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Advances in Geosciences, 5, 113–118, 2005 SRef-ID: 1680-7359/adgeo/2005-5-113 Advances in European Geosciences Union Geosciences © 2005 Author(s). This work is licensed under a Creative Commons License. Studies on data uncertainties of the local model of the German Meteorological Service in the catchment area of the river Ilm K. Pfannschmidt1,2 and H. Dohler¨ 2 1Thuringer¨ Landesanstalt fur¨ Umwelt und Geologie, Prussingstraße¨ 25, 07745 Jena, Germany 2Friedrich Schiller University of Jena, Ernst-Abbe-Platz 2, 07743 Jena, Germany Received: 7 January 2005 – Revised: 1 August 2005 – Accepted: 1 September 2005 – Published: 16 December 2005 Abstract. To assess the prognostic values concerning pre- ventive flood protection in Thuringia. Strong efforts were cipitation and air temperature in the river catchment area of made to improve the protection of the population and to the Ilm/Thuringia issued by the German Meteorological Ser- avoid flood damages, for example by modernising the flood vice, the differences between the prognostic values and re- warning and signalling service (Hack, 2003). In the Free lated measured values were statistically evaluated. The in- State of Thuringia, the monitoring network on the water- vestigations covered the period from November 1999 to De- courses comprises 176 gauges, 52 of which having been se- cember 2003. The spatial resolution corresponds to the LM lected and established as flood gauging and signalling sta- grid cells and the available data. The observed precipitation tions (Spanknebel et al., 1999). values were corrected with regard to systematic wind error in The technical developments of the last years implemented dependence of temperature. in the German Meteorological Service have led to an im- The forecast data were investigated comprehensively in or- provement of the climate forecast data. The numerical der to quantify the statistical reliability. Therefore, the 2nd weather prediction model (Local Model, LM) enables the moment as the case may be the standard deviation of the dif- model-based short-term discharge prediction (URL[3]). This ferences between observed precipitation values and forecast- constitutes the basis for the implementation of a flood warn- ing precipitation values have been calculated. The investi- ing and information service for a prototypical Thuringian gations were subdivided into the complete stretch of time, flood gauging and signalling station. The LM forecast data the hydrological summer and the hydrological winter. Each shall form the basis of a model-based operational discharge measured value was analysed with regard to the 12-, 24-, 36- prediction with the aim to detect looming flash-floods in and 48-hour forecast made by the DWD. good time and to be able to inform accordingly. As a result, empirical analytical functions were developed For this purpose, the gauge of Grafinau-Angstedt¨ located to describe adequately the statistical quantities for standard in the upper reaches of the Ilm river has been selected be- deviation of precipitation and temperature for the periods of cause of the experiences and practical results already ex- hydrological summer and hydrological winter in dependence isting for this partial catchment area (TEG) at the chair on forecasting time, mean ground level and roughness (frac- of Geoinformatics of the Institute for Geography of the tal dimension) of the LM-grid cell. Friedrich Schiller University of Jena (Staudenrausch, 2000; Fritzsche, 2001) For hydrological modelling of the TEG of Grafinau-Angstedt¨ the Precipitation-Runoff Modelling Sys- tem/Modular Modelling System (PRMS/MMS) (Leavesley 1 Introduction and Stannard, 1995) will be used based on the distributive approach of the Hydrological Response Units (HRUs). After the devastating flood disaster of August 2002 in Ger- The prognostic uncertainty calculated for the hourly pre- many, the Czech Republic, and Austria, many activities in cipitation and temperature forecast values will be used as σ- the field of flood protection were launched. borders in the operational working of a prognostic discharge This event showed clearly that early warning systems are model for the TEG of Grafinau-Angstedt.¨ the most effective instrument to prevent disasters due to the generation of a valuable gain in time for far-sighted ac- tion (URL[1]). New impetus was also given to the pre- Correspondence to: K. Pfannschmidt ([email protected]) 114 K. Pfannschmidt and H. Dohler:¨ Studies on data uncertainties • Fig. 1. Ground level andFig. selected 1 stationsGround ( ) with level catchment and boundaryselected of thestations Ilm (–). (•) with catchment boundary of the Ilm (-) 2 The local model of the German Meteorological Ser- tal climatic influences on the other hand (Thuringian Basin) vice (DWD) The mean annual temperatures(Bongartz, are 2001). around 5° C in the Thuringian Forest, 7° C in the Ilm- The mean annual temperatures are around 5◦C in the The LM is an operational,Saale-Ohrdruf hydrodynamical weatherzone forecastand 8° ThuringianC