Collisions between small precipitation drops. Part III: Laboratory measurements at reduced pressure

Collisions between drops in free fall were measured at atmospheric pressure s of 745 and 545 mb for sizes applicable to self-collection, the process th at controls the spreading of precipitation drop distributions in the warm r ain process. Orthogonal cameras were used to obtain the horizontal offset o f drops of 200-425-mum radii before collision and the outcome after collisi on. The effect of air pressure on collision outcomes for negligibly charged drops at high relative humidity was evaluated using three different drop s ize 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 m b for We less than or equal to 9.6. The collision outcomes in the collision cross section of the seven experime nts consisted of a central region of coalescence and an outer region of bou nce. For the three experiments with We > 9, the outcome pattern included a region of temporary coalescence at larger offsets that spread into the cent ral region of permanent coalescence for We > 12. The percentages of collisi on outcomes for the seven experiments were 14%-55% coalescences, 11%-77% bo unces, 0%-29% temporary coalescences, and 0%-13% of temporary coalescences producing satellites. A reduction in air pressure altered collision outcome s by promoting contact, thereby reducing bounce and increasing permanent an d/or temporary coalescence. The central cross section of coalescence, based on the observed minimum off set for bounce (epsilon (B)), was governed by X', a parameter composed of W eber number, size ratio, pressure, and temperature, originating from nondim ensional factors for excess kinetic energy, bounce time, and film drainage time of the air between the drops. A strong linear correlation (rho = 0.99) was obtained between X' and epsilo n (B), providing a reliable estimate of the coalescence efficiency for We < 10. At higher Weber number in the 545-mb experiments, the coalescence effi ciency was reduced by temporary coalescences because of higher relative rot ational energy. An empirical equation for this reduction was combined with <epsilon>(B) (X') so that the coalescence efficiency of epsilon = 10%-70% c ould be calculated for small precipitation drops over the range of Weber nu mber and size ratio in the experiments. The general trends in temporary coa lescences and satellites for negligibly charged drops were consistent with simple scaling parameters, but reliable formulas require additional experim ents at higher Weber number with more significant fractions of temporary co alescences and satellites.