AGU - Wildfire Smoke May Cut Rainfall; Also AGU - Dry Air At Night Over Western US Helps Drive Fires
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?’”
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https://news.agu.org/press-release/wildfire-smoke-may-lead-to-less-rain-in-the-western-us/
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Earth’s atmosphere is warming due to climate change and warming in many places has been greater at night. Warmer night air had been suspected as the culprit altering the daily pattern of wildfire activity, with burns continuing later into the night. The new study, however, shows it’s not just that the night air is warmer. The study found a dramatic shift from 1980 to 2019 in its drying power—how much moisture the nighttime air can carry away from the fuels—over much of the Western U.S. This shift is not captured in climate models, and the authors say it could be related to natural long-term cycles rather than to climate change.
“We paid special attention to the change in recent years compared to the conditions seen in the ’80s and ’90s, which is when many of the current firefighters started their careers, and presumably formed their ideas about what normal fire behavior should look like,” Chiodi said. “We tried to quantify the changes that we were hearing about from firefighters.” The study looks at the “vapor pressure deficit,” or the difference between the moisture in the air and the saturation moisture level at that air temperature. This difference is a measure of the air’s drying power.
“In the southern Sierra Nevada, the average summer nighttime vapor pressure deficit for the recent decade was 50% higher than the average in the ’80s and ’90s,” Chiodi said. “I was surprised—it’s unusual to see geophysical data change that dramatically.”
Some of this shift in vapor pressure deficit is happening because warmer nighttime air, caused by climate change, produces higher saturation values. But part of the drying power is happening because the nighttime air in some regions has less moisture, and that effect is not predicted by climate change models, at least this much or in this pattern. The authors find a possible connection to the Pacific Decadal Oscillation, a long-term cycle that can influence inland weather.
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https://news.agu.org/press-release/dryer-warmer-night-air-is-worsening-some-western-wildfires/