Kurz, Manfred (Deutscher Wetterdienst)
The Deutscher Wetterdienst (DWD) runs in its centre in Offenbach one of the Regional Spezialized Meteorological Centres (RSMC) of the "World Weather Watch" of the WMO. It provides analyses and forecasts for Europe and the North Atlantic for use by interested National Meteorological Services. For this task a model chain consisting of a Global Model (GM), a limited area model covering the whole area of responsibility (EM, grid size 55 km) and a high-resolution model covering Mid Europe with a horizontal resolution of 14 km and 20 layers in the vertical (DM) is used. In the paper the performance of this model chain in forecasting hazardous weather events in the Mediterranean and the Alps is demonstrated. With the aid of some well-known examples it is shown that EM and DM are able to produce forecasts of heavy rain with great skill well in advance so that these prognoses could be used to issue warnings and to organize measures for avoiding greater damage or even loss of life.
The verification of the forecasts also shows, however, that the models simulate in many cases similar forecasts which do not come true or have a great error with respect to time and date. These wrong forecasts may lead to superfluous alerts and as consequence to confusion of the users. In order to avoid this situation a careful monitoring of the forecasts has to be carried out. That can be done by a diagnosis of the special synoptic conditions which lead to the forecast of the hazardous weather events in the model and then a check of the real development as to whether these conditions will occur or not. This method is demonstrated with the aid of a DM forecast for the Alps in which very high totals of precipitation were simulated for a relatively small area, due to the development of a quasi- stationary convergence line at the windward side of the mountains. Since the observations showed that this convergence line was not stationary but moving, it became clear relatively soon that the forecast was wrong and a warning was not necessary.