Collisions between Small Precipitation Drops. Part III: Laboratory Measurements at Reduced Pressure

Abstract

Collisions between drops in free fall were measured at atmospheric pressures of 745 and 545 mb for sizes applicable to self-collection, the process that controls the spreading of precipitation drop distributions in the warm rain process. Orthogonal cameras were used to obtain the horizontal offset of drops of 200?425-?m radii before collision and the outcome after collision. The effect of air pressure on collision outcomes for negligibly charged drops at high relative humidity was evaluated using three different drop size pairs at reduced pressure over a range of impact Weber numbers (We = 5.7?16.6) as well as four drop size pairs from previous experiments at 1000 mb for We ≤ 9.6. The collision outcomes in the collision cross section of the seven experiments consisted of a central region of coalescence and an outer region of bounce. For the three experiments with We > 9, the outcome pattern included a region of temporary coalescence at larger offsets that spread into the central region of permanent coalescence for We > 12. The percentages of collision outcomes for the seven experiments were 14%?55% coalescences, 11%?77% bounces, 0%?29% temporary coalescences, and 0%?13% of temporary coalescences producing satellites. A reduction in air pressure altered collision outcomes by promoting contact, thereby reducing bounce and increasing permanent and/or temporary coalescence. The central cross section of coalescence, based on the observed minimum offset for bounce (εB), was governed by X?, a parameter composed of Weber number, size ratio, pressure, and temperature, originating from nondimensional factors for excess kinetic energy, bounce time, and film drainage time of the air between the drops. A strong linear correlation (? = 0.99) was obtained between X? and εB, providing a reliable estimate of the coalescence efficiency for We < 10. At higher Weber number in the 545-mb experiments, the coalescence efficiency was reduced by temporary coalescences because of higher relative rotational energy. An empirical equation for this reduction was combined with εB (X?) so that the coalescence efficiency of ε = 10%?70% could be calculated for small precipitation drops over the range of Weber number and size ratio in the experiments. The general trends in temporary coalescences and satellites for negligibly charged drops were consistent with simple scaling parameters, but reliable formulas require additional experiments at higher Weber number with more significant fractions of temporary coalescences and satellites.