Characterization of Extreme Storm Events Using a Numerical Model–Based Precipitation Maximization Procedure in the Feather, Yuba, and American River Watersheds in California

Abstract

mprovements on nonhydrostatic atmospheric models such as MM5 in the last few decades have enhanced our understanding of the precipitation mechanism affected by topography and nonlinear dynamics of the atmosphere. This study addresses the use of such a regional atmospheric model to estimate physical maximum precipitation rates for the next generation of flood management strategies under evolving climate conditions. First, 48 significant historical storm events were selected based on the continuous reconstructed precipitation conditions on the Feather, Yuba, and American River watersheds in California. Then, the boundary conditions of the numerical atmospheric model were modified with the fully saturated atmospheric layers (100% relative humidity) to generate the atmospheric conditions that maximize the precipitation over the three watersheds. Surprisingly, maximizing the atmospheric moisture supply at the model boundary does not always increase the precipitation in the watersheds of interest. A rain-shadow effect of the topography seemed to be intensified by the abundance of the atmospheric moisture in some cases. Consequently, although the linkage between the precipitable water in the atmosphere and the ground precipitation is generally proportional, the alignment of the topography and the wind field can modulate their relationship. Finally, a methodology to maximize the steady-state precipitation rate was discussed to characterize the conceptual continuous heavy storm event in the Feather, Yuba, and American River basins.