scholarly journals Stratified Inertial Subrange Inferred from In Situ Measurements in the Bottom Boundary Layer of the Rockall Channel

2010 ◽  
Vol 40 (11) ◽  
pp. 2401-2417 ◽  
Author(s):  
Pascale Bouruet-Aubertot ◽  
Hans van Haren ◽  
M. Pascale Lelong
Keyword(s):  
Boundary Layer ◽  
Energy Levels ◽  
Deep Ocean ◽  
Energy Dissipation Rate ◽  
Bottom Boundary Layer ◽  
Inertial Subrange ◽  
Buoyancy Frequency ◽  
Bottom Boundary ◽  
Wide Range ◽  
Taylor Hypothesis

Abstract Deep-ocean high-resolution moored temperature data are analyzed with a focus on superbuoyant frequencies. A local Taylor hypothesis based on the horizontal velocity averaged over 2 h is used to infer horizontal wavenumber spectra of temperature variance. The inertial subrange extends over fairly low horizontal wavenumbers, typically within 2 × 10−3 and 2 × 10−1 cycles per minute (cpm). It is therefore interpreted as a stratified inertial subrange for most of this wavenumber interval, whereas in some cases the convective inertial subrange is resolved as well. Kinetic energy dissipation rate ε is inferred using theoretical expressions for the stratified inertial subrange. A wide range of values within 10−9 and 4 × 10−7 m2 s−3 is obtained for time periods either dominated by semidiurnal tides or by significant subinertial variability. A scaling for ε that depends on the potential energy within the inertio-gravity waves (IGW) frequency band PEIGW and the buoyancy frequency N is proposed for these two cases. When semidiurnal tides dominate, ε ≃ (PEIGWN)3/2, whereas ε ≃ PEIGWN in the presence of significant subinertial variability. This result is obtained for energy levels ranging from 1 to 30 times the Garrett–Munk energy level and is in contrast with classical finescale parameterization in which ε ∼ (PEIGW)2 that applies far from energy sources. The specificities of the stratified bottom boundary layer, namely a weak stratification, may account for this difference.

Update on the Global Energy Dissipation Rate of Deep-Ocean Low-Frequency Flows by Bottom Boundary Layer

2018 ◽  
Vol 48 (6) ◽  
pp. 1243-1255 ◽  
Author(s):  
Chao Huang ◽  
Yongsheng Xu
Keyword(s):  
Boundary Layer ◽  
Kinetic Energy ◽  
Dissipation Rate ◽  
Low Frequency ◽  
Deep Ocean ◽  
Power Input ◽  
Energy Dissipation Rate ◽  
Bottom Boundary Layer ◽  
Geostrophic Currents ◽  
Bottom Boundary

AbstractThe global dissipation caused by bottom boundary layer drag is one of the major pathways for the consumption of kinetic energy in the deep ocean. However, the spatial distribution and global integral of the drag dissipation are still debatable. This paper presents an updated estimate of the dissipation rate, using the barotropic component of surface geostrophic currents and 632 in situ velocity measurements. Also, the seafloor roughness is proposed as a parameter of drag efficiency in the parameterized method. The results provide a map of the drag dissipation rate with a global integral of ~0.26 TW. Approximately 66% of this dissipation occurs in the Southern Ocean, which is consistent with the proportion of wind power input into this region. Building upon the work in previous studies on the bottom boundary layer drag, more long-period observations are used, eliminating the influence of the baroclinic contribution to the surface geostrophic currents in the construction of the bottom velocity, and taking topographic roughness into account. The estimates have implications for the maintenance of density structure in the deep ocean and understanding of the kinetic energy budget.

scholarly journals Estuarine Boundary Layer Mixing Processes: Insights from Dye Experiments*

2007 ◽  
Vol 37 (7) ◽  
pp. 1859-1877 ◽  
Author(s):  
Robert J. Chant ◽  
Wayne R. Geyer ◽  
Robert Houghton ◽  
Elias Hunter ◽  
James Lerczak
Keyword(s):  
Boundary Layer ◽  
Richardson Number ◽  
Flood Tide ◽  
Buoyancy Flux ◽  
Bottom Boundary Layer ◽  
Buoyancy Frequency ◽  
Entrainment Rate ◽  
Diapycnal Mixing ◽  
Bottom Boundary ◽  
Shear Production

