An Analysis of Sierra Nevada Winter Orographic Storms: Ground-based Ice-Crystal Observations

Abstract

Systematic observations of the sizes, shapes, and degrees of riming of ice particles falling at a downwind station of a major mountain barrier are presented. The observational station was equipped to measure ice-particle masses from 1 µg to a few milligrams, and to measure ice-particle dimensions, habits, degrees of riming, and degrees of aggregation. The results are shown to be useful in learning where ice nucleation and growth take place in the cloud system. The present study analyzed dissipating and developing winter orographic storm systems, which are representative of more than 60% of the storms observed over the study region. It suggests that most of the needles and columns observed at the ground may be formed by secondary ice production. Heavy riming was associated with light precipitation, while high precipitation rates were correlated with a high number fraction of aggregate crystals. Aggregation was found to be important in the process of precipitation development and the aggregate mass was mostly contained in the dendritic crystal growth region.