An important measure of the reliability of simulated precipitation fields by general circulation models will be its availability to reproduce the more important features of observed precipitation, including its spacial distribution, annual cycle characteristics, and the more salient features of its interannual variability.
Some important characteristics of the large-scale variability of observed precipitation fields during the past few decades over land, and the last 15 years over ocean areas are of the semiannual cycle in the tropics. A second is the strong influence of the El Niño/Southern Oscillation in modulating the rainfall patterns globally. A third salient feature is the decline of precipitation over the tropics since the mid-1970's, which in turn, appears to be connected to the prevalence of warm ENSO conditions in the Pacific during that time.
Henry F. Diaz