The Effects of van der Waals Attractions on Cloud Droplet Growth by Coalescence

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Abstract
The inclusion of van der Waals attractions in the interaction between cloud droplets has been recently shown to significantly increase the collision efficiencies of the smaller droplets. In the current work, these larger values for the collision efficiencies are used in a population dynamics model of the droplet size distribution evolution with time, in hopes of at least partially resolving the long-standing paradox in cloud microphysics that predicted rates of the onset of precipitation are generally much lower than those which are observed. Evolutions of several initial cloud droplet spectra have been tracked in time. Size evolutions are compared as predicted from the use of collision efficiencies computed using two different models to allow for droplet–droplet contact: One which considers slip flow effects only, and one which considers the combined effects of van der Waals forces and slip flow. The rate at which the droplet mass density function shifts to larger droplet sizes is increased by typically 20–25% when collision efficiencies which include van der Waals forces are used. The overall result is the more rapid formation of larger, rain-sized droplets, particularly from initial distributions with small values of the average radii and narrow initial distributions.

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