When does multiple scattering due to hydrometeors become relevant to space-borne radar measurements?

Space-borne radars are invaluable tools for characterizing clouds and precipitation. This ability has been successfully demonstrated by the tropical rainfall measuring mission (TRMM) precipitation radar in retrieving rain rate profiles at 13.8 GHz. Higher frequency space-borne radars are now being envisaged for up-coming missions such as the 35.5 GHz precipitation radar on-board the global precipitation measurement (GPM) satellite and its corresponding European version (EGPM) or those already deployed in on-going missions such as the 94 GHz cloud radar on board CLOUDSAT. At higher frequencies, attenuation due to hydrometeors increasingly becomes a relevant issue. Simultaneously, multiple scattering effects as well may become significant due to the increase of the single scattering albedo and the asymmetry parameter of the hydrometeors. Since radar measurements are typically interpreted on the basis of the radar equation with the fundamental assumption of the absence of multiple scattering, it is urgent to assess the relevance of multiple scattering effects to space-borne radar measurements. Appreciable multiple scattered radiation could enhance the received radar power (apparent reflectivity), which in turn could result in overestimating the retrieved rain rate profiles. In this study, we investigate multiple scattering due to rainfall, snow/graupel and ice crystals on radar returns for nadir observations at the frequencies of 13, 35 and 94 GHz. A numerical approach, based on the forward fully polarized Monte Carlo technique, which incorporates a Gaussian antenna pattern function with varying beam-widths, is adopted in the study. The evaluation of the multiple scattering effects as a function of antenna beam-width is also presented for different scenarios at the three frequencies. Results thus obtained reveal that by condidering realistic antenna beam-widths and minimum detection thresholds, multiple scattering effects are generally negligible compared with other typical uncertainties involved in retrieval problems.