Quantification of Atmospheric Moisture Flux on California Precipitation and Snow Water Equivalent Under Projected Climates

Precipitation and snow water equivalent (SWE) changes due to projected climate warming are simulated for California (CA) for two mean time periods, 2020-2029 and 2062-2071, using the National Center for Atmospheric Research (NCAR), Weather Research and Forecasting model version 3 (WRF3) coupled with the NCAR Community Land Surface Model version 3.5 (CLM3.5). Initial and lateral boundary conditions to the coupled WRF3-CLM3.5 are from the NCAR Community Climate System Model version 3 (CCSM3) and the Geophysical Fluid Dynamics Laboratory Climate Model version 2.1 (GFDL CM2.1), both with the relatively medium-high range A2 scenario from the Special Report on Emissions Scenarios (IPCC SRES). Analyses of model-based climate projections indicate pronounced drying in southern CA, with 5-25% decrease in November-March (NDJFM) precipitation, as compared to the simulated 1985-1994 precipitation. Our results indicate northern CA has slight decreased precipitation during the 2020s but insignificant change in the 2060s due to inconsistences between the two models. However, the projected surface warming results show consistently significant reductions in snow water equivalent (SWE) and snow cover area (SCA). During the NDJFM snow accumulation season in Sierra Nevada Mountains, reductions in SWE are approximately 41.3% in the 2020s and range from 45.3% to a potential upper limit of 70% in the 2060s, depending on the El Nino Southern Oscillation state under future climates. To find the mechanisms of projected CA precipitation change, we quantify the change in the local water balance for the CA domain by calculating the change in the large-scale zonal moisture flux within the lower troposphere over three defined water vapor source regions d01 (120W40N x 140W50N), d02 (120W30N x 140W40N) and d03 (120W20N x 140W30N). Results indicate the winter precipitation change in northern and southern CA is associated with the seasonal anomalies in the mean zonal moisture flux in subregions d01,d02 and d03, implying that zonal water transport change plays an important role in projected CA precipitation change. Further studies on the contributions of partitioned large-scale flow change into the mean circulation and transient eddies (i.e. storminess) to the sub-domain water sources expose the drying signals in d03 (representing southern CA) for the 2020s and 2060s is largely accounted for by an increase in the mean flow moisture divergence, and the relatively wet d01 (representing nothern CA) in 2060s is mainly caused by the increased transient-eddy moisture convergence. This intensified eddy moisture divergence causes an additional 5-10% drying in d03, while the increased transient-eddy moisture convergence caused by the increased frequency of winter storms offsets 10 to 25% of drying due to the increased divergence of mean flow. Keywords: California Climate Change, Moisture Flux, Dynamical Downscaling, WRF3-CLM3.5