Dynamics of thermally-driven upslope winds

Thermally-driven slope winds are mesoscale atmospheric circulations, known as breezes, that take place because of the heating (cooling) of the air layer close to the ground during daytime (nighttime). Mostly known to occur on days with weak synoptic forcing and under clear sky conditions, the wind blows up valleys and slopes during the daytime, and in the opposite direction during nighttime. A better comprehension of slope winds can improve the understanding of the soil-atmosphere turbulent exchange processes and of the energy budget over complex terrain, in addition to the evaluation of the along-slope transport of dangerous species (pollutants, pesticides), as well as water vapor (relevant for the development of convection). This research project aims to improve the knowledge of thermally-driven slope winds, with particular attention to the differences between the diurnal and nocturnal regimes. This is done through a multiple-way approach. Field data analysis, analytical solutions with realistic forcing, and numerical models are all employed to fulfill the objective. At first, data from two stations located on slopes were analyzed. Measurements were taken in the surroundings of the Alpine city of Innsbruck, as part of the i-Box field campaign, covering a period of 7 years (2013 to 2020). Observation indicates a marked seasonality of the phenomena, with warm season months being more prone to the occurrence of slope winds. Moreover, the results highlighted the key role played by the local topographical characteristics in the development of pure slope wind days, with both slope angle and orientation playing a major role in the interplay between valley and slope winds. Previous results suggested the development of an improved analytical model which uses the available net radiation at the surface as the forcing for slope circulations, in the form of a truncated Fourier series expansion. The net radiation model accounts for both the seasonality (day of the year) and the local topographic characteristics (latitude, slope angle, orientation, elevation). Therefore, differences in the properties of slope winds occurring in different seasons and on slopes with different slope angles and orientations are highlighted and studied. The last chapter of the thesis investigates the structure of the eddy viscosity and diffusivity employing numerical models. These parameters govern the mass, momentum, and heat turbulent exchanges from slope winds. A simple one-dimensional model was developed to test different turbulence closures. In particular, the attention focused on the so-called K-l closure, meaning that the eddy viscosity and diffusivity parameters are bounded to the turbulence length scale l, representing the distance a turbulent eddy can travel “carrying” heat, momentum, and mass. In the current work, different parameterizations of the turbulence length scale l are tested and compared. Results show how simple K-l closures are compared with other non-constant K profiles proposed in the literature for the case of katabatic winds. Nevertheless, such simple parameterizations for the turbulence length scale l still fail to properly discriminate between the daytime and nighttime regimes of slope winds.