Using dual-polarimetric radar moments to improve the precipitation estimate over Germany

Deep summertime convection associated with heavy rain and flooding endangers life and property. Radar derived quantitative precipitation estimates (QPE) play an important role as input for hydrological models and flash flood warnings. QPE is also one of the key inputs for nowcasting applications and seamless forecasting products as developed by the SINFONY (Seamless INtegrated FOrecastiNg sYstem)-project of Deutscher Wetterdienst (DWD).

Currently, the QPE at the DWD follows a hydrometeor-based approach, where reflectivity-rainfall rate relationships are selected depending on the results of a hydrometeor classification scheme. Each solid hydrometeor, such as hail, graupel or snow, has its own reflectivity-rainfall rate relationship, while the liquid phase also includes a proxy for convective or stratiform rain depending on the gradient in the reflectivity field.

According to literature, dual-polarimetric radar moments can directly be used to derive QPE. Gorgucci et al., 1994 showed an improvement in rain rate by combining the radar reflectivity and the differential reflectivity (ZDR) to derive the QPE. Chen et al., 2021 favored a combination of reflectivity and the specific differential phase (KDP) to estimate rain rates for the German C-Band radars. While heavy precipitation attenuates the reflectivity and thus leads to an underestimation of QPE, KDP is immune to attenuation. 

We compare these two methods to the current operational algorithm and perform a verification with ground stations (ombrometer) based on hourly precipitation accumulations. The analysis will focus on a case study of the flooding event in Germany in July 2021 and might extent to a full convective season. First results show an improvement in QPE when KDP is used for stronger liquid precipitation. 

References
•    Chen et al., 2021: Assessing the benefits of specific attenuation for quantitative precipitation estimation with a C-band radar network, Journal of Hydrometeorology, 22, 2617–2631.
•    Gorgucci et al., 1994: A robust estimator of rainfall rate using differential reflectivity, Journal of Atmospheric & Oceanic Technology, 11, 586-592