Sierra Nevada_1

From Saharan dust comes California snow

CARNEGIE MELLON (US) — Snow in California’s Sierra Nevada mountains can get its start as airborne dust particles that travel from deserts in Asia and Africa, scientists say.

Because snow in the Sierra Nevada provides a significant source of water to the region, finding ways to predict ice-initiated precipitation is crucial for water resource management, researchers say.

“The timing of when snow falls and how much snow falls is important to the Sierra Nevada’s vegetation and water supply, much as it is in many parts of the world,” says Ryan Sullivan, assistant professor of chemistry and mechanical engineering at Carnegie Mellon University. “Despite its importance, we cannot accurately predict snowfall.”

Clouds are made up of small droplets of water. When the droplets in the clouds freeze, they fall to Earth in the form of snow, ice, hail and graupal. These ice crystals can only form at extremely cold temperatures—around negative 38 degrees Celsius. In order to freeze at more moderate temperatures, the droplets need to attach to a catalyst called an ice nucleus, which can be any of a variety of particles, but are most often mineral dust or biological particles. Ice nuclei are very rare, typically ranging from one to 10 particles per million in the atmosphere.

“We’ve shown that the detection of ice nuclei might be a key variable in predicting the amount and type of precipitation an area can expect,” Sullivan says.

In February and March, heavy winds create clouds of dust in Asian deserts and the Sahara that become lofted high into the free troposphere. According to satellite images, those dust clouds travel over the Pacific Ocean, arriving at the West Coast of the United States about a week or two later.

Analysis of residue from snow that fell in Sierra Nevada during that time revealed evidence of Asian mineral dust, leading researchers to believe the snow was nucleated on mineral dust ice nuclei.

To test this theory, researchers used a combination of air- and ground-based techniques to gather data about the interactions between aerosol particles, clouds and precipitation over Sierra Nevada for one month in winter of 2011. Their research, published in the journal Science Express, was completed as part of the CalWater field campaign.

Scientists flew over the Sierra Nevada in a Department of Energy Gulfsream-1 aircraft equipped with instruments that allowed them to measure the cloud particles and aerosol particles surrounding the clouds. Sullivan and Paul DeMott, a senior research scientist in Colorado State University’s Atmospheric Chemistry Program, used an ice simulation chamber called a continuous flow diffusion chamber to analyze particles from outside the aircraft.

In the chamber, they exposed particles to controlled ice cloud formation conditions and counted the number of ice particles that were formed. From this, they determined how many ice nuclei were present in the atmosphere outside the aircraft.

Other group members used a technique called aerosol time of flight mass spectrometry (ATOFMS) to determine what types of particles were present in the air and clouds. From this combined dataset they were able to prove that the clouds that had the most ice nuclei also had the most Asian mineral dust—and that those clouds were most likely to produce precipitation.

The researchers also found that heavy precipitation was caused by a “seeder-feeder” mechanism, in which the ice crystals nucleated on dust particles would fall from their upper-level glaciated clouds and through a lower layer of supercooled liquid clouds. The water droplets in the lower layer of clouds would freeze and precipitate, creating an increased amount of snow. These lower liquid clouds likely would not have precipitated had they not been seeded by the ice crystals sedimenting out of the upper ice clouds.

Researchers from the University of California, San Diego, the Hebrew University of Jerusalem, Colorado State University, National Oceanic and Atmospheric Administration (NOAA) Earth System Research Laboratory, Scripps Institution of Oceanography, NASA Langley Research Center, and Pacific Northwest National Laboratory contributed to the study that was funded by the California Energy Commission.

Source: Carnegie Mellon University

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