A Passive Microwave Algorithm for Tropical Oceanic Rainfall

Abstract

This study discusses a rainfall algorithm utilizing six channels of microwave radiance data from the Nimbus-7 Scanning Multifrequency Microwave Radiometer. The algorithm is intended for short-term climate studies over the ocean at low latitudes. To find a set of functional relationships, rain ratios are regressed on brightness temperatures for each channel. Next, these functions are integrated over a class of rain-rate distributions to find relations between mean brightness temperatures and mean rain rates. This step accounts for beam filling. Finally, weights are obtained for combining the rain rates from the individual channels. The weights vary with the rain rates, so that the optimum combination of channels is always used. Results are stored in a database grid 1° latitude ? 1° longitude by one month. To test the algorithm, three years (1979?81) of data from the Indian Ocean are processed. These show spatial patterns very similar to previous climatological studies and to expected seasonal variations as determined at climatological observing stations. In addition, the results are compared with another microwave algorithm and with an infrared threshold method substantially the same as the GOES precipitation index.