Growth rates for liquid drops

Abstract

An expression for the growth rate, R, of a drop, radius R, is obtained in terms of the total local condensation rate in a cloud. It shows the redistributive effects arising from surface tension, dissolved species, and the interaction with radiation. In the continuum region, where growth is controlled by diffusion or conduction, an expression for R is derived which removes previous minor inaccuracies. In the free molecule regime the form obtained for R is analysed in terms of limitations to growth from heat and mass transfer. For water droplets growing in air, there is an unresolved conflict in the literature in experimental ‘sticking probabilities’ for water molecules which strongly affects the results. This also affects the simple expression proposed for R in the transition regime. The calculations performed for water droplet growth show to what supersaturations these linearised expressions for R are accurate, and the boundaries of the four regions representing droplet growth control by heat or mass transfer and continuum or kinetic control are drawn in the temperature- R plane. We show that radiation can strongly influence droplet growth and, not only in the atmosphere as was previously pointed out by Barkstrom, induce changes in size distributions in clouds by transferring mass from small to large droplets in radiatively cooling clouds which have no net growth, the magnitude of the effect being much greater for larger droplets than the Ostwald ripening process.