Along-wind dispersion by horizontal turbulent jets in the upper ocean

2021 ◽  
Author(s):  
Tobias Kukulka ◽  
Todd Thoman
Keyword(s):  
Turbulent Jets ◽  
Surface Boundary ◽  
Surface Boundary Layer ◽  
Near Surface ◽  
Idealized Model ◽  
Eddy Simulation ◽  
Wind Dispersion ◽  
Shear Dispersion ◽  
Coherent Jet ◽  
Local Shear

AbstractDispersion processes in the ocean surface boundary layer (OSBL) determine marine material distributions such as those of plankton and pollutants. Sheared velocities drive shear dispersion, which is traditionally assumed to be due to mean horizontal currents that decrease from the surface. However, OSBL turbulence supports along-wind jets; located in near-surface convergence and downwelling regions, such turbulent jets contain strong local shear. Through wind-driven idealized and large eddy simulation (LES) models of the OSBL, this study examines the role of turbulent along-wind jets in dispersing material. In the idealized model, turbulent jets are generated by prescribed cellular flow with surface convergence and associated downwelling regions. Numeric and analytic model solutions reveal that horizontal jets substantially contribute to along-wind dispersion for sufficiently strong cellular flows and exceed contributions due to vertical mean shear for buoyant surface-trapped material. However, surface convergence regions also accumulate surface-trapped material, reducing shear dispersion by jets. Turbulence resolving LES results of a coastal depth-limited ocean agree qualitatively with the idealized model and reveal long-lived coherent jet structures that are necessary for effective jet dispersion. These coastal results indicate substantial jet contributions to along-wind dispersion. However, jet dispersion is likely less effective in the open ocean because jets are shorter lived, less organized, and distorted due to spiraling Ekman currents.

Horizontal Transport of Buoyant Material by Turbulent Jets in the Upper Ocean

2020 ◽  
Vol 50 (3) ◽  
pp. 827-843
Author(s):  
Tobias Kukulka
Keyword(s):  
Center Of Mass ◽  
Turbulent Jets ◽  
Jet Flow ◽  
Turbulent Jet ◽  
Surface Boundary ◽  
Surface Boundary Layer ◽  
Near Surface ◽  
Horizontal Transport ◽  
Idealized Model ◽  
Convergence Zones

AbstractCurrents in the ocean surface boundary layer (OSBL) determine the horizontal transport of submerged buoyant material, such as pollutants, plankton, and bubbles. Commonly, the mean horizontal transport, that is, the transport that changes the horizontal position of the material’s center of mass, is assumed to be accomplished by horizontal mean currents. However, surface convergence zones due to OSBL turbulence organize both wind-driven horizontal currents and near-surface concentrated buoyant material. In such surface convergence zones, concentrations of buoyant material are enhanced (e.g., apparent as windrows) and collocate with increased horizontal turbulent currents, here referred to as turbulent jets. In turn, the correlation of turbulent jet flow and material concentrations leads to a net mean horizontal transport due to turbulent motion. To examine this turbulent jet transport, an idealized model is devised for a wind-driven flow that is perturbed by prescribed cellular flow structures with crosswind surface convergence zones. Model solutions of the jet flow and material concentrations reveal that turbulent jet transport is comparable to the transport by horizontal mean currents for sufficiently strong cellular flow and material buoyancy. To test this model, we also perform more realistic turbulence-resolving large-eddy simulations (LESs) of wind and wave-driven OSBL turbulence. LES results are consistent with many features of the idealized model and suggest that the commonly overlooked turbulent jet transport is about 20%–50% of the traditional transport by horizontal mean currents. Thus, turbulent jet transport should be taken into account for accurate transport models of buoyant material in the OSBL.

scholarly journals Evolution of Oceanic Near-Surface Stratification in Response to an Autumn Storm

