Evolution of Peaks in the Spectral Distribution of Raindrops from Warm Isolated Maritime Clouds

Abstract

Two and one-half years of observations and measurements of isolated maritime clouds in Guadeloupe (Lesser Antilles) are presented. Raindrop spectra are measured on the ground with a Joss device and an Epson PX8 Analyser. The greatest rainfall rate R is about 60 mm h?1. In the raindrop spectral distributions, localized drop diameter peaks are present at 0.6, 1.0, 1.8 and 3.0 mm. These diameters are compared to those measured and calculated by other authors. The high humidity in the subcloud layer and a negligible effect of the collisional breakup mechanism make rain spectra on the ground very representative of those at the cloud base level. When the rainfall rates increase, the spectra shift towards larger diameters. Thus, different spectra distributions correspond to different values of the rainfall rate. During a given shower, spectra with small drops are first observed, followed by ones with larger drops, and small drops reappear at the end of the shower. This shift cannot be attributed to the coalescence-breakup mechanism. It corresponds to a sorting of the drops. The time evolution of these spectral distributions and the negligible effect of collisional breakup during a shower allow to propose a simplified model of single maritime clouds. Cloud thickness, water contents and the updraft speeds are related to the rainfall rates. The existence of preferred drop diameters makes the relationship simple but this result can be generalized to spectra that do not show peaks. A fit to the shape of one spectral peak is suggested with a practical application. A determination of R from the maximum values of the peaks is proposed.