Last year, Neil Lareau, an atmospheric scientist at the University of Nevada, Reno, was browsing weather data for the then-raging Dixie Fire in Northern California when he decided he would be heading there as soon as as possible. It would look for Fire Generated Tornado Vortexes (FGTVs), more commonly known as Fire Tornadoes.
“Whirlwinds are a ubiquitous feature of wildfires,” Lareau said. Smaller whirlpools, called fire whirlpools, are common during forest fires, extending a few meters and dissipating within minutes.
The largest vortices can reach a kilometer in diameter and generate tornado-force winds. Although eddies large and small can be a dynamic agent affecting the behavior of wildfires, the forces that create and propel them remain mysterious.
Lareau partnered with a team from the San Jose State University (SJSU) Fire Meteorology Research Laboratory at a highway rest area 18 miles from the Dixie fire. They expected the fire to roll down the slopes after a few hours, but instead a brand falling from the sky ignited a spot fire on a nearby ridge.
Fiery field work
In 2020, the National Weather Service issued its first fire tornado warning, during the Loyalton Fire in California. Later that summer, the Bear and Creek fires, also in California, also spawned FGTVs.
Lareau was researching FGTVs long before Loyalton’s warning and continues to study the phenomenon. In an article recently published in the Bulletin of the American Meteorological SocietyLareau used open access Next Generation Weather Radar (NEXRAD) data to create a conceptual model identifying common characteristics of FGTVs.
Like water flowing around a rock in a stream, he found, the wind is forced to split around the plume of superheated air and ash rising from a wildfire. And just like water flowing around a rock, “these two vortices develop,” Lareau said, “which we call counter-rotating vortices. [CRVs]. Between these CRVs, there’s this kind of wake – the air blowing back toward the fire in opposition to what the wind would do if the fire wasn’t there.
Lareau recognized the limitations of the conclusions he could draw from this data, he said. The NEXRAD resolution was coarse and he could only observe the plume aloft, not near the ground. That’s where SJSU’s Truck Mounted Mobile Search Radar came in.
“One truck has our radar mounted on it, and the other truck actually houses our lidar,” said Kate Forrest, who was there that day. Forrest is an SJSU graduate student and operator of the Fire Weather Research Laboratory’s research radar, which produces very high resolution data. Because the truck-mounted radar can be driven toward the fire, it can take measurements from the flames skyward.
“That’s when the fire really hit on the downwind side of the Sierra,” Forrest said. “We actually started seeing these spinning plumes on either side; one of them started spinning, and it went super ashy. The wind has completely reversed.
Lareau had hoped to capture data of CRVs forming several kilometers away. Instead, he and the SJSU crew watched them form with their own eyes. “And boom, there they are,” Lareau said.
Lareau and Forrest noted that the vortices of the Dixie Fire were not true fire tornadoes because they did not generate tornadic winds. But the vortices confirmed that the principle of current vortices, which Lareau says came from the anecdotal knowledge of firefighters, can predict fire behavior in real time. What is becoming clearer through Lareau’s research is the ubiquity of the current vortex phenomenon and the process of turning a small CRV into a true FGTV.
However, even when vortices don’t become tornadoes, they “can become the primary driver of fire spread,” Lareau said. “If we want to understand the coupled fire-atmosphere system, how a fire spreads through the landscape, how it consumes fuel, understanding these vortices is of crucial importance.”
—Emilie Berger (@emilyshep1011), science writer