Measuring Drop-Size Distributions in Clouds with a Clear-Air-Sensing Doppler Radar

The advent of Doppler clear-air radars for wind-height profiling opens the way for their use in a variety of other applications. The author uses knowledge of the clear-air Doppler spectrum from a zenith-pointing radar together with the measured water droplet Doppler vertical velocity spectrum to calculate spectra of drop number density through clouds of droplets having substantial fall velocity. The method has been applied by Japanese scientists to measure drop size distributions to precipitation particles from data acquired at the VHF MU radar facility. Here, the method is applied to records obtained with a 915-MHz wind profiler located at Denver, CO., and the resulting spectra are presented and compared with the spectra that would have been obtained if the clear-air information were ignored. From the number density drop size distribution, the corresponding liquid water distribution can be calculated. It is concluded that failure to consider turbulence in the medium can result in order-of-magnitude errors in number density and liquid water. The requirements and limitations of a radar remote sensing drop spectrometer are discussed.