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This is to certify that the

thesis entitled

AIR MASS CLIMATOLOGY ‘OF THE
NORTH CENTRAL UNITED STATES

presented by

Mark Donald Schwartz

has been accepted towards fulfillment
of the requirements for

M - S - degree in Geogrépily

 

Date May 12, 1982

 

0-7639 MS U is an Affirmative Action/Equal Opportunity Institution

 

bV1ESI_} RETURNING MATERIALS:

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AIR MASS CLIMATOLOGY OF THE
NORTH CENTRAL UNITED STATES

By
Mark Donald Schwartz

A THESIS

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

MASTER OF SCIENCE

Department of Geography

l982

ABSTRACT

AIR MASS CLIMATOLOGY OF THE
NORTH CENTRAL UNITED STATES

By

Mark Donald Schwartz

This study is concerned with the geographic distribution of the
major air masses that affected the North Central United States from
1970-l979. These air masses, and the transition zones between them,
were delimited through map analysis of 850mb air and surface dewpoint
temperatures. The results show that Continental air was dominant in
all seasons in the north but was less influential in the south, whereas
Pacific air was prominent in the west. Also, Dilute Tropical/Tropical
air was important in the southeast, and Dry Tropical air was primarily
a summer season phenomenon, influential only in the western portions
of the study area. Furthermore, the occurrence of each air mass in
the study area appears to be associated with a particular 500mb pattern.
These flows may be helpful in explaining the mean and modal patterns

of the various seasons.

ACKNOWLEDGMENTS

The author wishes to express his gratitude to the individuals
who assisted in the successful completion of this study. Special
thanks should go to Dr. J. Harman, for his patience, advice, and
editorial comments as the study progressed.

Among the other individuals who contributed to the study are:
Dr. F. V. Nurnberger and the staff of the Michigan Office of Clima-
tology, who assisted in the assembling of the data base; my wife, Ann,
who proofread the manuscript; and Mr. Gerald w. Takis, Mr. Steven J.
Shaw and Mr. John R. Marien who donated the use of their computer

text editing system.

ii

TABLE OF CONTENTS

CHAPTER Page
LIST OF FIGURES ....................... . ................. vi

I. INTRODUCTION ............................................ l
The Problem .......................................... 1

II. REVIEW OF THE LITERATURE ................................ 4
III. METHODS ................................................. 10
Determination of the Study Area ...................... 10

Air Mass Names ....................................... 12

Source Regions ....................................... 12

Fall, Winter, and Spring .......................... 12
Continental .................................... 12

Pacific ........................................ 12

Tropical ....................................... 13

Dry Tropical ........... . ........................ 13

Summer ............................................ 13

Polar .......................................... 13

Tropical ....................................... 13

Dry Tropical ................................... l4

Assumptions .......................................... 14
Operational Definitions .............................. 15

Data Sources ......................................... 16

Data Analysis ........................................ 16

Air Mass Criteria .................................... 17
Continental ....................................... 20

Polar ............................................. 20

Pacific ........................................... 21

Dilute Tropical ................................... 21

Tropical .......................................... 21

Dry Tropical ...................................... 21

Map Preparation ...................................... 21

IV. RESULTS ................................................. 23
Outline of Presentation .............................. 23

CHAPTER Page

IV. Continued

Air Mass Distribution ................................. 23

Continental ........................................ 23

Winter .......................................... 23

Spring .......................................... 25

Summer .......................................... 25

Fall ............................................ 25

Polar .............................................. 25

Pacific ............................................ 29

Winter .......................................... 29

Spring .......................................... 29

Summer .......................................... 29

Fall ............................................ 29

Dilute Tropical/Tropical ........................... 33

Winter .......................................... 33

Spring .......................................... 33

Summer .......................................... 33

Fall ............................................ 39

Dry Tropical ....................................... 39

Winter .......................................... 39

Spring .......................................... 39

Summer .......................................... 39

Fall ............................................ 44

Air Mass Distribution Summary by Season ............... 44

Winter .......................................... 44

Spring .......................................... 44

Summer .......................................... 44

Fall ............................................ 46

Jenks Classification Technique ........................ 46

Results from Flint Data ............................ 46

Winter .......................................... 46

Spring .......................................... 46

Summer .......................................... 47

Fall ............................................ 47

V. DISCUSSION OF RESULTS .................................... 48

Relationship Between Upper Air Flow Patterns and

General Air Mass Climatology ....................... 48

Modal Upper Air Patterns .............................. 49
Relationship Between Upper Flows and Monthly Air Mass

Percentages ........................................ 51

Winter .......................................... 51

Spring .......................................... 58

Summer .......................................... 65

Fall ............................................ 69

iv

CHAPTER Page

V. Continued

Special Topics ....................................... 72
Transition Conditions .......................... 72

Increased Occurrences of "Pacific" Air in Ohio
During October .............................. 76
Synoptic Agreement ............................. 77
Validity of the Limits Technique ..................... 77
VI. SUMMARY, RECOMMENDATIONS, AND CONCLUSIONS ............... 80
Summary .............................................. 80
Recommendations ...................................... 81
Conclusions .......................................... 83
BIBLIOGRAPHY ................................................... 84

APPENDIX--Air Mass Percent Occurrence by Station for January,
April, July, and October 1970-1979 ................... 88

FIGURE

2a.
2b.

10.
11.
12.
13.
14.
15.
16.
17.
18.

tDCDNOSUT-bw

LIST OF FIGURES

. Station locations in the study area ......................

Air mass limits in January and April .....................

Air mass limits in July and October ......................

Percentage
Percentage

Percentage

. Percentage

Percentage
Percentage
Percentage
Percentage
Percentage
Percentage
Percentage
Percentage
Percentage
Percentage
Percentage

Percentage

of Continental air in January, 1970-1979 ......
of Continental air in April, 1970-1979 ........
of Continental air in October, 1970-1979 ......
of Polar air in July, 1970-1979 ...............
of Pacific air in January, l970-l979 ..........
of Pacific air in April, l970-1979 ............
of Pacific air in October, 1970-1979 ..........
of Dilute Tropical air in January, l970-l979..
of Tropical air in January, 1970-1979 .........
of Dilute Tropical air in April, 1970-1979....
of Tropical air in April, 1970-1979 ...........
of Tropical air in July, 1970-1979 ............
of Dilute Tropical air in October, l970-1979..
of Tropical air in October, l970-1979 .........
of Dry Tropical air in April, 1970-1979 .......
of Dry Tropical air in July, l970-l979 ........

vi

Page

1 1
18
13
24
26
27
28
30
31
32
34
35
36
37
38
4o
41
42
43

FIGURE Page

19. Percentage of Dry Tropical air in October, l970-1979 ...... 45

20. Mean configuration of the 500mb surface in January (show-
ing areas of high (H) and low (L) pressure height) ........ 52

21. Configuration of the 500mb surface for 002 on Monday
17 January 1977 associated with a Continental air occur-
rence in the study area (showing areas of high (H) and
low (L) pressure height) .................................. 53

22. Configuration of the 500mb surface for 002 on Wednesday
17 January 1973 associated with a Pacific air occurrence
in the study area (showing areas of high (H) and low (L)
pressure height) .......................................... 55

23. Configuration of the 500mb surface for 002 on Saturday
11 January 1975 associated with an occurrence of Dilute
Tropical/Tropical air in the study area (showing areas of
high (H) and low (L) pressure height) ..................... 56

24. Configuration of the 500mb surface for OOZ on Wednesday
16 January 1974 associated with a Dry Tropical Air occur-
rence in the study area (showing areas of high (H) and
low (L) pressure height) .................................. 57

25. Mean configuration of the 700mb surface for the period
15-19 January 1973 (showing areas of high (H) and low (L)
pressure height) .......................................... 59

26. Mean configuration of the 700mb surface for January 1977
(showing areas of high (H) and low (L) pressure height)... 60

27. Mean configuration of the 500mb surface in April
(showing areas of high (H) and low (L) pressure height)... 61

28. Configuration of the 500mb surface for 002 on Monday
29 April 1974 associated with a Tropical air occurrence
in the study area (showing areas of high (H) and low (L)
pressure height) .......................................... 63

29. Mean configuration of the 500mb surface in July
(showing areas of high (H) and low (L) pressure height)... 66

30. Mean configuration of the 500mb surface in October
(showing areas of high (H) and low (L) pressure height)... 70

vii

FIGURE Page

31. Percentages of Tropical (solid lines) and Dry Tropical
(dashed lines) air in July l970-1979 ...................... 74

32. Frontal positions and Midwestern dewpoints for 12002 on
Wednesday 25 April 1979 ................................... 78

viii

CHAPTER I

INTRODUCTION

North American weather patterns are difficult to generalize
because they are produced by an extremely complicated system. However,
generalization is necessary in climatology in order to begin to under-
stand how the different components of a weather-producing mechanism
create the specific combination of environmental conditions on the
earth's surface known as climate. Usually the main variables examined
are the characteristics and movements of air at the earth's surface
and at higher altitudes, and how often different types of air occur in

the area being studied.

The Problem

 

Different types of air with relatively uniform characteristics
are usually referred to as "air masses", and are a convenient way of
analyzing, describing, and thereby understanding the variability of
weather from day to day. Unfortunately, there is no widely accepted
method of classifying air masses. Many systems exist that group air
mass types on the basis of their upper air characteristics (Godson

1950), region of origin (Brunnschweiler 1952,1957; Bryson 1966, 1981),

or both (Showalter 1939; Willett 1940), without giving special atten-
tion to the range of surface conditions associated with each air mass
type. Developing a reliable quantitative air mass classification
scheme is not a simple task because extensive mixing occurs, making

the boundaries between air masses difficult to locate. Also, individual
air masses are modified by contact with the earth's surface as they
move outward from their source regions.

