News•February 28, 2013

Dusty Springs in Asia, Africa Can Increase Snow in Calif.

By Lauren Morello

A dusty spring in Asia and Africa can increase snowfall thousands of miles away in California’s Sierra Nevada mountains, according to a new study.

The process begins when winds stir up tiny particles of dust, pollution, bacteria and heavy metals from the Taklimakan and Gobi deserts in Asia and the Sahara in northern Africa.

In a matter of days, those particles — known to scientists as aerosols — travel halfway around the world, carried high above the Earth by the Jet Stream. When they run into moist air over the Sierras in late February and early March, the collision dumps snow on mountain peaks, said Kimberly Prather, a professor of atmospheric chemistry at the University of California, San Diego, who led the new research.

A remote forest near the Sugar Pine Dam ground monitoring site during CalWater 2011.
Credit: Jessie Creamean

“We realized the dust is actually seeding these mid-level clouds to make ice,” she said. “That ice falls through lower-level clouds loaded with water, and you end up with a massive amount of snow on the ground.”

Those results, published online Thursday by the journal Science, could be welcome news for California. Rising temperatures have reduced the size of the snowpack that accumulates each winter on the state’s peaks, and the flow of meltwater off those mountains each spring.

The stakes are high: Runoff from the Sierra Nevada provides about one-third of California’s water and helps generate about 15 percent of the state’s electricity by hydropower.

Climate models project that precipitation in California’s mountains will continue to decrease as the planet warms. Over time, scientists believe, more of that precipitation will fall as rain instead of snow, and the snow that remains will melt earlier each year, reducing runoff in late spring and summer and wreaking havoc with a system of manmade reservoirs built for a different climate.

Those projections are worrisome enough by themselves. But research by Daniel Rosenfeld, an atmospheric scientist at the Hebrew University of Jerusalem, found evidence that dust and particles of manmade pollution produced in California’s Central Valley could further reduce precipitation in the Sierra Nevada.

That helped spark the three-year, $3 million field campaign that discovered a very different link between dust and precipitation in those Californian peaks.

The project, known as “CalWater,” began in 2009 with a few deceptively simple questions: How do aerosols affect cloud formation and precipitation? Where are the aerosols that reach the Sierra Nevada coming from? And how will atmospheric rivers — weather systems that transport huge amounts of water across the Pacific into California — change as the climate warms?

Rosenfeld’s research suggested that pollution provided more surfaces that water vapor can condense on, but those tiny seeds would not grow large enough to fall out of the sky as drops of rain or flakes of snow.

The researchers sampled clouds using special instruments loaded aboard the Energy Department G-1 aircraft shown here.
Credit: Jessie Creamean

But CalWater scientists — a group that includes Rosenfeld — discovered early in their field campaign hints that dust was actually increasing snowfall in the Sierras.

“We went out to look for things that led to less precipitation,” Prather said. “But that’s not what we ended up finding.”

The researchers sampled air in the Sierras from a ground station near Sugar Pine Dam in the Tahoe National Forest and high in the sky, using a research plane owned by the Energy Department on dozens of flights out of Sacramento.

Similar instruments on the ground and in the air — which Prather helped design — were able to analyze a single cloud drop or ice crystal in a fraction of a second, spitting out a chemical fingerprint that revealed the presence of contaminants like dust, pollution, heavy metals or bacteria.

Those measurements helped explain stark differences in snowfall produced by two similar storms that hit the Sierras within a week of each other in February 2009.

One arose in an atmosphere rich with dust from China, measurements show. It dumped 40 percent more snow on the mountains than the second storm, which developed in an environment without much dust.

Researchers were able to trace that dust back to its source using data from NASA’s CALIPSO satellite. The particles that helped fuel a February 2011 snowstorm, for example, traveled from Oman to California over 10 days, the satellite data shows, passing through China along the way.

This graphic shows the path of aerosols that reached California in 2011. Circled numbers indicate locations in which dust was captured in CALIPSO satellite images.
Click to enlarge the image. Credit: Science

“Studies like this will help us improve our regional climate models,” said Guido Franco, technical lead for environmental research at the California Energy Commission, which sponsored the research effort. “We want to understand how climate change may affect energy production, and we want to understand how climate change may affect energy demand.”

But mysteries remain.

“The thing we’re looking at right now is, how common has this been?” Prather said. “We’ve found it every year we’ve gone out. Is it because it’s a more regular event now because deserts are getting drier? Or is this something that has always been there?”

The study’s authors aren’t sure how widespread the dust-snow link is, or whether dust is affecting precipitation in the Sierras at other times off the year. They also don't know what happens if dust transported across the Pacific by the Jet Stream passes through California without running into a developing storm system.

Would a similar combination of dust and atmospheric moisture produce more snow in the Rocky Mountains of Colorado, or the Cascades in the Pacific Northwest?

And what happens when dust, encapsulated within snowflakes, hits the ground? Research in the mountains of Colorado and Utah suggests that dark dust particles increase the melting of snow because they reduce snow’s ability to reflect sunlight.

Prather and her colleagues are already plotting new ways to help answer those questions. They’re gearing up for a massive new research effort, CalWater 2, set to begin in winter 2015. The scientists hope to fly out over the ocean to meet the dust before it comes ashore, hundreds of miles before it hits the snow-capped peaks of the Sierra Nevada.

As Prather said, it is just “one tiny little window” shedding light on the complex interplay between weather and climate, but it's an important one.