SATELLITE RETRIEVAL OF TROPICAL PRECIPITATION USING COMBINED INTERNATIONAL SATELLITE CLOUD CLIMATOLOGY PROJECT DX AND SSM I DATA/

A new precipitation retrieval scheme for use in the tropics is develop ed using a combination of visible (VIS)/IR and microwave satellite obs ervations. The new scheme combines the advantages of both the ample co verage of VIS/IR sampling and the physical directness between precipit ation and microwave radiances. A VIS/IR algorithm is ''trained'' using Special Sensor Microwave/Imager (SSM/I) derived values of precipitati on. Cloud top temperatures and visible optical depths provided by the International Satellite Cloud Climatology Project (ISCCP) level DX ana lyses are ''binned'' by every 10 K and every 10 unit optical depth, re spectively. Lookup tables for the probability of rain and mean rainfal l rate are constructed for each cloud top temperature/optical depth ce ll whenever the optical depth is available (daytime). For the nighttim e, the tables are based only on cloud top temperature. The instantaneo us rainfall rates are obtained by multiplying the mean rainfall rate b y the probability of rain, both characterized by the cloud top tempera ture and/or visible optical depth associated with the satellite pixel. The satellite retrieval always retains results from optimal parameter s, which means SSM/I results are used whenever SSM/I samplings are ava ilable; otherwise, VIS/IR results are used during daytime, while IR-on ly results are used during nighttime. The final precipitation product can have resolution as high as 3 hours in time and about 0.3 degrees i n space. However, averaging in space and/or time improves the statisti cs of the derived rainfall rates, since the statistics of the algorith m are more fully utilized. The satellite-derived rainfall product comp ares well with ship rain gauge data in terms of rain/no rain determina tion, and agrees well with daily rainfall accumulation derived from a large-scale moisture budget. The satellite retrieved rainfall rates ar e biased high relative to radar observations. In the absence of satisf actory data with which to validate the satellite-derived rainfall rate s, it can be concluded that there is useful information in the ISCCP d ata set that relates to precipitation and that the inclusion of visibl e optical depth information improves the daytime retrieval of precipit ation relative to IR-only methods.