My intent in this study is to examine the geographic distribution
of the major air masses that affect the North Central United States by
delimiting these air masses using 850mb air and surface dewpoint
temperatures. The validity of these limits will be assessed by analyzing
the upper air pattern associated with each delimited air mass type.
Because of the overlapping characteristics of some air masses, the
boundaries between them are often not sharp lines but gradual transition
zones, complicating recognition of the boundaries and reducing the
confidence with which they can be drawn. Therefore, I will examine only
the Continental, Pacific, Tropical, and Dry Tropical air masses because
these are relatively discrete types, and will not attempt to analyze
the many sub-classes and combinations associated with each major air
mass. I will examine the distribution of these air masses in each of
the four seasons (using January, April, July, and October to represent
their respective seasons) and also identify the changes in their numeri-
cal limits during different seasons. The validity of my "limits"
procedure as a method of air mass classification will be addressed by

comparing the results of this study to previous research.

Using the number of commonly recognized major air mass types in
my study area as a reference, I also intend to employ a statistical
classification procedure on the data from one station. A comparison
of the class limits from this statistical grouping with the numerical
limits I find through map analysis will help to determine how well the
statistical groupings appear to agree with physical reality. If the
results from these two methods compare well, the statistical method of
classification may have a future as a refinement of the necessary
subjectivity present in standard climatological air mass research at
this time.

This problem has broad implications for many aspects of climato-
logical research. If a quantitative classification system could be
devised, it would increase our empirical understanding of climate.

The quantitative system would allow air masses to be defined by a
definite range of temperature and moisture conditions, and the air
mass climatology of a region could be analyzed using daily surface and
upper air information. If a relationship between upper air patterns
and specific numerical distributions of air masses in a given geo-
graphical location is demonstrable, a more detailed description of
expected temperature and moisture conditions could accompany an upper
air pattern forecast. Such forecasting is not now routine in part
because of the lack of an adequate description of the range of surface
conditions associated with individual air masses in specific geographi-
cal locations. I believe that this study could be part of the effort
to develop the "better quantitative models" that will be necessary in
order to develop reliable long range weather and climate predictions

(Mather et a1. 1980).

CHAPTER II

REVIEW OF THE LITERATURE

North America and its sub-regions have been the subject of several
air mass studies (Burke 1945, Burbidge 1951, Harley 1962, Henry 1979,
McIntyre 1950, Miller 1953, Weedfall 1970) and studies of other regions
can be found in European meteorological/geographical literature as well
(Belasso 1952, Craddock 1951, Crowe 1965, Frisby and Green 1949, Geb
1975, James 1969;1970, McDonald 1975, Myachkova 1979, Soronking 1976,
Stessel and Van Isacker 1975, Stopa 1970). Burke (1945) studied the
modification of continental polar air over a water surface, and Burbidge
(1951) complemented that study with an examination of the modification
of continental polar air over Hudson Bay. Henry (1979) studied the
problem of separating tropical air from return flow polar air. Miller
(1953) wrote a general reference article concerning air mass clima-
tology, and Weedfall (1970) examined applications of air mass classifica-
tion to West Virginia climatology.

European air mass literature has placed a greater emphasis on
the quantification of air masses. McDonald (1975) used eigenvector
analysis as an aid to air mass recognition, and Stopa (1970) studied
the probability of extreme temperatures and definite diurnal amplitudes
in various air masses. These studies are typical of most of the air

mass literature. In general, research has been directed toward

detailed studies of one or two air masses in a specific area, a general
examination of all air masses in a review fashion, or the use of
commonly recognized air masses in an attempt to better understand some
aspect of local or regional climate. Few have attempted to devise new
methods for separating air masses, and an even smaller number have
tried new methods to describe the complete air mass climatology of a
region. The remainder of my review will be devoted to those articles
that have contributed to this last area of air mass research.

Dodd (1965, pp. 113-122) produced average dewpoint maps for the
contiguous United States for each month of the year based on an average
of ten years of record for each of the approximately 200 stations that
have psychrometric data available. The author also presented standard
deviation patterns for each of the months, and suggested that frequency
of air mass change, diurnal range of temperatures, and the relationship
between the absolute atmospheric water content and temperature should
be considered when interpreting these patterns. Harman and Harrington
(1978, pp. 402-413) addressed the problem of identifying tropical air
in the summertime. They concluded that a dewpoint of 65°F (18°C) at
the surface is the lower threshold of maritime tropical air in the upper
Midwest (p. 404).

Wendland and Bryson (1981) identified "near surface airstream
regions" in the Northern Hemisphere using 16-year mean resultant winds
from 3° latitude by 3° longitude grids. The authors proposed 19 dif-
ferent sources during the various seasons of the year and determined
the period of time each was present over the Northern Hemisphere.

Favored areas for frontal development (those areas subject to incursions

by the most air streams) were described, and they revealed several
north-south bands that represent "the mean leading edge of continental
air masses" (p. 225). Wendland and Bryson postulated that mean dew-
points "demonstrate the character of the moisture discontinuity across
several mean frontal boundaries."

In an earlier study Bryson (1966) analyzed July air mass fre-
quencies from 1945-1951 and 1954-1956 using daily computation of tra-
jectories back to source regions. He compared these results to an
independent analysis of the July air mass frequency distribution during
1948-1957 obtained by resolution of the daily maximum temperature
frequency distribution into partial collectives, i.e., component normal
distributions. Bryson concluded that these two methods produced similar
results, and performed a final analysis using monthly resultant wind
stream lines (produced from ten year mean geostrophic winds over North
America) near the surface which indicated that "mean airstreams and
confluences between airstreams define climatic regions with a distinc-
tive annual march of airstream and air mass dominance" (p. 228). The
final conclusion of the study was that the analyses "strongly suggest
that the boreal forest occupies the region between mean or modal south-
ern boundaries of Arctic air in winter and summer," and that a "distinc-
tive cornbelt climate" existed which "coincides with the corn belt as
defined by land use" (p. 257).

Barry (1967) used the 850mb level to determine the mean position
of the Arctic front during each season of the year from 1961-1965.

Air mass characteristics and boundaries were identified by analysis of

aerological data in the form of tephigrams, hodographs, and vertical

cross sections in order to determine the lines of intersection of frontal
surfaces with pressure levels (pp. 19-80). In comparing his results
with those of Bryson, Barry concluded that the position of the continen-
tal Arctic front in winter appeared to agree with Bryson's Arctic frontal
zone, and in summer the agreement continued, with some problem areas
developing east of 90°W longitude.

Willett (1940) is the author of one of the early studies of North
American air masses. The major types of air masses were classified by
the relative temperature (polar or tropical) and moisture content
(continental or maritime) characteristics of their source regions.
Processes of air mass modification, including surface contact, subsidence,
and turbulence, were explained, and the average characteristics (tempera-
ture °C, absolute and relative humidity) of several of the major
winter and summer air masses were presented. The author concluded that
the investigation of the characteristics of the principal air mass types
could be of great assistance to the synoptic meteorologist and fore-
caster (p. 73). Showalter (1939, pp. 204-218) covered similar topics
and concluded that ten air mass types (Polar Continental, Polar Atlantic,
Polar Pacific, Polar Moist, modified Polar Continental, modified Polar
Pacific, modified Polar Moist, modified Polar Superior, Tropical Mari-
time, and Superior) were of practical synoptic significance for the
United States during most of the year, with fewer types (modified Polar,
modified Polar Moist, modified Polar Superior, Tropical Maritime, and
Superior) in midsummer (p. 217).

Godson (1950) studied the relationship of air masses to frontal

positions and concluded that "an air mass contiguous to a given front

should be given the same designation at all points along the front
regardless of the possible differences in trajectory which might exist"
(p. 90). He noted that air masses that would be classed "maritime"

from their trajectory can be relatively dry at higher levels, thereby
suggesting that the trajectory method alone is not a satisfactory method
for air mass classification. The author postulated that "the number of
air masses in summer should be less than in winter" (p. 91) because of
the disappearance of continental arctic air in the summertime. After a
discussion of the typical temperatures at various upper levels in winter
and summer air masses and the processes of upper level cold dome and
warm pocket creation and destruction, he concluded by proposing that
three-dimensional analysis be included in the preparation of forecast
charts.

Two works by the same author have the closest relationship to my
study. Brunnschweiler (1952) examined North American air masses and
determined the actual daily position of each air mass boundary derived
from daily weather maps (p. 42). The author investigated the three
winter months (December, January, February) and the three summer months
(June, July, August) for the period 1945-1949. Two final maps were
drawn that indicated the average distribution and tendencies of expan-
sion of various air masses (p. 42). Three zones were established:
regions dominated by an air mass at least 80% of the days during the
season concerned (source regions), regions in which the particular air
mass prevailed 20-80% of the period (conflict regions), and regions
invaded by a particular air mass only occasionally, i.e., less than 20%

frequency (p. 42). Three types of air mass boundaries were recognized:

geographic, frontal, and transitional. The author found a "distinctive
dominance of arctic and polar air masses ... in winter" (p. 48), and
in summer "a less uniform air mass regime than that of winter“ (p. 48).
In a later study, Brunnschweiler (1957, pp. 167-195) applied the same
air mass classification methods as above to the entire Northern Hemi-
sphere, and suggested that his method, "aerosomatic", Could be used to
genetically classify climates, with air masses as "the major causative
factors of the climatic differentiation of the Northern Hemisphere"

(p. 195).

If air mass research is to be successfully integrated into the
mainstream of modern c1imatological/meteorological research, it must
become increasingly quantitative. This quantification is necessary if
air masses are to be described precisely so that their frequencies can
be compared to other meteorological variables. Air masses should be
exactly delimited whenever possible. Nearly all of the aforementioned
articles recognized air masses by their average conditions instead of
the range of conditions associated with each type in a given geographical
area. Those studies that did recognize air masses as a range of condi-
tions did not include a quantitative indicator of moisture content
along with their thermal criteria. Therefore, a study is needed that
not only examines the complete air mass climatology of a region but
also provides a workable method for recognizing the range of thermal
and moisture characteristics associated with each air mass type.
Further, the method should identify the transition zones that exist
between air masses. Such a study would facilitate the quantification of
air masses and might open possibilities for air mass research to improve

the detail of long range weather and climate predictions.

CHAPTER III

METHODS

Determination of the Study Area

 

The North Central United States (Figure l) was chosen as my study
area not only because I am familiar with the regional air masses, but
also because the air mass contrasts are pronounced in this region.

