Separation of Propagation and Backscattering Effects in Rain for Circular Polarization Diversity S-Band Radars

Abstract

Because precipitation particles are generally nonspherical, not only will microwave radiation be depolarized when reflected by precipitation, but also the polarization state of the transmitted wave will change as the radar beam penetrates the region of precipitation. The intrinsic scattering properties of the hydrometeors are, therefore, coupled with the properties of the propagation medium, and both effects contribute to establish the return signal. In this paper it is shown that these effects can be separated when S-band circular polarized radiation is transmitted and the copolar and cross-polar power, and the magnitude and phase of the copolar and cross-polar signal correlation, is available. The equations of the radar observables for a model medium containing nonspherical raindrops are presented. This model takes into account the distribution of canting angle but assumes the uniformity of the distribution function along the propagation path. Assuming the raindrops to have the same orientations throughout the region of precipitation and using two empirical relationships relating the mean and the differential attenuation to the differential phase shift, a set of equations for separation of propagation and backscattering effects is developed. Application of this to a model convective cell verifies that the use of the circular polarization technique at S band can also provide very good estimates of the intrinsic scattering properties of precipitation in regions of heavy rain rates. Data from a circularly polarized S-band radar system are used to confirm that this separation may be performed and to illustrate the microphysical information that can now be extracted.