AN ANALYSIS OF SIERRA-NEVADA WINTER OROGRAPHIC STORMS - GROUND-BASED ICE-CRYSTAL OBSERVATIONS

Systematic observations of the sizes, shapes, and degrees of riming of ice particles falling at a downwind station of a major mountain barri er are presented. The observational station was equipped to measure ic e-particle masses from 1 mu g to a few milligrams, and to measure ice- particle dimensions, habits, degrees of riming, and degrees of aggrega tion. The results are shown to be useful in learning where ice nucleat ion and growth take place in the cloud system. The present study analy zed dissipating and developing winter orographic storm systems, which are representative of more than 60% of the storms observed over the st udy 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 rate s were correlated with a high number fraction of aggregate crystals. A ggregation was found to be important in the process of precipitation d evelopment and the aggregate mass was mostly contained in the dendriti c crystal growth region.