Cloud‐droplet growth by condensation in cumulus

Abstract

AbstractThe growth of droplets by condensation upon a spectrum of salt nuclei is followed as they are carried up in a small cumulus which mixes and exchanges heat, momentum and water vapour with its surroundings. The vertical velocity, water content, supersaturation and cumulative drop‐size distributions are plotted as functions of time and height above cloud base. Growing cloud droplets are continually being transferred from the cloud to the drier environment and replaced by new condensation nuclei. The fact that some droplets remain in the cloud longer than others is largely responsible for the evolution of a broad drop‐size distribution which much more closely resembles observed spectra than those obtained from previous calculations for a closed parcel. Regarding the formation of large droplets, the calculations show that droplets exceeding 20 μ in radius arise on salt nuclei of mass 10−10 g and droplets larger than 30 μ radius arise on nuclei of 10−9 g within ten minutes of the air passing through the condensation level. Continued growth in a succeeding thermal and further deformation of the droplet spectrum by coalescence will produce even larger drops, but nevertheless it is difficult to explain the appearance of droplets larger than 30 μ in concentrations of order 1 cm−3 in terms of their growth on reported concentrations of giant hygroscopic nuclei.