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 simulations to regional scales, is evaluated for the California region. Regional climate simulations of wet and dry winters compare well with available observations, especially in comparison to global simulations. While the global model 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 over the Sierra Nevada and coastal regions. The regional model is especially advantageous for intense precipitation events at regional spatial scales. Although the regional simulations do not significantly alter the imposed large-scale features, they do significantly enhance the spatial as well as temporal variations of the hydrologic variables. Basically, downscaling by a regional model is a dynamically consistent method, which extends the usefulness of global climate forecasts. Still, current regional simulations are not perfect. Even higher resolution and further improvement in physical parameterizations are still needed.