Dynamic forcing and mesoscale variability of heavy precipitation events over the Sierra Nevada mountains
Heather Reeves, North Carolina State University and NCAR
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 (hereafter PNF), during heavy precipitation events. A brief climatology of 56 heavy precipitation events reveals that the synoptic conditions during which a maximum at PNF forms can vary widely but can be generalized as having either predominantly zonal or predominantly meridional flows. In all of the 56 events, the low-level flow upstream of PNF was from the south, southwest, or west. Two cases from the climatology, one predominantly zonal and one predominantly meridional, were chosen for more detailed investigation. Numerical simulations of the two cases reveal that upstream gradients in moisture and wind speed led to convergence in the approximate vicinity of PNF, which likely acted to enhance precipitation at PNF relative to adjacent locations. The sensitivity of the precipitation distribution to smaller-scale terrain features was tested in additional simulations. These tests show that moisture depletion by the Coastal Range acted to decrease precipitation accumulations at PNF but the degree to which this depletion affected precipitation was case-dependent with the more zonal case having the greater sensitivity. Directional changes in the local mountain-slope orientation from westerly to southwesterly at PNF were also found to be of importance. In both the predominantly zonal and the predominantly meridional cases, the incidence angle of impinging airstreams was decreased by this change and the rainrates at PNF were accordingly decreased. Removing all terrain inhomogeneities resulted in large decreases to the rainrates at PNF and, in the more meridional case, no isolated maximum.
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