North America is the only continent extending from polar to tropical
regions which does not present physical barriers to air mass movement

in a north-south direction. This unique physiography, along with the
central location of the study area on the landmass, allow easy movement
of the air masses from four source regions into the study area. Further,
other studies of this area have yielded results that do not always seem
consistent with daily synoptic patterns. The study area was defined so
as not to include the Rocky Mountains because of the complex air mass
climatology of that region. The South and East were not added to the
study because of the reduced air mass contrast of these areas. Since

my study is concerned with both upper air and surface data, I determined
which weather stations had these data available for the period 1970-1979.
These stations are shown in Figure 1. One station, Monette, MO, had
data from only 1971-1979; however, I considered this sufficient to be

included.

10

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Air Mass Names

 

In this study I am defining air masses using methods that are not
directly comparable to other research, and, as previously mentioned,
only a few relatively discrete types are being analyzed. Therefore, I
decided to use generic names for the air masses that referred to their
source region, conveying a general description of their relative charac-
teristics without directly associating them with names used in previous

research.

Source Regions

 

Fall, Winter, and Spring

 

Continental--Persistent upper level ridging in the western part

 

of North America induces areas of subsidence on the east side of the
ridge during passage of short wave troughs with their negative vorticity
advection areas. This subsidence favors anticyclogenesis, particularly
over north central Canada where cold surface conditions reinforce the
dynamic anticyclones. Taking on the cold, dry characteristics of their
source region, these high pressure air masses frequently travel south-
eastward along the mean flow lines behind moving troughs and affect
the United States. Barry (1967) used the 850mb temperature to distin-
guish these air masses from warmer types of air.

Pacific--During periods of zonal flow, air from the north Pacific
is advected landward behind moving troughs and forced to traverse the
Rocky Mountains where its moisture content is reduced and temperature

increased. This mild, relatively dry air can be distinguished from

13

Continental air by using Barry's (1967) 850mb temperature criteria, and
from moiSter types by the surface dewpoint.

Tropical--Particularly during periods of mean upper level trough-
ing in the western United States, tropical moisture is drawn northward
into the central United States. Cyclogenesis under the modal positive
vorticity advection area that would be located in the lee of the
Rockies would produce storms that move northeastward ahead of short
waves moving out of the mean trough. These storms bring with them a
great amount of tropical moisture from the Gulf of Mexico into the
central and eastern United States in the southerly low-level circulation
on their eastern flanks. This warm moist air was distinguished by
Harman and Harrington (1978) using the surface dewpoint temperature.

Dry Tropica1-—as described below.

 

meet:
Pglar--As the northern regions of North America warm in summer
and lose part of their snow cover, their temperature equals or surpasses
that of nearby oceans. Air occupying the land assumes characteristics
similar to air over the oceans. Therefore air from these two regions
(the Continental and Pacific source regions) that moves into the North
Central United States is indistinguishable during the summer. The name
"Polar" was used to designate this cold, relatively dry mixture of
Continental and Pacific air, which can be distinguished from Tropical air
by Harman and Harrington's (1978) surface dewpoint criteria.
Tropical--During the summer, the strongest mean tropospheric winds
withdraw to an average position north of the study area. This situation,

coupled with the weak circulation over the Texas coast, allows tropical

l4

moisture originally directed northward by high pressure over the
Caribbean Sea to move into the southern United States. The air is drawn
farther north in the southwesterly flow preceding traveling short wave
troughs and their associated cyclones crossing the midwest. As in other
seasons, this air can be distinguished by the surface dewpoint.

Dry Tropical-~During the warmer part of the year the vast conti-

 

nental area in northern Mexico and the southwest United States is heated
to extreme temperatures by the plentiful solar radiation. Air in this
region, also greatly heated, tends to be confined by the Rocky Mountains
to the west and undergoes limited mixing with Tropical air to the east.
Based on my preliminary analysis, this air can be distinguished from
cooler and moister types by using the 850mb temperature and surface

dewpoint.

Assumptions

 

The sources of my assumptions are threefold: Barry (1967), which
relates to 850mb limits; Harman and Harrington (1978), which addresses
surface dewpoint limits, and my own preliminary observations.

1. "Transition conditions" exist between air masses and these
account for a percentage of the total number of days in any season.

2. The months of January, April, July, and October in the years
l970-l979 inclusive provide a sufficient data base from which to deter-
mine the average frequencies of individual air masses in specific
geographical regions.

3. January, April, July, and October represent the best meteoro-

logical characterizations of winter, spring, summer, and fall, respectively.

15

4. The ranges of temperature and moisture content used to identify
the air masses have remained unchanged throughout the period of study.

5. The characteristics of an air mass from a given source region
change as the air mass moves away from the source region. Therefore the
limits defining an air mass may change as distance from the source

region increases.

Operational Definitions

 

1. Air mass--a body of air with a relatively uniform range of
temperature and moisture characteristics observable at or near the sur-
face of the earth. There are several types which originate from differ-
ent source regions on the earth's surface.

2. Transition conditions--groups of days that have meteorological
conditions which fall between air mass types and cannot be accurately
classified. These conditions can occur during or just after the passage
of a frontal zone through the station location. Since a front is a
broad transition zone, it takes time for the atmosphere to assume the
characteristics of the new air mass behind the front. Another source
of transition conditions is air mass mixing; this occurs when two or
more air masses are combined and modified before they reach the station.

3. Dilute Tropical-~air that is clearly of tropical origin, but
has had its moisture content reduced by mixing with other air masses.
This can occur through both dilution and modification. Modification
occurs in the cold months of the year when warm tropical moisture is
advected northward in a thin band over cold land surfaces that have been

recently occupied by Continental air. Dilution is produced when air

16

from a continental anticyclone is mixed with tropical moisture in the
"return flow" on the west side of an eastward moving anticyclone.
I will refer to both forms as Dilute Tropical.

4. Northern group-~stations at Flint, Sault Ste. Marie, Green Bay,
International Falls, Saint Cloud, Bismarck, and Rapid City.

5. Southwest group--stations at North Platte, Omaha, Dodge City,
Topeka, and Monette. I

6. Southeast group--stations at Peoria, Salem, and Dayton.

Data Sources

 

The data required for this investigation were obtained from the
National Weather Service's Daily Map Series. These maps were available
to me:

1. on microfilm at the Michigan Office of Climatology (Michigan
Weather Service), and

2. on themofax paper from 1972 to the present at the Department
of Geography, Michigan State University.

Data Analysis

 

Temperature data recorded at 12002 were used in this study.
Missing values were estimated from the surrounding stations. The limits
were used by a computer program which assigned each day to either an air
mass type or "transition conditions" and then tabulated totals and per-

centages for all types in each of the four seasons.

17

The above method is necessarily somewhat subjective, so the Jenks
statistical classification procedure was used on one station (Flint) in
order to determine whether the data distributions suggest limits which
agree with those found through map analysis. This procedure classifies
by maximizing inter-class variance and minimizing intra-class variance.
Because of computer memory limitations I was able to use only seven of
my ten years of data, but based on my preliminary examination this

number is representative.

Air Mass Criteria

 

The 850mb level is close enough to the surface (1200-1500 meters,
or 4000-5000 feet) to reflect surface conditions, but enough removed so
that it is not substantially influenced by diurnal changes in temperature.
These qualities make it an ideal indicator of surface air mass tempera-
ture, and the 850mb temperature has been established as a reliable way
to differentiate cold continental air from other types (Barry 1967).
Likewise, the surface dewpoint temperature has been regarded as an
acceptable way to distinguish tropical air (which can be recognized by
moisture characteristics alone) from cooler and drier types (Harman and
Harrington 1978). These two measures will be the criteria I will use
to separate and delimit each season's air masses.

The limits (depicted in Figure 2) were determined through map
analysis of air masses of known origin at the surface and 850mb level.
For example, in establishing the upper Continental limit in January, I
identified a number of Continental air masses over their source region,

and then noted the temperatures associated with wind shifts and other

18

.gmnopuo vcm apzw c. muwsw. mmme gw<

 
 

Co. 2.2.33 32:5

33 3.. og. 5....) 3:30. 3 so... 3 3.3» 3.6...

AN 8:3...

...La< ecu xgmaccw :. mu_E_F mmme g_<

 

 

0050 Dawn 0‘. 6.8:, 60:000. :0 5%.... 0‘ 30—50? 036.4 x

.mm wgampd

 

 

...... 2.332. 32.5

 

 

 

.338... q 3:35... _ .0039; to
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smxwm Exam
8... 83836: 3.000
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>4 3..

 
 

 

Co. 2.2.33 383m
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32.5.30 3:35.30
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3.; 8383.3... ...-.000 Go. 83835.; as. one
.338... So _ 3.32.8. _ 0.33.. . 8:768. q 322.28 .338 ... ~54 3.32.8. — v.23“. — 3......8. _ 3.35.30
a. o. o N m. .n. «mm .omr.
munchoo .:¢a<

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3.338. _

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m. m. .v n:

.339; to

 

 

 

19

discontinuities as they moved into and through my study area. After
observing an increasing number of these air masses I became confident
that specific numerical limits could be used to define each air mass.
Essentially the same procedure was followed in defining limits for
other air masses and seasons.

I realized that sharp boundaries may not exist between air masses;
therefore, daily temperature observations that did not clearly fall into
one air mass category or another were designated "transition conditions",
and were not included in any air mass total. Even with these precau-
tions, I suspect that in a few instances a mixture of different air
masses could produce a temperature that would normally be assigned to an
air mass category by my limits. The exact number has not been compiled,
but these days probably account for less than five percent of the days
in any category.

Differential limits dependent on season were established for the
individual air masses because the characteristics of the source regions
varied throughout the year. These changes affected the temperature and
moisture content of air masses in the study area. For example, the
temperature of Continental air increases in response to the shedding of
ice cover over a large part of its northern source region and the develop-
ment of longer days. Tropical air becomes warmer and more moist because
of the warming of the Gulf of Mexico. In general the 850mb limits
applied to the air masses were warmest in the summer, next warmest in
fall, cooler in the spring, and coldest in the winter. The surface dew-
points were highest in summer, lower in fall, still lower in spring, and

lowest in the winter.

