Thunderstorm formation in the Mediterranean: a cloud modelling sensitivity analysis to SST anomalies

Windstorms constitute nowadays one of the most dangerous hazards worldwide, representing the costliest natural hazard in Europe between 1980 and 2015 and ranking second for overall losses and fourth in terms of the number of human casualties. Among all the extreme phenomena associated to intense deep convective storms, they have the largest impacts on human health, structures, natural environments, as well as transport and energy infrastructures. Local non-synoptic phenomena as tornadoes and thunderstorm outflows are often responsible for the strongest winds, and according to IPCC projections these episodes are expected to further worsen soon in many parts of the world because of global warming. The warming of the Earth’s surface temperature, indeed, is expected to enhance in the future the convective activity at the base of thunderstorms, and therefore some studies project increasing intensity, for example in the Mediterranean region, with a forecast trend of more frequent and more severe synoptic and convective windstorms. However, given the great unpredictability of thunderstorm events, the study of their behaviour is still a challenge, especially from a numerical point of view. In this context, a study based on the explicit cloud modelling of thunderstorms is performed to investigate the mechanisms that underlie deep convective development and intensification in the Ligurian Sea, which is one of the areas in the Mediterranean most prone to severe convective episodes, particularly during the autumn. High-resolution simulations, performed using the Bryan Cloud Model Version 1 (CM1), are focused on the intense thunderstorm event and its associated downburst that hit the city of Genoa in the morning of the 14th of August 2018. The processes responsible for its development and that triggered its intensification are studied, as well as the forcing due to the complex topography, which is thought to have played a role in the enhancement of the thunderstorm outflow at the ground. Preliminary results show some discrepancies between idealized simulations without and with the orography of this area, in terms of intensity and duration of the event. Additionally, the possible role of the sea surface temperature (SST) discontinuities in the convection triggering, through the downward momentum mixing (DMM) mechanism, is investigated as a contributing factor to the thunderstorm development, together with the SST anomalies, observed warmer than average during the occurrence of this severe weather episode by satellite measurements.