The role of atmospheric aerosols on severe convective precipitation in a Mediterranean coastal region

Liguria, located in the Northwest of Italy, is one of the Italian regions hit in the recent past by some of the most severe precipitation events among the ones observed in Italy. From a synoptic point of view, similar configurations characterize the extreme events that affected Liguria, i.e. the simultaneous presence of a deep pressure minimum west of the region and a strong high pressure over eastern Europe. Such conditions are favorable to the triggering of a quasi-stationary mesoscale V-shaped convective system over the Ligurian Sea. Furthermore, this kind of configuration is favorable to the formation and intrusion of wide plumes of aerosol, mainly mineral dust from the Sahara Desert and sea salt aerosols generated under high wind conditions in the Mediterranean basin. The present study aims at evaluating the impact that these aerosol plumes can have on meteorological fields during the triggering and evolution of the deep convective systems responsible for the Liguria flooding events. This study is carried out through numerical simulations performed with the WRF-Chem model, version 4.0. In particular, our main aim is to investigate the influence that the so-called direct (aerosol-radiation) and indirect (aerosol-cloud) interactions may have on mesoscale V-shape convective systems and on the associated rainfall events. In the simulations in which the direct aerosol effects are switched on, a consistent weakening signal is identified in the wind speed in all simulations; a major role seems to be played by the increase of downward longwave radiation in the presence of aerosol, that, by reducing the land-sea temperature gradient, causes the weakening of surface winds. More complex are the consequences of aerosol on the amount/intensity of the resulting precipitations. Unexpectedly, a reduced convergence is not necessarily associated with a weakening of precipitation and in half of the studied cases the interaction between aerosol, moisture and radiation increases the instability despite the reduced dynamical forcing due to the weakening of the convergence.