Already a quick glance at regional climatic maps, for instance the ones prepared for the State of Baden-Wuerttemberg , shows that near surface temperature, wind and precipitation can vary strongly over distances of about ten kilometres and less. In order to assess the regional impact of climatic change, we therefore need information at such small spatial scales. The only way to obtain information on the future climate is by using global climate models. Their resolution, being in the order of more than 100km even for the most recent models (the presently used ECHAM5 model at the Max-Planck-Institut for Meteorology has a resolution of about 150km in our latitudes), however, is much too coarse for impact studies, lest for detailed planning purposes. At such resolutions, terrain height is smoothed resulting in over-/ underestimation of valley/mountain heights (e.g. in the Black Forest) by several 100 m; this can result in large errors for quantities which are closely related to terrain height, like temperature, precipitation and wind. Furthermore, subgrid-scale features like urban areas, valleys and mountains which can have considerable influence on regional climate are not taken into account. The same is true for subgrid-scale climatically relevant processes, like convective summer precipitation (thunderstorms), which have to be parameterised. This problem is similar to the closure problem in turbulence, perhaps with the difference that the cloud physical processes involved are less known and the parameterisations are even more uncertain.
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