The growth of cloud droplets by coalescence

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Abstract
A simple model of a cloud in which drops of different sizes are assumed to be falling at their terminal velocities is used to study the development of drops with radii up to 40 μ by the coalescence mechanism.It is not clear what initial distributions of drop sizes should be used in this study and therefore the development of larger drops from several different distributions which are thought to be characteristic of the early stages of various types of cumuli are compared. In this part of the paper Hocking's (1959) values of the collection efficiencies are assumed to apply. It is found that large drops develop rapidly in maritime clouds, and for clouds characteristic of continental climates it is shown that the maximum drop radius needs to be greater than about 22μ for the coalescence mechanism to be effective. There must also be an adequate supply of drops with radii between 14 and 20 μ.The effects of small increases in the collection efficiencies which may be caused by various factors, such as turbulence or electric fields, are studied with the model using two different initial drop size distributions. It is found that the most important single parameter in determining the onset of the coalescence mechanism is the minimum drop size capable of capturing still smaller drops. Generally, a significant effect is found if the change in the collection efficiency is equivalent to increasing the radii of the interacting drops by more than about 2μ. It is suggested that any future computations or experiments to determine changes in the collection efficiencies should look for effects of this magnitude.

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