Collisions between Small Precipitation Drops. Part I: Laboratory Measurements of Bounce, Coalescence, and Temporary Coalescence

Abstract

Self-collection efficiencies were measured for isolated drop pairs failing at terminal velocity using orthogonal cameras to obtain the horizontal offset of the drops before collision and the collision outcome. Data were obtained on four different drop-size pairs over a range of impact Weber number (1?10) and size ratio (0.45?0.73). Collision offsets and outcomes were recorded during 45 experiment runs as a function of drop charge. The collision results from all 4200 events were tabulated by offset and charge, and the coalescence efficiency was determined for each run as a function of charge. Collision results revealed a coalescence region for small offset and a bounce region at intermediate-to-large offset and low-to-intermediate charge. The critical offset that separated the regions of coalescence and bounce was independent of charge. At higher values of charge, increasing charge was found to induce permanent and/or temporary coalescence from smaller and larger offsets until bounce was completely eliminated. In the offset range for temporary coalescence, the filament connecting the separating drops often collapsed into one and, occasionally, two satellite drops. Mean satellite sizes of 58?81-µm radius were generally consistent with previous measurements using colliding drop streams. The production of satellite drops by colliding precipitation drops should provide precipitation embryos that would accelerate the accretion of cloud water in warm-base convective clouds. Coalescence efficiencies of 15%?55% at minimal charge were significantly lower than previously reported for smaller drops; therefore, the results indicate a further reduction in the growth rate of precipitation drops. The efficiencies did not vary in a simple way with either Weber number or size ratio. For a constant size ratio (p ≈ 0.7) the coalescence efficiency decreased with increasing Weber number, whereas for a constant Weber number (We ≈ 4.2) the coalescence efficiency decreased with increasing size ratio. An excellent fit 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 growth of radar reflectivity.