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Analysis of satellite, radar data show how elements caused ‘fire tornado’

by Lisa M. Krieger, Bay Area News Group |

What caused this summer’s rare apocalyptic “fire tornado”?

As residents fled the deadly Carr Fire on July 26, a massive 17,000-foot spinning column of smoke and fire tore through Redding, like a scene out of a disaster movie. It’s been directly linked to four deaths, numerous injuries and substantive loss of property.

A new analysis of satellite and radar data by a team led by atmospheric scientist Neil Lareau of the University of Nevada in Reno describes the factors that combined at just the right time and place to catalyze the vortex, dubbed a “firenado.”

A key factor in the vortex formation was the development of a fire-generated ice-topped cloud — known as a pyro-cumulonimbus — which reached as high as 39,000 feet, the scientists report in the scientific journal Geophysical Research Letters.

This cloud helped stretch the underlying column of air, concentrating its rotation and causing the tornado-strength winds, estimated at 143 miles per hour.

The “firenado” was formed when wind shear emerged during the hour before the vortex, he added.

The wind shear was created by a sudden and steep plume of convective air — hot air rising and cold air sinking. This plume, triggered by the expanding northeastern flank of the fire, grew from four to eight miles high in just 15 minutes.

“This paints a clear picture of the sequence of events leading to the vortex development and intensification,” said Lareau, assistant professor of physics, in a prepared statement.

Various weather factors also contributed to the vortex: very low humidity, record high temperatures, and terrain-channeled winds due to low atmospheric pressure.

It’s a process similar to the development of an ordinary thunderstorm or smaller “fire-thirls,” according to the team.

What distinguished this large Carr Fire vortex from these more frequent events was its link to the tall cloud, according to Lareau. The only other documented case of a “firenado” is during the Canberra Firestorm of 2003 in New South Wales, Australia.

The Carr Fire, which burned in Shasta and Trinity Counties, started July 23 after a tire blew out on a trailer and the rim made sparks on the pavement, igniting dry brush. It killed eight people, destroyed 1,079 homes and burned 230,000 acres – 359 square miles – making it the seventh largest fire in California history.

These observations may help forecasters and scientists identify – and potentially warn – for future destructive fire-generated vortices.

In the future, Lareau said, local meteorologists could scan radar data for evidence of rotation in wildfire plumes, and satellite data for indications of fire-cloud formations and storms.

“The availability of high-resolution radar and satellite observations,” he said, “provide advance indications for vortex formation — such that watches, or even warnings, may have been possible.”