Lau95a Lau, W. K.-M, Y. Sud, and J.-H Kim, 1995a: Intercomparison of hydrologic processes in global climate models. NASA Technical Memorandum 104517, NASA Goddard Space Flight Center, Greenbelt, MD, 161 pp.

Preliminary results of the Atmospheric Model Intercomparison Project (AMIP) diagnostic subproject on "Intercomparison of Hydrologic Processes in Global Models" are presented. The objective of the subproject is to evaluate the ability of atmospheric general circulation models (GCM) in simulating the global hydrologic cycle and to explore means of validating GCM precipitation and hydrologic processes with space-based and ground-based observations. Based on this evaluation, we hope to identify generic and/or specific strengths and weaknesses of the participating climate models which will help to formulate a strategy for model improvement. In this report, we address the intercomparison of precipitation (P), evaporation (E), and surface hydrologic forcing (P-E) for 23 AMIP GCMs including relevant observations, over a variety of spatial and temporal scales. The intercomparison includes global and hemispheric means, latitudinal profiles, selected areal means for the tropics and extratropics, ocean and land, respectively. In addition, we have computed anomaly pattern correlations among models and observations for different seasons, harmonic analysis for annual and semiannual cycles, and rain-rate frequency distribution. We also compare the joint influence of temperature and precipitation on local climate using the Koeppen climate classification scheme. Results show that the models collectively portray an Earthlike climate with respect to the observed land-only global mean surface temperature (=14.8°C ) and precipitation (=2.4 mm/day ) to within 10%. The model consensus indicates a cold-wet bias of about 1.5 degrees C and 0.5 mm/day. Most of the models conserve atmospheric water up to about 5%. While most models show a rain-rate intensity distribution similar to that of the observed, almost all models underestimate the frequency of occurrence of light rain (<1 mm/day), suggesting some fundamental problems in the treatment of nonconvective rainfall in models. The major areas of deficiencies in global rainfall distribution are in the eastern Pacific Intertropical Convergence Zone (ITCZ), the South Pacific Convergence Zone (SPCZ), and the Asian monsoon. The main discrepancy in the ITCZ and the SPCZ is in the rainfall amount. In the Asian monsoon region, the problem seems to be more severe. None of the models are able to reproduce the East Asian monsoon rainbelt. Perhaps, not by happenstance, the disparities among observed estimates are also large in the above regions. The model depiction of the hydrologic cycle tends to be more realistic where there is a strong annual cycle and where local moisture balance is the key operating mechanism, i.e., over large interior land mass in the extratropics. In regions of strong dynamical control (P-E >>0), i.e., over the tropical western Pacific, the monsoon region and the ITCZ, the differences among models tend to be large. In the interannual time scales, all models show enhanced rainfall prediction over the tropics because of sea surface temperature (SST) forcing, i.e., during El Nino Southern Oscillation (ENSO). However, the models do not show any useful skill for rainfall prediction in the extratropics from tropical SST forcing.