in the Forest, Inner-Thuringian 7◦C in the Ilm-Saale-Ohrdruf lowland. zone andThese differences are caused model solving a prognostic equation for the vertical move- 8◦C in the Inner-Thuringian lowland. These differences are 2 ment. A square grid composedby the ofdifferent elementary areas altitudes of 49 km andcaused windward/lee by the different effects altitudes and with windward/lee the prevailing effects south-west winds in the covers the whole arealow of Central mountain Europe. (URL[2]) range. The With Lo- regardwith the prevailingto precipitation, south-west winds the in study the low mountainregion has to be subdivided into a cal Model forecast started in November 1999. The model is range. With regard to precipitation, the study region has to be able to resolve weather-relevantlow mountain local landscape and details a summer and subdivided rain into type. a low mountainIn the and cool a summer and rain damp type. In zones of the ridge of the small-scale meteorological structures. Additionally to the the cool and damp zones of the ridge of the Thuringian For- horizontal and verticalThuringian wind components, Forest, pressure andprecipitation soil est, precipitation of up of to up to1200 1200 mm/year mm/year is reached, is on reached, the lee on the lee side of the moisture, the LM provides air temperature values and hourly side of the Thuringian Forest in the direction of the central liquid and solid precipitationThuringian totals. In Forest this connection in the 48- directionThuringian of Basin the the central annual precipitation Thuringian is below 500Basin mm the annual precipitation is hour model calculationsbelow are carried 500 out mm that provide (Fig. current 2). The(Fig. mean 2). The meanannual annual snow snow cover cover depth isdepth around 400–is around 400-500 mm in the forecast data, refreshed every 12 h. These forecast data shall 500 mm in the Thuringian Forest – a region with abundant form the basis of a model-basedThuringian operative Forest discharge - a predic- regionsnowfall with andabundant mostly with snowfall settled snow conditionsand mostly – around with settled snow conditions tion with the aim to detect looming flash-floods and to pro- 100–300 mm in the mountain foothills and <100 mm in the vide adequate information-, around for early warning100-300 systems. mm in theThuringian mountain Basin – foothills an area with littleand snowfall<100 (Bongartz,mm in the Thuringian Basin - an area with little snowfall. [Bong01]2001). 3 The river catchment area of the Ilm As prototypical river catchment area (FEG) the Ilm river 4 Analysed parameters and investigation period catchment in Thuringia, in particular the partial catch- ment area (TEG) of Grafinau-Angstedt¨ located in the upper The prognostic LM data were analysed with regard to their reaches, has been selected. statistical quality for the catchment area of the Ilm river in With a flowing length of about 135 km, the Ilm is a rela- Thuringia. The main focus was put on precipitation and air tively small river draining an area of about 1025 km2. temperature as main input parameters for hydrological mod- The difference in altitude between the source and the els. The investigations covered the period from the beginning mouth is 848 m (see Fig. 1). The mean annual discharge of the LM forecast in November 1999 to December 2003. of the Ilm is 6.59 m3/s. From the climatic point of view, For the study area, a representative selection – concerning Thuringia is located within the temperate zone of transition, the spatial distribution – of LM grid cells to be analysed was characterized by the maritime coastal climate with Atlantic determined, taking into consideration the measurement sta- influences on the one hand (Thuringian Forest) and continen- tions available. Fig. 2 Mean annual precipitation in the Ilm catchment area with significant LM-grid cells Analysed Parameters and Investigation Period The prognostic LM data were analysed with regard to their statistical quality for the catchment area of the Ilm river in Thuringia. The main focus was put on precipitation and air temperature as main input parameters for hydrological models. The investigations covered the period from the beginning of the LM forecast in November 1999 to December 2003. For the 3 Fig. 1 Ground level and selected stations (•) with catchment boundary of the Ilm (-) The mean annual temperatures are around 5° C in the Thuringian Forest, 7° C in the Ilm- Saale-Ohrdruf zone and 8° C in the Inner-Thuringian lowland. These differences are caused by the different altitudes and windward/lee effects with the prevailing south-west winds in the low mountain range. With regard to precipitation, the study region has to be subdivided into a low mountain and a summer rain type. In the cool and damp zones of the ridge of the Thuringian Forest, precipitation of up to 1200 mm/year is reached, on the lee side of the Thuringian Forest in the direction of the central Thuringian Basin the annual precipitation is below 500 mm (Fig.

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