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Rainfall Estimation from ESMR-5 Measurements and Application to El Niño

Hyo-Suk LimDepartment of Meteorology, Texas A&M University, College Station, Texas

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C. E. GravesDepartment of Earth and Atmospheric Sciences, Saint Louis University, St. Louis, Missouri

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G. R. NorthClimate System Research Program, Department of Meteorology, Texas A&M University, College Station, Texas

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T. T. WilheitClimate System Research Program, Department of Meteorology, Texas A&M University, College Station, Texas

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Abstract

An algorithm to estimate monthly 5° × 5° area-averaged rain rate over the oceans from January 1973 to December 1976 using single-channel microwave data from the Nimbus-5 satellite has been developed. This study extends the work of Shin et al. by including the full width of scan angles (from −50° to 50°) in order to reduce sampling error. The scan-angle dependence of the estimated rain rate due to variable antenna sidelobe effects, surface emissivity, and propagation pathlength is eliminated using a statistical method. A globally uniform beam-filling correction factor of 2.2 is applied 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 South Pacific convergence zone (SPCZ) but a low correlation in the extratropics and equatorial western Pacific. It is also proved that the retrieved rain rates are statistically significant.

The rainfall deviations from non-El Niño years April 1973 to December 1975 reveal the temporal and spatial variations produced by the 1972–73 and 1976–77 El Niño episodes. We observe an increase of rainfall over the eastern and central equatorial Pacific Ocean and a decrease over the equatorial western Pacific Ocean and eastern Australia during these events. Consistent with previous work, the rainfall anomaly of the 1972–73 El Niño was much stronger than that of the 1976–77 El Niño.

Abstract

An algorithm to estimate monthly 5° × 5° area-averaged rain rate over the oceans from January 1973 to December 1976 using single-channel microwave data from the Nimbus-5 satellite has been developed. This study extends the work of Shin et al. by including the full width of scan angles (from −50° to 50°) in order to reduce sampling error. The scan-angle dependence of the estimated rain rate due to variable antenna sidelobe effects, surface emissivity, and propagation pathlength is eliminated using a statistical method. A globally uniform beam-filling correction factor of 2.2 is applied 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 South Pacific convergence zone (SPCZ) but a low correlation in the extratropics and equatorial western Pacific. It is also proved that the retrieved rain rates are statistically significant.

The rainfall deviations from non-El Niño years April 1973 to December 1975 reveal the temporal and spatial variations produced by the 1972–73 and 1976–77 El Niño episodes. We observe an increase of rainfall over the eastern and central equatorial Pacific Ocean and a decrease over the equatorial western Pacific Ocean and eastern Australia during these events. Consistent with previous work, the rainfall anomaly of the 1972–73 El Niño was much stronger than that of the 1976–77 El Niño.

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