Measurement of Raindrop Size Distributions Using a Small Doppler Radar

Abstract

A small X-band bistatic Doppler radar originally developed for use in automated weather stations as a Precipitation Occurrence Sensor System (POSS) can also measure real time raindrop size distributions. In contrast to large-scale pulsed Doppler radars this system is continuous wave and the measurement volume is relatively small. The drop size distribution is retrieved from the Doppler power spectrum by using an iterative inversion method. The sampling requirements for a representative average power spectrum are estimated. The effect of winds on the measurement is discussed. The POSS measures rates that are consistent with conventional gauges assuming a 6 dB transmission loss due to water on the radomes. Average drop size distributions measured in stratiform rain are typically negative exponential for diameters greater than 0.7 mm. One-minute averages in rain showers show the multimodal character of the distribution.