|Coverage||Global (90N-90S, 0-360E)|
January 1979 - August 2002 (Version 9906)
August 2002 - Present (Version 0208)
|Resolution||2.5 x 2.5 degree, monthly|
Satellite infrared window channel measurements around the 11 to 12 micron band have long been used to estimate precipitation over the tropics. The availability of IR window channels on board geostationary satellites, which see through to the surface except when clouds are present, provides high temporal sampling (3-hourly or better) from space. Since clouds are opaque at IR frequencies, the resulting brightness temperatures over cloudy regions corresponds to the temperature of the cloud top, providing a measure of the cloud-top height. In the tropics where deep convection is prevalent, high-level cloudiness is reasonably well correlated with precipitation, at least when averaged over large space and time scales. Unfortunately the relationship between cloud-top height and precipitation changes with latitude and over land as well as over regions with significant amounts of non-precipitating cirrus or other clouds. As a result, IR only techniques suffer from significant regional and time dependent biases, which is why the merged techniques use passive microwave rain estimates to "adjust" the IR estimates.
V9906: Available January 1979 - August 2002
V0208: Available August 2002 - Present
The Outgoing Longwave Radiation (OLR) Precipitation Index (OPI) is computed from NOAA polar-orbiting IR window channel data using the technique developed by Xie and Arkin . The OPI is produced by calibrating OLR values using rain estimates from the CPC Merged Analysis of Precipitation (CMAP) data [Xie and Arkin . The OPI dataset is one of the component rainfall estimates used in the merged CAMS-OPI dataset [Janowiak and Xie, 1999].
This dataset is available through the CAMS-OPI web site at NOAA's Climate Prediction Center