest, north, or northeast
of the Caspian Sea produced northerly or northeasterly winds
which seldom affected the southeastern coasts. The lack of
sea effect in the other seasons was explained by the lessening
or even reversing temperature difference between sea and over-
lying air which is required for atmospheric destabilization.
During fall and early winter the sea was much warmer than was
the air, but it became cooler as the season progressed until,
in spring and summer, the land and overlying air masses be-
came warmer than the sea.

Although the findings of this study are in agreement
with those of Khalili (l), in contrast to his conclusion this

study did not show the establishment of a frontal zone between

 

125

the Caspian air mass and the interior continental air mass.
He had suggested that this front is the main cause for the
precipitation maximum of the region. This suggestion was
not supported by this study because Anzali, located on the
coast, received the highest precipitation of any station in
the country, north of the suggested front; if the Caspian
air mass yielded precipitation because of frontal uplift,
the region of maximum precipitation should have also shifted
farther south than Anzali, which was not the case in this
study. The occurrence of sea effect in Anzali indicated that
the destabilization by the sea surface alone was adequate for
lifting the air and hence yielding precipitation. However,
this study did not have data from areas inland from the coastal
zone to determine the exact extent of sea effect.

The southern coastal areas did not experience this
process because, as a result of their southerly location, the
air-water temperature difference was never adequate and most

of the cold period winds were off-shore.

Surface Heating

This mechanism contributed the lowest percentage of
annual precipitation days and showed the highest spatial vari-
ation among all mechanisms (Table 1). Despite the high dew
points over the coastal regions, an important precondition for
precipitation-stimulating convection, this mechanism occurred

most frequently in the northwestern highlands and in the

126

Southeast. As was mentioned earlier, these patterns apparent-
ly reflect the dynamically-induced stability of the atmosphere,
as the lower contribution in the southern coastal areas
suggests that the atmosphere was normally too stable to be
overturned by surface heating alone.

The spatially irregular contribution by the surface
heating mechanism could be explained differently for different
parts of the country. In the Northwest, where moisture from
the Mediterranean Sea was present much of the time, convectional
showers occurred in spring when high insolation on the mountain
slopes appeared to increase the lapse rate, as suggested by
Scorer (2). But in the hot weather of the Southeast, surface
heating gave rise to showers only when modified monsoon air
was drawn in from the east (Figure 19).

Despite the widespread and intense surface heating of
summer, it caused significant precipitation only in the Caspian
region; in the others summer precipitation was inconsequential
(Figure 16). The suppression of summer precipitation in these
regions might be related to the northward shift of the sub-
tropical jet stream in summer. As was suggested by other
researchers (3, 4), during summer the subtropical jet stream
shifts northward over Turkey and northern Iran, most frequently
placing the greater part of the country under subsiding condi-
tions. Under these circumstances only occasional well-
developed short waves invade Iran, particularly the northern

parts, resulting in uplift. This stable condition may also be

127

disturbed by very intense heating of modified monsoon air
in the Southeast, as evident by- the summer precipitation

at Iranshahr (Figure 16).

Surface Disturbances

Surface disturbances were particularly frequent across
the middle portions of the country, extending from Ahwaz in
the West to Birjand in the East (Figure 5). Along each
parallel, these disturbances contributed less in central Iran
than along the east and west parts of the country. This pat-
tern developed apparently because, first, some of the disturb-
ances dissipated as they moved eastward from western Iran, and
second, the eastern parts of the country received disturbances
from both the northwest and southwest. Thus, both the western
and eastern portions were affected by more surface disturbances
than were the central parts. This spatial distribution is in
agreement with the findings of Alijani (5), who found that the
main cyclonic track crossed Iran north of the Persian Gulf
(see also Table 3).