20

Geographical location also affected the limits in several seasons.
The characteristics of an air mass in one part of the study area did
not necessarily hold throughout the entire region because in some cases
the air masses were slightly modified as they traversed the study area.
For example, in spring Continental air warmed as it moved southward,
necessitating an upward adjustment in the 850mb limit from -1°C to 0°C.
Differential limits were established by subdividing the study area into
groups that were closer to particular source regions, and then observing
the limits associated with each air mass type in these various sub-
regions.

In light of these geographical differences the following adjust-
ments in the limits were made.

Continenta1--In spring, differential limits were established

 

because Continental air was consistently warmer in the southern part of
the study area. Northern group--(850mb temperature §_-1°C, surface
dewpoint 5_45°F), other stations--(850mb temperature 5_0°C, surface
dewpoint 5_45°F).

Pglar--In summer, the Continental and Pacific air masses became
indistinguishable and are designated together as "Polar". Polar air
was consistently warmer in the southwest portion, and moister in the
southern portion of the study area.

Northern group--(850mb temperature §_15°C, surface dewpoint_g 60°F).

Southwest group--(850mg temperature 5_l7°C, surface dewpoint

§_61°F)

Southeast group--(850mb temperature : 15°C, surface dewpoint

5_61°F).

21

Pacific--In spring differential limits were established because
Pacific air was found to be warmer in the southern part of the study
area.

Northern group--(3°C §_850mb temperature §_15°C, surface dewpoint

5_45°F)

Other stations--(4°C g 850mb temperature §_15°C, surface dewpoint

5_45°F)

Dilute Tropical--This air mass was not recognized as a separate

 

category in summer because in my preliminary analysis I was not able to
reliably separate days of this type from days of "transition conditions"
in this season.

Tropical--In summer, Tropical air was found to have consistently
higher dewpoints in the southeastern part of the study area.

Southeast group--(surface dewpoint 3_67°F)

Other stations--(surface dewpoint 3_66°F)

Dry Tropical--In summer, Dry Tropical air was found to be particu-

 

larly warmer in the southwest, and moister in the southern part of the
study area.
Northern group--(850mb temperature 5 19°C, surface dewpoint 5 60°F).
Southwest group-~(850mb temperature 5_20°C, surface dewpoint g_6l°F).

Southeast group--(850mb temperature §_19°C, surface dewpoint 5 61°F).

Map Preparation

 

The Surface 11 computer mapping routine was used to prepare the

isoline maps. The isoline interval was set so that the resulting maps

22

would accurately portray the range of the data distributions. Air mass
distributions with maximum values greater than 20% were assigned an
interval of ten, and those between 10% and 20%, an interval of five.
Likewise, distributions with maximum values between 8% and 10% were

assigned an interval of 2.5, while those less than 8% were given an

interval of one.

CHAPTER IV

RESULTS

Outline of Presentation

 

In order to share my results in a systematic manner, air mass
types will be described according to their relative importance in the
study area, the most important air mass being presented first. Within
each air mass discussion, the general distribution over the entire
year will be addressed initially, followed by the specifics of the

seasonal distributions in chronological order starting with winter.

Air Mass Distributions

 

Continental

 

This air mass was by far the most important in all seasons in the
northern part of the study area, and it also exerted a marked influence
in the southern region. Percentage occurrence was greatest in the
northeast section and smallest in the southwest.

'Migtgr--(Figure 3)--Most of the study area was occupied by the
air mass fifty percent or more of the time. Importance of this air mass
increased northward across the study area in general, except in the
Dakotas where it was greater toward the east. The fact that the eighty-

percent line included most of Minnesota, Wisconsin, and upper Michigan

23

24

.mnmpuonmp ..umzcmw cw me Fmpcmcwucou we mmmpcoocwa

.m mean.a

 

 

 

 

 

I"'l",".'ll I A

“g“.
-..-”-
----.‘f\- \
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25

was interpreted to mean that these areas have conditions in winter
similar to those of the Continental source region.

Spring--(Figure 4)--As in winter, the percentage occurrence was
greater toward the north and, in the Dakotas, toward the northeast.
The overall frequency was reduced approximately twenty to thirty percent
from winter, which left the influence on the southern regions substan-
tially weakened.

ngmer—-See Polar.

fall--(Figure 5)--Although the autumn percentage contribution of
Continental air was only slightly reduced from that of spring overall
(approximately ten percent), the pattern was more erratic in the south-
ern portion of the study area. Two areas of lessened frequency appeared
in the eastern Nebraska/western Iowa area and in southern Indiana and

Illinois.

Polar

As previously mentioned, air masses designated "Polar" (Figure 6)
in this study were recognized only in summer when air streams from the
Continental and Pacific source regions were indistinguishable. Thus,
"Polar" air may originate from one or both source areas and is the only
cool summertime air mass recognized in this study. It was most important
in the northeast section of the study area (Michigan, Ohio, Indiana,
Illinois, Wisconsin, and Minnesota), where it had a frequency of fifty
to seventy percent. Its impact on the southwest was substantially less

(ten to thirty percent).

.mumwlonmp ..wga< cw awn Faucmcwpcoo mo mmmucoogmm .¢ mgzmwm

 

26

 

 

 

    

,v‘w- ~‘---‘ .. ......
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----------

   

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27

.mumplommp .Lwnouuo Cw me Faucmcwuzou $0 mmmpcwucpwm

.m mgzmwd

 

 

 

 

 

 

 

 

 

 

 

28

.mam_-o~m. .».=o c. a.~ ampoa .0 mmaacmoama

.o mgzmwd

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

29

Pacific

The western part of the study area was influenced by this air
mass throughout the year; however, it did not attain even here the
degree of dominance exhibited by Continental air in the north during
winter. In general, frequencies of the air mass lessened toward the
east, and no contribution in any season was greater than fifty percent.

.fligtgr--(Figure 7)--Highest frequencies were found in the
southwest corner of Kansas (forty percent), with lower values toward
the northeast from that point. The air mass was uncommon in most of
Minnesota, Wisconsin, and Michigan, where it occurred less than ten
percent of the time.

ISpripg--(Figure 8)--The range of percent occurrence across the
region was only about twenty percentage points during this season, from
the mid-forties in the southwest to the mid-twenties in the northeast.
Overall the frequencies increased from winter and were spatially quite
uniform, with only a gradual increase toward the southwest.

Spmmg[--See Polar.

‘fall--(Figure 9)--Whi1e this season was marked by about the same
range of frequencies as was spring, the overall pattern of the distribu-
tion shifted to gradients in a more east-west orientation. In general,
values are greatest in the western part of the study area and lessen
toward the east. Also worth noting is the anomalous area of greater

occurrence that was present in eastern Indiana and Ohio.

30

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31

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32

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33

Dilute Tropical/Tropical

 

As was previously discussed, both of these air types originate
from the same source region; thus, Dilute Tropical is no more than a
sub-class of Tropical. Throughout the year these air masses were
important only in the south and east portions of the study area, and
dominated (greater than fifty percent) even in summer in but a small
portion of southern Illinois, Indiana, and eastern Missouri.

fliptgr--(Figures 10 and ll)--During January the dilute air mass
was found only occasionally in the south and east portions of the study
area (less than five percent of the time). True Tr0pical air was even
rarer, and occurred with a frequency of slightly more than one-half
percent in southern Illinois, Indiana, and southeastern Missouri. In the
remaining portions of the study area, these air masses occurred less than
one-half percent of the time during this season.

Spring--(Figures 12 and 13)--The dilute air mass influenced most
of the southern study area at least ten percent of the time. Tropical
air in this season was somewhat more common than in winter, but greatest
frequencies (less than ten percent) were still found in the southeast
portion of the study area. Across the remainder of the region these air
masses had a smaller influence.

.Summgr--(Figure l4)--As indicated before, the dilute air mass was
not recognized as a separate category in July. Even though the influence
of Tropical air was much greater in summer than in any other season in
parts of the southeast, the extreme west and northwest portions of the

study area were affected to a much smaller degree (less than ten percent).

34

. or...
o mmmpcmogma op mg: .
. c. LFm Fmomaogp mu=.wo m
.mnmplonmp mumscmq . .

 

 

 

 

 

 

 

 

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35

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36

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37

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38

 

 

 

 

 

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39

This air mass dominated (more than fifty percent of the time) in only a
small portion of the southeast during this season.

{ally-(Figures l5 and l6)--During the fall, Dilute Tropical air
occurred throughout the study area ten percent or more of the time
except in most of the Dakotas, Minnesota, and western Nebraska, where
the frequency was less. Overall the number of Tropical air occurrences
in this season was less than summer and comparable to spring, with the
northeast portions of the study area being the least influenced by this

air mass.

Dry Tropical

 

This air mass affected only slightly the portions of the study area
east of the Mississippi River, and was most prominent in the states of
North and South Dakota, Nebraska, and Kansas. Primarily a summer season
phenomenon, it rarely occurred in the other seasons and was least evident
in winter.

‘Njntgrf-No parts of the study area were affected one-half percent
or more of the time in this season by the air mass, so a map was not
prepared.

.Springg-(Figure l7)--Dry Tropical air did not occur in this season
more than five percent of the time in any part of the study area,
although it was more prominent than in winter. Kansas, Nebraska, and
South Dakota were the states most affected. 1

§ummgrf-(Figure 18)--Although the frequency of Dry Tropical air
dramatically increased in summer, the distribution was essentially

limited to the western four states (North and South Dakota, Nebraska, and

40

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41

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42

 

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43

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44

Kansas). Within these states, frequencies decreased rapidly to the east,
and the air mass was dominant (greater than fifty percent) in only a
small area of southwestern South Dakota.

[Elle-(Figure l9)—-Frequencies declined again in fall, with highest
values approaching ten percent. Influence of the air mass was limited

to the four western states, particularly western Kansas and Nebraska.

Air Mass Distribution Summary by Season

 

yfifltgrg-The air mass of primary importance in this season across
most of the study area was Continental. Pacific air was of equal
importance to Continental air in only the extreme southwest. Dilute
Tropical/Tropical air had a small influence on the southeastern portion
of the study area, and Dry Tropical was unimportant anywhere.

.SEEingg-The overall frequency of Continental air declined from the
winter maximum, but the air mass remained dominant in the northeast.
Pacific air became the most important air mass in the southwest, and the
influence of Dilute Tropical/Tropical air increased from winter in the
southwest. Dry Tropical air became recognizable in this season but had
only a small impact in the extreme southwest.

