Atmospheric Rivers and Heavy Precipitation From a Hierarchy of Climate Simulations

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. Associated with the warm sector of extratropical cyclones over the Pacific Ocean, atmospheric rivers (ARs) provide enhanced water vapor transport from the tropics. Upon landfall in the mountainous terrain along the U.S. west coast, ARs produce heavy precipitation that accounts for 25 – 50% of annual precipitation in the western U.S. Due to the narrow structure of the ARs and the complex terrain along the west coast, simulations of ARs and the associated precipitation are sensitive to model resolution. Using a suite of idealized aqua-planet simulations and AMIP simulations at multiple resolutions ranging from 30km to 240km, we investigate the sensitivity of simulated AR frequency and heavy precipitation to model resolution and dynamical core. Additionally, the impacts of global warming on ARs and heavy precipitation are investigated using model outputs from the Community Earth System Model Large Ensemble Project (CESM-LE) and the multi-model ensemble of the Coupled Model Intercomparison Project Phase 5 (CMIP5) to evaluate the thermodynamical and dynamical contributions to changes in extreme precipitation. Lastly, simulations of ARs observed during the CalWater – 2015 field campaign in the North Pacific and California are being evaluated with the campaign data and will be presented.