Large-eddy simulation of wind-driven circulation in a peri-alpine lake: implications of complex surrounding orography and thermal stratification on hydrodynamics of Lake Ledro

In the present thesis I investigate the wind-driven circulation and mixing in a peri-alpine lake (Lake of Ledro) located in Trentino Alto Adige (Italy). It represents an interesting case study because of two main reasons: it is surrounded by mountains, which affect wind distribution and consequently produce a non homogeneous wind stress of the lake surface; in recent years the lake has suffered for intense blooms of the harmful cyanobacterium \textit{Planktothrix Rubescens}, occurring in the winter season, characterized by the absence stratification. The study is carried out numerically, using a high-resolution, eddy resolving Large Eddy Simulation (LES) model (LES-COAST). The model resolves directly the large scales of motion and parametrizes the small ones by means of the Anisotropic Smagorinsky Model (ASM). The complexity of the coastline and bathymetry is treated by the Immersed Boundary Method which, combined with a curvilinear grid, provides a feasible tool for the study of the hydrodynamics of semi-closed or closed areas like a peri-alpine lake. To the best of author's knowledge this work represents the first application of a LES model to an actual lake as whole, considering its shape and its dimensions. Two types of boundary condition for the time changing wind stress are used: spatially homogeneous and non-homogeneous. The former is reproduced by applying to the lake's surface the wind velocity and the direction measured by a floating meteo station located in the center of the lake during the week 16-22 of January 2012, whereas the latter is obtained by the regional scale Weather Research and Forecasting model (WRF) for the same week, making a first attempt of coupling air-water models. The analysis of the wind-driven circulation, as well as of the high-order statistics, has been performed with the aforementioned wind conditions. The analysis shows the presence of downwelling/upwelling areas along the windward/leeward coastline respectively, providing an explanation of the bloom of cyanobacterium at the lake surface observed under winter conditions. Moreover, peculiar superstreaks, which span over the entire depth of the lake, have been simulated, enhancing the vertical mixing in the water column. The bottom boundary layer has been studied in both unstratified and stratified case. The role of the Coriolis force in such a small lake is highlighted, together with the effect of the simplification of the computational domain with respect to the actual geometry. The eddy viscosities values have been computed and they have shown, among other analysis, the presence of a minimum stress plane which has been confirmed by the study of the Turbulent Kinetic Energy. The evolution in time of the TKE and its dissipation rate in both wind regimes indicate the presence of equilibrium turbulence even under the unsteady conditions herein studied, moreover it provide an estimation of phase lag of the two quantities between the surface and the bottom of the lakes. Under wind inhomogeneity, turbulent mixing appears enhanced with respect to the simplified homogeneous wind case. In presence of thermal stratification the eddy viscosities, the turbulent kinetic energy and TKE dissipation rate are smaller with respect to the non stratified case, and evidences of the presence of the BBL in the stratified case are provided. Convergence and divergence areas confined in the surface mixed layer are shown, in particular in presence of strong wind squalls and inhomogeneity they appear aligned with the isotherms. The present thesis has to be considered as a first step toward the study of the hydrodynamics and of the internal waves in stratified lakes with different wind conditions.