On the Role of Coalescence between Raindrops in Shaping Their Size Distribution1

Abstract

Numerical computations of the modification of raindrop size distribution by coalescence are presented. The initial distribution is defined by giving the drop concentrations at a discrete set of drop sizes of which the volumes are integral multiples of an elementary volume; in this way, the drop sizes produced by coalescence remain confined to the discrete set. It is found that an initial exponential distribution of the Marshall-Palmer type undergoes comparatively little change. On the other hand, much narrower initial distributions, consisting of drops of two sizes only, are found to undergo considerable change and tend toward an exponential distribution. The possible role of coalescence in generating observed raindrop size distributions from narrow initial distributions is discussed.