PREFERENTIAL CONCENTRATION OF CLOUD DROPLETS BY TURBULENCE - EFFECTS ON THE EARLY EVOLUTION OF CUMULUS CLOUD DROPLET SPECTRA

A mechanism is presented, based on the inherent turbulent nature of cu mulus clouds, for the broadening of cloud droplet spectra during conde nsational growth. This mechanism operates independent of entrainment a nd, therefore, can operate in adiabatic cloud cores. Cloud droplets of sufficient size are not randomly dispersed in a cloud but are prefere ntially concentrated in regions of low vorticity in the turbulent flow field. Regions of high vorticity (low droplet concentration) develop higher supersaturation than regions of low vorticity (high droplet con centration). Therefore, on small spatial scales cloud droplets are gro wing in a strongly fluctuating supersaturation field. These fluctuatio ns in supersaturation exist independent of large-scale vertical veloci ty fluctuations. Droplets growing in regions of high vorticity will ex perience enhanced growth rates, allowing some droplets to grow larger than predicted by the classic theory of condensational growth. This me chanism helps to account for two common observations in clouds: the pr esence of a large droplet tail in the droplet spectrum, important for the onset of collision-coalescence, and the possibility of new nucleat ion above cloud base, allowing for the formation of a bimodal droplet spectrum.