Large-Aspect-Ratio Structures in Simulated Ocean Surface Boundary Layer Turbulence under a Hurricane

AbstractA large-eddy simulation (LES) initialized and forced using observations is used to conduct a process study of ocean surface boundary layer (OSBL) turbulence in a 2-km box of ocean nominally under Hurricane Irene (2011) in 35 m of water on the New Jersey shelf. The LES captures the observed deepening, cooling, and persistent stratification of the OSBL as the storm approaches and passes. As the storm approaches, surface-intensified Ekman-layer rolls, with horizontal wavelengths of about 200 m and horizontal-to-vertical aspect and velocity magnitude ratios of about 20, dominate the kinetic energy and increase the turbulent Prandtl number from about 1 to 1.5 due partially to their restratifying vertical buoyancy flux. However, as the storm passes, these rolls are washed away in a few hours due to the rapid rotation of the wind. In the bulk OSBL, the gradient Richardson number of the mean profiles remains just above (just below) 1/4 as the storm approaches (passes). At the base of the OSBL, large-aspect-ratio Kelvin–Helmholtz billows, with Prandtl number below 1, intermittently dominate the kinetic energy. Overall, large-aspect-ratio covariance modifies the net vertical fluxes of buoyancy and momentum by about 10%, but these fluxes and the analogous diffusivity and viscosity still approximately collapse to time-independent dimensionless profiles, despite rapid changes in the forcing and the large structures. That is, the evolutions of the mean temperature and momentum profiles, which are driven by the net vertical flux convergences, mainly reflect the evolution of the wind and the initial ocean temperature profile.

Download Full-text

Observations of Turbulence in the Ocean Surface Boundary Layer: Energetics and Transport

Journal of Physical Oceanography ◽

10.1175/2008jpo4044.1 ◽

2009 ◽

Vol 39 (5) ◽

pp. 1077-1096 ◽

Cited By ~ 82

Author(s):

Gregory P. Gerbi ◽

John H. Trowbridge ◽

Eugene A. Terray ◽

Albert J. Plueddemann ◽

Tobias Kukulka

Keyword(s):

Boundary Layer ◽

Diffusion Coefficient ◽

Kinetic Energy ◽

Wave Breaking ◽

Ocean Surface ◽

Surface Boundary ◽

Langmuir Turbulence ◽

Rigid Boundary ◽

Surface Boundary Layer ◽

Buoyancy Forcing

Abstract Observations of turbulent kinetic energy (TKE) dynamics in the ocean surface boundary layer are presented here and compared with results from previous observational, numerical, and analytic studies. As in previous studies, the dissipation rate of TKE is found to be higher in the wavy ocean surface boundary layer than it would be in a flow past a rigid boundary with similar stress and buoyancy forcing. Estimates of the terms in the turbulent kinetic energy equation indicate that, unlike in a flow past a rigid boundary, the dissipation rates cannot be balanced by local production terms, suggesting that the transport of TKE is important in the ocean surface boundary layer. A simple analytic model containing parameterizations of production, dissipation, and transport reproduces key features of the vertical profile of TKE, including enhancement near the surface. The effective turbulent diffusion coefficient for heat is larger than would be expected in a rigid-boundary boundary layer. This diffusion coefficient is predicted reasonably well by a model that contains the effects of shear production, buoyancy forcing, and transport of TKE (thought to be related to wave breaking). Neglect of buoyancy forcing or wave breaking in the parameterization results in poor predictions of turbulent diffusivity. Langmuir turbulence was detected concurrently with a fraction of the turbulence quantities reported here, but these times did not stand out as having significant differences from observations when Langmuir turbulence was not detected.

Download Full-text

A global perspective on Langmuir turbulence in the ocean surface boundary layer

Geophysical Research Letters ◽

10.1029/2012gl052932 ◽

2012 ◽

Vol 39 (18) ◽

Cited By ~ 167

Author(s):

Stephen E. Belcher ◽

Alan L. M. Grant ◽

Kirsty E. Hanley ◽

Baylor Fox-Kemper ◽

Luke Van Roekel ◽

...

Keyword(s):

Boundary Layer ◽

Ocean Surface ◽

Global Perspective ◽

Surface Boundary ◽

Langmuir Turbulence ◽

Surface Boundary Layer

Download Full-text

LESbrary: A library of large eddy simulation data for the calibration and uncertainty quantification of ocean surface boundary layer turbulence parameterizations

10.5194/egusphere-egu21-16199 ◽

2021 ◽

Author(s):

Gregory Wagner ◽

Andre Souza ◽

Adeline Hillier ◽

Ali Ramadhan ◽

Raffaele Ferrari

Keyword(s):

