Wildfire smoke could reduce rain in the western United States

Cumulus is mixed with smoke from the August 2018 Cougar Creek fire in the Okanoganwanachi National Forest in Washington State. Credit: Emily V. Fischer

Concerns about drought have increased as wildfires and heat waves stress the western United States. Dry landscapes are more flammable, and rain helps to extinguish already raging fires. But wildfire smoke may prevent its essential rain.

New research found Small particles Wildfire smoke affects the road splash In the form cloud,as a result rain Exacerbates the dryness of burning fire.

When a wildfire releases smoke into the atmosphere, small particles fly with it. Water droplets can condense on particles in the clouds.

The authors of the study expected an increase in numbers Water drops More particles form in the clouds as a result of a wildfire as they create more droplets. However, the difference between smoky and clean clouds was greater than expected, and smoky clouds hosted about five times as many droplets as clean clouds. The smoky droplets were also half the size of the original droplets.

The difference in size is what prevents the drops from falling. Wildfires in the western United States mean less rain during the wildfire season, as small droplets are less likely to grow and eventually fall as rain, according to a new study published in the AGU Journal. There is a possibility Geophysics Research LetterHas published a short, influential report with immediate implications across all earth and space sciences.

Cynthia Tuhee, an atmospheric scientist and lead author at the Northwest Research Associates and Scripps Institution of Oceanography, said: “What is the long-term effect of this? There are droughts, there are a lot of wildfires, and there are more wildfires over time. How clouds work on this painting.” I started thinking. “

Twohy and a team of atmospheric chemists spent the summer of 2018 on a C-130 Hercules research aircraft, sampling mid-altocumulus clouds, burning fire on in-flight equipment in the western United States, and gas released from wildfires. And the particles were measured and the droplets were sampled. The chemistry Twohy was analyzed in the lab.

This study provides direct new insights into the microphysics and chemistry of wildfire-related clouds and helps scientists understand the potential causes and effects of atmospheric changes during wildfires.

Wildfire smoke could reduce rain in the western United States

From the perspective of a C-130 research aircraft, a thin layer of cumulus clouds covers the heavy smoke from the Kiawah-Rabbit Foot fire that broke out in eastern Idaho in August 2018. Credit: Emily V. Fischer

The complexity of smoky clouds

For clouds that reach high altitudes in the atmosphere, the addition of particles can activate the clouds and cause rain, but the opposite is true for low-altitude cumulus clouds such as those studied by Twohy. Previous studies unrelated to the current study confirmed similar changes in the size and concentration of Amazon smoke-related droplets, supporting new findings.

“What really thrilled me with this treatise was the connection to the water cycle,” said Anne-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 affects the water cycle. In my experience, making cloud-related discoveries very robust is a kind of thing. It’s unusual. “

Cloud microphysics is complex, and Twohy states that there are factors other than droplet size to consider regarding the overall impact of smoky clouds on the local climate. A new study focused on small cumulus clouds that cover about a quarter of the western United States in the summer, but other types of clouds, such as high-altitude thunderstorms, can behave differently. In shallow clouds, more and smaller droplets can also be more reflective, which can have a slight cooling effect on the surface.

Due to the reduced summer rains in the region, Twohy believes the drought effect outweighs the factors that may increase rain, such as cloud activation.

“Over the last few decades, summer precipitation has decreased and temperatures have risen. Cloud effects can be an important part of all of these. These results are the net effects of smoke. We hope to spur in-depth regional modeling studies that will help us understand the region’s clouds and climate. “

Feedback between wildfire smoke, dry spells, and more wildfires can become more common in the future if the wildfire smoke makes it less likely to rain. Cloud physics is complex, so it may be only a matter of time before these relationships become clear.Anyway, when connecting Forest fire Twohy’s new research on smoke-to-cloud changes, and tentatively precipitation, is driving atmospheric physics and chemistry to keep up with climate change.

“There is all this feedback and interaction we don’t know about because humans have confused the composition of the atmosphere,” Carlton said. “This experiment we are doing on planet Earth is changing the cloud and water cycle, at least regionally. I think this treatise is scratching the surface of what we don’t know.”

Sowing of ice clouds with wildfire release

For more information:
Cynthia H. Twohy et al, Biomass Burning Smoke and Its Impact on Western Clouds, Geophysics Research Letter (2021). DOI: 10.1029 / 2021GL094224

Quote: Wildfire smoke from rains in the western United States (August 11, 2021) taken on August 11, 2021. May be reduced

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Wildfire smoke could reduce rain in the western United States

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