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

Date:
August 12, 2021
Source:
American Geophysical Union
Summary:
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.



FULL STORY ==========================================================================
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.


==========================================================================
A new study finds tiny particles in wildfire smoke affect the way droplets
form in clouds, potentially resulting in less rain and exacerbating dry conditions that fuel fires.

When wildfires send smoke up into the atmosphere, tiny particles fly up
with it. Water droplets can condense on the particles in clouds.

The study's authors expected an increase in the number of water droplets forming in clouds as a result of wildfires, because more particles create
more droplets. But the difference between smoky and clean clouds was
bigger than expected, with smoky clouds hosting about five times the
number of droplets than their clean counterparts. Smoky droplets were
also half the size of pristine droplets.

That size difference is what could stop the drops from falling. Because
small droplets are less likely to grow and eventually fall out as rain, wildfires in the western U.S. could mean less rain during wildfire season, according to the new study published in the AGU journal Geophysical
Research Letters, which publishes high-impact, short-format reports with immediate implications spanning all Earth and space sciences.

"We were surprised at how effective these primarily organic particles
were at forming cloud droplets and what large impacts they had on
the microphysics of the clouds," said lead author Cynthia Twohy,
an atmospheric scientist at NorthWest Research Associates and Scripps Institution of Oceanography. "I started thinking, 'What are the long-term effects of this? We have drought, and we have a lot of wildfires, and
they're increasing over time. How do clouds play into this picture?'"
Twohy and a team of atmospheric chemists spent the summer of 2018 in a
C-130 Hercules research plane, sampling mid-altitude altocumulus clouds
while fires burned across the western U.S. Instruments on board the plane measured gases and particles emitted by wildfires and sampled droplets,
whose chemistry Twohy analyzed back in the lab.



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The work provides direct new insight into the microphysics and chemistry
of wildfire-linked clouds that can help scientists understand potential
causes and effects of atmospheric changes during wildfires.

SMOKY CLOUD COMPLEXITIES In clouds that reach high into the atmosphere,
adding more particles can invigorate the clouds and cause rain, but the opposite is true for lower- altitude cumulus clouds like those Twohy
studied. Previous work, unrelated to the present study, found similar
changes in droplet size and concentration related to smoke in the Amazon, supporting the new findings.

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.



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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." ========================================================================== Story Source: Materials provided by American_Geophysical_Union. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Cynthia H. Twohy, Darin W. Toohey, Ezra J. T. Levin, Paul J. DeMott,
Bryan Rainwater, Lauren A. Garofalo, Matson A. Pothier, Delphine K.

Farmer, Sonia M. Kreidenweis, Rudra P. Pokhrel, Shane M. Murphy, J.

Michael Reeves, Kathryn A. Moore, Emily V. Fischer. Biomass
Burning Smoke and Its Influence on Clouds Over the Western
U. S.. Geophysical Research Letters, 2021; 48 (15) DOI:
10.1029/2021GL094224 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/08/210812092731.htm

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