COLLISIONS BETWEEN SMALL PRECIPITATION DROPS .1. LABORATORY MEASUREMENTS OF BOUNCE, COALESCENCE, AND TEMPORARY COALESCENCE

Self-collection efficiencies were measured for isolated drop pairs fal ling at terminal velocity using orthogonal cameras to obtain the horiz ontal offset of the drops before collision and the collision outcome. Data wc rt obtained on four different drop-size pairs over a range of impact Weber number (1-10) and size ratio (0.45-0.73). Collision offse ts and outcomes were recorded during 45 experiment runs cls a function of drop charge. The collision results from all 4200 events were tabul ated by offset and charge, and the coalescence efficiency was determin ed for each run as a function of charge. Collision results revealed a coalescence region for small offset and a bounce region at intermediat e-to-large offset and low-to-intermediate charge. The critical offset that separated the regions of coalescence and bounce was independent o f charge. At higher values of charge. increasing charge was found to i nduce permanent and/or temporary coalescence from smaller and larger o ffsets until bounce was completely eliminated. In the offset range for temporary coalescence, the filament connecting the separating drops o ften collapsed into one and, occasionally, two satellite drops. Mean s atellite sizes of 58-81-mu m radius were generally consistent with pre vious measurements using colliding drop streams. The production of sat ellite drops by colliding precipitation drops should provide precipita tion embryos that would accelerate the accretion of cloud water in war m-base convective clouds. Coalescence efficiencies of 15%-55% at minim al charge were significantly lower than previously reported for smalle r drops; therefore, the results indicate a further reduction in the gr owth rate of precipitation drops, The efficiencies did not vary in a s imple way with either Weber number or size ratio. For a constant size ratio (p approximate to 0.7) the coalescence efficiency decreased with increasing Weber number, whereas for a constant Weber number (We appr oximate to 4.2) the coalescence efficiency decreased with increasing s ize ratio, An excellent tit to the laboratory coalescence efficiencies , using the theoretical scaling for inelastic collisions, is presented in a companion paper. The resulting formulas for precipitation drops will allow application of these findings to self collection, a process that controls the spreading of raindrops to larger sizes and the grow th of radar reflectivity.