The GPI, which was developed by Arkin and Meisner (1987, Mon. Wea. Rev. , 115, 51-74) is a precipitation estimation algorithm. The GPI technique estimates tropical rainfall using cloud-top temperature as the sole predictor. The estimation procedure is rather simple:

Precipitation (mm) = FRAC x RATE x TIME where

.....FRAC is the fractional coverage of IR pixels < 235K over a reasonably large domain (50 km x 50 km and larger)

.....RATE is 3 mm/hour

.....TIME is the number of hours over which "FRAC" was compiled

Numerous studies have shown that the GPI yields useful results in the tropics and warm-season extratropics. The major advantage of the technique is that it is based on IR data which is available frequently over most areas of the globe from geostationary and polar orbiting satellites. The major weakness of the method is that estimation of precipitation from cloud-top temperature is relatively far removed from the physics of precipitation generation process.

Monthly precipitation estimates for 40N - 40S for the period January 1986 through the present month are available from the CPC ftp server.