Numerical Study of Effects of Atmosphere Temperature Profile on Wildfire Behavior

The vertical temperature profile and hence the stability in the atmosphere near the ground varies significantly between day and night. Typically, the potential temperature at the surface is higher than that above the ground during the day and lower than that above the ground during the night. Such differences in the vertical temperature profile might act to accelerate or slow down wildfire propagation accordingly. It is believed that low-level atmospheric stability influences wildfire behavior. Both observational studies (e.g., Werth & Ochoa 1993; Potter 1996) and numerical studies (e.g., Jenkins 2001) have been performed to compare with and verify the Haines Index (Haines 1988), which is a severity index for wildland fires based upon the stability and moisture content of the lower atmosphere. However, few of these studies isolated the effects of low-level atmospheric stability on fire behavior (e.g., spread rate, intensity, plume structure and evolution). In the present study, a coupled atmospherefire model, HIGRAD/FIRETEC, developed at the Los Alamos National Laboratory (Linn 1997; Reisner et al. 2000; Linn et al. 2002), is employed to examine the effects of the atmospheric potential temperature profiles on the rate of spread (ROS) of fire, in addition to the potential temperature and velocity fields in the domain. For this purpose, we studied five different types of vertical potential temperature profile. One of them represents the neutral stability case; two of them represent stable cases with potential temperature increasing with height; the other two represent the unstable cases with potential temperature decreasing with height. Linear and piecewise linear temperature profiles are assumed for an ambient wind speed equal to 2.22 m s (corresponding to 5 mph).