RAINFALL ESTIMATION FROM ESMR-5 MEASUREMENTS AND APPLICATION TO EL-NINO

An algorithm to estimate monthly 5 degrees X 5 degrees area-averaged r ain rate over the oceans from January 1973 to December 1976 using sing le-channel microwave data from the Nimbus-5 satellite has been develop ed. This study extends the work of Shin et al. by including the full w idth of scan angles (from -50 degrees to 50 degrees) in order to reduc e sampling error. The scan-angle dependence of the estimated rain rate due to variable antenna sidelobe effects, surface emissivity, and pro pagation pathlength is eliminated using a statistical method. A global ly uniform beam-filling correction factor of 2.2 is applied in this st udy. Comparison with island station rainfall measurements over the Pac ific shows a remarkably high correlation between two data in the equat orial dry zone and South Pacific convergence zone (SPCZ) but a low cor relation in the extratropics and equatorial western Pacific. It is als o proved that the retrieved rain rates are statistically significant. The rainfall deviations from non-El Nino years April 1973 to December 1975 reveal the temporal and spatial variations produced by the 1972-7 3 and 1976-77 El Nino episodes. We observe an increase of rainfall ove r the eastern and central equatorial Pacific Ocean and a decrease over the equatorial western Pacific Ocean and eastern Australia during the se events. Consistent with previous work, the rainfall anomaly of the 1972-73 El Nino was much stronger than that of the 1976-77 El Nino.