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Nonuniform beamfilling (NUBF) is a major error source in physical retrieval algorithms for estimating rain rates using satellite-borne passive microwave radiometers. The NUBF effects over the ocean within the beam effective field of view in the 10 GHz channel (FOV10) for the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) are investigated from simultaneous measurements made by the TMI and the Precipitation Radar (PR) aboard the TRMM satellite. They are investigated with respect to variability of a non-uniform parameter defined by the lognormal assumption from January to December 2000.  The parameter computed using surface rain rates estimated from PR data tends to be small (large) for stratiform (convective) rainfall in the FOV10. The parameter computed using surface rain rates estimated from brightness temperature (Tb) at 85 GHz is systematically lower in stratiform rainfall than the reference derived by the PR. Systematic differences are not found for the convective cases. To evaluate the effects on rain retrievals due to the systematic differences of the non-uniform parameters, a simple adjustment is applied, and rain retrievals from the observed TMI Tb at 10 GHz vertical polarization are performed over the ocean. Relatively large increases in rain rates retrieved using corrected parameters are found in the tropics, while the increases of the corrected parameters are similar for all latitudes. Effects of the non-uniform parameter differences on the rain retrievals are nonlinear and could be closely related to high background values of the non-uniform parameter in the tropics and more frequent high rainfall intensities.

Development of Nonuniform Beamfilling Correction Method in Rainfall Retrievals for Passive Microwave Radiometers over Ocean Using TRMM Observations