The Microstructure of Cumulus Cloud: Part IV. The Effect on the Droplet Spectrum of Mixing Between Cloud and Environment

Abstract

Calculations have been made of the effect on the droplet size distribution of mixing between a cloudy parcel and its clear air environment, with attention being concentrated on the first few hundred meters above cloud base where condensation is the dominant process. If the environment is nucleus-free, we conclude that mixing broadens the spectrum only slightly, while the mean droplet size and total concentration are reduced. If the environment contains nuclei which are activated to produce droplets after mixing has occurred, the spectrum is broadened considerably but in a way which 15 not observed in natural clouds. In natural clouds we find that the dispersion of the droplet size distribution is independent of the amount of mixing that has taken place if, as a measure of the mixing, we use the ratio of the observed liquid water content to its adiabatic value at the position of the droplet sample. Thus, from both theory and observation we must conclude that simple mixing between cloud and environment is unimportant in determining the drop size distribution, at least in the early stages of cloud growth.