Aircraft measurements of ice in Florida cumuli

Abstract

AbstractAircraft observations of cloud and precipitation particles just below the tops of Florida cumuli at temperatures near −4 and −9°C show that fresh, vigorously growing towers (updraughts of order 10 ms−1) tend to contain low concentrations (<0.11−1) of graupel, whereas older towers with lower updraught speeds contain much higher concentrations (>101−1). The ice phase, mostly as graupel, was often found to appear within a period of five minutes. Cloud drops of diameter >25μm were always found to be present in concentrations of 10 to 100cm−3 between −4 and −6°C, a condition, according to criteria established by laboratory studies, necessary for secondary ice crystal production by the riming of graupel particles. Vapour‐grown columns in concentrations as high as 501−1 were sometimes observed at the colder level, an observation consistent with the concept of a copious production zone below. Supercooled raindrops in concentrations > 11−1 occur in many regions of the clouds, often concurrently with graupel and crystalline ice. It is shown by means of a simple analytical model that aerodynamic capture of secondary ice particles by the supercooled raindrops and the subsequent transformation of the raindrops to new splinter‐producing graupel may be the main mechanism responsible for the rapid increase in observed graupel concentrations. The coalescence processes are therefore seen to be instrumental in the evolution of the ice phase. This process is favoured in lower updraught regions where the particles can be retained in the −4 to −6°C production zone long enough (≃100s) for the entire feedback process of splinter production, capture, drop freezing and renewed splinter production to take place.