Observational and numerical study of a giant hailstorm in Attica, Greece, on 4 October 2019

Abstract

On 4 October 2019, giant hailstones of 11 cm were reported in northern parts of Attica in southern Greece. During the same day, multiple large hail reports of hailstones larger than 3 cm and 5 tornadoes were reported in the European Severe Weather Database along the track of a long lived severe thunderstorm that formed over northeastern Peloponnese and moved northeastwards to Attica and Euboea. In this study, we investigate the synoptic and mesoscale weather conditions that led to this rare event by using upper-air measurements from the Athens International Airport, geostationary satellite retrievals, and reanalysis data. Furthermore, the predictability of this rare event is studied through high-resolution simulations performed with three numerical weather prediction models, which are also used operationally by the National Observatory of Athens. The models were able to reproduce the mesoscale environment associated with these severe weather events, showing a highly unstable environment in Saronic gulf with large amounts of convective available potential energy overlapped by a strong deep layer shear. However, the models were not able to fairly reproduce the triggering, track and timing of the severe thunderstorm formation highlighting the great uncertainties associated with the initiation of deep moist convection over areas with complex terrain. Here, we attempt to constrain these uncertainties by applying a diagnostic tool for predicting hail size using an ensemble of high resolution simulations. Our findings reveal similar hail sizes predicted by the numerical weather prediction models to the observed ones and illustrate the usefulness of such hail size diagnostics when applied to a set of high resolution simulations for early warning systems.