Initiation of Precipitation in Vigorous Convective Clouds

Abstract

The origin of a severe local storm is traced back to a cluster of three shower cells, each of which produced a fist radar echo close to the −30C level. This level is much higher than that associated with the majority of convective clouds studied by other workers. The great height of the first echoes is attributed to the presence of strong updrafts which carry the cloud particles to high levels in the time taken for them to grow to radar detectable sizes. Because of their low temperatures, the first echoes were probably due to ice particles. Echo intensification in each cell was fairly rapid during the minute or so after first detection and corresponded to the growth of these particles by gravitational accretion in the presence of a liquid water concentration equal to about half the adiabatic value. In each cell the reflectivity core due to the growing particles was balanced by the updraft at a constant level for a 5-min period following first detection, after which it descended to the ground with very little further growth. The low reflectivity of the echo core in relation to its rate of descent is interpreted as being due mainly to a smaller-than-usual particle concentration.