Ambient atmospheric environment affects the growth and spread of wildland fires, whereas heat and moisture release from the fires and the reduction of the surface drag in the burned areas can significantly alter local atmospheric conditions. Previous studies have investigated this interaction between the fire and the surrounding atmosphere, but the majority of these experiments collected in-situ data at the fire-atmosphere interface using only a few instrumented towers in typically a large burn plot of at least a few acres or several thousand square meters. This study reports results from a recent prescribed surface fire experiment conducted on a small, densely instrumented burn plot of one hundred square meters (10 m by 10 m). The fuel in the burn plot was pine needles with a fuel loading of about 0.5 kg m-2 and fuel moisture of 5.5%. At the time of the burn, the ambient wind speed at the fuel bed level was about 2 m s-1. In-situ meteorological observations were collected using a 4 by 4 array of three-dimensional sonic anemometers mounted on four trusses at 2.5 m right above the fuel bed level. The analysis of the 10-Hz velocity and temperature data from the 16 sonic anemometers focuses on fire-induced atmospheric turbulence. By comparing the observations collected before, during and after the fire, the study reveals how the fire can alter the heat and momentum exchanges between the combustion zone and the atmosphere above. Besides confirming some of the general findings about fire-induced turbulence from previous studies, the results reveal the existence of substantial heterogeneity in the fire-atmosphere interactions across the burn plot. Even for a plot as small as this, the perturbations of the fire to the ambient atmosphere depends strongly on the downwind distance from the initial fire line and the specific position relative to the fire front. This key finding further highlights the necessity for fire behavior models to have 1-2 m grid spacing to resolve the heterogeneities and capture fire-atmosphere interactions that are relevant to turbulence. The results also have important implications for modeling smoke dispersion, as atmospheric dispersion characteristics in the vicinity of a wildland fire are directly affected by fire-induced turbulence.
Read
