The "CAMS_OPI" (Climate Anomaly Monitoring System ("CAMS") and OLR Precipitation
Index ("OPI") is a precipitation estimation technique which produces real-time monthly analyses of global precipitation. To do this, observations from
raingauges ("CAMS" data) are merged with precipitation estimates from a satellite algorithm ("OPI"). The analyses are on a 2.5 x 2.5 degree latitude/longitude
grid, are updated each month, and extend back to 1979. This data set is intended primarily for real-time monitoring. For research purposes, we refer users to the
GPCP and CMAP products which are more quality-controlled and use both IR and
microwave-based satellite estimates of precipitation.
The CAMS_OPI data files contain, for each month:
- raingauge/satellite merged analysis
- gauge-only precipitation analyses
- the number of gauge reports in each gridbox
- OPI-only precipitation estimates
- gauge/satellite merged analysis anomalies (1979 - 1995 base period)
- anomalies expressed as a percentage of the Gamma distribution
The merging technique is very similar to that described in Xie and Arkin (1997), and
the CAMS_OPI technique has also been published recently (Janowiak and Xie 1999). Briefly, the
merging 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 propostional 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.
CAMS_OPI monthly estimates area available from January 1979 to present at the CPC
Huffman, G. J. and co-authors,1997:The Global Precipitation
Climatology Project (GPCP) combined data set.Bull. Amer. Meteor. Soc., 78, 5-20.
Janowiak, J. E. and P. Xie, 1999: CAMS_OPI: a global satellite-raingauge merged product for real-time precipitation
monitoring applications. J. Climate, 12, 3335-3342.
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.