The CPC 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).
It is important to note that the input data sources to make these analyses are
not constant throughout the period of record. For example, SSM/I (passive
microwave - scattering and emission) data became available in July of 1987;
prior to that the only microwave-derived estimates available are from the MSU
algorithm (Spencer 1993) which is emission-based thus precipitation estimates
are avaialble only over oceanic areas. Furthermore, high temporal resolution
IR data from geostationary satellites (every 3-hr) became available during 1986;
prior to that, estimates from the OPI technique (Xie and Arkin 1997) are used
based on OLR from polar orbiting satellites.
The merging technique is thoroughly described in Xie and Arkin (1997).
Briefly, the methodology is a two-step process. First, the random error is
reduced by linearly combining the satellite estimates using the maximum
likelihood method, in which case the linear combination coefficients are
inversely proportional to the square of the local random error of the
individual data sources. Over global land areas the random error is defined
for each time period and grid location by comparing the data source with the
raingauge analysis over the surrounding area. Over oceans, the random error
is defined by comparing the data sources with the raingauge observations over
the Pacific atolls. Bias is reduced when the data sources are blended in the
second step using the blending technique of Reynolds (1988). Here the data
output from step 1 is used to define the "shape" of the precipitation field and
the rain gauge data are used to constrain the amplitude.
Monthly and pentad CMAP estimates back to the 1979 are available from CPC
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.