THE GLOBAL PRECIPITATION CLIMATOLOGY PROJECT - 1ST ALGORITHM INTERCOMPARISON PROJECT

The Global Precipitation Climatology Project (GPCP) was established by the World Climate Research Programme to produce global analyses of ar ea- and time-averaged precipitation for use in climate research. To ac hieve the required spatial coverage, the GPCP uses simple rainfall est imates derived from IR and microwave satellite observations. In this p aper, we describe the GPCP and its first Algorithm Intercomparison Pro ject (AIP/1), which compared a variety of rainfall estimates derived f rom Geostationary Meteorological Satellite visible and IR observations and Special Sensor Microwave/Imager microwave observations with rainf all derived from a combination of radar and raingage data over the Jap anese islands and the adjacent ocean regions during the June and mid-J uly through mid-August periods of 1989. To investigate potential impro vements in the use of satellite IR data for the estimation of large-sc ale rainfall for the GPCP, the relationship between rainfall and the f ractional coverage of cold clouds in the AIP/1 dataset is examined. Li near regressions between fractional coverage and rainfall are analyzed for a number of latitude-longitude areas and for a range of averaging times. The results show distinct differences in the character of the relationship for different portions of the area. In general, to the so uth and east of the mountainous axis of Japan, rainfall and fractional coverage are highly correlated for thresholds colder than 245 K, and correlations can be increased by averaging in space and in time up to the dominant period of the precipitation events. To the north and west of the axis, the correlations between rainfall and fractional coverag e, while generally smaller for all scales, are highest for thresholds warmer than 245 K. The proportional coefficients relating rainfall to fractional coverage at cold thresholds, however, differ greatly betwee n the two periods and both differ significantly from those found for t he GARP (Global Atmospheric Research Program) Atlantic Tropical Experi ment. These results suggest that the simple IR-based estimation techni que currently used in the GPCP can be used to estimate rainfall for gl obal tropical and subtropical areas, provided that a method for adjust ing the proportional coefficient for varying areas and seasons can be determined.