| Abstract: | An algorithm to estimate monthly 5° x 5° area averaged rainrate over the oceans January 1973 to December 1976 using single channel microwave data from the Nimbus 5 satellite has been developed. This study is an extension of the work of Shin et al. (1990) by including whole scan angles (from -50° to 50°) in order to reduce sampling error. Scan angle dependence of the estimated rainrate due to variable antenna sidelobe effects, surface emissivity and propagation path length is suppressed using a statistical method. A globally uniform beam filling correction factor of 2.2 is employed in this study. Comparison with island station rainfall measurements over the Pacific shows a remarkably high correlation between two data in the equatorial dry zone and Southwest Pacific Convergence Zone (SPCZ), but low in the extratropics and equatorial western Pacific. It is also proved that the retrieved rainrates are statistically significant. The rainfall deviations from non-El Nino years April 1973 to December 1975 are derived to reveal the temporal and spatial variations produced by the 1972-73 and 1976-77 El Ninos. An increase of rainfall over the eastern and central equatorial Pacific Ocean and a decrease of precipitation is observed over the equatorial western Pacific Ocean and eastern Australia for these events. It is very consistent with previous works that the rainfall anomaly of 1972-73 El Nino is much stronger than that of 1976-77 El Nino. Nonoverlapping 5-day averages of rainfall from September 1973 to March 1975 (580 days) are retrieved and used to detect the 40-50 day oscillation. A strong spectral peak in the 30-60 day period is observed over the equatorial Indian Ocean and the western and eastern equatorial Pacific Ocean. Unlike many previous works, the eastward propagation of 40-50 day oscillation is not detected. On the other hand, the propagation was observed using Special Sensor Microwave/Imager (SSM/I) inferred rainfall data. |
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