Western U.S. to See Less Rain as Direct Consequence of Wildfires, Study Says

Dense smoke from eastern Idaho's August 2018 Kiawah-Rabbit Food fires pushes up against thin cumulus clouds hovering overehead. Image shot from a C-130 research plane which was part of the current study. Photo: Emily V. Fischer, Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado, used with permission

Under normal circumstances, rainclouds often provide short-term respite from small wildfires breaking out in dry regions on the ground.

A new study led by Cynthia Twohy, an atmospheric scientist at Northwest Research Associates and the Scripps Institution of Oceanography, just delivered the bad news that bigger wildfires and the much larger amount of smoke particles injected into clouds overhead will likely make that critical rainfall much less common. Less rainfall means the ground and forests below stay even drier even after the wildfires burn out, exacerbating the drought and fire-prone nature of the land for an even longer time.

When there are no fires, relatively pristine water droplets form in the clouds above. Those droplets, which are about half the size of water droplets formed around smoke particles, fall more readily to the ground than the smoky ones. That was understood as part of known science.

But when lead author Twohy and her associates studied what happens within clouds filled with wildfire smoke from below, they discovered there were roughly five times the number of these larger droplets than the previously “clean” ones. That surprise, combined with the knowledge that the bigger drops which condensed around the smoke particles would tend to stay in the air rather than fall, pointed to a serious feedback loop from large wildfires which had not been known before.

“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,” Twohy said in a recent statement to the press about her group’s investigation. “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.

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.

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.

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.”

“Biomass Burning Smoke and Its Influence on Clouds Over the Western U.S.,” by Cynthia H. Twohy, et. al., was published as a Research Letter in the peer-reviewed journal Geophysical Research Letters. Because of its importance to the scientific community, it is available for free download in full for all interested, for a period of roughly 30 days from now.