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A hot topic: Modeling the links between fire and wind

by Robert Henson
UCAR Communications

In some parts of the United States, summer means forest fires. Already this year, fires have ravaged parts of Alaska and the Southwest; in the tinder-dry plains and forests, more are expected. Winds play a critical role in fire spread, but a fire itself can modify local winds, helping it grow even more quickly. Now NCAR and university scientists have created one of the first computer models to trace 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 Univer-sity, Canada) and David Packham (Monash University, Australia).

Output from the fire-atmosphere coupled model shows the development of two fingers along a fire line. The line is just under two kilometers long, with the fingers developing at top and bottom. Arrows depict horizontal winds. Vertical motions (which roughly correspond to fire location) are shown in the contoured areas. (Illustration courtesy of Janice Coen and Terry Clark.)
Clark specializes in modeling 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 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 number 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:

  • A fire's growth pattern 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; such a blow-up killed 14 firefighters near Glenwood Springs, Colorado, 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 and the fire's spread may be more predictable.

  • Air temperatures near a fire are lower than one might 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 up to 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 degree C, even as the fire itself burns at more than 800 degree C.

  • The model helps to explain a commonly observed trait of wind-driven fires: the growth of fingers of flame, 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.

    Calculations for the coupled model were performed on NCAR's CRAY Y-MP supercomputer with support from NSF. Clark and colleagues 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. Clark may be reached at (303-497-8978 or clark@ucar.edu).

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