Bora wind and heavy persistent precipitation: atmospheric water balance and role of air-sea fluxes over the Adriatic Sea

The Adriatic basin is regularly affected by cold, strong and gusty Bora winds blowing from the northeast, especially during winter. These events are characterized by intense air-sea interactions and produce important meteorological effects not only over the eastern Adriatic basin, where Bora originates and attains its maximum intensity, but also downstream over the Italian peninsula where heavy rainfall and snowfall can occur. The present study aims at evaluating the impact of surface fluxes of sensible and latent heat, that characterize air-sea interaction during Bora episode, on wind strength and profiles over the Adriatic Sea, in relation with intense precipitation affecting the Apennines and the Italian coast. High-resolution numerical simulations are used in order to assess the role of surface fluxes in modulating the atmospheric water balance, modifying the thermodynamic characteristic of the boundary layer and, in turn, the dynamics of the orographic flow regime. Results show that while surface evaporation is responsible for a relatively small contribution to the total atmospheric water budget over the Adriatic area, surface fluxes still have a remarkable impact on precipitation via dynamical processes. Both sensible and latent heat fluxes modify the speed, temperature and moisture profiles of the low-level Bora wind, sensibly changing the dynamical characteristics of interaction of the flow with the downstream orography. The orographic flow regime determines the intensity and location of orographic-induced uplift and hence precipitation. Therefore, the picture that directly associates the precipitation amount upstream and over the Apennines with the degree of moistening of air during its passage over the sea and with orographic uplift is shown to be too simplistic. The variations of the wind speed and static stability due to surface fluxes involve complex and non-linear effects, changing the flow regime in response to the orographic forcing and thus determining amount and location of heavy precipitation.