SSM/I Rain Retrievals within a Unified All-Weather Ocean Algorithm

A new method for the physical retrieval of rain rates from satellite microwave radiometers is presented. The method is part of a unified ocean parameter retrieval algorithm that is based on the fundamental principles of radiative transfer. The algorithm simultaneously finds near-surface wind speed W, columnar water vapor V, columnar cloud liquid water L, rain rate R, and effective radiating temperature T U for the upwelling radiation. Comparisons with radiosondes demonstrate that the algorithm is able to retrieve water vapor when rain is present. For rain rates from 1 to 15 mm h 1 , the rms difference between the retrieved water vapor and the radiosonde value is 5 mm. A novel feature of the rain retrieval method is a beamfilling correction that is based upon the ratio of the retrieved liquid water absorption coefficients at 37 and 19 GHz. This spectral ratio decreases by about 40% when heavy and light rain coexist within the SSM/I footprint as compared to the case of uniform rain. This correction increases the rain rate when the spectral ratio is small. However, even with this beamfilling correction, tropical rainfall is still unrealistically low when the freezing level in the Tropics (5 km) is used to specify the rain layer thickness. Realism is restored by reducing the assumed tropical rain-layer thickness to 3 km. This adjustment is probably compensating for two processes: 1) the existence of warm rain for which the rain layer does not extend to the freezing level and 2) very heavy rain for which the 19-GHz channels saturate. Global rain rates are produced for the 1991-94 period from two SSM/Is. The authors find that approximately 6% of the SSM/I observations detect measurable rain rates (R 0.2 mm h 1 ). The global rain maps show features that are, in general, similar to those reported in previously published rain climatologies. However, some differences that seem to be related to nonprecipitating cloud water are apparent.