July 31, 1996
This summer has already proven to be a dangerous one in the tinder-dry forests of the southwest United States and Alaska. Winds play a critical role in fire spread, but a fire itself can modify local winds, helping it grow even more quickly. Now a senior scientist at the National Center for Atmospheric Research in Boulder, Colorado has created one of the world’s first computer models that traces the interplay over time between fire behavior and winds, pointing the way toward future models that might aid in fire prediction and management. First results from this model were published in the May 1996 issue of the Journal of Applied Meteorology.
“A Coupled Atmosphere-Fire Model: Convective Feedback on Fire-Line Dynamics” was written by NCAR scientists Terry Clark and Janice Coen with Mary Ann Jenkins (York University, Canada) and David Packham (Monash University, Australia).
Clark specializes in using supercomputers to model small-scale atmospheric phenomena. His work has analyzed severe thunderstorms, downslope windstorms, and the dynamics near fronts. For the fire-atmosphere study, one of Clark’s atmospheric models was coupled, or connected, with a model of dry eucalyptus forest fires (a major threat in Australia). Although forests vary in how they burn, the authors expect that their main findings will translate to a variety of settings.
Most previous studies on fire and wind have assumed a straightforward relationship between large-scale winds and fire behavior. However, the authors note, “Forest fires are very complex phenomena. . . . Interactions between forest fires and airflow are highly nonlinear [unstable], and radiation and combustion properties are not fully understood.” Using the coupled model, the scientists were able to examine a variety of wind speeds and observe–at resolutions as fine as 20 meters–how a fire’s development can alter the circulation around it. Among their findings:
1). A fire’s pattern of growth depends not only on large-scale winds but on the balance between those winds and a fire’s heat output. If the winds relative to an advancing fire line are weak, and the heating is particularly strong, a fire can force its own circulations, possibly resulting in unstable, “blow-up” fire conditions. (It was a sudden blow-up that killed 14 firefighters near Glenwood Springs, Colo., in 1994.) On the other hand, strong winds relative to the fire line–though literally fanning the flames–tend to produce a more stable regime in which the fire is less likely to create its own circulation pattern. Thus, the fire’s spread may be more predictable.
2). Air temperatures near a fire are lower than one might normally think. In the first several minutes of a new fire, the model shows surface temperatures soaring, which creates a chimney-like plume of rising air. Shortly thereafter, the atmosphere establishes a balance between the updraft (blowing at near-hurricane speeds as high as 30 meters per second) and the heat provided by the fire. In the model, the updraft strengthens and pulls in surrounding cooler air as a fire’s heat output increases. This keeps air temperatures near the fire in the range of 60 to 100 degrees Celsius (C), even as the fire itself burns at more than 800 degrees C.
3). The model helps to explain a commonly observed trait of wind-driven fires: the growth of fingers of flame, spaced about a kilometer apart, that form the main fire line. Previous researchers had proposed that the fingering was due to variations in either the fire’s fuel or the local geography. However, the coupled model suggests that, when winds are weak, a fire line several kilometers or more in length is inherently unstable and very likely to break up into fingers.
The calculations for the coupled model were performed on NCAR’s CRAY Y-MP supercomputer with support from the National Science Foundation . Clark and his colleagues plan to continue their fire-atmosphere modeling. They are now investigating a second, smaller-scale type of fire fingering that occurs through a process roughly similar to the one that causes supercell thunderstorms to rotate. Preliminary model results show the development of a tornado-like vortex within a fire, much like the vortices sometimes observed in actual fires.