Why Distant Dust Storms Matter to California Rainfall

Rain is what Prather and her team have been waiting for. They’ve set up a large truck trailer full of scientific equipment at the Bodega Marine Laboratory, about an hour north of San Francisco.

Clouds are made up of water vapor, but that’s not the only ingredient needed for rain. Tiny particles like pollution, sea spray, dust and smoke, are the seeds of a rainstorm. The water inside a cloud condenses on these aerosols, growing larger and larger until it becomes a raindrop or snowflake that’s heavy enough to fall.

Large pumps on the trailer next to us are pulling in millions of these particles, so Prather and her team can examine them one by one.

“We didn’t expect there to be much dust here, but in fact, there are parcels of dust continuously coming across the Pacific right now,” she says.

The dust has made its way from the deserts of Asia and Africa. Dust storms send particles miles into the air, then they drift to California in 7 to 10 days.

Prather has studied dust like this before, when she flew through Sierra Nevada snowstorms in a small research plane.

“So days with dust, we found, were days where you had the most snow on the ground consistently,” she says. “And it’s not a lot of dust. It’s just the right amount of dust that comes in and seeds the very top of the clouds.”

In one study, Prather found that the right kind of dust storm could boost snowfall in the Sierra Nevada by 40 percent. It happens because dust helps ice crystals grow. Ice formation appears to be the magic recipe for producing lots of precipitation.

“Ice is a very picky process,” Prather says. “Ice only likes to form on certain surfaces. Dust seems to be very good. Bacteria—biological particles seem to be very good.”

Yep—bacteria. “Dust will come out of the ground with microbes on it and so there are microbes that are still alive in those clouds,” she says.

Other particles appear to have the opposite effect. Urban air pollution, both from California and coming over from Asia, seems to reduce precipitation in the Sierra. The particles are small, creating a lot of water droplets that aren’t quite heavy enough to fall.

Prather says there’s still a lot to understand about the effects that particles have on the weather, but the information could help improve weather forecasting.

“Are we getting more precipitation?” she says. “Are we getting less precipitation? The ultimate goal is to be able to feed this into weather forecast models and improve those models, where they actually take into account the seeds. Right now, they don’t.”

Updating Weather Models

“From an operational perspective, we are giving that very serious consideration,” says Bill Lapenta, who runs the weather forecasting division of the National Weather Service.

Daily weather forecasts come from giant simulations that run on supercomputers. Adding more dynamics, like the behavior of dust storms, increases costs for the agency.

“It adds an additional amount of complexity which then requires more computational resources,” he says.

Dust and Particles Transported Around the Globe://www.youtube.com/watch?v=kQyxFpQCs7U Weather models are updated generally only once a year, because changes can have far-reaching consequences. “The models have to be run on time, every time,” Lapenta says. The information is key for preparing the public for extreme weather and floods. Still, Lapenta says he’s hoping to start including dust storms and other sources of particles in the next five years. “I do believe that in the end, including the effects of dust particles in these complex models will help improve the precipitation forecast,” he says. Storing More Water for Drought

An improved forecast could help California manage its water supplies better, leading to fuller reservoirs that help buffer California against drought.

“The storms that provide the beneficial water that we really need badly this year?” says Marty Ralph, director of the Center for Western Weather and Water Extremes at the Scripps Institution of Oceanography. “It often comes in just a few events each winter, and that’s really the make or break thing for the season.”

California’s network of reservoirs stores that runoff. But reservoirs are required to release a lot of stored water in the fall and early winter, in order to make room for runoff from extreme storms and protect against floods.

“In the future, it may be possible if we can predict these storms accurately enough ahead of time, we could maybe keep a little extra water in there, knowing that if we had three days lead time we could release that water in a safe and appropriate way,” Ralph says.

Then, if the storms turn out to be small, reservoirs wouldn’t be releasing water that California needs later in the summer.

Dynamically managing reservoirs this way would require a major change in the operating rules. A bill recently introduced in the U.S. House by Sonoma County Democrat Jared Huffman would provide some of that flexibility.

Given the improvements in weather forecasting science and California’s propensity for drought, many say it’s a change whose time has come.