Measured Collection Efficiencies for Cloud Drops

Abstract

The collection efficiency has been measured for 15 size pairs of relatively uncharged drops in over 400 experimental runs. The results indicate that collection efficiencies fall in a narrow range of 0.60 to 0.70 even though the collector drop was varied between 63 and 100 ?m radius and the collected drop from 11 to 26 ?m radius. The measured values of collection efficiencies were consistently below collision efficiencies based on calculations using rigid sphere hydrodynamics. The coalescence efficiencies computed from the ratio of the theoretical collision efficiencies to the measured collection efficiencies were between 0.6 and 0.8. Our data show fair agreement with one previous coalescence model result, with an existing semi-empirical formula for the coalescence efficiency, and with the predicted trend of the coalescence efficiency to decrease with the size of the collected drop. A plausible interpretation of our experimental findings, consistent with previous coalescence studies, is that contact is prevented during a grazing trajectory by a hydrodynamic deflection which is enhanced by a slight deformation of the collector drop.