Determination of Rain Intensity from Doppler Spectra of Vertically Scanning Radar

A new method is given to determine the rain intensity from data collected by a vertically scanning Doppler radar. The method is based on relating a theoretical velocity spectrum derived from a gamma drop size distribution to the measured Doppler spectra. A considerable reduction in calculation time is obtained by characterizing a Doppler spectrum by its total reflectivity and three velocities, and relating these parameters directly to the rain intensity, vertical air velocity and drop size distribution parameters. The method is verified with observations of a high-resolution radar. A good agreement with the resulting rain intensity is found when large raindrops are present, but the accuracy of the method appears to reduce sharply in the absence of large raindrops. The Doppler spectrum shows a sudden decrease in reflectivity near the maximum fall velocity of raindrops when enough large drops are present. Without large drops, the Doppler spectrum appears hardly sensitive to the mean drop size, and any method that is based on an interpretation of the shape of the Doppler spectrum should be restricted to situations with large raindrops to produce accurate results