Pre-storm environments and storm-scale properties of the major hailstorms of 2021 and 2022 in Europe.

Both 2021 and 2022 broke records in terms of the amount of large (≥ 2 cm) and very large (≥ 5 cm) hail reports across Europe. 24 June 2021 featured the highest number of large hail reports per day (860) in the history of the European Severe Weather Database and giant (≥ 10 cm) hail was reported in three countries. In 2022, the insured damage exceeded € 4 billion in France alone while 215 people were injured that year. Furthermore, the Catalonian hailstorm on 30 August 2022 caused the first direct hail fatality in Europe since 1997.  

In this work, we studied storm-scale processes of severe hailstorms and their larger-scale environment in relation to the maximum observed hail diameter and hailstorm lifetime. The storm-scale properties include storm type, the occurrence of storm mergers, and the type of storm dissipation.  The larger-scale environment was addressed using CAPE-shear parameter space, hodograph properties (shape, longest segment in the hodograph, and storm-relative winds), and the presence of boundaries near the storms. We selected the most impactful hailstorms of 2021 and 2022, all of which featured very large hail and caused considerable damage to property or agriculture, or caused injuries. 79 hailstorms were selected from both years, spanning maximum hail diameters of 5 to 14 cm and hailstorm lifetimes of 10 to 420 minutes.

We found that most hailstorm hodographs had a straight shape with the longest segment between 1 and 3 km and storm-relative inflow typically exceeded 10 m/s. Hodograph properties and the amount of CAPE had no relation to the duration of the hailstorm. Hailstorms forming near boundaries had average lifetimes twice as long as hailstorms forming elsewhere. For hail > 5 cm, CAPE had the strongest correlation with the observed diameter, even higher than the CAPE-shear product. Hodographs suggest that the inflow magnitude into the deviant moving storms stays almost the same (around 10 m/s) for 10 to 22 m/s of 0-6 km bulk shear. In some cases, very large hail occurred in marginally favorable environments only after a storm merger occurred. This shows that storm-scale processes (merger, deviant motion of the storm) and interaction with boundaries can be as important as the background environment.