Giant hail in Poland produced by a supercell merger in extreme instability – A sign of a warming climate?
Krzysztof Piasecki, Patryk Matczak, Mateusz Taszarek, Bartosz Czernecki, Filip Skop, Adrian Sobisiak
Abstract
Severe thunderstorms that occurred on 11 June 2019 over eastern Germany and western Poland caused severe material losses primarily due to large hail. This work documents a detailed inventorization of 79 large hail reports collected in the densely populated area of Gorzów Wielkopolski in Poland, including the largest with a diameter of 12 cm and heaviest with a weight of 380 g. Evaluation of environmental and Doppler radar data indicate that three unique aspects characterize this event. Firstly, the storm benefited from a favorable convective environment including a record-high CAPE exceeding 4000 J kg−1 that was the highest ever measured value at a proximity rawinsonde station of Lindenberg, and the highest for this region according to ERA5 since 1950. Secondly, moisture pooling along the convergence zone led to convective initiation of 3 isolated cells that merged together. Two of them with already embedded rotation. This merger subsequently evolved into a single powerful mesocyclone that was a main cause of giant hail. Subtle differences in the orography within Warta river valley could have played a role in the merging process. Third, a supercell had the biggest intensity over a densely populated area, which led to considerable material losses, but also allowed collection of a large number of hail reports. Distribution of these reports indicates that peak hail size varied significantly on very small distances. Storm has been producing giant hail for around 20-min over a distance of around 10 km and was preceded by a well-developed velocity couplet, bounded weak echo region and hook-echo signatures on the Doppler radar scans.