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NSF PR 96-36 - June 20, 1996

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NSF Scientist's Computer Model Links Fire and the Atmosphere

Winds play a critical role in fire spread in tinder-dry forests, but a fire itself can modify local winds, helping it grow even more quickly, according to scientist Terry Clark of the National Center for Atmospheric Research (NCAR) in Boulder, Colorado. NCAR is funded by the National Science Foundation (NSF).

"Clark has created one of the world's first computer models to trace the interplay over time between fire behavior and winds, pointing the way toward future models that will aid in fire prediction and management," said Jewel Prendeville, coordinator of NSF's lower atmospheric facilities section.

Using supercomputers to model small-scale atmospheric phenomena, Clark has analyzed severe thunderstorms, downslope windstorms, and the dynamics near fronts. For the fire-atmosphere study, one of Clark's atmospheric models was connected with a model of dry eucalyptus forest fires (a major threat in Australia). Although forests vary in how they burn, the findings 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, Clark points out that forest fires are very complex phenomena. "Interactions between forest fires and airflow are highly unstable," he said.

Among Clark's findings:

• 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, 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 situation in which the fire is less likely to create its own circulation pattern. Thus, the fire's spread may be more predictable.
  
  
• Air temperatures near a fire are lower than one might normally think. In the first several minutes of a new fire, Clark's model shows surface temperatures soaring, creating a chimney-like plume of rising air. Shortly thereafter, the atmosphere establishes a balance between the updraft (blowing at near-hurricane speeds) and the heat provided by the fire. 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 Centigrade, even as the fire itself burns at more than 800 degrees Centigrade.
  
  
• The model helps to explain a commonly observed trait of wind-driven fires: the growth of fingers of flame, spaced about a mile or more 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, Clark's model suggests that, when winds are weak, a fire line several mile's or more in length is inherently unstable and very likely to break up into fingers.

Clark and his colleagues are now investigating a second, smaller-scale type of fire fingering that occurs through a process 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 often observed in actual fires.

Note to television editors: B-roll footage is available from Nita Razo at NCAR Visual Communications, 303-497-8606.

Note to print editors: Photos of forest fire research are available from Anatta at NCAR Communications, 303-497-8604.

 
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