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In ice-covered lakes, penetrative radiation warms fluid beneath a diffusive boundary layer, thereby increasing its density and providing energy for convection in a diurnally active, deepening mixed layer. Shallow regions are differentially heated to warmer temperatures, driving turbulent gravity currents that transport warm water downslope and into the basin interior.We examine the energetics of these processes, focusing on the rate at which penetrative radiation supplies energy that is available to drive fluid motion. Using numerical simulations that resolve convective plumes, gravity currents, and the secondary instabilities leading to entrainment, we show that advective fluxes due to differential heating contribute to the evolution of the mixed layer in waterbodies with significant shallow areas. A heat balance is used to assess the relative importance of differential heating to the one-dimensional effects of radiative heating and diffusive cooling at the ice-water interface in lakes of varying morphologies.

In stratified natural waters, convective processes tend to form nearly homogeneous mixed layers. However, shear‐driven turbulence generated by large‐scale background flow often rapidly smooths them through mixing with the stratified surroundings. Here we studied the effect of background turbulence on convectively driven mixed layers for the case of bioconvection in Lake Cadagno, Switzerland. Along with microstructure measurements, a diffusive‐shape model for the mixed layers allowed us to define (i) mixed layer thickness and (ii) diffusive transition length. Further microstructure analysis was performed allowing estimation of convective turbulence in the mixed layer and shear‐driven turbulence quantified by eddy diffusion in their surroundings. Based upon these results, we propose a Péclet number scaling that relates mixed layer shape to the opposing effects of convection and diffusion. We further validate this quantitative approach by applying it to two other distinct convective systems representative of double‐diffusive convection and radiatively driven under‐ice convection.

Gravity currents are buoyancy-driven flows having a significant impact on the environment and human life. They can be observed in a vast range of natural and anthropogenic scenarios, such as seawater and freshwater, the atmosphere, or industrial processes. Gravity currents transport sediments, nutrients, and oxygen in water bodies but they are also often dramatically involved in the spreading of contaminants and can represent a severe hazard to humans and infrastructures. The relevance of these phenomena has motivated extensive research on gravity currents such as the present study which addresses two main different research objectives. The first objective of this research concerns the modeling of gravity currents with Shallow Water Equations (SWE). Large Eddy Simulations data of continuously-fed gravity currents are used to discuss how the chosen definition of current depth affects the value of the coefficients appearing in the SW continuity and momentum equations due to the non-uniformity of the vertical distribution of flow variables. Moreover, the implications that different definitions of current depth have on research findings and entrainment parameterization are discussed. This analysis showed that coefficients appearing in the momentum equation might significantly (up to $30\%$) differ from one, depending on the chosen current depth definition. On the contrary, the coefficients appearing in the continuity equation were shown to be very close to one. The second main research objective was to study how the mean and turbulent structure of a density current changes when it propagates over a porous substrate and to characterize the volume exchanges occurring at its lower interface. Experiments were carried out in the Laboratory of Hydraulic Constructions at EPFL to reproduce continuously-fed density (saline) currents, which propagated first over a smooth horizontal bed and then over a porous substrate of limited length. Inflow discharge, initial excess density, substrate porosity, and downstream confinement of the substrate were systematically varied. An image analysis technique was developed to monitor the flow density evolution above the substrate and within its pores. Simultaneously, an Acoustic Doppler Velocimetry Profiler acquires a quasi-instantaneous 3D vertical velocity profile over the substrate. Experimental results showed that, after an initial phase during which the current sinks into the substrate, freshwater entrainment from the substrate pores starts. Consequently, a mixing layer gradually forms at the lower boundary of the current. Its presence affects the mean density, mean velocity, and turbulent field primarily in the near-bed region, where buoyancy contributes to turbulent kinetic energy generation together with shear. Moreover, at the porous substrate interface, buoyancy instabilities enhance the bed-normal momentum flux, increasing the bulk flow resistance. As a consequence, although the no-slip boundary condition results in lower friction losses, the shear velocity increases with the porosity of the substrate. In addition, experimental measurements allowed to quantify the current volume loss and the ambient-fluid entrainment from the substrate pores. The main drivers of these exchanges are discussed. The study of the entrainment of ambient-fluid from the bottom is fundamental to understand and model the resuspension of substances, such as nutrients and pollutants, present in the porous substrates.