Improved Non-Uniform Beam Filling Parameterization for the GPM Combined Algorithm

The operational GPM Combined Algorithm has been designed to reduce uncertainties in precipitation retrievals by effectively incorporating complementary information from the GPM radar and radiometer observations. The application of Version 4 of the algorithm to more than two years of GPM data and the analysis of results revealed that, although the algorithm performs overall satisfactorily, improvements could be achieved through more accurate physical and statistical modeling. Among the improvements, a more accurate quantification and parameterization of the Non-Uniform Beam Filling (NUBF) impact on the estimation of the differential Path Integrated Attenuation (PIA) using the surface reference technique (SRT) is crucial in reducing uncertainties in the retrieval of convective precipitation, especially over land. This is because NUBF may significantly reduce the apparent PIA (i.e. the PIA estimated from the SRT) at both frequencies, but to a larger degree at Ka-band than at Ku-band. A NUBF parameterization based on the downscaling of the observed reflectivity profiles is already used in V4. This parameterization significantly improves the consistency between simulated and observed apparent PIA at both frequencies, but it results in a systematic increase of the effective PIA (defined as the ratio of effective reflectivity to measured reflectivity). Ground validation data suggest that the increase in the effective PIA induced by the V4 NUBF parameterization is unrealistically high. Therefore, an improved NUBF parameterization, based on a more complex reflectivity downscaling scheme, is under development. More specifically, the new NUBF parameterization employs a more complex methodology for generating sub-footprint reflectivity profiles based on the variability of reflectivity at larger scales (spanning several observed profiles in both along-track and cross-track). Initial tests of this parameterization suggest better performance than that of the V4 scheme.