NOAA NCEP Climate Prediction Center Merged Analysis of Precipitation 

PC Merged Analysis of Precipitation ("CMAP") is a technique which produces pentad and monthly analyses of global precipitation in which observations from raingauges are merged with precipitation estimates from several satellite-based algorithms (infrared and microwave). The analyses are are on a 2.5 x 2.5 degree latitude/longitude grid and extend back to 1979. These data are comparable (but should not be confused with) similarly combined analyses by the  Global Precipitation Climatology Project which are described in Huffman et al (1997).

"Please be advised that the CMAP data set may contain an artificial downward trend for period after 1996. Care must be taken when using the data set for accurate quantitative applications." Questions concerning the method used to merge the precipitation estimates should be directed to Pingping Xie ( Pingping.Xie_at_noaa.gov ).
Important information to be read before using the data is on the JISAO website for the monthly (http://jisao.washington.edu/data_sets/cmap_precip/) and for the pentad (http://jisao.washington.edu/data_sets/cmap_precip/pentad/) datasets.

There are two merged precipitation data sets:
1) Rain gauge, 5 satellite estimates( GPI, OPI, SSM/I scattering, SSM/I emission and MSU), and numerical model predictions.
2) Rain gauge, 5 satellite estimates( GPI, OPI, SSM/I scattering, SSM/I emission and MSU)
where OPI stands for OLR-based precipitation index (Janowiak and Xie, 1999).

rain rate (mm/day) (with model input) - cmaprain.<monthly/pentad>.nc  
rain amount (cm/mon) (with model input) - precipamountcmap.monthly.nc 
rain rate (mm/day) (no model input) - cmaprain2.<monthly/pentad>.nc 
rain amount (cm/mon) (no model input) - precipamountcmap2.monthly.nc 
More information and the the data in ascii format are available at the CMAP page on the NOAA-CPC website (http://www.cpc.ncep.noaa.gov/products/global_precip/html/wpage.cmap.html).

References:
Huffman, G. J. and co-authors, 1997: The Global Precipitation Climatology Project (GPCP) combined data set. Bull. Amer. Meteor. Soc., 78, 5-20.
Reynolds, R. W., 1988: A real-time global sea surface temperature analysis. J. Climate, 1, 75-86.
Spencer, R. W., 1993: Global oceanic precipitation from the MSU during 1979-91 and comparisons to other climatologies. J. Climate, 6, 1301-1326.
Xie P., and P. A. Arkin, 1996: Global precipitation: a 17-year monthly analysis based on gauge observations, satellite estimates, and numerical model outputs. Bull. Amer. Meteor. Soc., 78, 2539-2558.