Dynamic forcing and mesoscale variability of heavy precipitation events over the Sierra Nevada mountains

The aim of this research is to investigate the causes for an isolated maximum in precipitation that is typically found along the northern half of the Sierra Nevada mountains of California, in the vicinity of Plumas National Forest (PNF), during moderate to heavy precipitation events. Particular attention was paid to the role various mesoscale (i. e. < 200 km) terrain features may have played in localizing the precipitation at PNF. Numerical simulations and sensitivity experiments for two cases of heavy precipitation at PNF reveal that the extent to which terrain acts to focus precipitation is case sensitive. In the first case, the upstream flow was characterized by a strong horizontal gradient in wind speed and moisture. This gradient led to differential deflection of airstreams incident to the range and, consequently, localized convergence and enhanced rainrates at PNF. This localized enhancement occurred regardless of whether any terrain variations were present in the simulations or not. The second case was characterized by more homogeneous upstream flow and showed a much stronger sensitivity to terrain variations, in particular, short and long wavelength undulations along the leading edge of the Sierra Nevada range. When these undulations were removed, localized maxima in precipitation did not occur and the distribution along the slope of the mountain was nearly two-dimensional.