Hydrodynamics of a periodically wind-forced small and narrow stratified basin: a large-eddy simulation experiment

We report novel results of a numerical experiment designed for examining the basin-scale hydrodynamics that control the mass, momentum, and energy distribution in a daily wind-forced, small thermally-stratified basin. For this purpose, the 3-D Boussinesq equations of motion were numerically solved using large-eddy simulation (LES) in a simplified (trapezoidal) stratified basin to compute the flow driven by a periodic wind shear stress working at the free surface along the principal axis. The domain and flow parameters of the LES experiment were chosen based on the conditions observed during summer in Lake Alpnach, Switzerland. We examine the diurnal circulation once the flow becomes quasi-periodic. First, the LES results show good agreement with available observations of internal seiching, boundary layer currents, vertical distribution of kinetic energy dissipation and effective diffusivity. Second, we investigated the wind-driven baroclinic cross-shore exchange. Results reveal that a near-resonant regime, arising from the coupling of the periodic wind-forcing ($T=24$ h) and the V2H1 basin-scale internal seiche ($T_{V2H1}\approx 24$ h), leads to an active cross-shore circulation that can fully renew near-bottom waters at diurnal scale. Finally, we estimated the bulk mixing efficiency, $\Gamma$, of relevant zones, finding high spatial variability both for the turbulence intensity and the rate of mixing (${10}^{-3}\le \Gamma \le {10}^{-1}$). In particular, significant temporal variability along the slopes of the basin was controlled by the periodic along-slope currents resulting from the V2H1 internal seiche.