Boundary Layer ◽

Turbulent Mixing ◽

Large Scale ◽

Ocean Surface ◽

Limited Area ◽

Surface Boundary ◽

Model Errors ◽

Surface Boundary Layer ◽

Large Eddy ◽

Free Parameters

<p>Parameterizations of turbulent mixing in the ocean surface boundary layer (OSBL) are key Earth System Model (ESM) components that modulate the communication of heat and carbon between the atmosphere and ocean interior. OSBL turbulence parameterizations are formulated in terms of unknown free parameters estimated from observational or synthetic data. In this work we describe the development and use of a synthetic dataset called the &#8220;LESbrary&#8221; generated by a large number of idealized, high-fidelity, limited-area large eddy simulations (LES) of OSBL turbulent mixing. We describe how the LESbrary design leverages a detailed understanding of OSBL conditions derived from observations and large scale models to span the range of realistically diverse physical scenarios. The result is a diverse library of well-characterized &#8220;synthetic observations&#8221; that can be readily assimilated for the calibration of realistic OSBL parameterizations in isolation from other ESM model components. We apply LESbrary data to calibrate free parameters, develop prior estimates of parameter uncertainty, and evaluate model errors in two OSBL parameterizations for use in predictive ESMs.</p>

Download Full-text

A review of studies on turbulent mixing in ocean surface boundary layer

Oceanography in Japan ◽

10.5928/kaiyou.26.5_239 ◽

2017 ◽

Vol 26 (5) ◽

pp. 239-250 ◽

Cited By ~ 1

Author(s):

Yutaka Yoshikawa ◽

Takahiro Endoh

Keyword(s):

Boundary Layer ◽

Turbulent Mixing ◽

Ocean Surface ◽

Surface Boundary ◽

Surface Boundary Layer

Download Full-text

Buoyancy-induced, columnar vortices

Journal of Fluid Mechanics ◽

10.1017/jfm.2016.541 ◽

2016 ◽

Vol 804 ◽

pp. 712-748 ◽

Cited By ~ 3

Author(s):

Mark W. Simpson ◽

Ari Glezer

Keyword(s):

Boundary Layer ◽

Kinetic Energy ◽

Ground Plane ◽

Vortical Flow ◽

Surface Boundary ◽

Surface Boundary Layer ◽

Columnar Vortex ◽

Heated Air ◽

Thermally Driven ◽

Entrained Air

A buoyancy-induced, columnar vortex is deliberately triggered in the unstably stratified air layer over a heated ground plane and is anchored within, and scales with, an azimuthal array of vertical, stator-like planar flow vanes that form an open-top enclosure and impart tangential momentum to the radially entrained air flow. The columnar vortex comprises three coupled primary flow domains: a spiraling surface momentum boundary layer of ground-heated air, an inner thermally driven vertical vortex core and an outer annular flow that is bounded by a helical shear layer and the vanes along its inner and outer edges, respectively, and by the spiraling boundary layer from below. In common with free buoyant columnar (dust devil) vortices that occur spontaneously over solar-heated terrain in the natural environment, the stationary anchored vortex is self-sustained by the conversion of the potential energy of the entrained surface-heated air layer to the kinetic energy of the induced vortical flow that persists as long as the thermal stratification is maintained. This conversion occurs as radial vorticity produced within the surface boundary layer is tilted vertically near the vortex centreline by the buoyant air to form the core of the columnar vortex. The structure and dynamics of the buoyant vortex are investigated using high-resolution stereo particle image velocimetry with specific emphasis on the evolution of the vorticity distributions and their effects on the characteristic scales of the ensuing vortex and on the kinetic energy of the induced flow.

Download Full-text

Horizontal Dispersion of Buoyant Materials in the Ocean Surface Boundary Layer

Journal of Physical Oceanography ◽

10.1175/jpo-d-18-0020.1 ◽

2018 ◽

Vol 48 (9) ◽

pp. 2103-2125 ◽

Cited By ~ 11

Author(s):

Jun-Hong Liang ◽

Xiaoliang Wan ◽

Kenneth A. Rose ◽

Peter P. Sullivan ◽

James C. McWilliams

Keyword(s):

