A Comprehensive 20-Year Evaluation of radar-detected Hail Cell Severity in Germany

Radar technology has been demonstrated to be a reliable remote sensing method for detecting hail occurrence. Nevertheless, the determination of the severity of detected hail cells remains challenging. The tracking algorithm TRACE3D was applied to a 3D radar composite (from 16 single-polarized C-band radars operated by the German Weather Service, DWD) in order to detect potential hail cells at a high level of spatial and temporal homogeneity over twenty years (2005 - 2024). To go a step beyond purely cell tracking, various attributes of each detected cell were utilized as proxys for hail severity: Area and size of high reflectivity values, echotop height, and integrated reflectivity in the hail growth area. Additionally, insurance and lightning data were used for a more detailed classification of entire hail streaks in the radar data.

This relatively long dataset of real-time measurements of potential hail-bearing cells and their attributes allows us not only to conduct an in-depth examination of climatologies with high resolution, but also to carry out severity-dependent trend analyses. Furthermore, the relationship between the frequency and intensity of the tracks and various environmental parameters well-known to be highly relevant for hail occurrence, including Convective Available Potential Energy (CAPE), Lifted Index (LI), and storm-relative helicity (SRH), was analyzed.

Regionally differentiated climatologies for Germany depending on hail severity as well as trend analyses depending on cell attributes are presented for the first time. Those analyses provide deep insight into systematic changes in hail formation, especially in the context of climate change.