Preliminary Laboratory Results on the Coalescence of Small Precipitation-Size Drops Falling Freely in a Refrigerated Environment

Abstract

Results from laboratory observations of isolated collisions between small precipitation-size drops falling freely at terminal velocity in a refrigerated collision chamber are presented. The average radii of the size pair studied were 353 and 306 ?m. Air temperatures ranged from 20° to ?15°C. Drop temperatures ranged from 20° to approximately 2°C. Experimentation revealed that the coalescence efficiency increased from approximately 42% for mean drop temperatures between 20° and 10°C to about 81% for mean drop temperatures between 10° and 2°C. A particularly interesting finding was an abrupt, rather than gradual, increase in coalescence efficiency at a mean drop temperature of about 10°C. A reduction in drop deformation during impact due to a substantial increase in viscosity with decreasing temperature is considered as a mechanism that can act to promote coalescence. The apparent abrupt increase in coalescence efficiency requires further investigation. The extent to which these results may be extended to collection processes in clouds remains uncertain because of the effect that reduced pressure can have on deformation through drop fall speed and interaction time and because the drops were not at thermal equilibrium. The results of this experiment point to the need for further investigation in which free-fall drop collisions are produced at thermal equilibrium and at lower pressures and suggest that precipitation processes involving drizzle and raindrops may be considerably more complicated than previously suggested by experiment.