Weather Conditions Inform Timing of Prescribed Fire

Model identifies conditions for smoke plume formation in the Southeast

fire-smoke-plumes
These well developed smoke plumes from a prescribed fire in the southeastern U.S. are similar to those needed for FASMEE. Photo by Yongqiang Liu, USFS.

Prescribed fires generate smoke, which can harm human health – especially in areas where humans and forests are close together.

“Weather conditions are critical for prescribed fire, especially the effects of wind and humidity on smoke plume formation,” says Yongqiang Liu, USDA Forest Service research meteorologist.

Liu is the lead author of a modeling study that sought to identify specific weather conditions that allow formation of a large, well developed smoke plume that travel long distances during and after prescribed burning. This study is part of a national project called FASMEE, the Fire and Smoke Model Evaluation Experiment.

FASMEE is a comprehensive field campaign to better understand and predict smoke generated by wildland fires. The project is working to tighten the connections between fuel, fire behavior, smoke, and emissions models. Its predictions will be useful for fire and land managers in the Southeast, where heavy fuel loads can result in high-intensity fires and significant smoke production.

The research team also included SRS research meteorologist Scott Goodrick and retired scientist Gary Achtemeier, who developed two smoke models used in the study.

The researchers modeled smoke plumes during the day and fog at night. They analyzed the different weather conditions – including temperature, relative humidity, and wind – that contributed to plume formation. The models used hypothetical prescribed burns at Fort Stewart Army Base in southern Georgia – one of the sites where future FASMEE field measurements will be conducted.

The study findings were published in a special issue of Atmosphere.

The scientists used a tool called Daysmoke to model smoke plume formation. Daysmoke was developed specifically for prescribed fire and has been applied across the Southeast. It predicts the maximum height of a smoke plume.

If a plume rises above the atmospheric boundary layer – which hovers anywhere from a thousand feet to a mile above the ground, depending on the time of day and year – then smoke can be transported long distances, potentially to cities or other densely populated areas.

For nighttime smoke drainage and fog formation, the team applied the model PB-Piedmont, or PB-P. It predicts where and how smoke particles might travel overnight. Nighttime smoke formed on one night of the simulation.

As a fire smolders into the night, a haze of fog can form in the warm, moist portion of a system or front. If the air is still, super-fog can form.

“The southeastern U.S. often has a moist atmosphere, which is very conducive for super-fog. It’s a problem because it can affect visibility and traffic along roads,” says Liu.

wildfire-smoke-texas
Smoke from large wildfires in Oklahoma and Texas in 2017 traveled long distances, increased local concentrations of particulate matter, and degraded air quality. Photo courtesy of Jeff Schmaltz, NASA.

The researchers evaluated their models by comparing results with smoke plumes measured in the field at Eglin Air Force Base in northwestern Florida. The simulated plume rise and transport direction compared well with those measured at this site.

The study found that the best weather conditions for forming, and thus measuring, smoke plumes would be between two low-pressure systems. Well-developed plumes formed on two of the four simulated days, according to the model predictions.

The days without well-developed plumes had higher relative humidity and wind activity, which blew the smoke clouds around at ground level rather than lofting them upward.

Prescribed fires are often conducted during the dormant season, from late winter into early spring. In the Southeast, this is a period of highly variable weather conditions.

FASMEE plans to conduct a prescribed burn in the Southeast in the spring of 2020. In the meantime, Liu and his colleagues hope to link Daysmoke and PB-P in order to simulate the smoke from a single burn event continuously, from the day into the night.

“A major goal of FASMEE is to improve public information about fire and smoke warnings. Both land managers and local residents will benefit if we can provide more accurate and timely guidance about prescribed burning,” adds Liu.

The researchers are also interested in how climate change and warmer air temperatures may further reduce the number of days suitable for prescribed burning. This would make it even more critical for forest managers to understand the effects of different weather conditions on their burn plans.

“Landowners and managers burned more than 6.5 million acres of forestland in the Southeast in 2017, which is two-thirds of the total area burned by prescribed fire in the U.S. They need to use every possible opportunity to burn. We don’t want them to miss an ideal time to burn because they lack information about the best weather conditions,” says Liu.

Read the full text of the article.

For more information, email Yongqiang Liu at yongqiang.liu@usda.gov.

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