Hydrometeor Evolution in Rainbands over the California Valley

Abstract

Hydrometeor distributions were measured in two rainbands that passed over the California Valley. The ground radar was used to vector the University of Wyoming's instrumented King Air aircraft to the top of the rainband at which time an onboard computer algorithm was used to make multiple penetrations of an ensemble of ice particles that were assumed to descend at 1 or 2 m s?1 while drifting downwind. Distribution parameters were calculated for each penetration and the changes in these parameters were used to infer the different modes of growth that the particles undergo. It was found that there were five distinct regions of particle growth. Nucleation and depositional growth were the dominant processes near the top of the rainband. The hydrometeors then fell through a region of aggregation from the ?20°C level to the ? 10°C level. Ice crystal multiplication was the dominant process from the ?10°C level to the ?4° level. From ?4 to O°C, aggregation was again the dominant processes. Below the 0°C level, melting and collisional coalescence were the main processes. Bulk ire densities were calculated by comparing radar reflectivities and reflectivites calculated store the hydrometeor distributions. Bulk ice densities were very low, reaching only 0.1 g cm?3 just above the melting layer.