Scientists John Hubbe, left, and Celine Kluzek do preflight checks in a… (Peter DaSilva / For The Times )
A Gulfstream turboprop sits on the McClellan Airport runway under gray, gloomy skies. Kim Prather has waited two weeks for this day.
"I can't believe there are finally clouds," she says gratefully as she and her research team check and calibrate several million dollars' worth of equipment stacked in the plane's cabin.
After the plane takes off, it slices through a 9,000-foot-thick layer of storm clouds, zigzagging up the western slope of the Sierra Nevada to probe the mysteries of California's rain and snow.
Onboard, a special instrument that Prather invented and named "Shirley" will blow apart atmospheric particles with a laser and map their chemical composition, all in real time. Other devices will count and measure millions of cloud droplets, record water content and analyze gases.
On the ground, in the Tahoe National Forest, another array of equipment will simultaneously sample Sierra air masses.
Prather's team is trying to figure out why some clouds give up their moisture and others don't as they roll across the mountain ranges that provide much of the state with water.
They wonder: Is urban pollution reducing precipitation in Northern California's high country? Is Gobi Desert dust blown thousands of miles across the Pacific Ocean boosting the Sierra snowfall? Will atmospheric rivers — the moisture-laden bands in the sky that drenched the state in December and March — dump even more rain with global warming?
The answers will help resolve some of the unknowns of California's future water supply.
In 2009, federal and state agencies launched a research project called CalWater. It has two aims: to gain a better understanding of the mechanics of atmospheric rivers when they slam into the state's mountain ranges and how computer models of climate change should account for the rivers, and to determine if and how precipitation is influenced by the tiny particles called aerosols that are at the center of every cloud droplet.
Prather and her project colleague Daniel Rosenfeld are studying the aerosols, which can be liquid or solid, the products of nature, such as sea salt or windblown desert dust, or the products of man, such as urban soot or nitrates from farm operations.
Five years ago, Rosenfeld, an atmospheric scientist at the Hebrew University of Jerusalem, conducted research suggesting that man-made pollution particles from the Bay Area and Central Valley hindered the formation of rain clouds over the Sierra.
By creating more surfaces on which water vapor can condense, the theory goes, certain types of pollution may promote the formation of a lot of tiny droplets that never get big enough to fall as rain.
But the idea that aerosols influence the nature and amount of precipitation is controversial.
Meteorologists believe "that if a cloud wants to rain/snow, it is going to regardless of the type/amount of aerosol seeds," said Prather, a professor of atmospheric chemistry at UC San Diego who worked out of McClellan during the project's five-week field campaign last winter.
"There are so many factors that ultimately affect precipitation — meteorology, atmospheric dynamics, cloud microphysics and chemistry — and their effects are all intertwined and often change at the same time, so sorting them out is very, very difficult," she added.
The CalWater team hopes to settle the matter.
Today, while Rosenfeld and other scientists soar above her, Prather heads up the I-80 corridor to Sugar Pine Dam in the Tahoe forest. Halfway into her drive, she gets a call on her cellphone from McClellan that the plane was forced to turn back two hours into the flight because of power problems. "Shoot," she says.
It is lightly raining at Sugar Pine, where a small trailer is packed with 14 instruments — including "Laverne," Shirley's big sister — that are taking air samples. Nearby, a huge radar drum is collecting data from the same cloud layer the Gulfstream flew through.
By combining the ground and plane measurements, researchers can compile a vertical profile of the air masses that drop — or don't drop — rain and snow over the Sierra on this stormy afternoon.
Prather gets an update from the Sugar Pine staff and checks the instrument monitors. She studies a data stream that provides an instant chemical snapshot of the particles sampled by a periscope-like intake atop the trailer.
She heads back to McClellan, eager for the details of Shirley's aerial meal.
"It's really happy," Prather says as she scans Shirley's readings on a laptop computer in a second-floor conference room of the airport's corporate jet center. "It really had a lot of particles."
Prather identifies them with a practiced glance. "Organics, dust, sea salt." She notes that the samples taken outside the cloud have a different composition. "That's cows," she says, recognizing ammonium and nitrate from Central Valley dairy operations.