The CalWater 2 - ARM Cloud Aerosol Precipitation Experiment (ACAPEX)
Authors
Chris W. Fairall — NOAA- Earth System Research Laboratory
L. Ruby Leung — Pacific Northwest National Laboratory
Beat Schmid — Pacific Northwest National Laboratory
Paul J. DeMott — Colorado State University
Daniel Rosenfeld — The Hebrew University of Jerusalem
Jiwen Fan — Pacific Northwest National Laboratory
Kimberly Prather — Scripps Institution of Oceanography
Samson M Hagos — Pacific Northwest National Laboratory
Marty Ralph — National Oceanic and Atmospheric Administration
Chun Zhao — Pacific Northwest National Laboratory
Ryan Spackman — NASA - Ames Research Center
Description
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.
The ARM Cloud Aerosol Precipitation Experiment (ACAPEX) field campaign took place in northern California between 15 January – 10 March, 2015. As part of CalWater 2015, 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 implemented an observational strategy consisting of the use of land and offshore assets from DOE, NOAA, NASA, and NSF 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, present recent findings from modeling experiments to investigate long range transport of aerosols to the western U.S. and simulations of AR and heavy precipitation from global climate models, and highlight some preliminary analyses of data obtained from the field campaign.
