Dissecting California Precipitation

On a stormy February Friday, researchers and crew scurried to and from a Gulfstream-1 aircraft on the tarmac of an airfield northeast of Sacramento, hats pulled down and laptops tucked under raincoats as protection from the rain and wind.

“I love it!” exclaimed Beat Schmid, technical director of the Department of Energy’s Atmospheric Radiation Measurement Aerial Facility, which operates the Gulfstream-1.

Schmid was not the only one thrilled by the rain. He was in California with other researchers from DOE, additional federal agencies, and several universities to take part in a two-month, $10 million field research campaign called CalWater 2015. Since January, the scientists had been studying so-called atmospheric rivers that carry moisture from the tropics to western North America, where the moisture becomes rain and snow. It was an effort that has gained urgency as California faces its fourth year of a historic drought.

Depending on the year, atmospheric rivers can deliver as much as 50% of California’s precipitation, contributing significantly to the reservoirs and Sierra Nevada snowpack that provides much of the state’s summer water supply. But the storms can also bring flooding—in the February storm, a town 200 miles north of San Francisco got 13.4 inches of rain in 48 hours. For flood management, California reservoir levels are kept conservatively low. Those water levels could be safely higher if meteorologists understood and could predict how much precipitation atmospheric rivers carry. And in the longer term, how atmospheric rivers evolve in a warming climate will be equally key to managing California’s water supply.

The CalWater effort is geared toward solving those short- and long-term prediction challenges. “It’s a unique opportunity for atmospheric chemists and meteorologists to collaborate,” says Allen B. White, a research meteorologist at the National Oceanic & Atmospheric Administration (NOAA). “In the atmosphere there is all this stuff—water vapor and aerosols and dust—that is working together to produce the precipitation that we observe. If we’re going to get climate and weather models to more accurately predict precipitation, then we need to understand how these things work.”

A previous iteration of CalWater, for example, indicated that there is less heat transfer and evaporation from the ocean into storms than expected. That finding is critical for understanding the water budget and how much water can precipitate, says Christopher W. Fairall, a NOAA physicist who studies air-sea interactions.

Additionally, over land, pollution did not significantly affect precipitation. Airborne pollution particles tend to seed lots of small droplets that don’t get big enough to fall. “We went into it looking for how our local pollution was getting up into clouds and suppressing precipitation,” says Kimberly A. Prather, a professor of atmospheric chemistry at the University of California, San Diego; director of the Center for Aerosol Impacts on Climate & the Environment; and one of the CalWater leaders. What they found was the opposite: In severe storm conditions local pollution stayed close to the ground and didn’t affect precipitation.

Instead, dust from North Africa and Asia transported across the Pacific Ocean appears to play a far more important role, by promoting rain and snow (J. Geophys. Res. 2011, DOI: 10.1029/2010jd015351; Science 2013, DOI: 10.1126/science.1227279). Inorganic dust and biological particles such as bits of phytoplankton and bacteria lofted from the ocean are particularly effective at seeding large droplets and, in particular, ice crystals. “If you care about precipitation, you care about ice,” Prather says. “If you can get ice to form in a cloud, a lot of times that will influence the efficiency by which water is removed from that cloud. Once ice forms, it steals water from everything else”—and then that ice captures even more water from droplets it encounters as it falls to the ground.

While lab studies are playing an important role in understanding the interplay between particles such as aerosols and water, it’s also important for researchers to see what happens in the field. “It’s really complex chemistry and processes that are difficult to replicate in the lab,” Prather says.

This year’s CalWater campaign brought together an array of platforms and instruments to study atmospheric river meteorology, water transfer, aerosol chemistry, and how together they influence precipitation in winter storms. The NOAA research vessel Ronald H. Brown cruised the sea, taking position under storms. A ground site just north of San Francisco, at the UC Davis Bodega Marine Laboratory, as well as others around the state sat positioned to study storms as they came over land. Four aircraft—one from DOE, two from NOAA, and one from the National Aeronautics & Space Administration—flew in and around atmospheric rivers at different altitudes. And instruments such as NASA’s newly launched Cloud-Aerosol Transport System (CATS) peered from far above.

Instruments deployed by the researchers gauged the exchange of moisture between sea and air; measured cloud water content; and evaluated cloud droplet and ice crystal number, size, and chemical composition. Investigators also looked at aerosol particles’ chemical composition as well as their ability to nucleate water droplets and ice crystals. Probes, or sondes, released from planes and the ship further helped to track wind, humidity, and temperature. Dropsondes from planes parachuted to the surface, while radiosondes from the ship floated up by balloon. One plane also dropped Airborne Expendable Bathythermographs (AXBTs) provided by the Naval Research Laboratory to measure ocean surface temperature.

And during that early-February storm, the teams did something unprecedented: All four aircraft simultaneously flew above the ship at different altitudes to get what will undoubtedly be the best picture yet of an atmospheric river in action. “It was a huge coordination effort,” NOAA’s White says. “There will be just fantastic data sets to analyze.”

As great as those data sets will be, however, the CalWater teams did not get to study atmospheric rivers quite as much as they had hoped. California had the hottest and driest January on record, and only one other atmospheric river reached the state during the CalWater effort. Some atmospheric rivers stretched north to Oregon and Washington. The Ronald H. Brown was able to catch them en route, and the two planes with the greatest range—a NOAA Gulfstream-IV and NASA ER-2—ventured out to study them.

Otherwise, researchers took advantage of the time in the field to get a better understanding of nonstorm conditions. “We’re turning lemons into lemonade,” Prather says. They studied aerosol sources and characterized the differences between what’s on the surface and what’s in the upper atmosphere that ultimately ends up seeding clouds. In particular, it appears at higher altitudes, long-range transport brings pollution as well as dust across the ocean from Asia. The amount of pollution flowing across has been more than the scientists expected, Prather says. She notes that there appears to be a correlation between the conditions leading to atmospheric rivers and the types of aerosols that make it into the skies above California—but the researchers need more data to be sure.

As the scientists turn from field activities to data analysis, they’re also requesting resources to continue studying atmospheric rivers over the next several years. Even after such a comprehensive effort, there will still be lots to learn and confirm about atmospheric river systems and the chemistry of the particles that seed precipitation, says Lai-yung Ruby Leung, a climate modeler at Pacific Northwest National Laboratory.

Earlier in her career, Leung didn’t think much about chemistry, she says. But as time has gone on, she and other modelers have realized that uncertainty in predictions often comes down to chemistry. “There are a lot of debates around how much warming do we expect in the future and how warming will affect precipitation,” she says. “That’s all intimately related to aerosols and chemistry. We are learning about that now.”