Summgre-During this season, Polar air (a mixture of Continental
and Pacific) was dominant in the northeast. Tropical air increased in
importance in the southeast, and even dominated a small portion of this
area. The frequency of Dry Tropical air dramatically increased from

spring, but this air mass was most important in only the extreme west.

.mmm~-ommp .gmeepeo cm ewe Feeweegk Ago we omegcmugmm

.mp menace

 

45

 

 

 

 

 

 

 

 

 

 

 

46

fallf-The frequency of Continental air returned to levels similar
to those of spring in this season, and Pacific air became most important
again in the west. The influence of Dilute Tropical/Tropical air on the
southeast was reduced to spring-like levels, and the frequency of Dry
Tropical air in the southwest decreased substantially from summer to

levels slightly greater than those common in the spring.

Jenks Classification Technique

 

Limits obtained by this method are compared to those found through
map analysis. In the discussion below, the comparable map analysis

limit appears in parentheses after each Jenks limit.

Results from Flint Data

.fligtgrg-Surface dewpoint limits were underpredicted in this season
in all cases; however, the 850mb temperature limits were quite similar
to those found through map analysis. For example, the Dilute Tropical
dewpoint limit was 30°F (40°F), while the lower Pacific 850mb limit was
0°C (0°C).

Springr-Dilute Tropical and Tropical dewpoint limits were pre-
dicted quite accurately; however, as in winter, those for the colder air
masses were underpredicted. As examples, the Tropical dewpoint limit
was 60°F (59°F), while the Continental/Pacific dewpoint limit was 37°F
(45°F). The 850mb limits for this season were similar to the map analy-
sis limits, but provided for a greater range of transition conditions

between the Continental and Pacific air masses. For example, the upper

47

850mb limit for Continental air was -3°C (-l°C), and the lower limit for
Pacific air was 6°C (3°C).

Summere-The limits produced by the technique for this season were
in close agreement with those found by map analysis at both the 850mb
and surface levels. As examples, the surface dewpoint limit for Tropical
air was 60°F (60°F), and the 850mb limit for Dry Tropical air was l8°C
(l9°C).

fallf-As in spring, the dewpoint limits for Dilute Tropical and
Tropical air were predicted with reasonable accuracy by the technique,
but the cold air mass limits were still underpredicted. For example,
the Tropical dewpoint limit was 64°F (63°F), while the Continental/
Pacific dewpoint limit was 4l°F (47°F). The 850mb limits in this season,
as in spring, were similar to those obtained through map analysis, but
provided for a greater range of transition conditions between the
Continental and Pacific air masses. As example, the upper 850mb limit
for Continental air was 3°C (2°C), and the lower limit for Pacific air
was 9°C (6°C).

In summary, a number of the Jenks limits agreed quite well with
those found through map analysis, although many were somewhat different.
The technique predicted best the map analysis limits of summer, and was
least accurate in winter. Also, the surface dewpoint limits were some-

what better predicted than the 850mb temperature limits.

CHAPTER V

DISCUSSION OF RESULTS

Relationship Between Dpper Air Flow Patterns and
General Air Mass Climatology

 

To place these results in a broader climatological context, the
percentage maps presented in the previous chapter will be considered in
relation to monthly upper air circulations. Specifically, the results
of the air mass classification were examined in order to determine a
representative day or group of days for each air mass type in each month
studied. The upper air pattern on one or more days was then extrapo-
lated as an example of the mean flow associated with the appearance of
the given air mass in the study area. Thus, I will demonstrate the
different circulation patterns I found to be associated with each type.
Also, mean and modal monthly patterns will be used in an attempt to
explain the relative distribution of the air masses in each season.

Within the westerlies, the disposal of air masses from source
regions is not random, and their trajectories can be related at least
in part to the position of mean planetary waves. As planetary flow
lines determine the path of traveling waves and surface pressure
features, they also guide the movement of the associated air masses.

In general, air movement in the upper atmosphere is parallel to the flow

48

49

lines, and a geographic area must be "downstream" in the flow from a
source region if it is to be affected to any degree by air masses from
that source region (which move outward in association with traveling
waves and pressure cells). Thus, particular flow patterns will facili-

tate the movement of specific air mass types into the study area.

Modal Upper Air Patterns

 

If the daily 500mb pressure heights in the Northern Hemisphere
are observed for an extended period, clear geographical preferences for
some of the features will be noted. For example, a mean ridge position
is common near the Rocky Mountains. This association of air flow with
certain physical features is not random, and both mountain ranges
(orographically) and oceans (thermally) influence the atmosphere by
sustaining certain modal patterns in a given geographical location
(Bolin 1950). Although the persistence of modal patterns is not diffi-
cult to explain, the causes of deviations from these preferred flows are
more complex.

The westerlies are a continuous flow of air that encircles the
Northern Hemisphere, and this arrangement allows "teleconnection" to occur.
Changes of amplitude in one part of the flow can be transferred down-
stream to other areas. Thus, changes of amplitude in the upper air
patterns over North America could be the result of events half-way or
more around the world. The concept of the "Stationary wavelength" adds
an additional dimension to the interdependency of the planetary wave

train. The faster the air is moving within the westerlies, the more

50

rapid the downstream movement of a short wave; however, longer wave-
length can compensate for higher wind speed and slow wave migration.
Therefore, for any given meridional thermal gradient and associated
zonal index and mean wind speed, a stationary wavelength can be calcu-
lated that will balance the effects of wind speed and wavelength and
allow long waves to remain "anchored" in favored positions determined
by thermal and topographic factors with "resonant" waves spaced sym-
metrically downstream.

The stationary wavelength changes seasonally with wind speed and,
further, is not often an integer, while the number of waves in the upper
air circulation must be an integer because of the continuity of the flow.
If the prescribed stationary wavelength is 3.5, for example, the
hemispheric circulation may modulate between three and four planetary
waves in an attempt to reach stability and thereby cause deviations from
the modal flow. These deviations can cause realignment of all the mean
wave positions, which can in turn change the relative frequency of
different air masses in a given region. Beyond seasonal changes, tele-
connection, and stationary wavelength considerations, Namias (l969)
presented yet another possible mechanism to account for some deviations
from the modal positions. He proposed that anomalous sea-surface
temperatures in the central and northern Pacific were responsible for
a variation of the upper air flow that in turn produced climatic fluctu-
ations in North America.

In summary, modal patterns can conveniently explain much of the
upper air circulation structure, but deviations from these most frequent

features do occur, and are produced by a number of processes. Deviations

51

are expressed as changes in the amplitude or location of the mean long
waves, which can then alter the distribution and frequency of air

masses in the study area.

Relationship Between Upper Flows and Monthly
Air Mass Percentages

 

 

.Nintgrr-The mean height of the 500mb surface in January (Figure 20)
is an areal average of the component patterns of the month as well as a
representation of the relative frequency of each pattern. This mean
chart depicts a low amplitude ridge just off the west coast and a trough
in the eastern United States. In review, Continental air was dominant
in this season, with Pacific air second in importance and Tropical and
Dry Tropical of only minor influence. Given that the occurrence of
specific air mass types is related to certain upper level circulations,
the statistical dominance by Continental air suggests that the upper air
pattern associated with it throughout the study area would be the modal
arrangement of winter, and consequently would most resemble the mean
pattern in terms of long wave location. The pattern associated with a
specific occurrence of Continental air (Figure 21) displays the same
ridge-trough system as the mean map except that both features have higher
amplitude. Amplification of a western ridge facilitates the movement of
Continental air from the central Canadian source region into the study
area. It accomplishes this by favoring anticyclogenesis under the nega-
tive vorticity advection area on the east side of the ridge, and then
directing the resulting anticyclones southeastward into the study area,

both in association with moving waves.

52

 

~\\:> ‘0

 

 

 

 

 

 

 

%\

 

 

Figure 20. Mean configuration of the 500 mb surface in January
(Showing areas of high (H) and low (L) pressure height).
Source: Adapted from Lahey et al. (l958).

53

 

 

 

 

 

Figure Zl. Configuration of the 500 mb surface for 002 on Monday
l7 January l977 associated with a Continental air occurrence in
the study area (showing areas of high (H) and low (L) pressure
height .

Source: Adapted from National Meteorological Center facsimile
product.

54

In contrast, the pattern of a single Pacific occurrence (Figure 22)
is characterized by a low amplitude Gulf of Alaska trough coupled with a
relative zonal flow across North America. This type of flow contains
Continental air north of the study area and allows Pacific air to move
into the region behind passing Alberta-track cyclones. A deep trough in
the central United States is the distinguishing upper air characteristic
of a single occurrence of Dilute Tropical/Tropical air (Figure 23).
If a surface low was associated with this trough, it would most likely
have formed in the Oklahoma panhandle region and then tracked north-
eastward following the moving vorticity maximum. The southerly surface
winds associated with this cyclone would bring Tropical moisture into
the southeast portion of the study area as the low moved northward.

Only one day in January during the study period was classified as
Dry Tropical (Figure 24). The associated pattern is characterized by a
low amplitude ridge over the west-central United States, and is similar
to the Pacific pattern (Figure 22) in that a Gulf of Alaska trough
exists (though amplified) and the flow is generally zonal. The occur-
rence of Dry Tropical air in this season is very uncommon because, in
order for the air mass to occur, the upper air ridging pattern must per-
sist long enough for heating to take place in the Dry Tropical source
region. This heating is necessary because during usual conditions in
the winter, the Dry Tropical source region is substantially cooled from
summer, such that it is seldom able to produce a recognizable air mass
quickly. Further complications may be posed by snow cover in the source

region, which must be melted before substantial heating can occur.

55

 

 

 

 

 

 

 

 

 

 

 

 

Figure 22. Configuration of the 500mb surface for 002 on Wednesday
17 January 1973 associated with a Pacific air occurrence in the
study area (showing areas of high (H) and low (L) pressure height).
Source: Adapted from National Meteorological Center facsimile
product.

 

56

 

 

 

 

 

Figure 23. Configuration of the 500mb surface for 00Z on Saturday
ll January l975 associated with an occurrence of Dilute Tropical/
Tropical air in the study area (showing areas of high (H) and low (L)

pressure height).
Source: Adapted from National Meteorological Center facsimile product.

