RETRIEVAL OF MONTHLY RAINFALL INDEXES FROM MICROWAVE RADIOMETRIC MEASUREMENTS USING PROBABILITY-DISTRIBUTION FUNCTIONS

An algorithm for the estimation of monthly rain totals for 5-degrees cells over the oceans from histograms of SSM/ I brightness temperatures has been developed. There are three novel features to this algorithm. First, it uses knowledge of the form of the rainfall intensity probability density function to augment the measurements. Second, a linear combination of the 19.35 and 22.235 GHz. channels has been employed to reduce the impact of variability of water vapor. Third, an objective technique has been developed to estimate the rain layer thickness from the 19.35- and 22.235-GHz brightness temperature histograms. Comparison with climatologies and the GATE radar observations suggest that the estimates are resonable in spite of not having a beam-filling correction. By-products of the retrievals indicate that the SSM/I instrument noise level and calibration stability are quite good.