The 17,000 foot vortex of flame and ash that accelerated the Carr Fire in Redding in July was a very rare atmospheric tornado event called a “pyro-genetic tornado,” according to atmospheric scientists.
The “firenado” was formed in much the same way a traditional tornado would be formed, setting it apart from more common “fire whirls” caused only by the heat of flames, said Neil Lareau, a scientist and professor at University of Nevada, Reno.
There has only been one other documented case of a “firenado,” in the Canberra Firestorm of 2003 in Australia making this an extremely rare event, Lareau said by phone Wednesday.
Lareau and two co-authors recently released a study documenting the fire tornado and the factors which contributed to its formation, according to a press release from UNR.
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The Carr Fire, which killed eight people and destroyed more than 1,000 homes in the Redding area in July, is the seventh most destructive fire in California history.
The tornado formed its spin because of a pre-existing wind condition called a shear, and because of clouds formed on top of the fire plume, Lareau told The Sacramento Bee.
A pyrocumulonimbus cloud – a fire-fueled thunderstorm cloud – that formed on top of the fire’s smoke plume released heat into the atmosphere, which allowed the plume to grow vertically very quickly, said Lareau. The plume grew from four to 8 miles tall in just 15 minutes.
While the plume was growing, a wind condition called shear was happening near the ground. Shear is a wind pattern in which wind is blowing in two opposite directions very close to each other – Lareau compared it to a busy freeway with a center divider.
The wind blowing in opposite directions is what formed the tornado’s spin, Lareau said. He said it’s similar to how a paddle wheel, placed in the center divider of the busy freeway, would begin to spin rapidly.
The fire pushed heat and flame up into the spin, and the rapidly growing plume pulled those flames upwards, concentrating the spin into a tornado core about 1,000 feet wide. The concentration of the spin made the winds inside the tornado grow faster and more severe, reaching wind speeds of about 143 miles per hour, the same intensity as an EF3 tornado.
Normal fire “whirls,” Lareau said, are caused by wind patterns the fire creates, not pre-existing ones; however, Lareau said, they can still be extremely destructive and dangerous.
The “climate whiplash” California has experienced over the last few years, with a heavy drought followed by a very wet rainy season and then a dry winter, allowed a lot of vegetation to grow and then dry out, Lareau said. These climactic conditions allowed for what Lareau called “extreme fire behavior” – in this case, a tornado.
Lareau said that much of the radar and satellite technology used in monitoring the event could have potential applications in a warning system.
“It’s not a panacea,” Lareau said. “There are a number of things to be done to mitigate fire damage in California. But minutes matter.”
Mass evacuations can be harrowing, as evidenced by the recent Camp Fire evacuations of more than 50,000 people, Lareau said. Early warning technology that could detect dangerous wind and weather patterns could possibly add precious minutes to evacuation orders.
“It’s a really complex problem, but this is a step,” Lareau said. “The hope is that we can learn from some of these events.”