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August 1999

Frostfire: Prescribed burn holds clues to wildfires

Fire whips up Helmer's Ridge between Caribou and Poker Creeks during the Frostfire controlled burn northeast of Fairbanks, Alaska. (Photo courtesy U.S. Forest Service Research.)

These days, where there's fire, there's Larry Radke (ATD), Terry Clark, and Janice Coen (both of MMM). The team headed to Alaska in early July to participate in Frostfire, a controlled burn of 2,200 acres (880 hectares) approximately 25 miles (40 kilometers) northeast of Fairbanks. Frostfire is described as the first landscape-scale research burn in terrain dominated by permafrost.

The multiagency experiment in the Bonanza Creek/Caribou-Poker Creeks Research Watershed began on 8 July with "blacklining" to burn a protective 100-meter (330-foot) perimeter around the fire. Interior aerial ignition began the next evening about 7:00 and ended about 9:00 p.m. when the relative humidity got too high and "many of the 'starts' went out like a bad match," Larry reports. The 10 July plan called for more firing from the helicopter, but with the sun rising, the wind was blowing in just the right direction to spread the fire to most of the targeted plots. Stands of black spruce caught fire "in a rather amazing fashion," and spread rapidly. By 13 July the NCAR team was headed for home.

Larry and Terry flew over the fire, collecting data with NCAR's Thermacam imager, in a U.S. Forest Service Piper Navajo aircraft. The Piper's home base is the NASA Ames Research Center in California. The Thermacam is a digital, high-resolution infrared imager with a sensing range between -10° and 1,500° Celsius ( 14° and 2,732° Fahrenheit). The instrument, built by Inframetrics, produces color video images of hot, swirling air and flames, detailing their motion, size, structure, and temperature. Complementing the Thermacam was a fire-imaging spectrometer operated by NASA investigator Robert Higgins. The Forest Service sponsored NCAR's participation in Frostfire.

"I personally dislike flying in small aircraft, but they make great observing platforms for fires," says Terry. "The data we collected with the Piper Navajo are exciting. We were able to get quite close to the fire."

On a low mountaintop a bit less than two kilometers from the fire, Janice operated a second Thermacam with a telephoto lens for a CU research team. Co-principal investigators Shankar Mahalingam and John Daily (CU Joint Center for Combustion and Environmental Research) and graduate student Yottana Khunatorn staffed the ground operation. They will collaborate on data analysis and model refinement with Terry, who is co-PI on their NSF-funded project. Co-PIs Jana Milford (CU) and Don Latham (Forest Service) round out the modeling team. "The combination of the CU ground-based observing perspective along with our airborne data will allow us to document important aspects of the morphology of fire wind vortices for the first time," Terry says.

The unexpected 10 July flare-ups were a boon to the wildfire study team. "We got several crown-fire runs of rather considerable intensity that burned several hundred acres in just a few minutes," Larry says. "Even though this was a prescribed fire, we got some elegant fire dynamics for free." In all, there were three good observing periods for the mountaintop team, and the aircraft team was in the air for two of those.

The Frostfire observations will augment data gathered last fall during the Wildfire Experiment (WiFE, see the August 1998 Staff Notes Monthly and the 20 August 1998 news release). In the modeling work, Terry and colleagues have been reproducing many of the fine-scale structures frequently observed in explosive fires. Both radiation and the convection that results from fire-atmosphere interactions affect fire spread. To understand those effects, the team has sought out wildfires and the Frostfire controlled burn to observe fire fronts lapping at, or "fingering," unburned fuel. Quantifying fire-finger structures may hold an important key. "We're increasingly convinced that as fires become more violent, the nonlinear vortex motions we see that look like fire fingers become more important," says Larry.

The Frostfire observations are "exactly the sort of data sets we need" to validate the atmosphere-fire model, says Larry. Right now NCAR's supercomputers run the calculations, but "ultimately we'd like to fit the model on a high-powered laptop that firefighters can take with them to the fire. That's not practical today, but with the current pace of computer technology we foresee more practical applications of this work in the early 21st century."

While the NCAR observations are over, the Frostfire experiment is not. Firing may continue within the watershed this summer, next summer, and beyond. About 60 researchers are involved in ongoing experiments.

Long-term impacts on wildlife, soil erosion, regional carbon and nitrogen budgets, and other ecosystem components are all part of the agenda at this Long Term Ecological Research site. Frostfire is coordinated by the Forest Service and the University of Alaska, Fairbanks. •Zhenya Gallon

To view an animation of the atmosphere-fire model, go to Simulating Forest Fires. See the Frostfire Web site.

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Edited by Bob Henson, bhenson@ucar.edu
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