A rain retrieval method examined using airborne radar measurements of tropical storms

A method to estimate rainrate profiles from a nadir-looking rain radar operating at a rain attenuating frequency is described. The method takes the various phases of the hydrometeors into account such as ice, melting layer and temperature variation in the rain region. It uses a numerical method to retrieve attenuation-corrected radar reflectivity and estimates the drop size distribution in rain by using a constraint for the total path attenuation derived from sea-surface echoes. The method is tested using airbome radar measurements made at 10 GHz and 35 GHz over a typhoon event. Retrieved profiles of rain intensity are derived at each of the two frequencies separately and compared with each other. Results show very good agreement for stratiform rain with moderate rainrates. For a convective cell, however, the Ka-band signal loses its surface echo signal due to high rain attenuation, thus emphasising that 35 GHz is too high a frequency for measuring intense tropical storms.