Extreme rainfall in sight

"Land under" or bone dry

Flood wall on the K lner Rhine promenade DLRG
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Rainfall is not only an important part of the global water cycle but also the most important weather element for the living world. Scientists from the German Meteorological Institutes are working hard to improve the prediction of extreme precipitation and its spatial distribution. This is important both for disaster risk management during floods and for the accuracy of climate models.


Together with the air temperature, the precipitation height determines the natural appearance of the earth's surface and the survival in the plant and animal world. Contrary to subjective perception, however, the time during which precipitation falls is comparatively short in all climatic zones of the earth. In central Europe, for example, the statistical average only produces rainfall every twentieth minute - but with strong differences in intensity and spatial distribution.

As part of the DFG Priority Program "Quantitative Precipitation Forecasts", scientists from the German Meteorological Institutes are investigating the conditions for the formation of extreme precipitation. By means of atmospheric measurements and the comparison with historical observation series, the parameters for the maximum regional precipitation heights are to be determined. The first results for the Erzgebirge, for example, show that in comb layers, they can fall more than 500 millimeters within 24 hours. This is far more than previously thought.

River basins crucial

Such calculations are needed, for example, to be able to improve disaster risk management in the event of impending floods. Because only with precise knowledge of the precipitation intensities in river catchment areas can reliable construction and protection works be built. Their height and security are so far oriented on the so-called events of the century, which can be derived with justifiable uncertainty from the existing series of observations. In addition, according to widespread opinion, they represent a residual risk that the user still finds "bearable." Display

Regardless of the level of this threshold, however, there are regionally extremely precipitous precipitations, which are much higher than in a centenary event. However, the amount of potential precipitation can not increase immeasurably, but is almost equally limited by the regional conditions of topography and climatic conditions. Therefore, it is possible and also necessary to determine the maximum possible precipitation per day for each river basin. Only in this way can the models for flood protection be effectively improved.

Precipitation Measurement

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Methodologically, there are two ways to determine the maximum precipitation height for a region. On the one hand, data and information from the pre-instrumental period serve as an important basis for orientation. In this way, the length of the purely instrumental series of observations can be significantly extended in Central Europe from about 150 years to more than 500 years. The analysis of such data has already yielded a number of important results on trends in extreme precipitation events for Central European large rivers.

Another method is to use the atmosphere models. The calculation is based on long-term observations of individual parameters such as temperature, humidity and wind speed in a three-dimensional structure. To what extent the combination of these parameters influence the maximum precipitation levels is the focus of the current study.

(Gerd Tetzlaff (University of Leipzig), 16.09.2005 - AHE)