[Truncated abstract] Observations are presented from Lake Biwa (Japan) and Lake Kinneret (Israel) showing the ubiquitous and often periodic nature of high-frequency internal waves in large stratified lakes. In both lakes, high-frequency wave events were observed within two distinct categories: (1) Vertical mode one solitary waves with wavelength ˜100-500 m and frequency near 103 Hz and (2) sinusoidal vertical mode one waves with wavelength ˜5-30 m and frequency just below the local maximum buoyancy frequency near 102 Hz. The sinusoidal waves were associated with shear instability and were shown to dissipate their energy sporadically within the lake interior. Conversely, the solitary waves were found to be capable of propagating to the lake perimeter where they may break upon sloping topography, each releasing ˜1% of the total basin-scale internal wave energy to the benthic boundary layer.

[Truncated abstract] Internal waves redistribute energy and momentum in stratified lakes and constitute the path through which the energy that is introduced at the lake scale is cascaded down to the turbulent scales where mixing and dissipation take place. This research, based on intensive field data complemented with numerical simulations, covers several aspects of the energy flux path ranging from basin-scale waves with periods of several hours to high frequency waves with periods of few minutes. It was found that, at the basin-scale level, the horizontal shape of the lake at the level of the metalimnion controls the period and modal structure of the basin-scale natural modes, conforming to the dispersion relationship of internal waves in circular basins. The sloping bottom, in turn, produces local intensification of the wave motion due to focusing of internal wave rays over near-critical slopes, providing hot spots for the degeneration of the basin-scale waves due to shear instabilities, nonlinear processes and dissipation. Different types of high-frequency phenomena were observed in a stratified lake under different forcing conditions. The identification of the generation mechanisms revealed how these waves extract energy from the mean flow and the basin-scale waves. The changes to the stratification show that such waves contribute to mixing in different ways . . . Detailed field observations were used to develop a comprehensive description of an undocumented energy flux mechanism in which shear-instabilities with significant amplitudes away from the generation level are produced in the surface layer due to the shear generated by the wind. The vertical structure of these instabilities is such that the growing wave-related fluctuations strain the density field in the metalimnion triggering secondary instabilities. These instabilities also transport energy vertically to the thermocline where they transfer energy back to the mean flow through interaction with the background shear.

This doctoral thesis provides the first comprehensive study on the physical processes controlling hydrodynamics and transport in Lake Garda. The investigation is carried out in parallel on three different levels: data collection and analysis, three-dimensional numerical modeling and theoretical study. On the first level, data are collected by building up a network of research institutes and local administrations in the lake area. New data are acquired through traditional field campaigns (CTD, thermistor chains, satellite imagery), while a citizen-science approach, based on local knowledge harvesting, is successfully tested to gather qualitative data on surface circulation. On the second level, a three-dimensional modeling chain is set up, by coupling one-way a mesoscale atmospheric model to a hydrodynamic model. Both models are validated on multiple temporal and spatial scales, allowing to identify the main interactions between the weather forcing and the hydrodynamic response of the lake. Circulations in Lake Garda are found to be very sensitive to the thermal stratification, to the spatial distribution of the wind forcing and to the Earth’s rotation. Surface cyclonic gyre patterns develop in the lake as a residual outcome of alternating wind forcing of local breezes and differential acceleration induced by Earth’s rotation, whereas unidirectional currents flow under a nearly uniform and constant wind. Both model and observations evidences show that, under weak thermal stratification, Ekman transport activates a secondary circulations in the northern part of the lake, driving surface water to the deep layers and possibly preconditioning the lake for subsequent buoyancy-driven deep mixing events. On the third level, the relevance of the Coriolis term in the equations of motion for relatively narrow closed basins is analytically addressed. The classical Ekman problem is solved by including the presence of lateral boundaries and a new analytical solution is formulated. The validity of the new solution is proved by numerical tests of idealized domains of different size, geographical location and turbulent regime, and on Lake Garda as a real test case. The meaningful length scales are discussed, and the significance of Rossby radious as a reference horizontal scale is disproved for steady-state circulations driven by wind and planetary rotation.