Tentative Utilization of a Raindrop Size Distribution Meter Specially Designed for the Observation of Tropical Precipitation in the TOGA/COARE Project

Abstract

To offer precise data for a more quantitative analysis of the data obtained by the dual Doppler radar system, a specially designed raindrop size distribution meter was utilized at Manus Island, Papua New Guinea, in the TOGA/COARE project. The measuring device consists of an optical system with a light source and two video cameras. The raindrops that fall through a slit are illuminated against a uniform surface light source by means of a frosted glass plate immediately next to them and their shadow images are photographed by two video cameras with different shutter speeds for simultaneously measuring size and velocity of fall of the same raindrop. Fall velocity is used to examine the size measured and provides a highly reliable measurement of size for even the smaller raindrops.In this paper, two different types of size distribution of raindrops are shown by a single modal Gamma function and a bimodal distribution corresponding to relatively weak and relatively strong convective clouds, respectively. The bimodal case is discussed in comparison with the radar data analyzed in a few instances of a series of rainfall periods and some of the results provide possible evidence for the warm-rain mechanism and evaporation processes.