 

 

 

 

 

Figure 24. Configuration of the 500mb surface for 002 on Wednesday

16 January 1974 associated with a Dry Tropical air occurrence in the
study area (showing areas of high (H) and low (L) pressure height).
Source: Adapted from National Meteorological Center facsimile product.

58

From this description of some of the components of the monthly
mean, it should be evident that the features of the flow associated
with Continental air are most similar to the mean condition and could
represent the modal circulation. During a typical January, Continental
air is the dominant air mass, but Pacific air also occurs for several
days. These Pacific occurrences (Figure 25) may be associated with the
mid-winter "January" thaw. Occasionally, Continental air flow is so
prevalent that it persists essentially unchanged for an entire month or
more, producing below normal temperatures across the central and
eastern United States because of the constant Continental influence.
January 1977 was an example of such a period (Figure 26). The occurrences
of Tropical and Dry Tropical air are rare, and likewise the patterns that
produce them are markedly dissimilar from the mean pattern.

.§E£flflgf-The mean 500mb height map for April (Figure 27) is somewhat
different from that of January, in that the western ridge has less ampli-
tude and lies slightly east of its January position, and the eastern
trough also displays less amplitude but generally maintains its position.
In comparison to winter the spring flow is more zonal, and the north-
south pressure height gradient is reduced. In review, during this season
the Continental air mass decreased in importance from winter throughout
the study area but remained common in the northeast. Pacific air was
prominent and Dry Tropical air increased its importance in the southwest,
while the influence of Dilute Tropical/Tropical air increased in the
southeast portion of the study area.

Examination of the major upper air flow patterns common in the

spring indicated that, with the exception of Tropical air, the flow

59

 

 

 

 

 

 

 

 

 

 

Figure 25. Mean configuration of the 700mb surface for the period
15-19 January 1973 (showing areas of high (H) and low (L) pressure
height .

Source: Adapted from Wagner (1973a).

 

60

 

 

 

 

 

Figure 26. Mean configuration of the 700mb surface for January
1977 (showing areas of high (H) and low (L) pressure height).
Source: Adapted from Wagner (1977).

61

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 27. Mean configuration of the 500mb surface in April
(showing areas of high (H) and low (L) pressure height).
Source: Adapted from Lahey et a1. (1958).

62

associated with each air mass type was essentially the same as in winter.
The new Tropical pattern (Figure 28) moves the deep trough from the
central United States into the desert Southwest and California. If a
surface low were associated with this new trough position, it would most
likely form in the Colorado area and track northeastward. Tropical
moisture, already directed northward by high pressure over the Gulf of
Mexico, would be incorporated into the southerly flow of this cyclone
and consequently be transported into the southeastern portion of the
study area.

Although it appears that most of the component upper air patterns
and associated surface pressure features of spring are essentially the
same as winter, the different air mass distributions remain to be
accounted for. Because the air mass criteria were seasonally adjusted
for spring warming of all the air mass categories, the different percent-
ages of spring in comparison to winter must reflect seasonal contrasts
in the flow aloft. For example, the upper air arrangement linked with
Continental air is probably less frequent in the spring though the pattern
itself is essentially the same as in winter, whereas flows associated
with Pacific, Tropical, and Dry Tropical may be expected to increase in
frequency (and thus in overall importance). This change in relative
frequency of the component patterns probably accounts for the new monthly
mean. The April mean cannot be explained in relationship to a modal flow,
as in winter, since there no longer appears to be a single predominant
pattern. Rather, it is probably best understood as the areal average

of several equally frequent flow arrangements.

63

 

 

 

 

\ \ r

 

Figure 28. Configuration of the 500 mb surface for 002 on Monday
29 April 1974 associated with a Tropical air occurrence in the
study area (showing areas of high (H) and low (L) pressure height).
Sougce: Adapted from National Meteorological Center facsimile

pro uct.

64

In the central United States during the spring and particularly
in April, upper level cyclonic flow (troughs and closed lows) occurs
more often than in any other season (O'Conner 1964, p. 306). The mean
storm track associated with these circulations brings surface lows
through the study area in substantial numbers (Reitan 1974). The in-
creased occurrence of these upper level and surface pressure features
is caused by increased troughing in the western United States which,
more importantly to this study, is associated with the same circulation
that brings Tropical air into the study area (Figure 28).

Why western troughing increases in the spring is not well under-
stood. Pyke (1973, p. 26) thought that modification in the west and
mid-Pacific wave position might be responsible. He postulated that, in
spring, western Pacific cyclones develop at a somewhat lower latitude
than in fall and move toward the northeast. Increased temperature
gradients at these lower latitudes cause the cyclones to deepen rapidly
and reach maturity in the Aleutian area instead of in the Gulf of Alaska.
"As the result of this tendency for intensification and northeastward
curvature of Pacific cyclones at longitudes farther to the west during
early spring, the downstream ridge and the next downstream trough should
be located further west than they are in late fall.’I Such a situation
would favor a mean trough over the western United States. Thus, the
frequent occurrence of the western trough pattern in spring produces a
change in the relative distribution of air masses in the study area and
also modifies the mean long wave pattern from winter.

Although a western trough is associated with northward invasion

of Tropical air, this type of circulation does not always result in the

65

occurrence of the air mass in the study area. Many times when this pat-
tern occurs, Tropical moisture moves northward at only upper levels,

and in other cases the cyclones associated with western troughing move
through too fast to allow time for Tropical air to be drawn northward
into the study area (Harman and Harrington 1978, p. 404). Therefore,
although the western trough is a common circulation feature of spring,
the occurrences of Tropical air in the study area remain comparatively
small.

Summgre-The mean 500mb circulation in July (Figure 29) is composed
of three main features: a trough off the west coast, a ridge in the
west-central United States, and another trough in the east. Compared
to spring, the summer flow is positioned farther to the north and great-
est wind speeds are generally north of the study area. In review, the
Continental and Pacific air masses were indistinguishable in this
season, and "Polar" was used as the name for this mixture of airstreams.
Polar air was an important factor in the north, but of limited influence
in the southern portion of the study area. Tropical air substantially
increased in frequency throughout the study area, while Dry Tropical
was dominant in a large area of the west.

A major change takes place in the circulation from spring to
summer. Increased insolation during the summer (particularly at high
latitudes) warms the polar atmosphere, which reduces the meridional
thermal gradient. The strongest winds in association with a reduced
north-south temperature differential would generally be located farther
to the north (toward the pole where the greatest temperature contrasts

are found) and the stationary wavelength would also be reduced, thereby

66

 

 

 

 

 

 

I
.
\
... I ' .
.
-._ ._,__‘._. I" _‘ _ .-.II :
~ .r" . 4-
r .‘ ~ ‘1
g b— "" ”"x \ '
a .
: \ . ,4 .
\ .
> . : /
v "'
' l
. ' a
} ~ : v .
~.., . . . __ ..
“ ._ : 3 y.-
“f” -. _ : ....' .... . -’
‘ .— __ _ ,_ ...—u ‘ .. _ . ~ .1
o..- .
~ .
- ,4' '
....._. I '''''' \
1 ...... \
.i \
~__..—.x
“-~._.o'
" fl - I' '1
....
..‘
—' .
'

 

 

 

Figure 29. Mean configuration of the 500mb surface in July
(showing areas of high (H) and low (L) pressure height).
Source: Adapted from Lahey et al. (1958).

67

increasing the number of planetary waves necessary for hemispheric
stability. Thus, an increase in the number of mean planetary waves is
consistent with a decrease in mean wavelength across North America from
spring to summer. Regarding the positioning of the flow in summer, the
arid and semi-arid regions of the western United States and Mexico are
heated to extreme temperatures in this season, which would cause pres-
sure heights to rise and favor ridging over these areas (Sutcliff 1951),
in strong contrast to the troughing frequent in spring. Therefore, the
summertime mean ridge over the central United States is most likely
thermally induced (University of Chicago l947). This seasonally forced
feature, coupled with the decrease in the stationary wavelength and con-
sequent increase in the number of mean waves, then helps account for the
transformation of the modal upper air circulation from the two-feature
spring pattern (ridge-trough) to the characteristic three-feature summer
pattern (trough-ridge-trough).

As in spring, the summer air mass criteria were seasonally adjusted
to allow for warming of all the air mass categories, so the different
percentages of summer in comparison to spring must reflect seasonal
contrasts in the flow aloft. Even with these summertime modifications
in the overall circulation, the patterns associated with the individual
air masses remained similarly distributed to that of spring, though
universally shifted to the north. Polar air occurrences throughout the
study area were associated with an amplification of the western ridge
and eastern trough, as in spring, but the air mass seldom influenced the
southern portions of the study area because of the northern position of

the westerlies.

68

During both Tropical and Dry Tropical occurrences, a trough was
located near the west coast with a ridge in the central United States
and another trough in the east, although the ridge associated with the
Tropical pattern was of greater amplitude. Since this trough-ridge-
trough pattern is similar to the mean, it could be the modal pattern of
summer, which would account for the relative prominence of both the
Tropical and Dry Tropical air masses in this season. The Dry Tropical
air mass could be pulled into the western portion of the study area in
association with the southwesterly flow to the south of a cyclone moving
eastward through the northern United States or southern Canada. Klein
(1957) has demonstrated that this trajectory is a mean storm track in
July, which would then account for the large number of Dry Tropical
occurrences in this season. Tropical air also could be transported into
the study area in the southerly flow of cyclones crossing the northern
Great Plains, but amplification of the central United States ridge would
change the direction of storm movement from east to northeast. This
change in trajectory would then divert Dry Tropical air northward into
Canada and allow Tropical air to move into the northern study area.

In summary, the northward migration of the westerlies is the
primary agent responsible for the changes in the distribution of air
masses across the study region from spring to summer. With most of the
study area positioned south of the strongest winds, the Dry Tropical and
Tropical air masses could be easily transported northward by cyclones
following the mean storm track. Likewise, Polar air (found generally
north of the strongest circulation) was prevalent in the north, but had

limited influence on the southern portion of the study area.

