A wind tunnel investigation of collection kernels for small water drops in air

The rate at which small water drops grow by collision and coalescence has been investigated experimentally by means of a wind tunnel constructed for cloud physics research. Individual water drops of radii 70 < A <300μ were freely suspended in the tunnel airstream carrying a continuous water cloud with droplets of radii 2 < a < 8μ and with a liquid water content between 0.8 and 1.2 g m−3. The rate at which the suspended water drops were growing by collision and coalescence was determined as a function of the drop size of the suspended drop, the liquid water content of the cloud and the size distribution of the drops in the cloud. From the observed parameters, specific collision kernels were computed and compared with those derived from theoretical and experimental collision efficiencies reported in literature. The results show that for p = a/A <0.1, i.e. for large cloud drops growing by collision with small cloud drops, it is justified to compute the growth rate of cloud drops of A < 136μ on the basis of the theoretical collision efficiencies of Shafrir and Neiburger as given analytically by Berry, and the growth rate of cloud drops of A > 136μ on the basis of the theoretical collision efficiencies given by Mason.