California Precipitation Impacted By Saharan Dust
March 2, 2013

Tracking Sahara Desert Dust’s Impact On California Precipitation

redOrbit Staff & Wire Reports - Your Universe Online

Researchers from the University of California, San Diego (UCSD) and the National Oceanic and Atmospheric Administration (NOAA) have demonstrated for the first time that dust and other aerosols from one part of the world can influence rainfall in regions thousands of miles away.

The study is part of the CalWater field campaign — an initiative funded by the California Energy Commission and designed to help states in the western US better understand how climate change can influence their water supply and hydropower generation in the years ahead.

The project was led by UCSD and NOAA scientists and worked to track the amount of dust that travels great distances, the frequency with which particles did so, and the impact it could have on precipitation halfway around the world.

A study detailing their findings, entitled "Dust and Biological Aerosols from the Sahara and Asia Influence Precipitation in the Western US," was published Thursday in the online version of the journal Science.

"We were able to show dust and biological aerosols that made it from as far as the Sahara were incorporated into the clouds to form ice, then influenced the formation of the precipitation in California," Jessie Creamean, co-author of the study, said in a statement.

"To our knowledge, no one has been able to directly determine the origin of the critical aerosols seeding mid-level clouds which ultimately produce periods with extensive precipitation typically in the form of snow at the ground,” Creamean, who is also a postdoctoral associate at NOAA's Earth System Research Laboratory (ESRL) in Boulder, Colorado, added.

Experts have long known that winds can carry dust, soot, sea salt and other aerosols from one continent to another at altitudes exceeding 16,000 feet, and one study even tracked Asian dust as it completely encircled the Earth in less than two weeks time. The presence of those particles can cause water vapor and droplets to freeze at temperatures that would have been too warm for that to occur otherwise, causing either rain, snow or hail to begin falling.

By studying winter storms that occurred two years ago, the UCSD and NOAA researchers discovered that dust and biological aerosols tended to “enhance precipitation-forming processes” in the Sierra Nevada mountain range located in California and Nevada. Past research had suggested that those particles had the opposite effect.

The CalWater team collected most of their data using instruments known as aerosol time-of-flight mass spectrometers (ATOFMS) and the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite, the latter of which was used to track aerosols as they moved across continents.

“Using these tools, the researchers were able to determine that at least some of the dust and bioparticles detected by an aircraft-mounted ATOFMS unit during February 2011 flights through Sierra Nevada storm clouds were in the skies over Oman 10 days earlier, having likely originated in the Sahara a few days earlier,” explained the Scripps Institution of Oceanography at UCSD. “Along the journey, the Saharan dust and microbes mixed with other aerosols from deserts in China and Mongolia before wafting over the Pacific Ocean.”

“Upon arrival in California, the aerosols effectively seeded the storm clouds and contributed to the efficiency of clouds in producing precipitation,” they added. “Two other transportable ATOFMS units housed in trailers at Sugar Pine Dam just south of Interstate 80 in the Tahoe National Forest and other instruments made further measurements. They determined the chemical composition of aerosols at the end of their journey by looking at the particles present in precipitation samples that were collected during storms.”

Creamean and colleagues from UCSD, the Hebrew University of Jerusalem, Colorado State University, Carnegie Mellon University, NASA's Langley Research Center, and the Pacific Northwest National Laboratory said that future studies similar to the CalWater project will be required to clarify what types of aerosols are conducive to the formation of precipitation, as well as which ones will actually inhibit rainfall instead.