Analysis of dual-wavelength dual-polarization returns from frozen hydrometeors

Abstract

In most situations dual-wavelength radar returns in precipitation are functions of the cumulative path attenuation out to the observation range as well as the backscattering properties of the hydrometeors within the resolution volume. For spaceborne or airborne radars that view the storm from the top, there generally will be a region of frozen hydrometeors that is encountered before regions of partially or fully melted particles. In this region, where attenuation effects are small, the dual-frequency radar returns can be related to two parameters of the size distribution if the scattering at either wavelength is non-Rayleigh. Ambiguities in the estimation procedure remain, however, because of the unknown mass density of the scatterers as well as the effects of particle shape and orientation. Several approaches are available to resolve at least some of the ambiguities in the microphysical retrievals. One approach is the use of polarimetry. In particular, if the shape and orientation distributions of the particles are known, then the linear depolarization ratio, the dual-frequency ratio and the radar reflectivity factor can provide, in principle, estimates of the size distribution and average mass density of the particles at each range bin. The authors illustrate some of the advantages and limitations of the this approach by using data from the dual wavelength, dual-polarization airborne radar build by the Communications Research Laboratory (CRL) of Japan. For the CaPE experiment, the CRL instrument was installed in the NASA T-39 jet aircraft. In July 1991, 9 data flights were conducted in coordination with ground-based radars and multi-instrumented aircraft.