The seasonal fluctuations in the extent to which Iran
is affected by surface disturbances are consistant with the
variation in size of the Northern Hemisphere westerly vortex
as it envelopes the northern half of the country during fall
and expands to include all of Iran by late winter, bringing
strong short waves into even southern Iran. By summer the

westerlies have shifted northward, leaving the country under

128
the influence of the subtropical highs and beyond the reach
of mid-latitude cyclones. As a result, cyclonic activity
begins in fall, reaches its spatial maximum during winter,
retreats in spring, and is completely absent in summer (Table
3). Therefore, the results of this study confirm the find-
ings of earlier workers (5, 6) that the surface disturbances
were frequent in, and caused much precipitation during, the
colder period of the year.

Another interesting aspect of the surface disturbance
pattern is the number of cyclones without fronts during spring.
In this season, ”frontless" lows were more frequent than were
cyclones with fronts (Table 3). This may indicate that, in
spring, solar heating of the earth's surface modifies the air
masses to such an extent that they lose their surface iden-
tities, particularly at the warm front, and thus the fronts
disappear (7). This process often occurred as the cyclones
invaded Iran and the associated warm fronts dissipated (Table
4); during spring the frontless lows outnumbered the

depressions with frontal systems (Table 3).

Upper Level Disturbances
These disturbances contributed more precipitation days
than did any other mechanism over the country. The opposing
trend of occurrence and contribution along the north-south
direction deserves comment. Their highest absolute frequency

was in the Caspian region (Table 2), where they contributed

 

129
the lowest percentage to the mean number of annual precipi—

tation days (Figure 4); the opposite pattern developed in

the South along the Persian Gulf coast. This pattern appears
to be related to the number of uplift mechanisms which
occurred over the different parts of the country, in that in
the Caspian region almost all of the identified uplift mech-
anisms were present. In the southern region, however, some

of them, like sea effect, were absent, or they contributed a
lower percentage, such as surface disturbances and surface
heating (Table 5). Consequently, in the North the precipita-
tion days were divided between more uplift mechanisms than they
were in the South, resulting in a higher contribution by upper
-level disturbances in the latter region. The absence of sea
effect in the South was discussed earlier in this chapter; the
lower contribution by surface disturbances and surface heating
is related to the seasonal expansion and contraction of the
westerly vortex. As is apparent from Table 2, the northern
parts of the country experienced more upper level disturbances
than did the southern parts throughout the year, with fewer
strong upper level disturbances and, hence, surface disturb-
ances in the South. On the other hand, since the southern
limit of the mid-latitude westerly vortex is marked by the
position of subtropical high centers, the northward shift of
the westerlies in summer brings the southern parts under the
subsidence associated with the subtropical anticyclone. This

subsidence prohibits most convectional uplift, and only

130

occasional upper level disturbances can invade the area which
result in uplift and possible precipitation, as indicated
earlier.

The importance of upper level disturbances in the
northeastern region (Figure 4), the only region where upper
level disturbances were frequent and important through all
seasons (Tables 7, 8), is explained by the influence of the
Siberian anticyclone and the continental situation of the
area. During much of the winter the Siberian anticyclone

dominated this region and, consequently, diverted surface dis-

 

turbances along a more southerly track; in the warmer season
because of the continental situation of the region, convection
probably occurred with inadequate low-level moisture to produce
significant amounts of precipitation. As a result, only upper
level disturbances disrupted the stable conditions caused by
the Siberian high in winter and were responsible for importing
moist air from distant sources which led to precipitation in
both seasons. V

One important winter characteristic of upper level
disturbances was the strength of their contribution in the
northwestern, southwestern, and southeastern regions. This,
once again, indicates that in winter strong upper level dis-
turbances affected nearly all the country in association with
surface disturbances.

In all regions, upper level disturbances produced very

high precipitation amounts because they were frequent. But in

131

the northwestern region they yielded the most total summer-
time precipitation even though surface heating was the most
frequent mechanism (Figures l3, 17). An explanation of this
pattern lies in the moisture availability for the uplift
mechanisms. Convectional uplift operates on local moisture,
whereas upper level disturbances bring moist air from outside
sources as well. Consequently, because of the continental
situation of the northwestern region, upper level disturbances
imported more moisture and generated more precipitation than

did surface heating.

