Raindrop interactions and rainfall rates within clouds

Experiments showed that when a pair of water drops of raindrop dimensions collide and separate in air at their relative terminal velocities satellite drops are generally produced. Although the numbers and sizes of satellites resulting from individual interactions are highly variable a typical event produces about 3 satellites each having a volume of 0.04 V_iV_j/(V_i + V_j), where V_i and V_j are the volumes of the parent drops. A full stochastic treatment is presented of the development of rainfall in a volume within a cloud where water is being released by condensation at a constant rate J. The production of satellites and an experimentally verified expression for the coalescence efficiency ε are incorporated into these calculations. Computations are made of the variation with time of the cloud water content, C, the rainwater content, L, the radar reflectivity, Z, the rainfall rate, P, and the drop‐size distribution. The calculations show that the crucial parameter in governing the intensity and duration of rainfall is J, and that the microphysical processes are of secondary importance; C, L and P are all insensitive to ε and the production of satellite drops. The influence upon rainfall development of the break‐up of large raindrops on attaining their maximum size within a cloud is found to be even less than that of the satellites. The drop size distribution after several minutes of growth is insensitive to the initial spectrum but is markedly dependent upon ε and the production of satellites, which tend eventually to produce a bimodal spectrum. The radar reflectivity increases very rapidly with time (typically by about an order of magnitude every 2 1/2 minutes) and depends quite strongly on ε, but is unaffected by the production of satellites.