GLOBAL MONTHLY PRECIPITATION ESTIMATES FROM SATELLITE-OBSERVED OUTGOING LONGWAVE RADIATION

The relationship between the flux of outgoing longwave radiation (OLR) estimated from satellite observations and precipitation is investigat ed using monthly OLR data from the NOAA polar-orbiting satellites and the merged analysis of precipitation of Xie and Arkin for the 8-yr per iod from July 1987 to June 1995. The mean annual cycle of OLR in the T ropics is dominated by changes in cloudiness and exhibits a strong neg ative correlation with precipitation, while in the extratropics the st rongest influence on the annual cycle of OLR is surface temperature an d a positive correlation with precipitation is found. However, the ano maly of OLR exhibits a negative correlation with precipitation over mo st of the globe. The regression coefficient relating the anomaly of pr ecipitation to that of OLR is spatially inhomogeneous and seasonally d ependent but can be expressed with high accuracy as a globally uniform linear function of the local mean precipitation. Based on these resul ts, a new technique is developed to estimate monthly precipitation ove r the globe from OLR data. First, the mean annual cycle of precipitati on is calculated from the merged analysis of precipitation for the 8-y r period. The precipitation anomaly is then estimated from the OLR ano maly field using the coefficient value appropriate for the mean annual cycle of precipitation at each location. Finally, the total precipita tion is estimated as the sum of the mean annual cycle and the anomaly. Verification tests showed that this estimate, which is referred to he re as the OLR-based precipitation index (OPI), is able to represent la rge-scale precipitation with globally uniform and temporally stable hi gh quality? similar to geostationary satellite IR-based estimates over the Tropics and to estimates based on microwave scattering observatio ns over extratropical areas. The OPI estimates are then produced for t he 22-yr period from 1974 to 1995 and are used to investigate the annu al and interannual variability of global precipitation. The mean distr ibution and seasonal variations as observed in the 22-yr set of OPI es timates agree well with those of several published long-term means of precipitation estimated from station observations, and the interannual variability in precipitation associated with the Fl Nino-Southern Osc illation phenomenon resemble those found in previous studies but with additional details, particularly over ocean areas.