Mgst Frequent Uplift Mechanism

During the year as a whole, upper level disturbances,
with or without surface disturbances, accounted for the great-
est percentage of precipitation days. However, during summer,
surface heating became the predominant mechanism, but, since
summer precipitation was very light in regions other than the
Caspian region, surface heating contributed a small absolute
amount of moisture to annual totals (Figure 16). This leads
to the conclusion that, at least in terms of precipitation,
Iran is under the influence of mid-latitude climate controls.
During the study period no type of tropical uplift mechanism
(tropical cyclones, monsoon depressions, easterly waves, or the
Inter-Tropical Convergence Zone) was detected, a finding in
agreement with the conclusions of Snead (7) and Tanaka (8).

Although whatever summer precipitation occurred was usually

 

132

’of convective origin, the uplift mechanism most common in
the tropics, I conclude that the country cannot be regarded
as being under the influence of tropical controls because
the contribution by convection was small in comparison to
uplift mechanisms associated with the baroclinic westerlies.
Also, the moisture influx through the monsoon low affected
only a small part of the country and accounted for low pre-

cipitation amounts.

REgions of Precipitation Mechanimns

The cluster analysis, based on all uplift mechanisms,

did not result in distinctive contiguous regions. This was
true because, first, the mechanisms were interrelated and
increases in the percentage contribution of one mechanism
resulted in decreases in the others. Second, resulting regions
made little spatial sense: region types were scattered across
the map with no apparent spatial logic; and many regions were
based on fewer than three stations.

However, the regions identified according to the single
variable, predominant (most frequent) uplift mechanisms were
significantly different from each other and, in addition,
showed distinctive characteristics through the subsequent
analysis of different uplift mechanisms.

Furthermore, the spatial distribution of the most fre-

quent mechanism displayed regional concentrations with sharp

133
gradient zones between them (Figure 10). The following
unique attributes were possessed by each region:

1) The southern region showed the highest contri-
bution by upper level disturbances, but the lowest contri-
bution by surface heating (Table 5). In all seasons except
summer, upper level disturbances were the most frequent and
important mechanisms; summer precipitation was unimportant.

2) The southeastern region exPerienced the great-
est frequency contribution by surface heating and the mini-
mum frequency contribution by surface disturbances. Also,
it had the lowest annual number of precipitation days.

This region was the only one that benefitted from Indian
monsoon air in summer (Figure 19).

3) The southwestern region received the highest
frequency contribution from surface disturbances among all
regions. It is the only region where surface disturbances
were the most frequent uplift mechanism and yielded much
precipitation during the cold period of the year.

4) The northwestern region was distinguished from
others because it had a spring maximum of precipitation
apparently associated with surface heating.

5) The northeastern region was second to the south-
ern region in percentage contribution by upper level disturb-
ances (Figure 4). It was the only region where, even in summer,
upper level disturbances were the most frequent mechanism and

gave more precipitation. This region was affected by the

134
Siberian high through much of the winter, also.

6) The Caspian region was the only region character-
ized by fall precipitation resulting from sea effect. It
was also the region with the highest annual number of pre-
cipitation days and lowest frequency contribution by upper

level disturbances.

Moisture Sources

Although the Caspian Sea was the moist air source
accounting for the highest percentage of combined regional
precipitation over the country (Table 16) and the Persian
Gulf was frequently the moisture source in the South, moist
air from the Mediterranean Sea was most widely involved in
precipitation falling over Iran. The effect of the Caspian
Sea was limited to a narrow coastal strip north of the
Elburz Mountains. Therefore, despite its high contribution,
it did not play an important role in the precipitation pro-
duction over most of the country. The southern and south-
eastern regions, where southern water bodies are the primary
sources but mean precipitation is low, also did not contribute
significant amounts to the national mean (Table 15). The
Mediterranean Sea was found to be the most frequent source
in the northwestern region and a secondary source in the
Caspian and northeastern regions. Because these regions
occupy a large portion of the country and are heavily agri-

cultural the Mediterranean Sea emerges as the most important

135

water source for the country as a whole.

