The Effect of Raindrop Interactions on Observed Drop Size Distributions

Abstract

A model for raindrop interactions incorporating satellite droplet production is used to determine drop size distributions at the ground. Assuming an exponential distribution at cloud base and using known meteorological conditions, the theoretical drop size distribution agrees well with distributions observed in the field; both predicted and observed distributions have reduced numbers of drops of radius near 0.35 mm. A time series analysis is facilitated by using a running-mean technique on each part of observed drop size distributions. A cross correlation is then performed between the time-series for large, medium and small drop concentrations. It is found that the smallest drops are associated in time with drops having a high satellite droplet production rate. On the other hand, the time delay at ground level between changes in large-drop concentration and small-drop concentration can be interpreted in terms of fallspeed differences. This allows the effective altitude of the active rain-producing region to be determined.