Spatial variability of physical properties induced by circulation and stirring remains unaccounted for in the energy pathway of inland waters. Recent efforts in microstructure turbulence measurements have unraveled the overall energy budget in lakes. Yet, a paucity of lake-wide turbulence measurements hinders our ability to assess how representative such budgets are at the basin scale. Using an autonomous underwater glider equipped with a microstructure payload, we explored the spatial variability of turbulence in Lake Geneva. Microstructure analyses allowed turbulent dissipation rates and thermal variances estimations by fitting temperature gradient fluctuations spectra to the Batchelor spectrum. In open waters, results indicate mild turbulent dissipation rates in the surface and thermocline (~10⁻⁸ W kg⁻¹), which weaken towards the deep hypolimnion (~10⁻¹¹ – 10⁻¹⁰ W kg⁻¹). The strong thermal stratification inhibited interior mixing in the thermocline. In contrast, measurements along the coastal slope reveal a notorious enhancement of turbulent dissipation (~5×10⁻⁸ W kg⁻¹) above the sloping topography way above the known extent of the bottom boundary layer. These distinct turbulence patterns result from differing large-scale dynamics in the interior and coastal environments. Current measurements in open waters show dominant internal Poincaré waves. On the coast, three-dimensional numerical results from meteolakes.ch suggest that enhanced bottom dissipations arise from the development of centrifugal instabilities. A process driven by coastal cyclonic circulation interacting with the sloping bottom reported for the ocean but so far overlooked in large lakes. The spatially-distributed turbulence measurements we report here highlight the potential of underwater glider deployments for further lake exploration.

Radiatively-driven convection is a physical process that occurs in freshwater below the temperature of maximum density wherein volumetric heating of surface waters by solar radiation creates a diurnal, spatially distributed, destabilizing buoyancy flux that drives penetrative convection. While this process has typically been studied under ice-covered conditions, it can also occur in open water during springtime warming leading up to overturn, and in such systems, it may serve as the dominant process driving mixing of nutrients and biota. Despite the ecological significance and unique physical dynamics of radiatively-driven convection, little is understood regarding the spatial heterogeneity and three-dimensional structure of the process. The addition of wind shear also modifies radiatively-driven convection dynamics in open water conditions, yet observations have not yet been used to quantify the relative scales and importance of these separate forcings in driving mixing and turbulence. This study examines data collected with a buoyancy-driven autonomous underwater vehicle (aka glider) during a period of active radiatively-driven convection and low surface wind shear in early springtime in Lake Superior. Conductivity, temperature and depth (CTD) measurements reveal distinct convective plumes of anomalously warm downwelling water with width scales on the order of 100 m and temperature anomalies of ~0.1 °C. Shear and temperature microstructure measurements indicate turbulence kinetic energy (TKE) dissipation rates exceeding 10-8 W/kg, orders of magnitude greater than laterally adjacent waters. This is the first known observation of lateral variability in TKE dissipation rates during radiatively-driven convection. Spatially and temporally averaged TKE budgets illustrate buildup, vertical transport, and dissipation of TKE, while the ~3 hr lag between buoyancy forcing and dissipation is consistent with the Deardorff convective timescale. These observations demonstrate that radiatively-driven convection can dominate vertical mixing dynamics even in deep, open water systems.