An Examination of Double-Plate Ice Crystals and the Initiation of Precipitation in Continental Cumulus Clouds

Abstract

Measurements within continental convective clouds in the Highveld region of South Africa indicate that the first appreciable nucleation of ice occurs between the ?9° to ?12°C levels as the cloud top rises through these levels. Moreover, these crystals usually take the form of double plates. Frozen drop centers were observed in 30% of the double crystals, with the diameter of the frozen drop between 10 and 20 ?m. The growth of these ice crystals is investigated using ice crystal measurements collected in situ as well as a modeling study. New information regarding axial dimensions, bulk densities, and riming characteristics of double-plate crystals is presented. Numerical simulations were performed to compare the growth characteristics of ice crystals nucleated at different temperature levels between ?6° and ?16°C in updrafts of 1 and 3 m s?1. Crystals nucleated at temperatures ≥?12°C are shown to start riming at smaller sizes than previously thought and these crystals are potentially important as graupel embryos in initiating precipitation in growing cumulus congestus clouds.