Improved Estimates of Tropical and Subtropical Precipitation Using the GOES Precipitation Index

Nine years (1986–94) of tropical and subtropical precipitation estimates based on the GOES precipitation index (GPI) are examined. The GPI, based on the results of studies relating fractional coverage of cold cloud to convective rainfall, uses IR observations gathered by geostationary and polar-orbiting satellites. Longitudinal discontinuities in mean GPI coincident with the boundaries of satellite coverage led to a comparison of GPI derived from each geostationary satellite in overlap regions. This study revealed both intersatellite calibration differences and satellite zenith angle dependence. Its goals are to remove these sources of systematic error within the GPI, investigate the climatology of the corrected GPI, and compare against other estimated rainfall datasets. To correct calibration differences, Global Precipitation Climatology Project geostationary satellite IR data are standardized to one satellite by temperature adjustments deduced by the International Satellite Cloud Climatology Project. The resulting GPI values are corrected for zenith angle dependence based on a comparison between GOES-7 and Meteosat-3 that found a systematic increase in GPI of 9% for every 10° of zenith angle beyond 25°. The corrections remove noticeable discontinuities in time-averaged GPI and are largest (>2 mm day−1) over the eastern Indian Ocean, the equatorial Pacific near the date line, and South America. The spatial correlation between corrected GPI and rainfall derived from rain gauges is greater than 0.8 in tropical regions with adequate gauge density. Empirical orthogonal functions of monthly anomalies of corrected GPI show the expected El Niño–Southern Oscillation spatial pattern.