Acoustic Sounding of Raindrop Size Distribution

Abstract

Radars have been successfully used for remote measurements of raindrop size distribution and rain intensity for many years. Over the last two decades, sodars have been increasingly applied to measure the parameters of precipitation. In the present study, a novel method of acoustic sounding of normalized raindrop size distribution (NRSD) and rain intensity is described based on processing of raw spectra of acoustic signals measured in rainfall with a continuous-wave (CW) bistatic Doppler sodar. This method of sodar data interpretation is based on differential acoustic scattering cross sections of solid spherical particles calculated by exact formulas of the Mie scattering theory. It is used to reconstruct NRSDs and rain intensities from raw spectra of acoustic signal power measured in rainfall with two high-frequency CW bistatic sodars developed at the Khar'kov Institute of Radio Electronics (KIRE), in Ukraine, and at the University of Auckland, in New Zealand. NRSDs were reconstructed from individual power spectra recorded every 20 s and averaged over 1-, 2-, 5-, 10-, and 25-min periods. This allows the NRSD transformation in steady-state and transient rainfall to be traced. It was found that for the steady-state rain, the NRSD decreased smoothly with an increase in the raindrop size. For transient rain, a clearly pronounced maximum of large raindrop number density was observed, the position of which shifted with time from 0.3 to 1 mm. The effect of sorting large raindrops was also revealed. The estimated rain intensities varied from 0.1 to 6.1 mm h?1.