Probability density of displacement and overturning length scales under diverse stratification

[1] Vertical overturns, produced by turbulence in density-stratified lakes and oceans, are often quantified by the Thorpe scale, L-T. The correlation between L-T and the Ozmidov (energy-containing) scale can be used to estimate rates of turbulent dissipation and vertical diffusivities. Based on temperature microstructure measurements from several stratified lakes, the probability density functions (pdf's) of Thorpe displacements and overturning length scales are distinguished and discussed. The stratification was varying over five orders of magnitude between the different data sets, resulting in Thorpe scales between 1 cm and 100 m. It is shown that the analyzed pdf's follow a universal form that can be empirically described by an exponential function and parameterized by a single length scale. The functional form of the pdf of overturning length scales can be related to the inertial subrange, which is found in temperature, as well as in length-scale fluctuation spectra. As a result, the ratio between the maximum displacement length scale, L-max, and the Thorpe scale depends on the rate of turbulent kinetic energy dissipation. Based on the universal pdf of overturning length scales, we show with numerical simulations that L-max is a more appropriate macroscopic length scale than L-T for the estimation of turbulence based on displacements in temperature profiles.

Application of k-epsilon turbulence models to enclosed basins: The role of internal seiches

Alfred Johny Wüest

[1] A numerical model was developed for the prediction of the density stratification of lakes and reservoirs. It combines a buoyancy-extended k-epsilon model with a seiche excitation and damping model to predict the diffusivity below the surface mixed layer. The model was applied to predict the seasonal development of temperature stratification and turbulent diffusivity in two medium-sized lakes over time periods ranging from 3 weeks to 2 years. Depending on the type of boundary condition for temperature, two or three model parameters were optimized to calibrate the model. The agreement between the simulated and the observed temperature distributions is excellent, in particular, if lake surface temperatures were prescribed as surface boundary condition instead of temperature gradients derived from heat fluxes. Comparison of different model variants revealed that inclusion of horizontal pressure gradients and/or stability functions is not required to provide good agreement between model results and data. With the aid of uncertainty analysis it is shown that the depth of the mixed surface layer during the stratified period could be predicted accurately within +/-1 m. The sensitivity of the model to several parameters is discussed.

Amer Geophysical Union2002

Bottom boundary mixing: the role of near-sediment density stratification

Alfred Johny Wüest

The turbulent dynamics and stratification of bottom boundary layers, as well as the net diapycnal buoyancy flux in the deep water, have been observed to vary strongly among lakes. The most relevant parameters governing the different regimes are the bottom current stress and the rate of release of dissolved solids from the sediment. The ratio of boundary-induced mixing to the density flux associated with the flux of ions from the sediment determines whether the bottom boundary layer is extremely stably stratified or well-mixed. The aim of this contribution is (1) to demonstrate these two boundary phenomena, (2) to give a physical criterion for assessing the two mixing regimes, (3) to present a potential model to quantify the boundary-induced buoyancy flux and the basinwide diapycnal diffusivity, and (4) to test the model with data from two representative lakes with significantly different deep-water mixing characteristics.

Am. Geophys. Union1998

Oxygen depletion in Lake Geneva

Robert Vincent Schwefel

Low oxygen concentrations remain a global concern for the ecological health of lakes. High nutrient inputs and climate-induced changes in stratification and mixing are anthropogenic threats which largely impact aquatic oxygen budgets and overall ecosystem health. In this thesis, the relevant processes for hypolimnetic oxygen depletion were investigated on different temporal and spatial scales in the deep perialpine Lake Geneva and the corresponding total oxygen budget was estimated. The short-term variability of sediment oxygen uptake (SOU) and its dependency on the bottom boundary layer currents were investigated using microprofile measurements. Sediment core analyses for reduced substances profiles allowed distinguishing between SOU caused by both oxic respiration and the flux of reduced substances out of the sediment. Long-term monitoring data were used to estimate the relative importance of SOU for the total oxygen depletion in the lake. Finally, one-dimensional numerical models were used to reproduce lake temperature and oxygen concentrations and to assess the impact of changing environments on the oxygen budget of the deep-water. The results of the microprofile measurements led to a new parameterization of turbulent diffusion close to the sediment and enabled a similarity scaling of diffusivity as well as oxygen close to the sediment. However, the comparison of microprofile measurements at different lake depths showed that SOU decreased consistently with depth from ~1 g m-2 d-1 at 40 m to ~0.2 g m-2 d-1 at 133 m independently from the small-scale variability due to hydrodynamic forcing. Similar vertical structures of SOU and total oxygen depletion have been found in other Swiss lakes. The decrease of SOU with depth was attributed to the greater amount of easily degradable organic matter available in the upper layers. The comparison between SOU and the reduced substances flux revealed that oxic respiration is by far the dominant pathway of organic matter mineralization. While the long-term monitoring data did not show a decreasing trend in either the areal hypolimnetic mineralization rate (1.34 g m-2 d-1) or the extent of hypoxia, a strong relationship between deep mixing in winter hypoxic conditions was found. Hence, deep-water oxygen concentrations were predominantly controlled by resupply during the unstratified period in winter. To assess the long-term changes of winter mixing in Lake Geneva, the one-dimensional model SIMSTRAT was used to reproduce lake temperature and stratification between 1981 and 2012 and was run afterwards under atmospheric conditions representative for the years 2045â2076 and 2070â2101, according to the IPPC scenario A1B. The simulations predicted (i) a decrease in winter mixing depth from an average of ~172 m to only ~127 m at the end of this century, and (ii) complete homogenization of temperature and oxygen in winter will decrease by ~50%. Hence, changes in mixing may have stronger impact than eutrophication on the deep-water oxygen. A simple oxygen model coupled to SIMSTRAT predicted an increase in hypoxic conditions in the deep part of Lake Geneva by ~25%. Additionally, a detailed oxygen model was developed based on the observational findings of this dissertation which takes the spatial variability of oxygen depletion and its dependency on lake turbulence into account. This model can be generalized to understand and predict climate-induced changes of future oxygen concentrations in other deep lak

EPFL2017

Oxygen depletion in Lake Geneva

Robert Vincent Schwefel

EPFL2017