Th2 Climatology of Heavy Storms

Precipitation in southern Iran was more sporadic
and in the form of heavier storms than it was in the North.
Westerly disturbances infrequently invaded the South because
of its lower latitude and the influence of the stable atmos-
phere. Thus, most of the time the region experienced dry
weather. However, the occasional strong westerly disturbances
lifted the moist air supplied by local sources, as indicated
by the high dew points, and produced heav ier rains (9,10,11).
In the North, with the exception of the Caspian region, both
the intensity and percentage contribution of the heavy storms
were less. For example, in the northwestern region their
mean intensity was 27mm per day and they contributed only 14%
of the region's annual precipitation (Table 16). They were
less important because, first, the moisture source is located
far from the region and therefore the air masses release
some of their moisture before reaching Iran, and, second,
these areas are usually affected only by uplift mechanisms
associated with westerly disturbances, leading to a pattern
of frequent precipitation of moderate intensity. Only very
rarely do these disturbances have access to very moist and
produce torrential precipitation. In the Caspian region,
the contribution of heavy storms to regional precipitation
was not very high although the intensity of the storms aver-

aged higher than elsewhere over the country. This results

136
because,first, the air is always moist and, second, the
uplift mechanisms have similar strength so that the total
precipitation is distributed rather evenly between all pre—
cipitation events; abnormal, strong mechanisms are very
rare.

In conclusion, proximity to water bodies seems to
affect the intensity of the precipitation events. That is,
uplift mechanisms produced heavier precipitation on coastal
regions because of the presence of moist air. This conclu-

sion is in complete agreement with other studies (12).

Original Contributions

The major contribution of this study is the clarifi-
cation of some aspects of the precipitation climatology of
Iran. Specific contributions are listed below:

1) In the past some authors have referred to the
climate of Iran as being very complex (13), and this assump-
tion was the main impetus for this research. As was mentioned
early in this work, Iran's subtropical location places it in
a transitional zone between the Mediterranean climate to the
west and the monsoon climate to the east, the Siberian anti-
cyclone to the north and the subtropical highs to the south.
The irregular spatial distribution of precipitation (Figure h+)
further suggests a complexity of causes. However, this
research has demonstrated that in spite of this spatial

complexity the precipitation dynamics in Iran can be explained

137
in terms of a relatively small number of repetitive precipi-
tation mechanisms. I

2) A high percentage of precipitation days was
caused by westerly disturbances. This percentage increased
progressively from north to south.

3) Seasonally, the invasion of Iran by westerly dis-
turbances began first in the North in fall, expanded to in-
clude much of the country in winter, and receded in spring,
so that by summer the country was almost free of these dis-
turbances.

4) The precipitation mechanisms in Iran showed region-
al differences.

5) Precipitation events were heavier and more sporadic
in the South than in the North; most often, upper level dis-
turbances with no surface disturbances were responsible for
these heavy storms.

6) Sea-effect precipitation in the Caspian region
resulted not only from the thermally-induced Siberian anti-

cyclone, but also from eastward-moving dynamic ones.

Research Design

The overall appropriateness of the research design is
reflected to some degree by the fact that only 14% of the
annual precipitation days remained unclassified, whereas 86%
were assigned to at least one mechanism (Table 1). Certain
research design decisions made during the course of the study

deserve comment here.

 

138
1) At the beginning of the study, the general

category of upper level disturbances was divided into three
sub-classes which included upper level divergence, trough
axes aloft, and upper cold lows. Subsequently, it became
apparent that the percentage contribution by trough axes
aloft and upper cold lows was very low (Table 2). Therefore,
in the later stages they were recombined into a single class
of upper level disturbances. This combination was justified
because all three mechanisms were types of uplift generated
by vorticity relationships, and the distinction seemed arti-
ficial.

