Measuring raindrops using a precipitation micro-physical characteristics sensor

Abstract

Aiming at the simultaneous measurement of the size, shape, fall velocity of precipitation particles, a ground-based optical instrument, the precipitation micro-physical characteristics sensor (PMCS) was developed, it can record the double-exposure image of each particle in a single frame, by which the size, equivalent diameter, axis ratio of raindrops can be calculated, its fall velocity can be calculated according to its displacement and time interval. The PMCS is calibrated by some glass balls with certain diameters in sunny, cloudy, and night conditions, based on which a fitted relationship between the threshold and image area is obtained and used to ensure the measurement accuracy of raindrop sizes. The field measurement of PMCS has obtained plenty of raindrops images, based on which the shape, axis ratio, and fall velocity of raindrops were calculated and discussed. The typical shape of large raindrop takes on an oblate ellipsoid with a flatter base and a more raised top, the axis ratio of raindrops decrease linearly with the increasing of diameter, the fall velocity of raindrops increases exponentially with the increasing of the diameter, and above observed data are in good agreement with the empirical model. These results validate the performance of PMCS, which can be widely applied for interpreting weather radar data, investigating regional precipitation features, and studying the physical process of precipitation.