Studies on data uncertainties of the local model of the German Meteorological Service in the catchment area of the river Ilm

Abstract. To assess the prognostic values concerning precipitation and air temperature in the river catchment area of the Ilm/Thuringia issued by the German Meteorological Service, the differences between the prognostic values and related measured values were statistically evaluated. The investigations covered the period from November 1999 to December 2003. The spatial resolution corresponds to the LM grid cells and the available data. The observed precipitation values were corrected with regard to systematic wind error in dependence of temperature.

The forecast data were investigated comprehensively in order to quantify the statistical reliability. Therefore, the 2nd moment as the case may be the standard deviation of the differences between observed precipitation values and forecasting precipitation values have been calculated. The investigations were subdivided into the complete stretch of time, the hydrological summer and the hydrological winter. Each measured value was analysed with regard to the 12-, 24-, 36- and 48-hour forecast made by the DWD.

As a result, empirical analytical functions were developed to describe adequately the statistical quantities for standard deviation of precipitation and temperature for the periods of hydrological summer and hydrological winter in dependence on forecasting time, mean ground level and roughness (fractal dimension) of the LM-grid cell.