2) Similarly, surface disturbances were first divided
into sub-classes of cyclones, frontless lows, and frontal
systems alone but later were combined into a single category
of surface disturbances. The combination was made because,
first, most surface disturbances were cold fronts (Table 4),
and second, all of them in fact are constituents of one major
type of synoptic-scale system in which uplift is concentrated
largely along the frontal zones (14). i

3) In order to regionalize the country, I relied
heavily on the patterns of westerly disturbances because
traveling synoptic-scale surface disturbances seldom.exist
without the support of upper level triggers; both are inter-
related through the vorticity principle. Thus, since most of
the precipitation in Iran was generated by both upper level
and surface disturbances, all related through the vorticity

principle, the results of this study convey the idea that the

 

139

long-term precipitation pattern in Iran is related to the
strength, frequency, and spatial distribution of upper
level vorticity centers in the baroclinic westerlies.

4) The designation of mechanisms accounting for
each precipitation day proceeded on an assumed order. In
other words, evidence was first sought for the presence of
upper level disturbances, followed by surface disturbances.
On the coastal regions, in the absence of either mechanism,
sea effect was deduced if the required conditions existed.
Surface heating was then invoked if an event was still un-
explained and the predetermined criteria were met. This
approach gave generally acceptable results. Further, this
procedure lowered the probability of overlap between the mech-
anism so that no precipitation day was counted more than once
in the case of multiple causation.

5) Another important point about the research methods
used in this study concerns the scale and coverage of the
syn0ptic weather maps. Although it is likely that the 24-
hour map interval and smaller scale obscured some short lived
or mesoscale mechanisms, they were useful, nonetheless, in
identifying at least one mechanism for each precipitation day
86% of the time throughout this study. This fact underscores
their usefulness, especially for areas where local meteoro-
logical data are not available. Since over at least part of
the world local meteorological networks are not extensively
developed, large—scale synoptic maps with short intervals are

not available, or there are very few local soundings, these

 

140

maps have important utility in the explanation and under-
standing of precipitation mechanisms. This study has shown
that maps of this kind can be used in a remote part of the

world to yield useful results.

Limitations of the Study
During this research some limitations inherent in the
methods were recognized, but they are not considered to be
factors affecting the validity of the results. They are

mentioned here as areas for future improvements. One poten-

 

tial source of error was the lack of synchrony between the
timing of the precipitation observations and the time of the
synoptic maps. In this study 12GMT synoptic weather maps
were used to investigate the synoptic origin of daily pre-
cipitation. It is possible that occasionally during each 24-
hour interval other mechanisms in addition to the one(s)
depicted on the map were responsible for that day's precipi-
tation, or that none of the responsible mechanisms were present
at the map time over the respective station. Because of I
this imperfect synchrony and the smaller scale of the maps,
this study was not able to demonstrate the effect of mountain
ranges on precipitation despite the mountainous nature of the
country. Unfortunately, these maps were the only current
data sources available, and the high percentage of explained
precipitation events confirms their value as a research tool

nonetheless. Also, the brevity of the study period (5 years)

141
did not constitute a serious limitation because each day
was treated as a separate observation, and it is likely
that all important synoptic precipitation-inducing situations
were encountered during the period.

The method used for assigning precipitation mechanisms
contained a few minor weaknesses. First, it underestimated
the contribution by surface heating because, as noted earlier,
even warm summer days were classified into a dynamic uplift
mechanism category when evidence so indicated, in spite of

possible assistance by surface heating. Second, the 80°F

 

minimum.threshold designated for surface heating was not
appropriate because during most summer days the temperature
was above this level. A similar subjective method was used
for determining sea effect, in that the minimum threshold was
a 15°F difference between windward and lee shore dew point
temperatures in the Caspian region. Although the general
research method would give the same overall results regard-
less of the order of steps used to assign each day to a
mechanism, it was not able to determine the relative influence
of each mechanism.during days when precipitation resulted from
multiple causes.

