Physically Based Estimation of Maximum Precipitation over Three Watersheds in Northern California: Atmospheric Boundary Condition Shifting

Abstract

Maximum precipitation during a historical period is estimated by means of a physically based regional atmospheric model over three watersheds in Northern California: the American River watershed (ARW), the Yuba River watershed (YRW), and the Upper Feather River watershed (UFRW). In Northern California, severe storm events are mostly caused by a high-moisture atmospheric flow from the Pacific Ocean, referred to as atmospheric river (AR). Therefore, a method to maximize the contribution of an AR on precipitation over each of the targeted watersheds is proposed. The method shifts the atmospheric boundary conditions of the regional atmospheric model in space with latitude and longitude so that the AR strikes each of the targeted watersheds in an optimal direction and location to maximize the precipitation over these watersheds. For this purpose, the fifth generation Penn State/National Center for Atmospheric Research (NCAR) Mesoscale Model (MM5) is used as the regional atmospheric model, and the NCAR/National Centers for Environmental Prediction (NCEP) Reanalysis I data set is used as the input data for the MM5. The 72-h basin-average maximized precipitation depths (MPs) are 600, 632, and 448 mm, respectively, over the ARW, YRW, and UFRW watersheds. They are 42, 38, and 18% larger, respectively, than the largest values obtained from the reconstruction of the historical storm events over the target watersheds by means of the MM5 model, which used the NCAR/NCEP historical reanalysis data for its synoptic initial and boundary atmospheric conditions. This methodology can be applied not only to MP estimation during the historical period, but also to the ones in the future if the future projection data by the global circulation model (GCM) are used as input to the MM5.