Influence of orography on supercells in Slovakia

The spatial distribution of supercells is strongly inhomogeneous across Slovakia with a higher frequency of occurrence in several regions in the eastern part of the country. The main aim of the work was to find reasons for this inhomogeneity. This is done by running high-resolution simulations of the ALARO model with a grid spacing of 1 km for all supercell cases in eastern Slovakia between 2017 and 2020. We studied the evolution of pre-convective environments, as well as the lower tropospheric flow patterns to establish the typical scenarios, in which supercells occur in Slovakia. The most important factor was found to be an interaction of large-scale flow with local orography, which affected a whole range of processes in the troposphere. As a result of the various orographic effects, such as blocking, wrapping, gap wind, lee cyclogenesis, and upslope flow, we have detected zones of enhanced convergence and spatial anomalies of ingredients important for the development of deep moist convection. In most of the analyzed cases, we noted a local increase in the vertical wind shear in the lee of the mountains, which increased the probability of supercell formation. In the cases with the prevailing southerly flow and warm-air advection regime, supercells typically formed to the south and east of the Western Carpathians. Convergence lines and upslope flow initiated convection and local enhancement of vertical wind shear was essential for the genesis of supercells. In the cases with the prevailing northerly flow and cold-air advection, blocking allowed for the maintenance of a warm and humid air mass (with non-zero CAPE values) and a generation of convergence zone on the southern flank of Western Carpathians.