Refilling California's Reservoirs-The Roles of Aerosols and Atmospheric Rivers

The coastal observatories will give detailed information about incoming storms such as winds and water content


As California enters another year of drought, scientists from Scripps Institution of Oceanography at UC San Diego, the Department of Energy's Pacific Northwest National Laboratory (PNNL), and NOAA will preview an upcoming field campaign in early 2015 that could accelerate scientists’ understanding of the natural and human-caused phenomena that determine when and how the state gets its precipitation.

The workshop to describe CalWater-2/ARM Cloud Aerosol Precipitation Experiment (ACAPEX), an interagency, interdisciplinary, field campaign starting in January, takes place at 10:30 a.m. Thursday, Dec. 18, at the Fall Meeting Press Conference Room (Room 3000, Moscone West, Level 3). CalWater2/ACAPEX mission scientists Marty Ralph and Kim Prather from UC San Diego, Ruby Leung from PNNL, and Chris Fairall from NOAA’s Earth Systems Research Laboratory will discuss the campaign and accomplishments to date.

“This will be an extremely important study in advancing our overall understanding of aerosol impacts on clouds and precipitation,” said Prather, who is a co-principal investigator on the portion of CalWater2/ACAPEX dealing with aerosols.  “It will build upon findings from CalWater1, adding multiple aircraft to directly probe how aerosols from different sources, local, ocean, as well as those from other continents, are influencing clouds and precipitation processes over California.”

“We are collecting these data to improve computer models of rain that represent many complex processes and their interactions with the environment,” said PNNL's Leung, principal investigator of ACAPEX. “Atmospheric rivers contribute most of the heavy rains along the coast and mountains in the West. We want to capture those events better in our climate models used to project changes in extreme events in the future.”

The key natural phenomenon of interest is called an atmospheric river (AR) – a veritable firehose of moisture channeled from the tropics to the U.S. West Coast. Where the AR hits the coast is where the precipitation occurs, but total accumulation and how much falls as rain as opposed to snow is influenced by aerosols.

The groundbreaking CalWater1 field campaign conducted in 2009-2011, which included research aircraft and ground-based sampling, has already yielded new insights into how precipitation processes in the Sierra Nevada can be influenced by different sources of aerosols that seed the clouds. 

In terms of aerosols, mineral dust transported intercontinentally and biological particles possibly from the ocean were broadly shown to correspond to greater precipitation, while the heightened presence of aerosols produced by biomass burning and smog correlated with less precipitation.  The upcoming experiment will use enhanced sampling at the coast and offshore to search for these processes “upwind” of the Sierra out over the Pacific Ocean and along the California coast.

The experiment offshore will observe the strength of ARs, which can transport water vapor in amounts 10-20 times the flow of the Mississippi River.  It will also explore phenomena that influence the predictability of landfalling ARs and precipitation.  These ARs produce up to 50 percent of California’s precipitation.  The role of evaporation from the ocean as an AR input as well as changes in the ocean after passage of an AR are also being studied. 

The workshop takes place in the midst of December atmospheric river events that are anticipated to bring several inches of rain and snow to the West.

“The recent AR event that struck California provided some drought relief bringing major precipitation and flooding in Northern California,” Ralph said.

This panel will describe how ground-based, multiple-aircraft, and ship-based measurements will help provide a better understanding of how California gets its rain and snow, how human activities are influencing precipitation, and how the new science provides potential to inform water management decisions relating to drought and flood.


A33Q-02 • Wednesday, Dec. 17, 1:55 ­– 2:10 p.m.  •  Moscone West 3006

Ryan Spackman, Marty Ralph, Kimberly Prather, Daniel Cayan, Paul DeMott, Michael Dettinger, Chris Fairall, Ruby Leung, Daniel Rosenfeld, Steven Rutledge, Duane Waliser, Allen White “CALWATER 2 ­– PRECIPITATION, AEROSOLS, AND PACIFIC ATMOSPHERIC RIVERS EXPERIMENT”


A34E-01 Wednesday, Dec. 17, 4 ­– 4:15 p.m.  •  Moscone West 3006

Ruby Leung, Kim Prather, Marty Ralph, Daniel Rosenfeld, Ryan Spackman, Chris Fairall, Paul DeMott, Jiwen Fan, Chun Zhao “CALWATER 2 – ARM CLOUD AEROSOL PRECIPITATION EXPERIMENT (ACAPEX)”


A33Q-05 Wednesday, Dec. 17, 2:40 – 2:55 p.m. Moscone West 3006



A33Q01 Wednesday, Dec. 17 1:40 – 1:55 p.m. Moscone West 3006


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