Deep Orographic Storms over the Sierra Nevada. Part II: The Precipitation Processes

Abstract

The thermodynamic and kinematic structure of two stable orographic storms were described in Part I based on instrumented aircraft data and single Doppler radar data. The precipitation processes in these storms are described in this paper. The storms were deep with cloud top temperatures of about ?25°C. Below the melting level the cloud droplet population was continental with a mean droplet diameter <10 ?m. Above the melting level the cloud droplet population was maritime with mean droplet diameters of 20 to 30 ?m. Near the ?5°C level a peak in ice crystal concentration of 30 to 200 L?1 was observed. Since most of the ice crystals were needles, are rime-splintering secondary ice crystal production processes as generally described by Hallett and Mossop was probably occurring. Calculations of the condensation supply rates were compared with the depletion rates by deposition and accretion. The depletion rates by deposition were less than half the condensation supply rates, and the liquid water contents remained low. Accretion is deduced to be the dominant process, which acts to deplete the condensate to near zero. Deep, stable orographic storms over the Sierra barrier, therefore, develop an efficient glaciation process.