The CalWater 2 - ARM Cloud Aerosol Precipitation Experiment (ACAPEX)
L Ruby Leung, Pacific Northwest National Laboratory, Richland, WA, United States, Kimberly A Prather, University of California San Diego, Scripps Institution of Oceanography, La Jolla, United States, F Martin Ralph, University of California San Diego, La Jolla, United States, Daniel Rosenfeld, The Hebrew University of Jerusalem, Jerusalem, Israel, J. Ryan Spackman, Science and Technology Corporation, Boulder, CO, United States, Chris W Fairall, NOAA Physical Sciences Laboratory, Boulder, United States, Paul J DeMott, Colorado State University, Department of Atmospheric Science, Fort Collins, United States, Jiwen Fan, Argonne National Laboratory, Lemont, United States and Chun Zhao, University of Science and Technology of China, School of Earth and Space Sciences, Hefei, China
Abstract:
The western U.S. receives precipitation predominantly during the cold season when storms approach from the Pacific Ocean. The snowpack that accumulates during winter storms provides about 70-90% of water supply for the region. Two elements of significant importance in predicting precipitation variability in the western U.S. are atmospheric rivers and aerosols. Atmospheric rivers (ARs) are narrow bands of enhanced water vapor associated with the warm sector of extratropical cyclones over the Pacific and Atlantic oceans. While ARs are responsible for a large fraction of heavy precipitation in the western U.S. during winter, much of the rest of the orographic precipitation occurs in post-frontal clouds, which are typically quite shallow, with tops just high enough to pass the mountain barrier. Such clouds are inherently quite susceptible to aerosol effects on both warm rain and ice precipitation-forming processes.
In January – March 2015, the ARM Cloud Aerosol Precipitation Experiment (ACAPEX) field campaign will take place in northern California. Joined with CalWater 2, the field campaign aims to improve understanding and modeling of large-scale dynamics and cloud and precipitation processes associated with ARs and aerosol-cloud interactions that influence precipitation variability and extremes in the western U.S. We will implement an observational strategy consisting of the use of land and offshore assets to monitor (1) the evolution and structure of ARs from near their regions of development, (2) long range transport of aerosols in eastern North Pacific and potential interactions with ARs, and (3) how aerosols from long-range transport and local sources influence cloud and precipitation in the U.S. West Coast where ARs make landfall and post-frontal clouds are frequent. This presentation will provide an overview of the science questions and hypotheses to be addressed by CalWater 2/ACAPEX, review key results from prior studies, and discuss recent findings from modeling experiments to investigate long range transport of aerosols to the western U.S. and the influence of cloud condensation nuclei and ice nuclei on precipitation.
