Dangerous Wildfire-Generated Pyrocumulonimbus Cloud Thunderstorms Are on the Rise

26 February 2019

Published by https://weather.com/

USA – As global warming causes more frequent and more intense wildfires, we’re likely to see more fire-generated thunderstorms that can spark more fires and carry smoke and particles high into the atmosphere, scientists warn.

These rare thunderstorms are called pyrocumulonimbus clouds, or “pyroCbs,” and they are cropping up in places where they have never occurred before, such as Texas, Portugal, South Africa and Argentina, according to Yale Environment 360, an online magazine published by the Yale School of Forestry and Environmental Studies.

Some studies have found that a single fire season in western North America can include more than 25 intense pyroCbs. Since 2000, an average of 73,200 wildfires have burned an average of 6.9 million acres yearly, according to the Federation of American Scientists. That’s nearly double the average of 3.3 million acres burned annually in the 1990s.

Mike Flannigan, director of the Canadian Partnership for Wildland Fire Science at the University of Alberta, explains that warmer temperatures are likely causing more intense fires. Those fires create huge black smoke plumes, carbon and water vapor, key ingredients in pyroCbs.

Flannigan told the Yale magazine that pyroCbs can whip up tornado-strength winds that fling embers for miles in all directions. Higher up, the clouds create lightning that can ignite new fires far from the original blaze.

“PyroCbs like the one that was associated with the Carr Fire in California in 2018 can be catastrophic because they can generate tornado-strength vortexes,” Flannigan said.

Scientists have long known that a wildfire can produce its own weather system, but they are still studying the mechanics of exactly how it happens.

What is known is that extremely hot air rises above the fire, carrying smoke, carbon and other noxious particles, like ammonia, hydrogen cyanide and ethane, and whatever water vapor has been sucked out of the ground and surrounding plants.

The moisture condenses on smoke particles as it rises and forms a big fluffy cloud like those that birth thunderstorms. But pyroCbs rarely produce rain, just a lot of lightning and high winds.

Pyrocumulus clouds are often seen over erupting volcanoes. Researchers like Mike Fromm, a meteorologist at the U.S. Naval Research Laboratory, and others are working to show that pyroCbs may affect weather and climate much like volcanic eruptions have in the past.

A study published last August found that pyroCbs pushed smoke and other particles much higher into the atmosphere than fires alone.

The particles linger in the stratosphere because there is no rain to wash the cloud away. Eventually, the particles fall back to the ground. In one case, chemical compounds consistent with those in a 2017 pyroCb event in British Columbia were found on Ellesmere Island, more than 2,000 miles away.

One pyroCB may not lower the planet’s temperature for more than a year like a 1991 Mt. Pinatubo eruption did, but Fromm says several in one year might.

“Figuratively, we have a map pinpointing the location of pyroCbs events,” Fromm told Yale Environment 360. “It’s been filling up fast. Maybe we’re better at detecting them, but I think they’re increasing.”

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