Precipitation Index Estimated over Chao Phraya River Basin in Thailand

Abstract

Precipitation Index, a formula to obtain estimates of precipitation on the ground as a function of infra-red radiation observations of a geostationary satellite was developed over the 111, 000km2 area of Chao Phraya river basin in Thailand. The satellite data used were 0.25° mesh basic histogram of GMS-IR observations and the rainfall data were the daily rainfall measured by the Royal Irrigation Department of Thailand. The various time and space scale correspondences between GMS-IR and precipitation were identified. For 10-day rainfall estimate over 2.5° mesh, a formula PI (mm)=12.1 FCND+3.3ND was derived where FC was a fractional coverage of clouds below 235K and ND is the number of days. The coefficient of determination was D=0.25. The estimates were found considerably smaller than that of GOES Precipitation Index (GPI) currently used at CAC/NOAA which was developed by Arkin (1979) based on GATE using GOES in 1974. They were also found smaller than those by Maruyama et al. (1986) and Eguchi (1988) obtained from GMS over Western Pacific. It was concluded that precipitation production over the land was prevailed by a different mechanism from that over the ocean, thus it was necessary to develop region by region estimation formulae for reliable global rainfall monitoring.