Wildfire smoke may lead to less rain in the western US

Particles from wildfires make small cloud droplets that are less likely to fall as rain


WASHINGTON - August 11, 2021 (www.waternewswire.com) As wildfires and heatwaves stress the western United States, concern over drought is rising: Dry landscapes burn more readily, and rain can help quell fires already raging. But wildfire smoke may keep that essential rain from falling.

Cumulus clouds mingle with smoke from the August 2018 Cougar Creek fire in Washington state's Okanogan-Wenatchee National Forest.
Credit: Emily V. Fischer

A thin layer of cumulus clouds caps dense smoke from the Kiawah-Rabbit Foot fires in eastern Idaho during August 2018, as viewed from a C-130 research plane.
Credit: Emily V. Fischer

"What really excited me about this paper were the connections to the hydrological cycle," said Ann Marie Carlton, an atmospheric chemist at the University of California-Irvine who was not involved in the new study. "They observe differences in cloud droplet size and precipitation, and cloud formation definitely impacts the hydrologic cycle. To have cloud-related findings so robust is sort of unusual, in my experience."

Cloud microphysics are complex, and Twohy notes that there are factors other than droplet size to consider for the overall impact smoky clouds have on regional climate. The new study focused on small cumulus clouds, which blanket about a quarter of the western U.S. in the summer, but other types of clouds, like higher-altitude thunderstorms, could behave differently. In shallower clouds, the more numerous, smaller droplets also can be more reflective, which could have a slight cooling effect at the surface.

With summer rain in the region decreasing, Twohy thinks the drying effects are winning out over factors that could increase rain, like cloud invigoration.

"Over the past couple decades, summer precipitation is down and temperatures are up. The cloud effects are likely an important part of all this. I'm hoping these results will spur detailed regional modeling studies that will help us understand the net impact of smoke on clouds and climate in the region," said Twohy.

If wildfire smoke is making rain less likely, feedback between smoke, dry spells and more wildfires could be more common in the future. Cloud microphysics are complex, so it may be a matter of time before these relationships are clear. Regardless, in connecting wildfire smoke to cloud changes and tentatively, precipitation, Twohy's new research pushes atmospheric physics and chemistry to catch up with climate change.

"As humans have perturbed the composition of the atmosphere, there are all these feedbacks and interactions that we don't even know about," said Carlton. "This experiment we're doing on planet Earth is altering clouds and the hydrologic cycle, at least regionally. I think this paper is scratching the surface of what we don't know."

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AGU press contact:

Liza Lester, +1 (202) 777-7494, news@agu.org

Contact information for the researchers:

Cynthia Twohy, Scripps Institution of Oceanography and NorthWest Research Associates, twohy@nwra.com

Paul J. DeMott, Colorado State University, paul.demott@colostate.edu

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