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 cumulus clouds, for the broadening of cloud droplet spectra during condensational growth. This mechanism operates independent of entrainment and, therefore, can operate in adiabatic cloud cores. Cloud droplets of sufficient size are not randomly dispersed in a cloud but are preferentially 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 concentration). Therefore, on small spatial scales cloud droplets are growing in a strongly fluctuating supersaturation field. These fluctuations in supersaturation exist independent of large-scale vertical velocity fluctuations. Droplets growing in regions of high vorticity will experience enhanced growth rates, allowing some droplets to grow larger than predicted by the classic theory of condensational growth. This mechanism helps to account for two common observations in clouds: the presence of a large droplet tail in the droplet spectrum, important for the onset of collision-coalescence, and the possibility of new nucleation above cloud base, allowing for the formation of a bimodal droplet spectrum.