Monitoring the data quality of the new polarimetric weather radar network of the German Meteorological Service

The German Meteorological Service DWD is currently replacing all radar systems with new dualpolarization radars. With the introduction of dualpolarization systems improved quantitative precipitation estimations and a better classification of meteorological and nonmeteorological targets are expected. Along with the radar hardware replacement the project “Radarmassnahmen” has been launched to develop a centralized data processing chain for the dualpol data and subsequent products. The radar processing chain “POLARA” consists of a data quality assurance module, a fuzzy logic hydrometeor classification scheme (HMC) and a quantitative precipitation estimate (QPE) module. An important component of this project is the development of operational data quality monitoring methods including the verification and monitoring of dualpol data quality. Here we focus on the data quality that can be affected by the radar hardware, in particular on the receive and transmit path of the radar signal. Further quality checks and an attenuation correction are implemented in the centralized data processing chain (Tracksdorf et. al. 2013, Steinert et al., 2013). It is well known that dualpol moments must satisfy requirements with respect to accuracy and uncertainty before the full benefit for QPE and HMC can be seen. The key moments we consider are the horizontal reflectivity factor Zh, the differential reflectivity ZDR, the cross correlation coefficient ρhv, the differential phase Φdp, and ∗Corresponding author address: Michael Frech, DWD, Meteorologisches Observatorium Hohenpeissenberg, D-82383 Hohenpeissenberg, Germany. Email: Michael.Frech@dwd.de Extented abstract, AMS Radar conference 2013, Breckenridge, CO, USA. based on latter moment, the specific differential phase Kdp. The accuracy of those moments is affected by various aspects. The sampling strategy to compute the moments has a strong influence on the resulting accuracies (Melnikov, 2004). Therefore the scan strategy must consider the requirements to obtain good radar data quality from a scanning radar (Seltmann et al, 2013). The effect of the radar hardware on the dualpol moments also has to be considered. Since we use the simultaneous transmit and receive mode we have to make sure that the analog signal path from transmit to receive is well characterized and monitored for the horizontal and vertical polarization state. The goal is to achieve an accuracy better than 1 dB in Z and 0.1 dB in ZDR. An important element here is the antenna (Frech et al., 2013). The antenna characteristics in H and V must match very well not only during the acceptance of a system but also during the lifetime of the radar system. For example the feed could defocus which may result into increased side-lobe levels and increased beam-squint. This in turn would affect the clutter suppression performance and the interpretation of differential reflectivity in areas with large reflectivity gradients. So monitoring the transmit and receive path is an important aspect of the overall data quality monitoring. Up to now the following aspects are considered in our monitoring scheme at the radar site: