Anything from 10 to 686 km, or what influences the hail swath length in supercells?

Recent years have brought devastating hailstorms across Europe with large economic losses and hundreds of injuries. We study 315 European hailstorm cases from 2021 to 2024 that produced at least two reports of hail 5 cm or larger (i.e., very large), using the European Severe Weather Database, their proximity environments using the ERA-5 reanalysis, and storm motion inferred from the composite radar data (such as OPERA). 

Supercells are often considered by the forecasters to be long-lived storms. However, as evidenced in our study, only 77 out of 315 studied hailstorms produced large hail for at least 100 km, and only 86 lasted at least one hour. All 4 supercells of 24 July 2023 that affected Italy, Slovenia, and Croatia had hail swaths longer than 200 km, including the storm that set the European hail record of 19 cm. On 13 July 2023, a single supercell tracked for at least 686 km and lasted more than 9 hours. Here we look at how the pre-convective environment influences the length of the hail swath, a characteristic less explored in the existing literature compared to the observed hail size.

We first considered the individual 1-D vertical profiles of temperature, moisture, and horizontal wind representing the conditions in the area with the largest observed hailstone per hailstorm. We found that the storms with longer hail swaths formed in environments of higher storm-relative helicity, stronger inflow, lower cloud bases, and weaker cold pool potential. Of the kinematic parameters, observed storm motion was best correlated with the hail swath length.

In the second part of the study, we investigate the spatial distribution of pre-convective environments parallel and perpendicular to the hailstorm path, using the radar-derived storm-motion vectors. We hypothesize that longer-lived hailstorms feature larger areas of favourable environments parallel to the path. We also wonder whether the longer-lived hailstorms stay near low-level boundaries or move away from them to prevent the upscale growth. Results of this study advance our understanding of the controls of the supercell and their hail production maintenance.