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Science Briefing

This summer promises 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 MMM senior scientist Terry Clark has adapted his mesoscale model to trace the interplay over time between fire behavior and winds. First results 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 Terry, Janice Coen (also of MMM), Mary Ann Jenkins (York University, Canada), and David Packham (Monash University, Australia).

Fingers of flame about a kilometer apart are evident in this photo of the Owens Valley, California, sagebrush fire of July 1987. (Photo by Charles George, courtesy International Fire Science Laboratory.)
For this study, Terry coupled a version of his mesoscale model 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. They 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:

• 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 regime 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, 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 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 C, even as the fire itself burns at more than 800 degrees C.

• 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.

Terry 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.

Two UOP programs have been combined to form a single office that will provide full technical and logistical support for field programs worldwide, as well as continuing previous international support for climate research. The Joint International Climate Program/Planning Office and the Office of Field Project Support were merged on 17 June into a new office (yet to be named) with Karyn Sawyer as director and Dick Dirks and Gus Emmanuel as associate directors.

In announcing the move, UOP director Bill Pennell said, "I believe that this action will significantly improve UOP's ability to serve its university and agency constituents by eliminating unnecessary confusion regarding the roles and missions of two closely related organizations; by improving the efficiency by which we maintain, plan, develop, and deliver present and future field project support to our constituents; and by simplifying the problem of oordinating UOP's and NCAR's field project support services."

A transition team drawn from the managers of both previous groups will guide the integration efforts this summer.

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Prepared by Jacque Marshall, jacque@ucar.edu, 303-497-8616