More than 40 000 PV cutoff life cycles are identified and analyzed in the almost 40-year data set. The novel method is applied to the ERA-interim dataset for the years 1979–2017 and the first global climatology of PV cutoffs is presented that is independent of the selection of a vertical level and identifies and tracks PV cutoffs as three-dimensonal features. As it is based on isentropic air parcel trajectories, the detailed evolution of the cutoffs on isentropic surfaces, including their associated cross-tropopause mass fluxes, can be analyzed. To address this gap, a method is introduced that allows tracking PV cutoffs as three-dimensional objects. While climatological frequencies of such upper-level cyclonic vortices are well known, their life cycle and in particular their three-dimensional evolution is poorly understood. The aim of this study is to explore the nature of potential vorticity (PV) cutoff life cycles. In contrast, thermodynamical proxies such as the lifted index or lapse rate show only small differences between the different intensity classes. Using composites of environmental conditions centered around the hailstorm tracks, we found that dynamical proxies such as deep-layer shear or storm-relative helicity become important when separating hail diameters and, in particular, their lengths 0–3 km helicity as a dynamical proxy performs better compared to wind shear for the separation. These events usually develop in a high-shear environment.
Frontal storm tracks associated with hail on average produce larger hailstones and have a longer track. Over complex terrains, such as in southern Germany, the proportion of frontal convective storms is around 10 %–15 %, while over flat terrain half of the events require a front to trigger convection. Our analyses reveal that on average about a quarter of all severe thunderstorms in the investigation area were associated with a front. This event set was further combined with eyewitness reports, environmental conditions, and synoptic-scale fronts based on the ERA-Interim (ECMWF Reanalysis) reanalysis. Around 26 000 severe convective storm tracks between 20 have been estimated from 2D radar reflectivity for parts of Europe, including Germany, France, Belgium, and Luxembourg.
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