Microphysical Structure and Evolution of a Central Sierra Nevada Orographic Cloud System

Abstract

This paper describes complex microphysical interactions observed within an orographic cloud system over the central Sierra Nevada of California. Measurements of the physical structure and microphysical characteristics of this cloud system were made during the Sierra Cooperative Pilot Project (SCPP) with a 5-cm scanning radar, an instrumented aircraft, two rawinsonde stations, and a network of precipitation gauges across the mountain. The cloud system extended 130 km upwind of the Sierra Nevada crestline, was 4 km deep, had a base temperature of +5°C, and a top temperature of approximately −15°C. Cloud droplets rising from cloud base grew to sizes >50 μm and first encountered dendritic ice particles descending from cloud top over the middle elevations of the Sierra Nevada. Within this region, rapid removal of the large droplets by riming occurred, and secondary ice-crystal production due to rime splintering was observed. Needles produced by secondary ice-particle production were incorporated into dendritic aggregates falling from cloud top farther toward the mountain crest. The nature of the microphysical interactions within this cloud system and their impact on the distribution, amount, and type of precipitation across the mountain range are discussed. Changes in the distribution of precipitation from the orographic cloud system due to the approach of a short-wave trough are also presented.