Friday, 3 August 2018

We're in unprecedented territories with FIRE TORNADOES


Redding's Fire Tornado Was Not Your Garden Variety Fire Whirl


30 July, 2018
 
On Thursday evening, the Carr Fire exploded into the outskirts of Redding, blazing through everything in its path.
By the next morning, through images and video posted online, the world could see the force that had driven the fire’s leap across the Sacramento River and rapid spread into the city: a massive, rotating cloud of smoke and ash and flame. ABC10, which posted one of the most widely shared videos on Twitter, said it wasn’t a tornado. But it sure looked like one.
It was a fire whirl, also known as a fire devil, fire vortex, fire tornado, or “firenado.”

A fire whirl is any intensely rotating column of heated air that rises from a wildfire, according to Neil Lareau, an atmospheric scientist at the University of Nevada, Reno.

It begins to form when air starts to rotate, usually near the surface of the earth. The air is very hot because of the fire, but also due to the hot ambient air temperatures that set the stage for the fire in the first place. Heat rises, so that spinning air at the surface gets stretched high into the atmosphere, forming a rotating column of hot air that pulls ash and flame along with it.

What causes the rotation in the first place?

That’s the part we don’t really know,” says Lareau.

Fire tornadoes are just one type of extreme fire behavior that may be exacerbated by climate change.
Rotation can be driven by differences in wind speed or temperature, caused by features of the local environment: the terrain or an obstacle on the ground. But researchers don’t understand this part very well, and it’s difficult to pinpoint what exactly started the rotation at the Carr Fire.

What happens when the rotation rises into the air is better understood. The rotation becomes concentrated in that column. Lareau likens it to a whirling figure skater who puts their arms up over their head to spin faster.


"They kind of concentrate that rotation, and make it stronger. And the fire is doing this huge vertical boost to that rotation that may have been at the surface," he explains, "It's starting to look more like a tornado than your garden-variety fire whirl."

Usually, meteorologists take issue with describing fire whirls as "fire tornadoes" because they form differently, and fire whirls are usually made up of much weaker winds than tornadoes. Last Thursday, though, seeing the aftermath with roofs ripped off of homes and large trees completely uprooted just outside the burn zone, even the weather experts were tempted to describe the Carr Fire whirl as a tornado.

Fire whirls are not an uncommon sight at a wildfire. But they're usually small, up to 1,000 feet high, and last only minutes or even seconds. On Thursday night, the fire whirl outside of Redding stretched to about 18,000 feet into the air and lasted for nearly an hour, and winds were clocked near the lower end of tornado-strength wind speeds.

"It's very rare as well to have these really persistent long-lived events like that, " says Lareau, "To get a big one like this is really scary."
Fire whirls are dangerous not only because of the strength of the winds involved but because they lift embers high into the air, where higher-altitude winds can carry them far from the flaming front. That's one way that fires get past what should be major barriers -- like the Sacramento River.
A 3D visualization of the 'fire tornado' at the Carr Fire on the evening of July 26. (Neil Lareau)

Lareau says that other fire whirls on this scale have occurred in Northern California in the recent past, but not in densely populated areas like Redding. Big fire whirls may have played a role in the spread of last year's Thomas Fire and North Bay Fires -- counted as the state's largest and most destructive on record, respectively -- but the fire whirls were not on the same scale, and those fires were mostly driven by regional Santa Ana and Diablo winds.
Scientists are really just starting to learn about fire whirls, and are a long way from being able to predict the occurrence of major fire whirl events -- but they can identify the ingredients that make such an event more likely. The topography of the area is part of it. And the heat, which reached 113 degrees in Redding last week.


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