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.