Alpine precipitation in a tripled CO2-climate

A statistical model is constructed to determine the change in wintertime monthly mean Alpineprecipitation due to a tripling of the CO₂ concentration. The model is based on optimallycorrelated pairs of predictand (Alpine precipitation) and predictor (500 hPa geopotential) patternswhich were determined from observational data by a canonical correlation analysis. Thestatistical model is applied to the change in large scale climate simulated by a general circulationmodel (GCM) for a 3×CO₂ concentration. The statistical model predicts a change of Alpineprecipitation of up to 6% of the observed monthly rainfall for a 3×CO₂ climate. A maximalincrease of wintertime precipitation is obtained for the stations in the south of the Alps andonly slight changes are predicted for the stations in the north of the Alps. It is found to beimportant to remove the area averaged change of the geopotential simulated by the GCM andto apply the statistical model only to the spatial variation of the geopotential. The regional climate change estimated with the statistical model depends only slightly on the exact set-upof the model. Thus the error of the statistical model seems to be small in comparison with the uncertainty in the large scale climate predicted by the GCM for 3×CO₂ conditions.