First climatological analysis of mountain venting using water vapour profiles up-wind and down-wind of the Alps

Mountain Venting is one of the processes that influence the exchange of air including water vapour and pollutants between the polluted boundary layer and the free troposphere. So far, this exchange could only be quantified for some days by intensive field campaign studies as performed for example in the former EU project VOTALP. In those studies, it was found that 3 times the boundary layer air mass within deep Alpine Valleys were injected per day into higher altitude layers (typically 2000 to 4000 masl) above the Alpine area during summer daytime. The amount of boundary layer air in this injection layer was found to be typically around 20-40%. It was suggested that a large fraction of this injection layer would leave the Alpine area and would be incorporated into the lower free troposphere. Only few Lidar observations with elevated aerosol layers down-wind of the Alps supported this hypothesis. In this study we now use 12 years of water vapour sounding data of Payerne, Switzerland, and Milano, Italy to assess the effects of mountain venting in a more climatological manner. We could show that under fair weather conditions, and more generally for the whole summer period, water vapour concentrations were strongly enhanced downwind of the Alps between 2000 and 4000 masl. It can also be shown that the average contribution of boundary layer air in this layer is on average around 30%. This climatological analysis is thus supporting the evidence gained from previous field campaign studies. Various additional quantitative analyses of the mountain venting exchange processes like the seasonality and the dependence on convection can now be discussed. The quantified meteorological processes will finally also influence continental pollution export to a larger scale.