In other studies of this type which are concerned
with determining the relative influence of precipitation
mechanisms in Iran, the following modifications are recommended:

1) Synoptic maps of shorter interval and larger scale

should be used to facilitate more detailed studies.

142

2) The degree of uplift associated with westerly
disturbances should be measured according to the amount
of vorticity advection.

3) The thermal instability of the atmosphere should
be measured through the use of local soundings.

4) The amount of daily precipitation should be
statistically related to the degree of vorticity advection,
atmospheric instability, and the amount of moisture in the

atmosphere.

 

10.

ll.

12.

13.

LIST OF REFERENCES - CHAPTER VI

Khalili, A. 1973. Precipitation Patterns of Central
Elburz. Arch. Met. Gepph. Biokl., Ser. B, 21:215-32.

Scorer, R.S. 1955. The Growth of Cumulus Over Mountains.
Arch. Met. Geoph. Biokl., Ser. A, 8:25-34.

A

Weickmann, L. 1961. Some Characteristics of the Sub-
tropical Jet Stream in the Middle—East and Adjacent
Regions. Iranian Met.Dept., Tehran, Ifan.

Meteorological Office. 1962. Weather in the Mediteran-
ean, Vol. 1, General Meteorology. HMSO, London.

 

Alijani, B. 1979. Cyclone Tracks in Relation to Upper
Flow Patterns in the MiddIe_East, 1964-67. ’MLA.
Thesis, MichiganState University, East Lansing.

weickmann, L. 1960. Haufigheit sverteilung und Zugbahnen
von Depresionen im Milteren Osten. Met. Rund. Berlin.
13(2):33-38.

Snead, R.E. 1968. Weather Patterns in Southern West
Pakistan. Arch. Met. Geoph. Biokl., Ser. B. 16:316-46.

Tanaka, M. 1980. Role of the Circulation at the 150mb
level in the Winter and'Summer Monsoon in the Asian
andeustralIan Regions. Climatological Notes No. 2,
Occasional Papers No.2, Inst. of Geosci., Univ. of
Tsukuba, Japan.

Douglas, H. 1970. A Sudden Squall at Sharjah, Trucial
States. Met. Mag., 99:336-41.

Murray, R. and G.A. Coulthand. 1957. A Thunderstorm at
Sharjeh, Persian Gulf, on 23 November 1957. Met. Mag.,
88:176-78.

Stevens, J.H. 1970. Rainfall at Buraimi Oasis in July
1969. ‘Met. Mag., 99:37—39.

Miller, J.E. 1955. Intensification of Precipitation by
Differential Advection. J. Met., 12: 472-77.

Fisher, W.B. 1971. The Middle East: A Physical, Social,
and Regional Geography. MeEhuen & Co. Ltd., London.

143

CHAPTER VII

SUMMARY AND CONCLUSIONS

The synoptic origin of precipitation in Iran was
investigated by utilizing daily precipitation totals of
40 stations distributed over the country, obtained from

the Meteorological Organization of Iran for the period 1963

 

to 69. Responsible uplift mechanisms for each precipitation
event at each station were determined from 12GMT surface and
500mb synoptic maps of the Northern Hemisphere. Importance
of uplift mechanisms was judged on the basis of their contri-
bution to the number of each station's annual or seasonal
precipitation days.

The results of this research strongly suggest that
upper level disturbances, alone, were the most frequent and
important uplift mechanism in Iran throughout the year; al-
.though surface heating produced a high percentage of summer
precipitation, its overall importance to annual totals was
low because summer precipitation is normally light. Second
to upper level disturbances in contribution to both precipi-
tation days and precipitation totals were surface disturbances,
which caused most of the cold-period precipitation over the

western highlands of the country. Sea effect contributed to

144

145

fall and early winter precipitation along the southwestern
coast of the Caspian Sea. The combined category of upper
level and surface disturbances (traveling westerly disturb-
ances) accounted for the highest percentage of the country's
precipitation totals. The results of the study led to the
conclusion that, first, Iran is under mid-latitude climate
controls in terms of precipitation climatology and, second,
a well-developed westerly flow pattern, strong enough to
generate surface disturbances, develops seasonally over the

northern parts of the country and only occasionally extends

 

over the southern portions of Iran.

