Simulation of the hydrological cycle over Europe: Model validation and impacts of increasing greenhouse gases

Different methods of estimating precipitation area means, based on observations, are compared with each other to investigate their usefulness for model validation. For the applications relevant to this study the ECMWF reanalyses provide a good and comprehensive data set for validation. The uncertainties of precipitation analyses, based on observed precipitation or from numerical weather forecasting schemes, are generally in the range of 20% but regionally much larger. The MPI atmospheric general circulation model is able to reproduce long term means of the main features of the hydrological cycle within the range of uncertainty of observational data, even for relatively small areas such as the Rhine river basin. Simulations with the MPI coupled general circulation model, assuming a further increase of anthropogenic greenhouse gases, show clear trends in temperature and precipitation for the next century which would have significant implications for human activity, e.g. a further increase of the sea level of the Caspian Sea and less water in the Rhine and the Danube. We have gained confidence in these results because trends in the temperature and precipitation in the coupled model simulations up to the present are partly confirmed by an atmospheric model simulation forced with observed SSTs and by observational data. We gained further confidence because the simulations with the same coupled model but using constant greenhouse gases do not show such trends. However, doubts arise from the fact that these trends are strong where the systematic errors of the model are large. (C) 1999 Elsevier Science Ltd. All rights reserved.