Multiple scattering effects due to hydrometeors on precipitation radar systems

Space- borne radars are invaluable tools for characterizing clouds and precipitation. At higher frequencies ( like those used for the TRMM PR or envisaged for GPM radars) attenuation due to hydrometeors increasingly becomes a relevant issue. Simultaneously when dealing with active sensors, multiple scattering effects could be significant due to the simultaneous increase of the optical thickness and the single scattering albedo of the hydrometeors with frequency. In this study, we investigate multiple scattering due to rainfall and graupel on radar returns for nadir observations at 13 and 35 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. Results reveal that multiple scattering effects are driven by the interplay between the antenna footprint, the medium scattering coefficient and the depth traveled inside the medium. The multiple scattering effects are generally negligible at 13 GHz for typical space-borne and air- borne systems while they are relevant to space-borne but almost negligible in air- borne configurations at 35 GHz.