Global to regional simulations of California wintertime precipitation

A global to regional modeling system, capable of making large-scale climate simulations and forecasts and then downscaling these forecasts and simulat ions to regional scales, is evaluated for the California region. Regional c limate simulations of wet and dry winters compare well with available obser vations, especially in comparison to global simulations. While the global m odel paints a broad swath of precipitation across the western United States , the regional model, forced by the large-scale background flow produced by the global model, more accurately simulates the increased precipitation ov er the Sierra Nevada and coastal regions. The regional model is especially advantageous for intense precipitation events at regional spatial scales. A lthough the regional simulations do not significantly alter the imposed lar ge-scale features, they do significantly enhance the spatial as well as tem poral variations of the hydrologic variables. Basically, downscaling by a r egional model is a dynamically consistent method, which extends the usefuln ess of global climate forecasts. Still, current regional simulations are no t perfect. Even higher resolution and further improvement in physical param eterizations are still needed.