COLLISIONS BETWEEN SMALL PRECIPITATION DROPS .2. FORMULAS FOR COALESCENCE, TEMPORARY COALESCENCE, AND SATELLITES

Collisions between small precipitation drops in free fall were analyze d for sizes applicable to self-collection, the process that controls t he spreading of precipitation drops to larger sizes. Results from 45 l aboratory experiments were generalized using dimensionless parameters to scale the coalescence efficiency, the temporary coalescence probabi lity, and the satellite occurrence frequency. The coalescence efficien cy for uncharged drops (epsilon(0)) was found to be highly correlated (rho = 0.99) with a simple combination of factors that scale the tende ncy for colliding drops to bounce apart as a function of the Weber num ber (We) and size ratio (p). Charge-induced coalescence was scaled by the electric field between the drops, assuming charged conducting sphe res. The coalescence efficiency was obtained as a function of the norm alized charge using a semiempirical formula (rho = 0.95) for the amoun t of charge required to eliminate bounce and temporary coalescence. Th e occurrence of temporary coalescence is predicted by p We > 4 with a lower limit of p We > 1 for charge-induced coalescence. The fraction o f collisions resulting in temporary coalescences increased with (1 - e psilon(0))p We, whereas the fraction of collisions producing satellite s increased with (1 - epsilon(0)) We(2). Both fractions were highly co rrelated with their respective scaling parameters (rho = 0.99). Satell ite drop radii were found to increase linearly with the geometric mean radius of the parent drops. Mass transfer in collisions involving tem porary coalescence and satellite generation was estimated for use in m odeling studies. Contour diagrams are provided for coalescence efficie ncy, temporary coalescence probability, and satellite occurrence frequ ency over a wide range of drop sizes for comparison with formulas base d on previous laboratory results in the accretion and breakup regimes. Recommendations are given for applying present formulas to self-colle ction, as well as extending our findings to accretion and breakup.