Precipitation Initiation Mechanisms and Droplet Characteristics of Some Convective Cloud Cores

Abstract

Samplings of the unmixed cores and the mixed edges of convective clouds at various heights were made in an aircraft using standard instrumentation and a Continuous Particle Collector (CPC). The unmixed cores are defined as regions where constant values are indicated on the hot wire type, liquid water content meter. The various correction factors relating these CPC replicas to the true droplet and drop situation in the cloud are not well established, but the observations are nevertheless sufficiently accurate 1) to give some useful data on the relative size distributions of small droplets, and 2) to show that drizzle-size drops can develop quickly within the cloud core. The evolution of the distributions of small droplets (<40? diameter) with height is shown. Droplet concentrations are of the order of 1000 cm?3 near cloud base, and some correlation between concentration and upcurrent strength at cloud base is noted. The large droplets from which drizzle-size drops evolve can be present, even at cloud base, in concentrations of about 20 liter?1. Possible sources of these large droplets are discussed. Graupel was encountered at ?5 and ?6 C, mostly outside the convective cores, presumably resulting from the freezing and riming of the larger liquid drops at relatively warm temperatures.