Abstract A series of dye releases in the Hudson River estuary elucidated diapycnal mixing rates and temporal variability over tidal and fortnightly time scales. Dye was injected in the bottom boundary layer for each of four releases during different phases of the tide and of the spring–neap cycle. Diapycnal mixing occurs primarily through entrainment that is driven by shear production in the bottom boundary layer. On flood the dye extended vertically through the bottom mixed layer, and its concentration decreased abruptly near the base of the pycnocline, usually at a height corresponding to a velocity maximum. Boundary layer growth is consistent with a one-dimensional, stress-driven entrainment model. A model was developed for the vertical structure of the vertical eddy viscosity in the flood tide boundary layer that is proportional to u2*/N∞, where u* and N∞ are the bottom friction velocity and buoyancy frequency above the boundary layer. The model also predicts that the buoyancy flux averaged over the bottom boundary layer is equal to 0.06N∞u2* or, based on the structure of the boundary layer equal to 0.1NBLu2*, where NBL is the buoyancy frequency across the flood-tide boundary layer. Estimates of shear production and buoyancy flux indicate that the flux Richardson number in the flood-tide boundary layer is 0.1–0.18, consistent with the model indicating that the flux Richardson number is between 0.1 and 0.14. During ebb, the boundary layer was more stratified, and its vertical extent was not as sharply delineated as in the flood. During neap tide the rate of mixing during ebb was significantly weaker than on flood, owing to reduced bottom stress and stabilization by stratification. As tidal amplitude increased ebb mixing increased and more closely resembled the boundary layer entrainment process observed during the flood. Tidal straining modestly increased the entrainment rate during the flood, and it restratified the boundary layer and inhibited mixing during the ebb.

scholarly journals Dissipation of Turbulent Kinetic Energy in the Oscillating Bottom Boundary Layer of a Large Shallow Lake

2020 ◽  
Vol 37 (3) ◽  
pp. 517-531 ◽  
Author(s):  
Aidin Jabbari ◽  
Leon Boegman ◽  
Reza Valipour ◽  
Danielle Wain ◽  
Damien Bouffard
Keyword(s):  
Boundary Layer ◽  
Kinetic Energy ◽  
Turbulent Kinetic Energy ◽  
Bottom Boundary Layer ◽  
Inertial Subrange ◽  
Order Structure ◽  
Inertial Method ◽  
Bottom Boundary ◽  
Law Of The Wall ◽  
Order Of Magnitude

AbstractMixing rates and biogeochemical fluxes are commonly estimated from the rate of dissipation of turbulent kinetic energy ε as measured with a single instrument and processing method. However, differences in measurements of ε between instruments/methods often vary by one order of magnitude. In an effort to identify error in computing ε, we have applied four common methods to data from the bottom boundary layer of Lake Erie. We applied the second-order structure function method (SFM) to velocity measurements from an acoustic Doppler current profiler, using both canonical and anisotropy-adjusted Kolmogorov constants, and compared the results with those computed from the law of the wall, Batchelor fitting to temperature gradient microstructure, and inertial subrange fitting to acoustic Doppler velocimeter data. The ε from anisotropy-adjusted constants in SFM increased by a factor of 6 or more at 0.2 m above the bed and showed a better agreement with microstructure and inertial method estimations. The maximum difference between SFM ε, computed using adjusted and canonical constants, and microstructure values was 25% and 50%, respectively. This difference was 30% and 55%, respectively, for those from inertial subrange fitting at times of high-intensity turbulence (Reynolds number at 1 m above the bed of more than 2 × 104). Comparison of the SFM ε to those from law of the wall was often poor, with errors as large as one order of magnitude. From the considerable improvement in ε estimates near the bed, anisotropy-adjusted Kolmogorov constants should be applied to compute dissipation in geophysical boundary layers.

Simulations of coastal upwelling on the Sydney Continental Shelf

2000 ◽  
Vol 51 (6) ◽  
pp. 577 ◽  
Author(s):  
Patrick Marchesiello ◽  
Mark T. Gibbs ◽  
Jason H. Middleton
Keyword(s):  
Boundary Layer ◽  
Coastal Upwelling ◽  
Deep Ocean ◽  
Bottom Boundary Layer ◽  
Return Flow ◽  
Pressure Gradients ◽  
Bottom Boundary ◽  
Eastern Australia ◽  
Flow Through ◽  
Good Agreement

Two-dimensional numerical simulations of the response of the coastal waters of Sydney, south-eastern Australia, to idealized upwelling-favourable winds are presented. The spin up of the upwelling circulation is investigated, in particular the structure of the nearshore circulation. The intensity of the final upwelling state is found to be strongly linked to the activation of the return flow through the bottom boundary layer, which is also related to the strength of imposed alongshore pressure gradients. Results from a simulation of upwelling forced by a deep-ocean alongshore-current jet also show the final upwelling state to be weak in comparison with upwelling states produced by the action of the local wind stress. Bottom boundary layer shut-down in the presence of such a forcing jet is also discussed. A simulation of a real upwelling event was also performed and good agreement was found between the simulation and observations from a field experiment performed during summer 1994 in the Sydney coastal ocean.

scholarly journals Buoyancy Arrest and Bottom Ekman Transport. Part II: Oscillating Flow

2010 ◽  
Vol 40 (4) ◽  
pp. 636-655 ◽  
Author(s):  
K. H. Brink ◽  
S. J. Lentz
Keyword(s):  
Boundary Layer ◽  
Mixed Boundary ◽  
Broad Band ◽  
Bottom Boundary Layer ◽  
Ekman Transport ◽  
Buoyancy Frequency ◽  
Bottom Boundary ◽  
Bottom Stress ◽  
Interior Flow ◽  
Lower Frequencies