69

f§11;-The mean 500mb circulation of October (Figure 30) is composed
of a low amplitude ridge in the west and trough in the eastern United
States. In comparison to summer, the fastest wind speeds in fall are
positioned farther to the south (University of Chicago 1947). Although
the October mean is similar to the April mean circulation, the fall
pattern displays less amplitude and is nearly zonal across the United
States. In review, Continental and Pacific air were again recognized as
separate air masses in this season. The frequency of Continental air
increased to spring-like levels, and Pacific air became important in the
west. The frequencies of Dilute Tropical/Tropical air in the southeast
and Dry Tropical air in the western portion of the study area were
reduced from their summertime maximum during the fall.

The fall mean circulation returns to the two feature (ridge-trough)
circulation that is typical of all seasons except summer. This change
is consistent with the fall cooling in high latitudes, which increases
the stationary wavelength in response to an increased meridional thermal
gradient. The fall air mass criteria were seasonally adjusted to account
for cooling of all the air mass categories, and the different percentages
of fall in comparison to summer must reflect seasonal contrasts in the
flow aloft.

Even though the fall mean circulation resembles that of spring,
the two appear to be produced from somewhat different component flows,
and the strongest winds in October are located north of their position
in the April circulation (University of Chicago 1947). The occcurrences
of Continental air in this season were associated with amplification of

the western ridge and eastern trough into a flow pattern similar to that

70

 

 

 

 

 

 

 

 

I
‘ ,
- —. _ ._‘_,, — -
...... .. - .... .-.—\_ \ /
a
\ """
\
.
I»-_ : .‘ v
....... : ‘ ' -
“Mu. : 0’
H‘ m ‘ - I
.0 ._ 4“ ...—......" . “ ... '
. ”-..—- ‘
g .‘ ‘ ‘
. ...... —. \
. ; o
. "fl . \
. ~ ,
. > .,..-
E 3 \
. .
. .
s I
~‘ .--fl-x
~.._.- .
.'-
‘r— '”
'.
‘h- _ ‘ v' "
— \ ~..- ,
l ’
.
.
.

 

 

 

 

Figure 30. Mean configuration of the 500mb surface in October
(showing areas of high (H) and low (L) pressure height).
Source: Adapted from Lahey et al. (1958).

71

associated with Polar air in the summer. The northern location of the
fall mean jet stream generally confined Continental air to the northeast,
and thus limited its influence elsewhere in the study area. Pacific air
occurrences in the study area were associated with a circulation composed
of a trough off the west coast of Canada and zonal flow across the north-
ern United States. If a surface low were associated with this circula-
tion it would most likely form near the Gulf of Alaska and track eastward
across central Canada in connection with a moving trough. Pacific air
would be pulled into the study area behind this system. Klein (1957)
found this trajectory to be a mean storm track in October, which would
account for the importance of Pacific air in this season throughout much
of the study area.

The upper air circulation associated with Dry Tropical and Tropical
air occurrence in the fall are similar to those of summer, although
shifted slightly southward. As in summer, during this season these air
masses moved into the study area in response to the circulation around
cyclones following a storm track eastward across the northern Great
Plains. In the fall, the modal North American circulation is a two
feature (ridge-trough) system, which reduces the number of central
United States ridge occurrences (a component of the Dry Tropical/
Tropical pattern) in favor of a mean ridge position farther to the west
(O'Conner 1964). This autumn decrease in central United States ridging
is consistent with the decreased frequency of the Dry Tropical and
Tropical air masses in the study area.

In summary, Continental air was important in the northeast but of

reduced influence elsewhere because of the northern location of the fall

72

mean jet stream. Pacific air was particularly important in this season
in the west, as passing cyclones following the mean storm track brought
this air mass into these portions of the study area. Dry Tropical and
Tropical air were reduced in importance from summer throughout the

study area in this season probably because their component flow patterns
became less frequent. This change in air mass distribution from summer
to fall is consistent with a mean circulation that has been modified
from a three feature (trough-ridge-trough) to a two feature system
(ridge-trough), with the highest wind speeds shifted somewhat to the

south.

Special Topics

 

Throughout my presentation and discussion, I have alluded to
several special problems that have resulted either as a direct conse-
quence of my analysis technique or as a possible product of the uncer-
tainty associated with specific air mass limits in certain locations.
Since these topics deserve additional discussion, I will present a more
detailed examination of each in this section.

Transition Conditions--One of the innovative aspects of my method

 

of air mass classification is the recognition of "transition conditions"
between the various air masses. Although the existence of these condi-
tions has been noted in previous research (Bryson 1966, p. 238), the
problem of their delimitation has been largely ignored. The usefulness
of the "limits system" employed in this study can be judged to a large

degree by its ability to correctly quantify these elusive boundaries.

73

The method should place days in their appropriate air mass categories
while at the same time excluding and identifying as "transition condi-
tions" all remaining days.

In general, it is difficult to pre-determine exactly what percent-
age of the total days at a station should be classed as transition
conditions. My initial quess was that perhaps seventy-five percent of
the days should be included in air mass categories, which leaves twenty-
five percent unclassed. After completing my study I found that in almost
all cases the number of transition days accounted for between about
twenty and thirty percent of the total days at a station in any season.

One of the largest exceptions to the above results was noted in
the southwest portion of the study area during July. In that area the
number of transition days exceeded forty percent and approached fifty
percent of the total in some cases. Clearly, some of this excess number
could be caused by inadequacies of my limits, and I decided to study
this situation in greater detail. An inspection of the characteristics
of these days revealed that many were warm enough at the 850mb level
to be classified as Dry Tropical but had too high a surface dewpoint to
be accepted into that category. Most of the others failed both tests,
i.e., their surface dewpoints were too high and their 850mb temperature
too low for them to be classed as Dry Tropical.

I suspect, since this area (Kansas and Nebraska) is situated
between regions that I found to be dominated by Dry Tropical and Tropical
air masses during July (Figure 31), and, as previously mentioned, both
of these air masses are drawn northward into the study area by similar

synoptics, that these transition days might represent actual mixtures of

74

.mmmpuommp apee cw ewe Amecwp emsmeev
Feeweegp men use AmeewF ewpemv Feuweech me mmmeucmegma .Pm exempt

 

 

 

 

 

 

 

.
-
."' "“-.’I".y

cafe

 

 

 

75

the two air masses. Using Dodge City as an example, I found that the
number of transition conditions days was inversely related to the
number of days classified as Dry Tropical, while unrelated to the other
air mass totals (Polar and Tropical). I identified the air mass dis-
tributions during July of 1973 and 1974 as examples of this relation-
ship. During July of 1973, the number of unclassified days was 54.8%,
while Dry Tropical air accounted for only 22.6% of the total. In con-
trast, July 1974 had 19.3% of its days unclassed, and 74.2% classified
as Dry Tropical. The climatic difference between these two summers
has been noted by Wagner (1973b;l974). July 1973 was distinguished by
cooler and moister-than-normal conditions across the central United
States caused by a westward displacement of the mean long wave ridge.
July 1974 found a mean ridge displaced eastward across the central
states (Dry Tropical pattern) with warmer and drier-than-normal condi-
tions prevailing there.

Based on this preliminary analysis it seems reasonable to conclude
that many of the large average number of "transition conditions" days
found in the southwest portion of the study area during July were in
fact produced by a mixture of Tropical and Dry Tropical air. During
summers when the mean long wave ridge is displaced to the west of its
normal position, the distribution of Dry Tropical air is moved west of
the study area. The area of Dry Tropical/Tropical mixing is also dis-
placed westward in this situation, which leads to a greater-than-normal
frequency of unclassed days in the southwest portions of the study area.
Conversely, during summers when the mean ridge is displaced to the east

(mean Dry Tropical pattern), the number of transition conditions days

76

decreases, since Tropical air is then synoptically inhibited from
moving into the southwest portions of the study area.

Increased Occurrences of "Pacific" Air in Ohio During October--
As was evident from Figure 9, my limits identified a larger percentage
of Pacific air occurrences in Ohio than in Indiana and Illinois during
October. Since this situation is inconsistent with the physical place-
ment of Ohio in relationship to the Pacific source region, the limits
used to identify Pacific air at this station may have been inappropriate.
A closer examination of days identified as "Pacific" at Dayton, but not
at Peoria, revealed two different synoptic arrangements. In the first,
a low pressure area stalled over the east coast of Virginia. This pres-
sure cell caused the wind flow at the 850mb and surface levels to be
from the northeast across most of the northeastern United States. This
arrangement favored mixing of air from the Atlantic with air of
Continental origin, thus producing a pseudo-Pacific air mass at Dayton.
In the second situation, a high pressure area positioned off the
Carolina coast directed air from the southeast toward Dayton. This air
was a mixture of Tropical and return flow Continental air, which was
not moist enough to be correctly identified as Dilute Tropical, but was
instead classed as "Pacific".

Therefore, the limits of Pacific air may be inappropriate at the
Dayton station in October, as they have incorrectly classified several
mixtures of other air masses as Pacific air. Because these days do not
account for more than approximately five percent of the total number of
days at the station in October, this problem does not represent a

serious limitation.

77

Synoptic Agreement--One of the problems encountered in numerical

 

air mass classification is that of agreement between the results of
numerical and synoptic analysis in air mass designation. The circula-
tion on each day must be consistent with the air mass category determined
for that day by the quantitative method. For example, in establishing
the limits of Tropical air, I discovered a large number of days that

were not being classed as Tropical because of low dewpoint temperatures
despite the fact that their associated circulation pattern appeared to

be similar to days that were classed as Tropical. An example of one of
these days is shown in Figure 32. The warm front has moved through
Michigan, and by frontal and wind analysis alone this area would be
classed as Tropical. In contrast, an analysis of dewpoints indicates
that Tropical air has reached only Illinois (dewpoints greater than 59°F).
I felt that days of this type should not be considered as "transition
conditions", since they were clearly associated with an identifiable
synoptic pattern. Further, since the upper air flow on these days was
essentially the same as days I identified as being "Tropical", these
days must be tropically influenced. Thus I applied the name “Dilute
Tropical" to conditions where Tropical synoptics were associated with

air masses that nonetheless had depressed dewpoint values.

