The role of breakup and coalescence in the three-peak equilibrium distribution of raindrops

Filament breakup produces drops in the small-drop peak of the 3PED and sheet breakup in the medium-drop peak, while disk breakup does not significantly affect the 3PED. Drops in the large-drop peak are produced by all types of interaction. A forty-dimensional phase space for number concentration, each dimension representing a different drop size, shows how equilibrium is approached and maintained. When reduced to seven dimensions, the complex processes of moving drops in and out of the peaks at equilibrium are clearly identified. The introduction of a measure for the difference between two spectra, the metric, shows that, starting from a Marshall-Palmer distribution with a rainrate of 54 mm h^-1, the distribution moved farther away from the 3PED between 120 s and 350 s before approaching the 3PED continuously. There is no evidence for any oscillation of the raindrop spectra about its equilibrium value. It appears that a significant population of both 1 mm and 2 mm drops are needed before a 3PED can occur. -from Authors