2019 ◽  
Vol 49 (11) ◽  
pp. 2961-2978 ◽  
Author(s):  
Natasha S. Lucas ◽  
Alan L. M. Grant ◽  
Tom P. Rippeth ◽  
Jeff A. Polton ◽  
Matthew R. Palmer ◽  
...  
Keyword(s):  
Heat Budget ◽  
Ocean Surface ◽  
Momentum Balance ◽  
Surface Boundary ◽  
Surface Boundary Layer ◽  
Near Surface ◽  
Shear Alignment ◽  
Eddy Simulation ◽  
Rate Of Dissipation ◽  
Bulk Model

AbstractUnderstanding the processes that control the evolution of the ocean surface boundary layer (OSBL) is a prerequisite for obtaining accurate simulations of air–sea fluxes of heat and trace gases. Observations of the rate of dissipation of turbulent kinetic energy (ε), temperature, salinity, current structure, and wave field over a period of 9.5 days in the northeast Atlantic during the Ocean Surface Mixing, Ocean Submesoscale Interaction Study (OSMOSIS) are presented. The focus of this study is a storm that passed over the observational area during this period. The profiles of ε in the OSBL are consistent with profiles from large-eddy simulation (LES) of Langmuir turbulence. In the transition layer (TL), at the base of the OSBL, ε was found to vary periodically at the local inertial frequency. A simple bulk model of the OSBL and a parameterization of shear driven turbulence in the TL are developed. The parameterization of ε is based on assumptions about the momentum balance of the OSBL and shear across the TL. The predicted rate of deepening, heat budget, and the inertial currents in the OSBL were in good agreement with the observations, as is the agreement between the observed value of ε and that predicted using the parameterization. A previous study reported spikes of elevated dissipation related to enhanced wind shear alignment at the base of the OSBL after this storm. The spikes in dissipation are not predicted by this new parameterization, implying that they are not an important source of dissipation during the storm.

scholarly journals Lagrangian Investigation of Wave-Driven Turbulence in the Ocean Surface Boundary Layer

2019 ◽  
Vol 49 (2) ◽  
pp. 409-429 ◽  
Author(s):  
Tobias Kukulka ◽  
Fabrice Veron
Keyword(s):  
Boundary Layer ◽  
Turbulent Transport ◽  
Ocean Surface ◽  
Inertial Subrange ◽  
Breaking Wave ◽  
Surface Boundary ◽  
Lagrangian Analysis ◽  
Surface Boundary Layer ◽  
Near Surface ◽  
Eddy Simulation

AbstractTurbulent processes in the ocean surface boundary layer (OSBL) play a key role in weather and climate systems. This study explores a Lagrangian analysis of wave-driven OSBL turbulence, based on a large-eddy simulation (LES) model coupled to a Lagrangian stochastic model (LSM). Langmuir turbulence (LT) is captured by Craik–Leibovich wave forcing that generates LT through the Craik–Leibovich type 2 (CL2) mechanism. Breaking wave (BW) effects are modeled by a surface turbulent kinetic energy flux that is constrained by wind energy input to surface waves. Unresolved LES subgrid-scale (SGS) motions are simulated with the LSM to be energetically consistent with the SGS model of the LES. With LT, Lagrangian autocorrelations of velocities reveal three distinct turbulent time scales: an integral, a dispersive mixing, and a coherent structure time. Coherent structures due to LT result in relatively narrow peaks of Lagrangian frequency velocity spectra. With and without waves, the high-frequency spectral tail is consistent with expectations for the inertial subrange, but BWs substantially increase spectral levels at high frequencies. Consistently, over short times, particle-pair dispersion results agree with the Richardson–Obukhov law, and near-surface dispersion is significantly enhanced because of BWs. Over longer times, our dispersion results are consistent with Taylor dispersion. In this case, turbulent diffusivities are substantially larger with LT in the crosswind direction, but reduced in the along-wind direction because of enhanced turbulent transport by LT that reduces mean Eulerian shear. Our results indicate that the Lagrangian analysis framework is effective and physically intuitive to characterize OSBL turbulence.

scholarly journals A Parameterization with a Constrained Potential Energy Conversion Rate of Vertical Mixing Due to Langmuir Turbulence

