Towards a Radar-Based Precipitation Climatology for Germany — First Results and Future Perspectives

Lately, the number of fire brigade operations linked to flash floods seems to increase in Germany. Especially small-scaled heavy rain events, for example in July 2014 in Münster, issue more and more challenges to policymakers of cities. They often occur on short notice, allow only short response times and demand a fairly good preparation. The preparation includes a carefully considered urban planning and infrastructure design, as well as an elaborate strategy of civil protection. For its cross-cutting character, four federal institutes (Bundesamt für Bevölkerungsschutz und Katastrophenhilfe – BBK, Bundesinstitut für Bau-, Stadt- und Raumforschung – BBSR, Umweltbundesamt – UBA, Technisches Hilfswerk – THW) and the Deutscher Wetterdienst (DWD) started a joint project to perform a homogeneous and quality-controlled high-resolution precipitation reanalysis to evaluate the recent changes in extreme precipitation patterns in Germany since 2001.

Compared to station-based measurements, radar measurements provide a higher spatial resolution, which is needed to perform a detailed investigation on heavy precipitation events. Since 2000, DWD has been operating a national weather radar network, which covers about 98% of the German territory. Today it consists of 17 C-band radar systems, 16 of them with modern simultaneous dual-polarization technology. Furthermore, DWD operates a precipitation analysis algorithm called RADOLAN (Radar Online Adjustment), which combines weather radar data with hourly surface precipitation observations of about 1300 automated rain gauges. The derived data are quality-controlled, high-resolution quantitative precipitation estimation (QPE) products for real-time hydrological applications like flood forecast or water resources management.

Meanwhile, a database, containing 15 years of radar data, has been accumulated, which provides valuable information on short-term climatological questions. In a first step, all radar data have been homogeneously re-processed with the RADOLAN algorithm. In this first version of the radar-based precipitation reanalysis some well-known radar artifacts, for example residual clutter or partial beam blockage, have still remained uncorrected. In the next step, we will pre-process the raw radar data with the new POLARA (Polarimetric Radar Algorithms) software framework comprising a set of about 35 detection and correction algorithms primarily designed for real-time application that can also partly be used on single-polarization data in the reanalysis mode.

After reprocessing, the time series will be examined by descriptive and extreme value statistical approaches. In addition, case studies of particularly hazardous events will be performed blending precipitation data with users' data like water levels of rivers, the number of operations of civil protection units or runoff paths in urban regions. All in all, the project enables a nationwide risk analysis as well as a classification of individual extreme events in terms of the climatological return period and the specific damage potential. Additionally, potential users of the project's results are already involved during early project stages to ensure an ideal application-specific processing of the scientific results.

This paper will give an overview of the project, starting with the re-processing which leads to the data basis for statistical investigations. Thereby, the correction approaches will be presented more detailed and finally application examples will be shown.