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Fall 1998

Wildfire Experiment

by Bob Henson and Anatta
UCAR Communications

Each year, wildfires typically burn 5 million acres in the United States at a cost of hundreds of millions of dollars. Only too often, they also claim the lives of firefighters. This September and October, NCAR scientists plan to fly the NSF/NCAR C-130 aircraft around and over wildfires in hopes of getting a better understanding of these frightening, deadly events.

"We're most interested in understanding the violent fires that kill firefighters and confound prediction efforts," says Lawrence Radke (NCAR Atmospheric Technology Division), co-principal investigator for the Wildfire Experiment, or WiFE. "We need to be able to predict the course of a dangerous fire to develop the most effect strategy for suppressing it."

The Thermacam is faithfully detecting the heat in Radke's upper face (shown in the screen image). The difference between his skin and glasses causes the difference in color in the image. (Photo by Carlye Calvin.)

Radke has experience sampling out-of-control fires, including the oil-fueled blazes in Kuwait during the Gulf War. He explains that large, violent wildfires often generate their own controlling weather. The released heat can spawn deep convection, even thunderstorms, with strong and dangerous winds. So-called fire whirls, similar to tornadoes, can hurl flaming logs and other burning debris a considerable distance away, setting other areas ablaze. Such vortices already occur at all scales in the atmosphere, and they intensify locally near fires.

Much of the inspiration for the NSF-funded WiFE comes from dramatic modeling results obtained by WiFE co-PI Terry Clark (NCAR Mesoscale and Microscale Meteorology Division) and his colleagues in recent years. After developing one of the world's most comprehensive atmosphere-fire models, Clark and MMM's Janice Coen uncovered small-scale structures in explosive fires. Now Clark is eager to observe these "fire fingers" in nature and to quantify their structures. Both radiation and the convection that results from fire-atmosphere interactions affect fire spread. To understand those effects, Clark must first see how fire fronts lap at or "finger" unburned fuel. WiFE data will be used to validate models like Clark's, which can then help improve the fire-spread models used by firefighters.

Experience and modeling have combined to show that wildfires are especially likely in negative wind shear: strong winds near the ground (moving at the leading edge of a microburst or gust front, or created by the fire itself) combined with lighter winds aloft. "Our simulations show that negative wind shear can produce extreme fire behavior, such as rapid intensification of fire-line dynamics, strong fire whirls, erratic spread rates, and strong updrafts that can loft burning embers ahead of the fires," Coen says. She is studying how these simulated blowups occur by examining intense horizontal vortices that form at the base of fire-line updrafts.

There's also a chemistry component to WiFE, coordinated by Hans Friedli (NCAR Advanced Study Program). Elliott Atlas (NCAR Atmospheric Chemistry Division) and members of ACD's stratospheric/tropospheric measurements group plan to sample methyl halides (bromine, chloride, and iodide) and other organic compounds emitted from forest fires. The methyl halides have properties similar to those of the infamous chlorofluorocarbons, which attack the ozone layer and also contribute to global climate change. Many human-produced halides have been banned, but the chemicals are still being emitted by natural sources, which include grassland and forest fires. Atlas hopes that WiFE will clarify how much wildfires contribute to the global budget of these chemicals.

In addition, ACD's Teresa Campos will be measuring carbon dioxide at two sampling rates. "Combined with methan and carbon monoxide information from Elliott's measurements, we can determine the fire's combustion efficiency," she says. These data will also help clarify the amount of total carbon emitted by fires.

"We chemists don't get to explore big fires very much," notes Atlas of ACD's involvement in WiFE. "This is a real opportunity for us."

The sensors

Last summer, Radke and Clark took part in a pre-WiFE engagement, the International Crown Fire Modeling Experiments. (Crown fires spread across the tops of trees.) Among the devices tested in that program was NCAR's new Thermacam, which has given the best insight to date into the small-scale dynamics of intense wildfires.

The Thermacam, a digital infrared imager built by Inframetrics Inc., is literally the coolest instrument in WiFE. The compact camera houses a minirefrigerator to keep its sensitive array of radiometers at -196 degrees C (-321 degrees F). The extreme cold provides a more stable reference point than room temperature. The array can map temperatures as high as 1,500 degrees C (2,700 degrees F) at a resolution of one meter (three feet) and better across a field of flame.

Complementing the Thermacam for infrared sensing will be ATD's airborne imaging radiometer, specially adapted by Craig Walther and Peter Hildebrand so that its microwave beams can sense higher temperatures than ever before--up to 2,000 degrees C (3,600 degrees F). Fuel type is a key variable in wildfire prediction, and the moisture content of biomass helps determine its combustability. The ATD radiometer, when used in its polarized mode, will be able to sense biomass amounts and perhaps their moisture contents.

Also providing instruments, scientists, and observers are NASA Ames Research Center and the U.S. Forest Service's Rocky Mountain Research Station and Riverside Fire Research Laboratory. The Rocky Mountain Area Aviation and Fire Coordination Center will help the scientists decide which fires to observe and will coordinate flights around the fires.

The C-130 aircraft is reserved for four or five flights over a six-week period starting 1 September. "The C130's speed, range, and endurance will allow us to be over any wildfire in the United States in less than a day," says Radke. The aircraft will circle the fires counterclockwise at 150 knots or less, cruising between the minimum safe altitude and 10,000 feet. For safely, all three NCAR pilots (Henry Boynton, Lowell Genzlinger, and Mike Heiting) will be on board for each marathon flight, and a full support team will assist from ground level.

This project may be NCAR's first WiFE of several, according to Radke. "Success in this test phase will not only encourage us to continue the program into coming years, but new knowledge also will guide us in further integration of observational tools." He points out that, like many other weather-related catastrophes, wildfires will take a greater toll over time. "As urbanized America continues to spread into the woods, the cost of such incidents is only going to rise."

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Edited by Carol Rasmussen, carolr@ucar.edu
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Last revised: Tue Apr 4 15:03:21 MDT 2000