The Evolution of Drop Spectra Due to Condensation, Coalescence and Breakup

Abstract

A numerical model of warm rain processes incorporating activation of cloud condensation nuclei, drop growth by condensation and stochastic coalescence, and drop breakup is described. A collisional breakup model is compared to a spontaneous disintegration model and found not only to dominate over the spontaneous disintegration model but to produce an exponential spectrum in fair agreement with average observed drop spectra. The steady-state spectrum was found to be quite insensitive to the number of satellite drops farmed by collision or their size distribution. The effect of the finite-difference solution to the collection and breakup equations is analysed. A properly stochastic formulation for finite time steps is presented but found to differ only slightly from the simpler, ?discrete? formulation. Time stops of 5 s and 45 size categories are found adequate to describe the essential quantitative features of more intensive treatments. The model results were found to be somewhat sensitive to values used for the collection efficiency.