Precipitation fields are known to be of much higher spatio-temporal variability than other meteorological variables. Therefore the analysis of precipitation fields based on GTS-type data with typical station distances of several tens of km are not adeaquate. Even the inclusion of the many non-GTS stations in the Alpine region with typical station distances of approximately 10 km is not sufficient for this purpose, e. g. model comparisons of very high resolution. We would like to reach a spatial resolution on the order of 1 km and a temporal resolution of, say 10 Minutes. Only RADAR information fullfills this criterion.

Over flat terrain the downscaling procedure with the help of RADAR is a promising approach. Much work is being done presently in this field. Over complex terrain, however, RADAR brings a lot of difficulties, e. g. due to shading effects. Nevertheless, a three dimensional incorporation of RADAR to downscale/disaggregate precipitation may be carried out. A variational approach is being discussed here.

An other method for downscaling seems to be applicable over complex terrain: The incorporation of very high resolution topographic information into the precipitation fields. Under stratiform precipitation as well as under convective precipitation conditions the sloping of the terrain, the height of ridges and peaks and the distance from ridges strongly influence the very small scale precipitation distribution. Also for this purpose a variational approach may be used, which basics will be shown. Some preliminary results will be presented in an other paper at this conference.