A New Optical Instrument for Simultaneous Measurement of Raindrop Diameter and Fall Speed Distributions

Abstract

This paper presents a recently developed optical spectro-pluviometer. The principle of the device is based upon the optical occultation of an infrared light beam by falling drops. This allows the retrieval of raindrop-size and velocity distributions. The characteristics of the instrument are described in detail. Possible errors resulting either from instrumental limitations or from environment conditions of measurement are reviewed. Preliminary results regarding a 20 min rainshower observed near Paris in September 1980 are presented and discussed. The deduced total precipitation amount is in good agreement with other independent measurements. The results reveal small but significant differences between the deduced fall velocities and Gunn and Kinzer's observations. The relationship between rainfall bulk parameters (rainfall rate, water content, reflectivity factor and mean fall velocity) are obtained with small relative dispersion.