Fire and rain and the smoke that lingers

Wildfire-driven thunderstorms, such as those created in Australia a year ago, produce lingering impacts that may affect much of the globe, scientists have told the virtual annual meeting of the American Geophysical Union.

Such storms occur when fires funnel hot air upward, much like a massive chimney, says David Peterson, a meteorologist at the US Naval Research Laboratory (NRL). When conditions are right, they can create giant pyrocumulonimbus clouds (fire-driven thunderstorms) that can punch all the way into the stratosphere.

Like ordinary thunderstorms, these can produce lightning and rain. But unlike ordinary thunderstorms, they can inject enormous amounts of smoke into the upper atmosphere, where it can linger for months or longer.

Wildfire-driven thunderstorms occur each year around the globe, but the ones created by Australia’s New Year’s fires were “off the chart”, Peterson says. “We’ve never seen anything like that before.”

In combination, he says, they were similar to a volcanic eruption in their ability to inject fine particles into the upper atmosphere.

Once in the upper air, their smoke not only lingers, but much of it doesn’t disperse. Rather, says Peterson’s NRL colleague Mike Fromm, a meteorologist, it forms swirling anticyclones (so named because they rotate the opposite direction from the cyclones that form tropical storms).

The effect, he says, is like spinning the flakes in a snow globe. “The smoke is trapped.”

Why this occurs isn’t entirely known, but most likely, Fromm says, it’s because the smoke absorbs sunlight. That heats not only the smoke itself, but also the surrounding air, inducing it to rotate as it spreads out, much like the air in ordinary high-pressure zones.

The effects of all of this on the weather lower down is unclear. There are no operational weather models that include the effect of pyrocumulonimbus clouds, Peterson says.

But persistent, smoke-driven anticyclones could affect the flow of upper-level air currents such as the jet stream, which shepherds the movement of high- and low-pressure systems Fromm says. “That can affect weather not only locally, but downstream.”

One thing that is known is that pyrocumulonimbus storms can create lightning that can spread wildfires by igniting surrounding vegetation.

They can also affect air travel. A storm in Portugal sent upper atmosphere smoke across much of northern Europe, Fromm says, where it got into the cabins of airplanes flying through it. That set off smoke detectors on several planes, forcing emergency landings because pilots thought the smoke was from onboard fires.

Meanwhile, scientists are working to understand the meteorology of these storms.

In August 2019, for example, a NASA team flew an airplane through two of them in Washington state, says Laura Thapa, a graduate student in atmospheric sciences at the University of California, Los Angeles. Once fully analysed, she says, the results will help meteorologists better understand how pyrocumulonimbus storms behave – something that might be increasingly important in the future.

That’s because climate change may affect the frequency and size of these storms in ways other than simply making fire seasons longer and more intense. “What we know is that you need the weather to be hot, dry and windy,” Fromm says. “The hotter, drier and windier it is, the better the chance of these is.”

And in some parts of the globe, hot, dry, and windy may be exactly what climate change has in store.

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