Atmospheric rivers, cloud-creating aerosol particles, and California water situation

“Climate and weather models have a hard time getting precipitation right,” said Ralph. “In fact, the big precipitation events that are so important for water supply and can cause flooding, mostly due to atmospheric rivers, are some of the most difficult to predict with useful accuracy. The severe California drought is essentially a result of a dearth of atmospheric rivers, while, conversely, the risk of Katrina-like damages for California due to severe ARs has also been quantified in previous research.”

For the next month or more, instrument teams will gather data from the NOAA research vessel Ronald H. Brown and two NOAA, one DOE, and one NASA research aircraft with a coordinated implementation strategy when weather forecasters see atmospheric rivers developing in the Pacific Ocean off the coast of California. NASA will also provide remote sensing data for the project.

“Improving our understanding of atmospheric rivers will help us produce better forecasts of where they will hit and when, and how much rain and snow they will deliver,” said Allen White, NOAA research meteorologist and CalWater 2015 mission scientist.  “Better forecasts will give communities the environmental intelligence needed to respond to droughts and floods.”

Most research flights will originate at McClellan Airfield in Sacramento. Ground-based instruments in Bodega Bay, California, and scattered throughout the state will also collect data on natural and human contributions to the atmosphere such as dust and pollution.

This data-gathering campaign follows the 2009-11 CalWater1 field campaign, which yielded new insights into how precipitation processes in the Sierra Nevada can be influenced by different sources of aerosols that seed the clouds.

“This will be an extremely important study in advancing our overall understanding of aerosol impacts on clouds and precipitation,” said Kimberly Prather, a CalWater lead investigator and Distinguished Chair in Atmospheric Chemistry with appointments at Scripps Oceanography and the Department of Chemistry and Biochemistry at UC San Diego. “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 this data to improve computer models of rain that represent many complex processes and their interactions with the environment,” said PNNL’s Leung.

“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.”

Prather’s group showed during CalWater1 that aerosols can have competing effects, depending on their source. Intercontinental mineral dust and biological particles possibly from the ocean corresponded to events with more precipitation, while aerosols produced by local air pollution correlated with less precipitation.

The CalWater 2015 campaign comprises two interdependent efforts. Major investments in facilities include aircraft, ship time, and sensors by NOAA. Marty Ralph, Kim Prather, and Dan Cayan from Scripps, and Chris Fairall, Ryan Spackman, and Allen White of NOAA lead CalWater-2. The DOE-funded ARM Cloud Aerosol Precipitation Experiment (ACAPEX) is led by Ruby Leung from PNNL. NSF and NASA have also provided major support for aspects of CalWater, leveraging the NOAA and DOE investments.