LABORATORY MEASUREMENTS AND ANALYSIS OF THE GROWTH AND COLLECTION EFFICIENCY OF CLOUD DROPLETS

Abstract

Water droplets initially 2 µ in radius were held stationary for observation in an upward streaming cloud and were found to grow by accretion. The rate of increase in the radius of individual droplets was measured from 4 to 65 µ. Experimental collection efficiencies obtained from the data agree within limits of experimental error with those calculated from a theory that is developed from intuitive physical arguments which are based on a superimposed random motion observed in the fall of droplets in the supporting cloud. The theory implies that a small-scale eddy diffusion transfers smaller droplets from the parent cloud to the growing droplet. The coefficient of droplet transport calculated from the theory agrees well with one evaluated experimentally. Past measurements of drops growing by accretion from 100 µ to 1.5 mm in radius have been combined with the results of the present investigation in order to compare the efficiencies calculated by the new theory with measured values over a more extensive range. The free water content and the temperature of the parent cloud were approximately 3.5 gm m⁻³ and 22C, respectively. Two types of distributions were employed: the first with 80 per cent of the free water contained in droplets 5.5 to 8.0 µ in radius; and the second with the free water distributed more widely among droplets 3.9 to 10.4 µ in radius. The droplets of the parent cloud were produced by atomization of water and were randomly electrified. The electrification was found to be of a magnitude similar to that measured recently on droplets in some types of natural cumuliform clouds. It is shown that the electrification was too small to produce dynamic effects of more than one-tenth of the order of magnitude of those associated with the eddy transfer, although coalescence may be promoted by electrical forces whenever eddy diffusion transports droplets to within 2 or 3 µ of the growing droplet.