Boundary Layer ◽

Three Dimensional ◽

Ocean Surface ◽

Turbulent Dispersion ◽

Surface Boundary ◽

Particle Trajectories ◽

Stokes Flows ◽

Surface Boundary Layer ◽

Horizontal Dispersion ◽

Shear Dispersion

ABSTRACTThe horizontal dispersion of materials with a constant rising speed under the exclusive influence of ocean surface boundary layer (OSBL) flows is investigated using both three-dimensional turbulence-resolving Lagrangian particle trajectories and the classical theory of dispersion in bounded shear currents generalized for buoyant materials. Dispersion in the OSBL is caused by the vertical shear of mean horizontal currents and by the turbulent velocity fluctuations. It reaches a diffusive regime when the equilibrium vertical material distribution is established. Diffusivity from the classical shear dispersion theory agrees reasonably well with that diagnosed using three-dimensional particle trajectories. For weakly buoyant materials that can be mixed into the boundary layer, shear dispersion dominates turbulent dispersion. For strongly buoyant materials that stay at the ocean surface, shear dispersion is negligible compared to turbulent dispersion. The effective horizontal diffusivity due to shear dispersion is controlled by multiple factors, including wind speed, wave conditions, vertical diffusivity, mixed layer depth, latitude, and buoyant rising speed. With all other meteorological and hydrographic conditions being equal, the effective horizontal diffusivity is larger in wind-driven Ekman flows than in wave-driven Ekman–Stokes flows for weakly buoyant materials and is smaller in Ekman flows than in Ekman–Stokes flows for strongly buoyant materials. The effective horizontal diffusivity is further reduced when enhanced mixing by breaking waves is included. Dispersion by OSBL flows is weaker than that by submesoscale currents at a scale larger than 100 m. The analytic framework will improve subgrid-scale modeling in realistic particle trajectory models using currents from operational ocean models.

Download Full-text

Assessing the Effects of Langmuir Turbulence on the Entrainment Buoyancy Flux in the Ocean Surface Boundary Layer

Journal of Physical Oceanography ◽

10.1175/jpo-d-17-0085.1 ◽

2017 ◽

Vol 47 (12) ◽

pp. 2863-2886 ◽

Cited By ~ 30

Author(s):

Qing Li ◽

Baylor Fox-Kemper

Keyword(s):

Boundary Layer ◽

Climate Model ◽

Ocean Surface ◽

Buoyancy Flux ◽

Entrainment Rate ◽

Surface Boundary ◽

Langmuir Turbulence ◽

Convective Turbulence ◽

Surface Boundary Layer ◽

The Impact

AbstractLarge-eddy simulations (LESs) with various constant wind, wave, and surface destabilizing surface buoyancy flux forcing are conducted, with a focus on assessing the impact of Langmuir turbulence on the entrainment buoyancy flux at the base of the ocean surface boundary layer. An estimate of the entrainment buoyancy flux scaling is made to best fit the LES results. The presence of Stokes drift forcing and the resulting Langmuir turbulence enhances the entrainment rate significantly under weak surface destabilizing buoyancy flux conditions, that is, weakly convective turbulence. In contrast, Langmuir turbulence effects are moderate when convective turbulence is dominant and appear to be additive rather than multiplicative to the convection-induced mixing. The parameterized unresolved velocity scale in the K-profile parameterization (KPP) is modified to adhere to the new scaling law of the entrainment buoyancy flux and account for the effects of Langmuir turbulence. This modification is targeted on common situations in a climate model where either Langmuir turbulence or convection is important and may overestimate the entrainment when both are weak. Nevertheless, the modified KPP is tested in a global climate model and generally outperforms those tested in previous studies. Improvements in the simulated mixed layer depth are found, especially in the Southern Ocean in austral summer.

Download Full-text

Wind- and Wave-driven Ocean Surface Boundary Layer in a Frontal Zone: Roles of Submesoscale Eddies and Ekman-Stokes Transport

Journal of Physical Oceanography ◽

10.1175/jpo-d-20-0270.1 ◽

2021 ◽

Author(s):

Jianguo Yuan ◽

Jun-Hong Liang

Keyword(s):

Boundary Layer ◽

Frontal Zone ◽

Surface Current ◽

Ocean Surface ◽

Buoyancy Flux ◽

Temperature Structure ◽

Surface Boundary ◽

Surface Boundary Layer ◽

Horizontal Mixing ◽

The Right

AbstractLarge-eddy simulations are used to investigate the influence of a horizontal frontal zone, represented by a stationary uniform background horizontal temperature gradient, on the wind- and wave-driven ocean surface boundary layers. In a frontal zone, the temperature structure, the ageostrophic mean horizontal current, and the turbulence in the ocean surface boundary layer all change with the relative angle among the wind and the front. The net heating and cooling of the boundary layer could be explained by the depth-integrated horizontal advective buoyancy flux, called the Ekman Buoyancy Flux (or the Ekman-Stokes Buoyancy Flux if wave effects are included). However, the detailed temperature profiles are also modulated by the depth-dependent advective buoyancy flux and submesoscale eddies. The surface current is deflected less (more) to the right of the wind and wave when the depth-integrated advective buoyancy flux cools (warms) the ocean surface boundary layer. Horizontal mixing is greatly enhanced by submesoscale eddies. The eddy-induced horizontal mixing is anisotropic and is stronger to the right of the wind direction. Vertical turbulent mixing depends on the superposition of the geostrophic and ageostrophic current, the depth-dependent advective buoyancy flux, and submesoscale eddies.

Download Full-text