Abstract The effects of a sloping bottom and stratification on a turbulent bottom boundary layer are investigated for cases where the interior flow oscillates monochromatically with frequency ω. At higher frequencies, or small slope Burger numbers s = αN/f (where α is the bottom slope, N is the interior buoyancy frequency, and f is the Coriolis parameter), the bottom boundary layer is well mixed and the bottom stress is nearly what it would be over a flat bottom. For lower frequencies, or larger slope Burger number, the bottom boundary layer consists of a thick, weakly stratified outer layer and a thinner, more strongly stratified inner layer. Approximate expressions are derived for the different boundary layer thicknesses as functions of s and σ = ω/f. Further, buoyancy arrest causes the amplitude of the fluctuating bottom stress to decrease with decreasing σ (the s dependence, although important, is more complicated). For typical oceanic parameters, arrest is unimportant for fluctuation periods shorter than a few days. Substantial positive (toward the right when looking toward deeper water in the Northern Hemisphere) time-mean flows develop within the well-mixed boundary layer, and negative mean flows exist in the weakly stratified outer boundary layer for lower frequencies and larger s. If the interior flow is realistically broad band in frequency, the numerical model predicts stress reduction over all frequencies because of the nonlinearity associated with a quadratic bottom stress. It appears that the present one-dimensional model is reliable only for time scales less than the advective time scale that governs interior stratification.

scholarly journals Submesoscale Baroclinic Instability in the Bottom Boundary Layer

2018 ◽  
Vol 48 (11) ◽  
pp. 2571-2592 ◽  
Author(s):  
Jacob O. Wenegrat ◽  
Jörn Callies ◽  
Leif N. Thomas
Keyword(s):  
Boundary Layer ◽  
Mixed Layer ◽  
Baroclinic Instability ◽  
Slope Angle ◽  
Deep Ocean ◽  
Bottom Boundary Layer ◽  
Surface Mixed Layer ◽  
Nonlinear Simulation ◽  
Bottom Boundary ◽  
Topographic Slope

AbstractWeakly stratified layers over sloping topography can support a submesoscale baroclinic instability mode, a bottom boundary layer counterpart to surface mixed layer instabilities. The instability results from the release of available potential energy, which can be generated because of the observed bottom intensification of turbulent mixing in the deep ocean, or the Ekman adjustment of a current on a slope. Linear stability analysis suggests that the growth rates of bottom boundary layer baroclinic instabilities can be comparable to those of the surface mixed layer mode and are relatively insensitive to topographic slope angle, implying the instability is robust and potentially active in many areas of the global oceans. The solutions of two separate one-dimensional theories of the bottom boundary layer are both demonstrated to be linearly unstable to baroclinic instability, and results from an example nonlinear simulation are shown. Implications of these findings for understanding bottom boundary layer dynamics and processes are discussed.

The tidally induced bottom boundary layer in the rotating frame: development of the turbulent mixed layer under stratification

2009 ◽  
Vol 619 ◽  
pp. 235-259 ◽  
Author(s):  
KEI SAKAMOTO ◽  
KAZUNORI AKITOMO
Keyword(s):  
Boundary Layer ◽  
Mixed Layer ◽  
Interfacial Layer ◽  
Rapid Development ◽  
Tidal Mixing ◽  
Bottom Boundary Layer ◽  
Buoyancy Frequency ◽  
Rotating Frame ◽  
Three Dimensional Model ◽  
Bottom Boundary

To investigate turbulent properties and the developing mechanisms of the tidally induced bottom boundary layer in the linearly stratified ocean, numerical experiments have been executed with a non-hydrostatic three-dimensional model in the rotating frame, changing the temporal Rossby number Rot = |σ/f|, i.e. the ratio of the tidal frequency σ to the Coriolis parameter f. After the flow transitions to turbulence, the entire water column can be characterized by three layers: the mixed layer where density is homogenized and the flow is turbulent (z < zm); the stratified layer where the initial stratification remains and the flow is laminar (z > zt); and the interfacial layer between them where the flow is turbulent but the stratification remains (zm < z < zt). Turbulence is scaled by the frictional velocity uτ and the mixed-layer thickness zm (uτ and uτ/N where N is the buoyancy frequency) in the mixed (interfacial) layer, and has similarity. The mixed layer is thickened by the process where light water of the upper stratified layer is mixed with the lower unstratified layer water through the interfacial layer. As Rot approaches unity, i.e. near the critical latitude, the mixed layer develops more rapidly according to the following mechanism. As becomes Rot closer to unity, the current shear in the interfacial layer is intensified, since the difference of velocity becomes larger between the lower turbulent mixed and upper laminar stratified layers, and this leads to thickening of the interfacial layer. As a result, density deviation of the water entrained from above becomes larger, and this causes more rapid development of the mixed layer. In terms of the energy conversion from the eddy kinetic energy (EKE) to the potential energy (PE), the efficiency factor β which is the ratio of the conversion rate from EKE to PE to that from the tidal shear to EKE increased from 0.25% for Rot = 0.5 to 3.5% for Rot = 1.05 on average. When the time is normalized by the period required for the mixed layer to be thickened to the unstratified turbulent boundary layer δ = uτ/|f+σ|, the mixed layer development occurred in a similar manner in all cases. This similarity suggests the possibility of universal formulation for the turbulent tidal mixing under stratification.

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