Small-Scale Precipitation Elements in Midlatitude Cyclones Crossing the California Sierra Nevada

Abstract

adar data in some frontal systems passing over the Sierra Nevada of California show large variance on scales of ~10 km. The most prominent features are a few kilometers in scale and are similar to small-scale precipitation cells embedded in fronts seen over other mountain ranges. Other frontal systems crossing the Sierras are characterized by more uniform air motions. Updrafts in large-variance storms have characteristics of shear-induced turbulence, although buoyant instability may also contribute. Large-variance storms occur under stronger upstream winds and vertically integrated cross- and along-barrier moisture fluxes. Rain gauges indicate that large-variance storms have precipitation greater than smaller-variance storms. Stronger horizontal moisture fluxes may provide greater mean upslope condensation rates; however, it is hypothesized that accelerated microphysical processes are needed to most efficiently convert the condensate into precipitation that falls out on the lower slopes before being carried downstream. Radar data indicate that the turbulence embodied in the cellular motions of the large-variance cases is consistent with microphysical enhancement resulting from updraft elements producing pockets of liquid water conducive to riming and coalescence. In addition, radar spectrum-width data show that the cells contain strong subcell-scale turbulence conducive to particle collisions and aggregation. Polarimetric radar data just below the 0°C level show large raindrops in the cells, consistent with aggregation occurring in cells just above the melting layer. It is hypothesized that such enhanced microphysical processes in large-variance cases hasten the growth and fallout in the regions of maximum condensation over the windward slopes.