A Tale of Two Storms

Researchers in a wide-ranging project to understand the climate forces that influence California's water supply found evidence that the character of aerosols carried across the Pacific affects how much precipitation winter storms produce.

Storms spaced one week apart from each other during February and March 2009 delivered one-fourth of the year's snowfall at a Sierra Nevada meteorological station set up as part of a project known as CalWater. The latter storm, however, deposited 1.4 times as much precipitation as the first.

The clouds in each storm contained a mix of particles from various sources. An analysis of the particles contained in the first storm revealed large quantities of potassium and other elements that created by the burning of biomass such as wood and grasses, most likely from sources within California. The aerosols contained in the second storm were comprised of iron, titanium and other elements typically sent airborne by dust storms in central Asia. The difference shows the influence that pollutants from the other side of the world can have on local climate, researchers said.

"We went into this looking to understand the impacts of local pollution sources on clouds and precipitation and surprisingly found that long-range transport appeared to be playing an important role," said Professor Kim Prather, Distinguished Chair in Atmospheric Chemistry, the lead principal investigator on the aerosol-cloud impacts portion of the CalWater study. Prather has a joint faculty appointment at Scripps Institution of Oceanography, UC San Diego as well as the UCSD Department of Chemistry and Biochemistry.

The endeavor, sponsored by the California Energy Commission, represents an ongoing multiyear project to investigate the possible effects of air pollution on California's precipitation that began in 2009. The Energy Commission's final goal is to improve the projections on how climate may change in California and this type of information is essential to improve and test the regional climate models use to develop these projections. California's hydroelectric generation depends heavily on precipitation falling in the Sierra Nevada.

The early measurements in 2009 were taken at ground level and therefore the inference that dust from Asia was substantially changing the nature and amount of precipitation was preliminary. However, measurements taken this past winter in 2011 with a Department of Energy Pacific Northwest National Laboratory (PNNL) G1 research aircraft showed that dust and other aerosols in long-range transport also occur inside the clouds, confirming the initial hypothesis.

Prather will discuss the relative roles of Asian aerosols, local Central Valley pollution, marine particles and other influencers of Sierra Nevada precipitation at the 2011 American Geophysical Union (AGU) Fall Meeting in San Francisco. (A23E-01 Tuesday, Dec. 6, 2, 1:40 PM, Room 3002). She will also co-chair the A23E session entitled "Aerosol, Cloud Properties, Atmospheric Rivers, and Precipitation in California: CalWater II" which will focus on CalWater findings to date.

The Sierra Nevada snowpack that supplies more than 30 million Californians with their water has shown a steady decline over the past century, possibly in part because of the state's air pollution. Projections of the Southwest's future suggest a further drying trend between 10 and 15 percent this century. Particulates in air pollution affect precipitation by changing the number and composition of cloud condensation and ice nuclei-the tiny particles of dust, sea salt, organic materials produced in combustion and human-produced pollutants such as diesel soot around which cloud droplets form. The moist droplets need to grow large enough to fall from the sky as rain or snow, so changes in the quantity of seed particles in the air can influence how much rain or snow a cloud is capable of producing.

Making analysis of these influences possible is an instrument co-developed by Prather known as an aerosol time-of-flight mass spectrometer (ATOFMS). Continuously pulling in air samples, the instrument can chemically characterize the individual particles present that form the cloud seeds. The fingerprints of each individual particle measured by ATOFMS reveal to scientists where particles likely originated and how long they have been airborne. In addition, UCSD graduate students Jessie Creamean and Andrew Ault collected rainwater and snow samples. The ATOFMS was used to analyze the chemistry of these samples to gain insights into which particles actually seeded the snow and rain.

Scientists from other institutions including PNNL, Colorado State University, NOAA, the Hebrew University of Jerusalem, Lawrence Berkeley National Lab and UC Davis participated in ground- and aircraft-based studies during CalWater.