The country was regionalized according to the spatial
varianCe of the Z-score values of the percentage contribution
by the predominant uplift mechanism (westerly disturbances)
to the stations' annual precipitation days. The resultant
regions were statistically distinct from each other and had
unique climatological characteristics, as follows:

1) The southern region experienced the greatest fre-
quency contribution by upper level disturbances among all
regions in all seasons except summer.

2) The southeastern region received the greatest per-
centage of its precipitation days from surface heating but
the lowest percentage from surface disturbances among the
regions.

3) Among all regions, surface disturbances caused the
greatest percentage of precipitation days in the southwestern

region.

146

4) Surface heating accounted for a greater percent-
age of spring time precipitation days in the northwestern
region than in any other northern region of the country.

5) The northeastern region was unique in that, even
in summer, upper level disturbances were the most frequent
and important mechanisms

‘6) The Caspian region had the lowest total contribu-
tion from.upper level disturbances among all regions and was
the only region with a significant contribution from sea
effect.

Important sources of atmospheric moisture were deter-
mined for days with 10mm or more precipitation at represent-
ative stations through the use of both surface and 700mb level
data. The results suggest that southern water bodies (the
Persian Gulf and the Gulf of Oman) were the most frequent
source for the southern, southeastern, and northeastern
regions, but that the Mediterranean Sea contributed the great-
est amount of moisture in the northwestern region. The Caspian
Sea contributed the highest percentage of moisture in the'
Caspian region; during summer the Bay of Bengal was the most
frequent moisture source in the southeastern region. In
terms of moisture contribution to the country as a whole, the
Caspian Sea led all sources, but its influence was limited to
a narrow coastal strip, whereas the Mediterranean Sea, though
with a lower percentage contribution, provided the moisture
for a larger portion of the country, particularly the agri-

cultural areas.

147

The nature of precipitation was very sporadic and‘
heavy in the South compared to the North. In the South
most of precipitation was produced from very heavy storms,
whereas in the North storms with moderate intensity were
most often responsible for the region's precipitation.

The results of this study suggest additional ques-
tions regarding the precipitation climatology of Iran. Some
of these are summarized below.

1) A high percentage of precipitation in Iran was

caused by westerly disturbances. These disturbances travel

 

eastward along the configuration of the upper-level mean flow
pattern. What is the relationship between the spatial distri-
bution of precipitation mechanisms in Iran and the position
of upper-level mean long wave troughs over the country and
adjacent regions?

2) The contribution by westerly disturbance was con-
centrated over the southern and northeastern parts of the
country, where subtropical highs and the Siberian anticyclone,
respectively, most often occur. A relationship may exist~
between the presence of these controls and the relative
importance of westerly disturbances compared to other mech-
anisms. To what degree do these anticyclones relate to the
occurrence and spatial concentration of precipitation mech-
anisms?

3) An assessment of the influence of mountain ranges

was not sought at the outset of this study because it was

148

assumed that the mountain ranges only enhance existing
uplift mechanisms. However, the concentration of preci-
pitation days along the mountains does suggest a possible
influence by the mountains themselves. Do these mountain
ranges exercise their influence because they interfere
with the influx of moisture, enhance the already-existing
uplift mechanisms, or generate additional uplift?

4) It was suggested that the prevalence of the sur-
face heating mechanism in the northwestern region was caused
by differential heating on the mountain lepes. Under what
synoptic circumstances does this process occur?

5) There was no trOpical effect detected in this
study. Is it likely that a longer study period would demon-
strate that uplift mechanisms of tropical origins affect
the country?

