Atmospheric Science ETH Hönggerberg, CH-8093 Zürich, Switzerland. frei@atmos.umnw.ethz.ch.

Air temperature, atmospheric moisture, precipitation

Europe

With the isealizd warm climate "WARM" the simulated mean evaporation and precipitation show spatial variations very similar to that of the CTRL but there is a significant overall increase. The enhancement of evaporation from the ocean amounts to 14%. This can be interpreted in terms of changes in the turbulent transfer through the atmospheric boundary layer over the ocean, which is in turn induced by the increased vertical gradient in the mean specific humidity. Over land the evaporation increase is somewhat lower (11%).

Averaged over themodel domain the ensemble-mean precipitation increases by between 12 and 15% for individual months of the ensemble. The increase is most evident for areas that already experience wet conditions in the CTRL simulation, i.e. the ocean, Southern Europe and the Alpine region. For the Mediterranean subdomain, the relative increase (17%) is larger than for the Alpine region (12%) and Central Europe (9%), however the limited sample size and the magnitude of the case-tocase variations militate against identifying statistically significant regional variations for this relative increase.

The simulated warming has a significant impact upon the intensity distribution of the precipitation events. For light-intensity events the effect is marginal, and the frequency of rainy days (>0.1mm per day) remains almost unchanged. However the effect progressively increases with precipitation intensity, and is most pronounced for strong events. Occurrences exceeding 30mm per day are more frequent by more than 20% for all three subdomains. Indeed it is the change in the frequency of high-intensity events that is the major contributor to the simulated increase in the mean precipitation.

The results suggest that the stipulated increase in the atmospheric moisture content has marginally affected the spatial extent of the simulated precipitation systems but resulted in a change of their intensity leading to substantial reductions in the return period of strong events.

The regional climate model utilized is the hydrostatic mesoscale weather forecasting model developed at the German Weather Service, driven by observed and modified lateral boundary conditions and sea-surface temperature distributions. In a first step a control simulation (referred to as CTRL) is conducted of the present-day climate by driving the model at its lateral boundaries with the observed weather evolution (perfect boundary conditions). In the second step a sensitivity experiment is conducted with the initial and boundaryelds of the first realisation modified by an uniform temperature increase of 2K. Also, consonant with inference from observations and GCM experiments, the boundary condition for relative humidity is left unchanged and this results in a domain-averaged 15% increase of the atmospheric moisture content. Results will be presented of month-long simulations for an ensemble of five October months (1987, -88, -89, -90, -92), representative for the fall-time climatic conditions over Central Europe.

The response in precipitation intensity is assessed by considering the simulated daily rainfall totals at gridpoints grouped in sub-areas of the model domain. The frequency distributions for the ensemble of the CTRL integrations compare well with those derived from an objective analysis of observed daily precipitation. The slight tendency to overestimate the occurrence of moderate to high precipitation events is associated with the overall wet bias.

(4) - Remarques générales