Hailstorm in Eastern Croatia - analysis and forecasting potential by convection-permitting numerical model

Warm seasons of 2021 was quite exceptional if we consider the hail occurrence in the continental part of Croatia which attracted a lot of public attention due to the announced changes in the Law on the Hail Suppression System. One of the events that sparked a great debate in the public occurred on 25 June near the town of Požega due to hail size and damage, although this event was very well forecasted, and orange warning for thunderstorm was issued one day in advance.

This work aims to explore the atmospheric conditions present during this hailstorm but also a forecasting potential of such events using the convection-permitting km-scale numerical models.

On that day ahead of an advancing trough that stretched from Scandinavia to the western Mediterranean all necessary ingredients for severe deep moist convection were recognized. Convective initiation started first in Bosnia and Herzegovina ahead of the surface cold front. For damage in the vicinity of town Požega, most important were convective cells that were advected into the area of Slavonian Posavina. They experienced "explosive" development, and then moved in the southwestern and southern flow across western Slavonia. The estimated convective available potential energy of the air parcel raised from the ground to the level of free convection (SBCAPE) in the Zagreb area, based on radiosounding at 12 UTC was about 2700 J/kg and the deep layer wind shear was estimated at 20-25 m/s, which favored the formation of a supercell thunderstorm.

Here, convection-permitting km-scale WRF model is utilized to inspect the ability of the WRF model to reproduce the atmospheric conditions leading to formation and evolution of an extremely damaging hailstorm. Additionally, the ability of HAILCAST and Lightning Potential Index (LPI) diagnostics to reproduce the main characteristics of observed hail and lightning is explored. HAILCAST is a one-dimensional hail growth model that forecasts the maximum hail diameter at the ground. Similarly, LPI highlights the areas with the potential for developing lightning activity. Moreover, the sensitivity experiments are performed to investigate the impact that convection parameterization has on the simulated timing and characteristics of convection.

The results reveal that the WRF model, when run at the km-scale resolution (1.5 km) can reproduce the synoptic and mesoscale conditions present during an extremely damaging hailstorm occurring over Croatia. Moreover, HAILCAST and LPI diagnostics reproduce hail and lightning characteristics comparable to those observed.