On the coalescence of water droplets in turbulent clouds.

Abstract

It is shown that, of the nine components of the flow velocity gradient tensor, only the two which represent the vertical gradient of the horizontal velocity components contribute significantly to increase the collision rate of small droplets above the still-air value. Hence, to predict the rate at which droplets collide, the turbulent motion within a developing cloud is modelled by a uniform vertical shear flow which varies randomly in time. (A)