Boundary mixing in lakes: 1. Modeling the effect of shear-induced convection

Becherer, Johannes; Umlauf, Lars

doi:10.1029/2011jc007119

Cited by 37 publications

(65 citation statements)

References 69 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…When calculated using the Shih et al (2005) parametrization, the average diffusivity for the bottom 10 m is K Shih = 6.5(5.6, 7.6) × 10 −5 m 2 s −1 , i.e., nearly an order of magnitude lower than when using Γ = 0.2. Previous studies have suggested such substantial reduction of the mixing efficiency (Γ ≪ 0.2) for energetic mixing near bottoms having slopes similar to that of our sampling site (Umlauf and Burchard, 2011;Becherer and Umlauf, 2011). However, these studies also predict considerably reduced stratification above the seafloor which is not the case in the present observations (Figure 9b suggest a slight increase of the stratification in the bottom 5 m).…”

Section: Mean Turbulent Quantitiessupporting

confidence: 76%

Behavior and mixing of a cold intermediate layer near a sloping boundary

2015

View full text Add to dashboard Cite

As in many other subarctic basins, a cold intermediate layer (CIL) is found during ice-free months in the Lower St. Lawrence Estuary (LSLE), Canada. This study examines the behavior of the CIL above the sloping bottom using a high resolution mooring deployed on the northern side of the estuary. Observations show successive swashes/backwashes of the CIL on the slope at a semi-diurnal frequency. It is shown that these upslope and downslope motions are likely caused by internal tides generated at the nearby channel head sill. Quantification of mixing from 322 turbulence casts reveals that in the bottom 10 m of the water column, the timeaverage dissipation rate of turbulent kinetic energy is ǫ 10 m = 1.6 × 10 −7 W kg −1 , an order of magnitude greater than found in the interior of the basin, far from boundaries. Near-bottom dissipation during the flood phase of the M 2 tide cycle (upslope flow) is about four times greater than during the ebb phase (downslope flow). Bottom shear stress, shear instabilities and internal wave scattering are considered as potential boundary mixing mechanisms near the seabed. In the interior of the water column, far from the bottom, increasing dissipation rates are observed with both increasing stratification and shear, which suggests some control of the dissipation by the internal wave field. However, poor fits with a parametrization for large-scale wave-wave interactions suggests that the mixing is partly driven by more complex non-linear and/or smaller scale waves.Frédéric Cyr Institut des sciences de la mer de Rimouski, Université du Québecà Rimouski, Rimouski (Québec) Canada.

show abstract

Section: Mean Turbulent Quantitiessupporting

confidence: 76%

Behavior and mixing of a cold intermediate layer near a sloping boundary

2015

View full text Add to dashboard Cite

show abstract

“…Observations of shear-driven convection in lakes stem from two different single point observations in Lake Alpnach [17] and Lake Constance [4]. The results from these point measurements are consistent with analysis of numerical simulations of shear-driven convection [18], [19], [23]. Nonetheless, to date there are only a few field observations in lakes leaving it unclear as to the relative importance of shear-induced convection versus breaking of NLIW in energizing benthic BBLs.…”

Section: Introductionmentioning

confidence: 57%

“…This process involves the differential advection of stratified waters near a sloping boundary during up- and downwelling events due to internal seiching [4], [17], [18], [19]. During periods of upslope flow, a velocity gradient forms near the boundary, so that away from the boundary layer water is moving faster.…”

Section: Introductionmentioning

confidence: 99%

“…This process is termed as shear-induced convection [17]. During downslope flow, lighter fluid is preferentially advected over dense fluid leading to an enhanced stratification [19] which can restabilize the BBL [17], [19], [20]. Thus shear-induced convection can cause a strong asymmetry in the level of BBL turbulence and stratification between up- and downwelling phases of the internal seiche.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

The Interaction of Large Amplitude Internal Seiches with a Shallow Sloping Lakebed: Observations of Benthic Turbulence in Lake Simcoe, Ontario, Canada

Cossu

Wells

2013

PLoS ONE

View full text Add to dashboard Cite

Observations of the interactions of large amplitude internal seiches with the sloping boundary of Lake Simcoe, Canada show a pronounced asymmetry between up- and downwelling. Data were obtained during a 42-day period in late summer with an ADCP and an array of four thermistor chains located in a 5 km line at the depths where the thermocline intersects the shallow slope of the lakebed. The thermocline is located at depths of 12–14 m during the strongly stratified period of late summer. During periods of strong westerly winds the thermocline is deflected as much as 8 m vertically and interacts directly with the lakebed at depth between 14–18 m. When the thermocline was rising at the boundary, the stratification resembles a turbulent bore that propagates up the sloping lakebed with a speed of 0.05–0.15 m s−1 and a Froude number close to unity. There were strong temperature overturns associated with the abrupt changes in temperature across the bore. Based on the size of overturns in the near bed stratification, we show that the inferred turbulent diffusivity varies by up to two orders of magnitude between up- and downwellings. When the thermocline was rising, estimates of turbulent diffusivity were high with KZ ∼10−4 m2s−1, whereas during downwelling events the near-bed stratification was greatly increased and the turbulence was reduced. This asymmetry is consistent with previous field observations and underlines the importance of shear-induced convection in benthic bottom boundary layers of stratified lakes.

show abstract

“…We consider, however, that in view of the near-rectilinear configuration of Windermere that our measurements were reasonably representative of the physical processes operating in a large proportion of the central area of the lake, but future investigations should consider additional observations to account for the spatial variability that may exist in the velocity structure (Hodges et al 2000;Rueda et al 2003;Appt et al 2004;Becherer and Umlauf 2011). Given these extensions of the measurements, there are good prospects for better determinations of energy budgets and consequent improvements in our understanding of lake and shallow sea processes.…”

Section: Summary and Discussionmentioning

confidence: 93%

Energy input and dissipation in a temperate lake during the spring transition

Woolway

Simpson

2017

Ocean Dynamics

View full text Add to dashboard Cite

ADCP and temperature chain measurements have been used to estimate the rate of energy input by wind stress to the water surface in the south basin of Windermere. The energy input from the atmosphere was found to increase markedly as the lake stratified in spring. The efficiency of energy transfer (Eff), defined as the ratio of the rate of working in near-surface waters (RW) to that above the lake surface (P 10 ), increased from ∼0.0013 in vertically homogenous conditions to ∼0.0064 in the first 40 days of the stratified regime. A maximum value of Eff∼0.01 was observed when, with increasing stratification, the first mode internal seiche period decreased to match the diurnal wind period of 24 h. The increase in energy input, following the onset of stratification was reflected in enhancement of the mean depth-varying kinetic energy without a corresponding increase in wind forcing. Parallel estimates of energy dissipation in the bottom boundary layer, based on determination of the structure function show that it accounts for ∼15% of RW in stratified conditions. The evolution of stratification in the lake conforms to a heating stirring model which indicates that mixing accounts for ∼21% of RW. Taken together, these estimates of key energetic parameters point the way to the development of full energy budgets for lakes and shallow seas.

show abstract

Boundary mixing in lakes: 1. Modeling the effect of shear-induced convection

Cited by 37 publications

References 69 publications

Behavior and mixing of a cold intermediate layer near a sloping boundary

Behavior and mixing of a cold intermediate layer near a sloping boundary

The Interaction of Large Amplitude Internal Seiches with a Shallow Sloping Lakebed: Observations of Benthic Turbulence in Lake Simcoe, Ontario, Canada

Energy input and dissipation in a temperate lake during the spring transition

Contact Info

Product

Resources

About