At the moment, the four major coupled atmosphere-ocean GCMs are the high-resolution climate model of the United Kingdom Meteorological Office (UKMO, Hadley Centre, Bracknell), the climate model of the National Center of Atmospheric Research (NCAR, Boulder, USA, without flux correction), the Geophysical Fluid Dynamics model (GFDL, Princeton, USA), and the MPI model (Max-Planck-Institut fŸr Meteorologie, Hamburg, Germany). These models qualitatively agree on the global patterns (IPCC, 1990). The most certain conclusion is that the average precipitation amounts increase globally. Annual changes have been estimated to lie within the range of +3 to +15% under doubled CO2 conditions*.
Increases are particularly expected at high latitudes throughout the year, and in much of the mid-latitudes during winter. More rainfall is predicted in southeast Asia (monsoon region). In the tropics, projections are uncertain, i.e. they differ from model to model. In the Southern Hemisphere, precipitation increases are predicted along the mid-latitude storm tracks.
In the IPCC report of 1992 results of several doubled CO2 experiments with GCMs and mixed-layer ocean models have been summarised (Table B2, pg. 111). In general, the simulated globally-averaged increases in precipitation are similar to those in the 1990 IPCC report. The annual percentage increases range from 2.5% to 8%.
One of the first papers on changing rainfall processes estimated by GCMs was written by Noda and Tokioka (1989). Their (mixed layer) model is developed at the Meteorological Research Institute of Japan (MRI-GCM). Under doubled CO2 concentrations, the increase of the global mean precipitation is attributed to the increase of precipitation of cumulus type causing a decrease in the global area of total precipitation. Furthermore, the frequency of intense convective rainfall increases while that of nonconvective rainfall decreases.
Parey (1994), analysing results from the high resolution global simulations at the LMD (Laboratoire du Meteorologie Dynamique, France), observed that the changes in precipitation between three simulations (1xCO2, 2xCO2, and 3xCO2) were small. She did find more rainfall in winter (and a decrease in snowfall) and less rainfall in summer over the high and middle latitudes of the Northern Hemisphere. The number of rain days increased due to snowfall became rainfall during the early spring season. The variability in rainfall appears to change in the same way as the mean rainfall amounts. No conclusion could be drawn on changes in the frequency of extreme rainfall events.
Gordon et al. (1992) investigated the variability of daily rainfall with the global model of CSIRO (Commonwealth Scientific and Industrial Research Organization, Australia). More heavy rainfall days and fewer light rainfall days are observed when CO2 has doubled: a heavy rain event which occurs once in five years becomes at least twice as frequent. A widespread increase in rainfall of convective origin is observed, and the mean intensity of rainfall rises correspondingly.
Fowler and Hennessey (1994) obtained comparable findings (UKMO high-resolution model): the mean intensity increases by 10-30% at most latitudes. Studies currently being conducted with the UKMO model confirm this: tendencies to heavier rain events, in connection with an increase of convectional activity.
In general, the results obtained with the MPI model (Cubasch et al., 1994) show agreement. Again, an increase in rainfall during the Northern Hemisphere mid-latitude winter period (Northwest Europe), and in the monsoon region in summer is observed. Cubasch proposed a Monte Carlo technique to obtain more representative results. An ensemble average of four different runs was calculated. The individual findings were compared to obtain a general idea of the stability of the result. A considerable between-experiment standard deviation was observed.
Meehl et al. (1993) attempted to estimate possible sensitivities of ENSO related effects with the global NCAR model. The observed rise in mean sea-surface temperatures leads to an increase in evaporation and precipitation. An intensification of atmospheric anomalies in the tropics is indicated: most dry areas became drier and wet areas wetter.