6) Under what specific synoptic situations does the
moisture influx from each moisture source affect Iran?

7) The frequent occurrence of heavy storms in the
South was attributed to the abundance of moist air in the
region. To what degree is moisture, rather than the relative
strength of uplift mechanisms, actually responsible for this
more intense precipitation?

This research was a preliminary and exploratory study
regarding the syn0ptic origin of precipitation in Iran. Al-
though it provided only a general image of the spatial and

seasonal variations of both the dynamics and moisture sources

 

149
of precipitation over the country, this study may be used
for both a basic information source and for planning more

detailed studies on the subject.

 

 

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APPENDIX 3

Stations included in the cluster analysis
with their Z-scores for the uplift mechanisms

 

 

Cluster Station Upper Surface Surface Upper ~ Sea
Level disturb- heating Level & effect
disturb- ances surface
ances disturb-
(only) ances

1 Abadan .19 1.23 -1 . 1.04 -1.1

Ahwaz .3 1.23 -.47 1.17 -1.1-
Bandar Abbas 1.88 .09 -.86 2.03 -1.1
Birjand -.15 1.61 -.27 .92 -1.1
Bushehr 1.05 .57 -2.51 1.41 -1.1
Dezful -.27 .85 -.27 .3 -1.1
Esfahan -.05 .66 -.85 .42 -1.1
KermanShah -.39 1.04 -.47 .3 -1.1
Sabzewar .91 .66 -l 1.41 -1.1
Saqez -.51 .85 -.27 .05 -1.1
Shahr Kord -.54 2.10 -1.2 .92 -1.1
Tabas .21 .66 -.86 .67 -1.1
Torbat Heydarieh .3 .85 -.86 .8 -1.1
Urmia -.66 .66 -.26 -.19 -1.1
Zabol .93 -.1 -l.2 .92 -1.1
Mean .21 .86 -.82 .81 -l.1
2 Ban .55 -.68 .31 .18 -1.1
Kashan .19 -.68 -.66 -.19 -1.1
Kerman .55 .09 .7 .67 -1.1
Khoy -.39 .28 .12 -.19 -1.1
Mashhad .55 -.3 -.47 .42 -1.1
Sanandaj -.39 -.3 .31 -.56 -1.1
Saman .45 -.68 .12 .05 -1.1
Shiraz 1.29 -.87 -1 .18 -1.1
Tehran -.15 .3 .51 -.31 -1.1
Mean .29 -.31 -.OO .02 -1.1

156

APPENDDC3 (Continued. . .)

 

 

Cluster Station Upper Surface Surface Upper Sea
level disth'b- heating level & effect
disturb- ances surface
ances disturb-
(only) ances

3 Chahbahar .45 -2.02 .12 -1.3 -l.1

Gorgan -1.02 -.68 1.69 -1.42 -.94
Hanadan -.78 -.49 1.1 -1 -1.1
Iranshahr .19 -1.6 2.27 -.8 -l.1
Ramsar -.65 -1.4 .7 -l.5 -.12
Yazd -.17 -.09 2.1 -.07 -l.l
Zahedan -.51 -.49 1.69 -.81 -l.1
Mean -.25 -.96 1.38 -.98 -.93
4 Anzali -1.98 -.87 -.1 -2.54 1.1
Arak -1.2 .66 .7 -.81 -1.1
Khorramabad -1.02 .47 -1 -.69 -1.1
Qazvin -1.2 .66 -.l -.81 -1.1
Shahroud -1.2 .47 .51 —.93 -1.1
Tabriz -1.38 .28 .9 -l. 18 -1. 1
Zanjan -1.26 .28 1.29 -1 -l.l
Mean -l.32 .27 .31 -l.13 -.78
5 Bandar Lengeh 3.08 -1.8 -1 2.08 -l.1
Jask 2.14 -3.35 -.47 .05 -l.1
Mean 2.97 -2.57 -.73 1.04 -1.1

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