|Coverage||Global (90N-90S, 0-360E)|
|Time Period||July 1987 - Present|
|Resolution||2.5 x 2.5 degree, monthly|
Window channel observations from passive microwave radiometers have been used to estimate rainfall since the launch of the Defense Meteorological Satellite Program (DMSP) Special Sensor Microwave/Imager (SSM/I) in July of 1987. Over radiometrically cold ocean regions, changes in brightness temperature due to the absorption/emission by liquid hydrometeors at frequencies below the 50-60 GHz oxygen absorption band is directly related to rainfall. While this approach doesn't work over radiometrically warm land surfaces, scattering by precipitation size ice particles at frequencies above the oxygen absorption band can be used to estimate rainfall. Microwave sounding channel data have also been used to estimate rainfall going back to 1979 from the Microwave Sounding Unit (MSU) on board NOAA's TIROS-N polar-orbiting satellite in October of 1978. Although MSU provides a longer data record than SSM/I, poor spatial resolution of the sensor along with limited precipitation information have limited it use primarily to enhance coverage prior to SSM/I in the merged CMAP product.
This dataset is one of the intermediate products from version 2 of the Global Precipitation Climatology Project (GPCP) merged rainfall dataset.
The SSM/I scattering dataset is based on the Grody  scattering index, which is supplemented over ocean regions with the Weng and Grody  emission technique. The microwave scattering technique infers the quantity of precipitation size ice in a cloud from depressions in the 85GHz channel brightness temperatures, which is then related to surface rainfall. This relationship is physically less direct than in the emission technique, but it works equally well over land and ocean in detecting deep convection. The SSM/I scattering dataset is one of the component rainfall estimates used in the GPCP merged dataset [Huffman et al., 1997].
This dataset and other GPCP intermediate datasets are available through either of the following sources.