On the Flat Size Distribution of Drops from Convective Rainclouds

Abstract

The flat size distributions of riandrops were observed in heavy rainfalls during the last stages of Baiu at Hitoyoshi in southern Kyushu. Most flat distributions are fairly frequently seen especially in the beginning stage of shower rain, but such flat shape remained as to appear in the medium size range of the average distribution of each individual convective rainfall.
Generation processes of those characteristic spectra were investigated by numerical experiments. Cloud models were considered with some assumptions from the results of the radar observations which were simultaneously made in the raindrop sampling. Intermittent supply of liquid water with a certain drop size spectra, which was assumed to be corresponding to the observation fact of combination of cell echoes by radar, was introduced in the well mixed cloud model by the process of drop disintegration and coalescence. The results represent well the average size distribution as observed in each rainfall at Hitoyoshi, which is characterized by the numerous space density of drops smaller than 1mm diameter, the high population of large drops, and the flat shape of the distribution curve in medium size range. Furthermore, the very flat distribution was obtained by the one dimensional sedimentation model in which the rain with the abovedescribed average distribution was released at the upper level.
The Z-R relation derived from the computational results may be well represented by Z=360R1.6 which was observed at Hitoyoshi.