From GCM scales rainfall variability

The variability of rainfall is a key component determining how the continental surfaces react to the atmospheric forcing. When studying the impact of climatic fluctuations onto the water resources, it is'thus of paramount importance to evaluate to which extent the atmospheric models used in this kind of studies are able to reproduce the variability of the rain process, both in space and time. First among these are the general circulation models (GCM) with coarse resolution, which has two consequences: (i) a simplified parametrisation of convection; (ii) a scale of representation of rainfields which is not adequate when it comes to use them as inputs to hydrologic models. Since linking GCMs and regional hydrologic models is the corner stone of impact studies, it is necessary to analyse the consequences of this gap in scales and to find ways of bridging it. As a preliminary step in that direction, a comparative analysis of the observed and of the GCM rainfall variabilities is carried out for a tropical semi-arid zone of West Africa displaying a high sensitivity to climatic fluctuations. Over tropical regions the GCM used here (LMD-6) has a space resolution of 1.6O in latitude and of 3.75' in longitude. The comparative study shows that the errors of the GCM rainfall outputs may be traced down to two fundamental shortcomings: (i) a wrong seasonal cycle, probably linked to problems in representing the large scale circulation; (ii) an unrealistic