2019 ◽  
Vol 49 (11) ◽  
pp. 2935-2959 ◽  
Author(s):  
Brandon G. Reichl ◽  
Qing Li
Keyword(s):  
Turbulent Mixing ◽  
Vertical Mixing ◽  
Stable Stratification ◽  
Surface Boundary ◽  
Langmuir Turbulence ◽  
Surface Boundary Layer ◽  
Eddy Simulation ◽  
Available Energy ◽  
Climate Simulations ◽  
Surface Buoyancy

AbstractIn this study we develop a new parameterization for turbulent mixing in the ocean surface boundary layer (OSBL), including the effect of Langmuir turbulence. This new parameterization builds on a recent study (Reichl and Hallberg 2018, hereafter RH18), which predicts the available energy for turbulent mixing against stable stratification driven by shear and convective turbulence. To investigate the role of Langmuir turbulence in the framework of RH18, we utilize data from a suite of previously published large-eddy simulation (LES) experiments (Li and Fox-Kemper 2017, hereafter LF17) with and without Langmuir turbulence under different idealized forcing conditions. We find that the parameterization of RH18 is able to reproduce the mixing simulated by the LES in the non-Langmuir cases, but not the Langmuir cases. We therefore investigate the enhancement of the integrated vertical buoyancy flux within the entrainment layer in the presence of Langmuir turbulence using the LES data. An additional factor is introduced in the RH18 framework to capture the enhanced mixing due to Langmuir turbulence. This additional factor depends on the surface-layer averaged Langmuir number with a reduction in the presence of destabilizing surface buoyancy fluxes. It is demonstrated that including this factor within the RH18 OSBL turbulent mixing parameterization framework captures the simulated effect of Langmuir turbulence in the LES, which can be used for simulating the effect of Langmuir turbulence in climate simulations. This new parameterization is compared to the KPP-based Langmuir entrainment parameterization introduced by LF17, and differences are explored in detail.

scholarly journals Horizontal Dispersion of Buoyant Materials in the Ocean Surface Boundary Layer

2018 ◽  
Vol 48 (9) ◽  
pp. 2103-2125 ◽  
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.

scholarly journals Symmetric Instability, Inertial Oscillations, and Turbulence at the Gulf Stream Front

2016 ◽  
Vol 46 (1) ◽  
pp. 197-217 ◽  
Author(s):  
Leif N. Thomas ◽  
John R. Taylor ◽  
Eric A. D’Asaro ◽  
Craig M. Lee ◽  
Jody M. Klymak ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Stability Analysis ◽  
Gulf Stream ◽  
Surface Boundary ◽  
Unstable Flow ◽  
Inertial Oscillations ◽  
Surface Boundary Layer ◽  
Eddy Simulation ◽  
Symmetric Instability ◽  
Shear Production

AbstractThe passage of a winter storm over the Gulf Stream observed with a Lagrangian float and hydrographic and velocity surveys provided a unique opportunity to study how the interaction of inertial oscillations, the front, and symmetric instability (SI) shapes the stratification, shear, and turbulence in the upper ocean under unsteady forcing. During the storm, the rapid rise and rotation of the winds excited inertial motions. Acting on the front, these sheared motions modulate the stratification in the surface boundary layer. At the same time, cooling and downfront winds generated a symmetrically unstable flow. The observed turbulent kinetic energy dissipation exceeded what could be attributed to atmospheric forcing, implying SI drew energy from the front. The peak excess dissipation, which occurred just prior to a minimum in stratification, surpassed that predicted for steady SI turbulence, suggesting the importance of unsteady dynamics. The measurements are interpreted using a large-eddy simulation (LES) and a stability analysis configured with parameters taken from the observations. The stability analysis illustrates how SI more efficiently extracts energy from a front via shear production during periods when inertial motions reduce stratification. Diagnostics of the energetics of SI from the LES highlight the temporal variability in shear production but also demonstrate that the time-averaged energy balance is consistent with a theoretical scaling that has previously been tested only for steady forcing. As the storm passed and the winds and cooling subsided, the boundary layer restratified and the thermal wind balance was reestablished in a manner reminiscent of geostrophic adjustment.

