Exploring hail and lightning diagnostics over the Alpine-Adriatic region in a km-scale climate model

Abstract. The north and south of the Alps, as well as the eastern shores of the Adriatic Sea, are hot spots of severe weather events, including hail and lightning associated with deep convection. With advancements in computing power, it has become feasible to simulate deep convection explicitly in climate models by decreasing the horizontal grid spacing to less than 4 km. These so-called kilometer-scale or convection-resolving models improve the representation of orography and reduce uncertainties associated with the use of deep convection parameterizations.

In this study, we perform convection-resolving simulations for eight observed cases of severe convective events (seven with and one without observed hail) over the Alpine-Adriatic region. The simulations are performed with COSMO-crCLIM, a climate version of the Consortium for Small-scale Modeling (COSMO) regional model that runs on Graphics Processing Units (GPUs) at a horizontal grid spacing of 2.2 km. For analyzing hail and lightning, we have explored the hail growth model (HAILCAST) and lightning potential index (LPI) diagnostics integrated with the COSMO-crCLIM model.

Comparison with available high-resolution observations reveals good performance of the model in simulating heavy precipitation, hail, and lightning. By performing a detailed analysis of three of the case studies, we identified the importance of significant meteorological factors for heavy thunderstorms that were reproduced by the model. Among these are the moist unstable boundary layer and dry mid-level air, the topographic barrier, as well as an approaching upper-level trough and cold front. Although COSMO HAILCAST tends to underestimate the hail size on the ground, the results indicate that both HAILCAST and LPI are promising candidates for future climate research.