In the western United States, winter precipitation is key to providing the water states like Colorado and California need to survive their dry summers. The snow and rain that comes in the cold season runs off into reservoirs, where it is stored for drinking water, agriculture, hydropower and other uses.
Now, researchers have linked airborne dust and other particles from as far away as the Sahara and Asian deserts with the precipitation that falls over California's Sierra Nevada mountains. The paper, published online Feb. 28 in the journal Science, was co-authored by Jessie Creamean, a postdoctoral associate at the National Oceanic and Atmospheric Administration's Earth System Research Laboratory in Boulder, Colo., and Kaitlyn Suski, a graduate student at the University of California, San Diego.
"Basically, we were able to show that dust and biological aerosols that were lofted from deserts all the way across the world in the Sahara and Asia were airlifted all the way across the world to make ice crystals in clouds in the western United States," Creamean said.
Before this study, researchers had no idea that dust from such distant places would show up in clouds in California. "Evidence of Saharan dust in precipitation along the U.S. West Coast is somewhat surprising. It is more than half a world of circumnavigation away," said Ryan Spackman, a NOAA atmospheric scientist familiar with the study.
The aerosols catalyze the formation of ice in the clouds, which could increase precipitation, although more research needs to be done on that linkage, Creamean said.
The researchers do know that the presence of certain aerosols plays a large role in the formation of ice that leads to precipitation, but they also need to tease out the importance of that with other factors, like the amount of available water vapor and vertical storm winds.
Although past research had showed that dust from Asia travels far -- even demonstrating that the aerosols have circled the globe in 13 days -- no one had yet shown that dust from far away influenced precipitation.
Tracking by satellite and airborne lab
In order to figure this out, the researchers had to put together a lot of different data, detective style. They used images from NOAA's Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite to track the dust, and confirmed it was traveling all the way to California. During storms, they flew a research aircraft with several powerful instruments, including one that can identify the type of particle in a cloud and determine whether it was dust or some other type of aerosol.
"It can do that really fast and in real time," Creamean said. "We can look at very detailed information about the chemistry of individual aerosols."
At the same time as the aircraft were taking measurements, Creamean and her colleagues also took samples of the precipitation on the ground and analyzed them to see whether that dust was in the rain or snow landing in the mountains. This, combined with the cloud sampling, helped them gain confidence that the dust was a factor in creating the precipitation.
Paul DeMott, an atmospheric scientist at Colorado State University in Fort Collins, was one of the scientists who flew through the clouds. When the plane got to the higher levels of clouds, where the dust was, DeMott took cloud samples.
While in the air, he tested the samples to see whether he could make the dust particles in them create ice nuclei again, inside his instrument. The ability to essentially re-create in the aircraft what the researchers believed was happening out in the cloud further confirmed that the dust particles were creating ice nuclei.
"I'm taking a particle that was in an ice crystal in a cloud and demonstrating that there was something in it that makes ice," DeMott said. "So we were making these measurements while people were on the ground collecting the precipitation and getting radar data," and together, they were able to paint a better picture of how what happened in the cloud influenced the rain or snow falling on the ground.
In 2009, a similar study had been conducted with just on-the-ground and satellite data, said DeMott, but because it lacked the cloud sampling it was more difficult to understand what was actually happening with the dust in the atmosphere.
Climate change a factor
Spackman, the NOAA researcher, said the scientific community often debates the quality of aerosol measurements taken from clouds, but because the authors used multiple data sources, he said, this strengthened their interpretation. "The complementary meteorological and satellite data and interpretation provided by the authors strengthen the case that dust from disparate global sources can enhance precipitation in certain meteorological environments," Spackman wrote in an email.
Overall, Spackman said, the study makes a strong contribution.
"This is a cutting-edge study in the field of cloud-aerosol-precipitation interactions that includes an interdisciplinary group of atmospheric chemists and meteorologists," he said. "It contributes directly to a number of key science questions on the impact of aerosols on clouds and precipitation."
Climate change is likely to influence rainfall patterns in the Sierra Nevada as well as the amount of dust that makes its way into the atmosphere, so the hope is that a better understanding of how aerosols affect precipitation will help water managers in the future.
"Understanding which factors lead to more or less [precipitation] and how this happens is critical to developing infrastructure to capture more water, reduce flooding, et cetera," Kimberly Prather, a study author who holds appointments at the Scripps Institution of Oceanography and the department of chemistry and biochemistry at UC San Diego, wrote in an email.
Prather added that the results from this study can improve climate models by clarifying the roles of aerosols on precipitation. As the climate warms, researchers expect more dusts to make their way aloft, possibly having impacts on precipitation by changing where rain or snow falls.
The study was part of the CalWater research program, whose goal is to help Western states better understand how their water supply might be altered under climate change. A follow-up will be done in the winter of 2015, Prather said.
"Hydropower is an essential source of electricity in California providing, on average, 15 percent of our annual generation," California Energy Commission Chairman Robert Weisenmiller said in a news release on the study. "This state-funded study in cooperation with NOAA will help us understand how small particles in the air affect precipitation and hydropower generation. Additionally, this information will be useful in estimating the effects of our changing climate."
Reprinted from ClimateWire with permission from Environment & Energy Publishing, LLC. 202-628-6500. E&E Publishing is the leading source for comprehensive, daily coverage of environmental and energy issues. Click here to start a free trial to E&E's information services.
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