Intense air-sea exchanges and heavy orographic precipitation over Italy: the role of Adriatic Sea during Bora and Sirocco events

The occurrence of strong winds over the Adriatic Sea is often associated with heavy precipitation over Italy. When cold, strong and gusty Bora winds blow from the northeast, especially during autumn and winter, heavy rainfall and snowfall can occur in particular over the eastern slopes of the Apennines and central Italy coastal areas. On the other hand, in case of intense moist southeasterly low-level winds (Sirocco), rainfall affects northeastern Italy and the Alpine chain. All these events are characterized by intense air-sea exchanges, due to strong low-level winds or relevant temperature differences between the atmosphere and the sea. High-resolution simulations of different Bora and Sirocco events leading to severe precipitation are performed using a convection-permitting model (MOLOCH). Sensitivity tests and a diagnostic analysis of atmospheric water balance and water vapour fluxes are carried out in order to evaluate the impact of surface fluxes of sensible and latent heat and the role of Adriatic SST in relation with intense precipitation. The sensitivity tests show that the picture that directly associates more intense precipitation upstream and over the orography with increasing surface heat fluxes or increasing SST, as a consequence of the degree of moistening of air during its passage over the sea, is too simplistic. The impact of SST and surface fluxes varies among the events and it mainly involves the modification of the PBL characteristics and thus the flow dynamics and its interaction with the orography, including complex non-linear effects.