scholarly journals Characteristics of the Near-Surface Boundary Layer within a Mountain Valley during Winter

2012 ◽  
Vol 51 (3) ◽  
pp. 583-597 ◽  
Author(s):  
Warren Helgason ◽  
John W. Pomeroy
Keyword(s):  
Boundary Layer ◽  
Heat Fluxes ◽  
Quadrant Analysis ◽  
Turbulent Heat ◽  
Surface Boundary ◽  
Wind Speed Profile ◽  
Surface Boundary Layer ◽  
Near Surface ◽  
Mountainous Regions ◽  
Mass Fluxes

AbstractWithin mountainous regions, estimating the exchange of sensible heat and water vapor between the surface and the atmosphere is an important but inexact endeavor. Measurements of the turbulence characteristics of the near-surface boundary layer in complex mountain terrain are relatively scarce, leading to considerable uncertainty in the application of flux-gradient techniques for estimating the surface turbulent heat and mass fluxes. An investigation of the near-surface boundary layer within a 7-ha snow-covered forest clearing was conducted in the Kananaskis River valley, located within the Canadian Rocky Mountains. The homogeneous measurement site was characterized as being relatively calm and sheltered; the wind exhibited considerable unsteadiness, however. Frequent wind gusts were observed to transport turbulent energy into the clearing, affecting the rate of energy transfer at the snow surface. The resulting boundary layer within the clearing exhibited perturbations introduced by the surrounding topography and land surface discontinuities. The measured momentum flux did not scale with the local aerodynamic roughness and mean wind speed profile, but rather was reflective of the larger-scale topographical disturbances. The intermittent nature of the flux-generating processes was evident in the turbulence spectra and cospectra where the peak energy was shifted to lower frequencies as compared with those observed in more homogeneous flat terrain. The contribution of intermittent events was studied using quadrant analysis, which revealed that 50% of the sensible and latent heat fluxes was contributed from motions that occupied less than 6% of the time. These results highlight the need for caution while estimating the turbulent heat and mass fluxes in mountain regions.

scholarly journals Measurements of Enhanced Near-Surface Turbulence under Windrows

2020 ◽  
Vol 50 (1) ◽  
pp. 197-215
Author(s):  
Seth F. Zippel ◽  
Ted Maksym ◽  
Malcolm Scully ◽  
Peter Sutherland ◽  
Dany Dumont
Keyword(s):  
Boundary Layer ◽  
Surface Flux ◽  
Breaking Waves ◽  
Surface Boundary ◽  
Surface Boundary Layer ◽  
Near Surface ◽  
Surface Turbulence ◽  
Order Of Magnitude ◽  
Convergence Zones ◽  
Shallow Bay

AbstractObservations of waves, winds, turbulence, and the geometry and circulation of windrows were made in a shallow bay in the winter of 2018 outside of Rimouski, Québec. Water velocities measured from a forward-looking pulse-coherent ADCP mounted on a small zodiac show spanwise (cross-windrow) convergence, streamwise (downwind) velocity enhancement, and downwelling in the windrows, consistent with the view that windrows are the result of counterrotating pairs of wind-aligned vortices. The spacing of windrows, measured with acoustic backscatter and with surface imagery, was measured to be approximately twice the water depth, which suggests an aspect ratio of 1. The magnitude and vertical distribution of turbulence measured from the ADCP are consistent with a previous scaling and observations of near-surface turbulence under breaking waves, with dissipation rates larger and decaying faster vertically than what is expected from a shear-driven boundary layer. Measurements of dissipation rate are partitioned to within, and outside of the windrow convergence zones, and measurements inside the convergence zones are found to be nearly an order of magnitude larger than those outside with similar vertical structure. A ratio of time scales suggests that turbulence likely dissipates before it can be advected horizontally into convergences, but the advection of wave energy into convergences may elevate the surface flux of TKE and could explain the elevated turbulence in the windrows. These results add to a limited number of conflicting observations of turbulence variability due to windrows, which may modify gas flux, and heat and momentum transport in the surface boundary layer.

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