The Coalescence process in raindrop growth

Abstract

Photographic and visual observations of a large number (> 104) of collisions between two water drops, one of cloud droplet size (70 μ diameter) and one of raindrop size (1.0–3.5 mm diameter) reveal the conditions under which a collision may result in coalescence, bouncing, or partial coalescence. Coalescence occurs for 85–95% of collisions at velocities equivalent to the difference in terminal velocities of the drop-droplet pair. Bouncing always results from collisions in which the droplet center trajectory does not intersect the collector drop surface. Partial coalescence occurs infrequently and is restricted to narrow ranges of velocities and incident angles. Barriers to coalescence are the existence of an air film between the approaching surfaces and the ability of the drop surface to deform on collision. Sufficient electrical attraction, long contact time, or high impact velocity can overcome these barriers and insure coalescence. Surface tension variations of ±15% from the value for distilled water did not affect the coalescence process at velocities equivalent to natural conditions. Electric charges of like or unlike sign on the drop pair are found to enhance coalescence once a threshold charge of 10−2 esu on the collector drop is attained (with a droplet charge of −10−5 esu); higher drop charges insure 100% coalescence. Values of coalescence efficiency E2 calculated from experiments, lead to an expression E2 = R2/(R + r)2, which is valid at least for drop radii 400 μ<R<2000 μ, and droplet radii 20 μ<r<100 μ. Sample calculations show that, because not every collision results in coalescence, previous growth rates are overestimates. In summary, it is no longer considered adequate to assume a coalescence efficiency of unity for all drop-droplet collisions.