Validity of the Limits Technique

 

Only two known previous studies (Bryson 1966, Brunnschweiler 1952)
produced results similar enough to mine to be effectively compared.
Unfortunately, neither of these studies addressed the air mass distribu-

tions of spring or fall, and only one (Brunnschweiler 1952) examined

78

 

 

 

 

 

Figure 32. Frontal positions and Midwestern dewpoints for lZOOZ
on Wednesday 25 April 1979.

Source: Adapted from National Meteorological Center facsimile
product.

 

79

winter. A comparison of my results with those of Bryson (1966) for
July showed general agreement between the two studies, particularly
regarding regions dominated (more than fifty percent) by an air mass.
Considering that I dealt with only major air mass types, that my "Polar"
category was a mixture of the Continental and Pacific airstreams, and
the limits I selected placed Dry Tropical air dominance farther north
than Bryson's method did, I believe the results of the two studies show
similar distributions. The greatest contrast between the results of
these two studies occurred with regard to the width of the transitional
areas between regions of single air mass dominance. Bryson's work
showed these transition zones to be rather narrow bands, while the
method I employed allowed for broader transition zones.

My results also compared well to those of Brunnschweiler (1952)
in most cases. For example, both methods revealed the dominance of
Continental air over most of the study area in January. In July, there
was low spatial agreement between Brunnschweiler's results and both mine
and those of Bryson. The generally favorable comparison between the
patterns yielded by my research and those produced by previous research
leads me to conclude that the "limits method" is a valid technique of

air mass classification.

CHAPTER VI

SUMMARY, RECOMMENDATIONS, AND CONCLUSIONS

mm

The method of air mass classification employed in this study
delimits air masses and the transition conditions between them through
the use of both surface and upper air temperature data. The limits
were determined through map analysis of surface and 850mb level condi-
tions of air masses of known origin. Temperatures that did not clearly
falfl into one air mass category were designated "transition conditions"
and were not included in any air mass total.

My results showed that:

1. Continental air was the dominant air mass in all seasons in

the north, and it was also influential in the southern por-

tions of the study area.

2. Pacific air was prominent in the western portions of the
study area, particularly in spring and fall, but of less

importance elsewhere.

3. The Dilute Tropical and Tropical air masses were important

only in the south and east portions of the study area.

4. Dry Tropical air was primarily a summer season phenomenon

affecting mainly the western portions of the study area.

80

81

Also, I demonstrated that a unique 500mb flow appears to be
associated with the occurrence of each air mass in the study region,
and that mean and modal flow patterns of the various seasons may be

explainable in terms of the frequency of these component patterns.

Recommendations

 

The results of this study were only a first approximation of air
mass distributions and there are a number of ways I recommend that the
method could be improved. First, the Jenks method of data classifica-
tion (or a similar system) should be used as another input into the air
mass delimitation problem. Although this statistical method does not
handle "transition conditions" well, it can be useful in finding
natural breaks in the data distributions. Secondly, a mean 500mb map
should be compiled for each air mass category in every season. I pro-
pose that these maps be a statistical composite of the 500mb surface
on all days when the particular air mass occurred in fifty percent or
more of the study area. In order to test the assumption that these mean
flow maps, when compiled, would reflect a single persistent or character-
istic pattern rather than an average of several dissimilar ones, a mean
map could be drawn from a randomly selected subset consisting of perhaps
ten percent of the days from each air mass category. If the map from
this subset is areally similar to the overall mean flow map of each air
mass, a single pattern was probably responsible for the mean, supporting
the contention that specific upper air flows are associated with the

occurrences of different air masses in specific geographical locations.

82

Another improvement of my method would be to decompose the
"transition conditions" days into sub-groups related to specific air
masses. For example, this procedure would allow the origin of unclassi—
fied days occurring between Continental and Pacific air masses to be
explored as a problem separate from unclassified days between other air
masses. Examination of the distribution of these component "unclassified"
days and their associated flow patterns may help in redefining the
numerical limits for better agreement with synoptic conditions.

One of the assumptions of the partial collective method employed
by Bryson (1966) is that different source regions produce air of differ-
ent mean characteristics, with a normal distribution of temperatures
about that mean. This assumption could be used to address the problem
of the adequacy of sample size in my study. Assuming that the frequency
distribution of an infinite number of temperatures associated with a
particular air mass is indeed normal, a sample air mass distribution
would not significantly differ from normal if the sample size is adequate.
Therefore, an analysis of the frequency distribution of temperature and
moisture in each air mass could be used to statistically determine the
adequacy of a particular sample size.

The results of a partial collective analysis similar to Bryson's,
but also using 850mb temperature and surface dewpoint data as input,
could be useful in developing an approximation of the appr0priate number
of "transition conditions" days at a particular station. Since Bryson's
method breaks a temperature frequency distribution into component
normal distributions associated with each air mass, the points where

these component distributions intersect should be within the numerical

83

transition zones. The limits themselves probably would be located
between the intersection points and where the component distributions
approach zero. Analysis of this type could thereby approximate the
number of transition days, as well as provide another independent verifi-

cation of the limits themselves.

Conclusions

 

In future research, the air masses of any region could be recog-
nized and delimited from surface and upper air data using the "limits
method" and utilizing the various improvements I have suggested. If
the relationship between 500mb flow patterns and various air mass
distributions could be statistically verified, a more detailed summary
of expected regional temperature and moisture patterns could commonly
accompany upper air forecasts. Also, the relative air mass dominance
in a region (or at a station) over a given time period could be inferred
from a known mean temperature deviation from normal by comparing the
deviated mean to the range of temperatures associated with the regional
air masses. Furthermore, once the limits for the four seasonal months
(January, April, July, October) are firmly verified, it would be possible
to interpolate the most appropriate limits for the months between,
thereby facilitating the study of monthly changes in the air mass dis-
tributions. Thus, this method could be easily applied to the problem
of air mass quantification and may facilitate the integration of air
mass research into the mainstream of.modern climatological/meteoro-

logical research.

BIBLIOGRAPHY

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APPENDIX

AIR MASS PERCENT OCCURRENCE BY STATION FOR
JANUARY, APRIL, JULY, AND OCTOBER
1970-1979

 

 

APPENDIX
JANUARY
Station Cont Pacif. Dil. Trop. Trop. Dry Trop. Unclassed
Bismarck 72.6 14.5 0 O O 12.9
Dayton 61.0 16.1 4.5 0.3 O 18.1
Dodge City 38.1 47.4 0.3 0 0.3 13.9
Flint 77.4 9.4 0.3 O 0 12.9
Green Bay 82.9 6.5 0 0 0 10.6
Int. Falls 89.6 3.6 O 0 0 6.8
Monette 36.2 35.8 3.2 0.4 O 24.4
North Platte 50.3 36.1 0 O 0 13.6
Omaha 57.1 25.5 0.3 O O 17.1
Peoria 60.0 19.4 1.3 0 O 19.3
Rapid City 55.2 28.7 0 O O 16.1
St. Cloud 84.5 10.0 0 O O 5.5
Salem 50.3 21.9 3.6 1.0 O 23.2
S. Ste. Marie 90.6 2.9 0 O O 6.5
Topeka 46.8 32.5 1.6 O 0.3 18.8
APRIL
Station Cont Pacif Dil. Trop. 'Trgp. (Dry Trop. Unclassed
Bismarck 40.0 32.3 1.0 O 0.7 26.0
Dayton 28.7 30.7 14.7 3.3 O 22.6
Dodge City 10.0 49.0 12.0 1.7 4.7 22.6
Flint 44.7 30.0 7.0 1.7 O 16.6
Green Bay 45.0 29.0 5.3 0 0 20.7
Int. Falls 61.0 22.7 0.7 0 0 15.6
Monette 10.4 43.3 24.1 5.2 O 17.0
North Platte 21.7 43.7 4.7 0.3 2.7 26.9
Omaha 23.7 41.3 11.0 2.0 O 22.0
Peoria 28.7 36.7 11.3 2.7 0 20.6
Rapid City 25.3 45.3 0.7 0 0.7 28.0
St. Cloud 44.3 26.7 5.0 0 0.7 23.3
Salem 17.7 32.0 23.7 9.3 O 17.3
S. Ste. Marie 61.3 19.3 3.0 O 0 16.4
Topeka 15.0 41.3 19.7 4.0 O 20.0

88

89

APPENDIX--continued

 

 

JULY

Station Polar Tropical Drerrppical Unclassed
Bismarck 42.9 1.0 23.7 32.4
Dayton 34.8 29.4 1.0 34.8
Dodge City 13.6 5.5 37.1 43.9
Flint 52.9 24.5 0.7 21.9
Green Bay 56.1 16.5 0.7 26.7
Int. Falls 72.6 2.6 1.3 23.5
Monette 15.4 42.3 1.8 40.5
North Platte 26.5 0.7 41.0 31.8
Omaha 21.7 35.8 6.1 36.7
Peoria 35.8 24.2 2.3 37.7
Rapid City 26.5 0.7 50.3 22.5
St. Cloud 48.1 11.6 4.2 36.1
Salem 19.0 57.7 0.7 22.6
S. Ste. Marie 69.7 6.8 1.9 21.6
Topeka 15.8 45.8 2.9 35.5

OCTOBER
Station Cont. Pacif. Oil. Trop. Trop. Dry Trop. Unclassed
Bismarck 38.4 37.1 0.7 O 2.3 21.5
Dayton 25.8 33.5 18.4 1.6 O 20.7
Dodge City 9.0 42.6 10.3 0 13.6 24.5
Flint 37.4 25.2 15.8 0.7 O 20.9
Green Bay 42.3 27.4 11.6 1.0 0.7 17.0
Int. Falls 53.3 23.2 3.6 O O 19.9
Monette 7.5 40.8 25.5 4.3 1.1 19.8
North Platte 21.3 44.5 1.9 O 10.3 22.0
Omaha 19.4 36.5 15.8 1.3 1.3 25.7
Peoria 23.6 30.0 18.4 2.6 O 25.4
Rapid City 27.7 42.9 0 0 9.7 19.7
St. Cloud 39.4 28.7 8.4 0.7 1.0 21.8
Salem 14.5 26.8 26.8 7.7 O 24.2
S. Ste. Marie 51.0 17.7 7.4 O O 23.9
Topeka 13.2 37.7 17.7 3.9 1.6 25.9

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