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.

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.

Ocean Surface Boundary Layer Response to Abruptly Turning Winds

2021 ◽  
Author(s):  
Xingchi Wang ◽  
Tobias Kukulka
Keyword(s):  
Boundary Layer ◽  
Wave Model ◽  
Ocean Surface ◽  
Stokes Drift ◽  
Navier Stokes ◽  
Surface Boundary ◽  
Navier Stokes Equation ◽  
Surface Boundary Layer ◽  
Eddy Simulation ◽  
Budget Analysis

AbstractTurbulence driven by wind and waves controls the transport of heat, momentum, and matter in the ocean surface boundary layer (OSBL). For realistic ocean conditions, winds and waves are often neither aligned nor constant, for example, when winds turn rapidly. Based on a Large Eddy Simulation (LES) method, which captures shear-driven turbulence (ST) and Langmuir turbulence (LT) driven by the Craik-Leibovich vortex force, we investigate the OSBL response to abruptly turning winds. We design idealized LES experiments, whose winds are initially constant to equilibrate OSBL turbulence before abruptly turning 90° either cyclonically or anticyclonically. The transient Stokes drift for LT is estimated from a spectral wave model. The OSBL response includes three successive stages that follow the change in direction. During stage 1, turbulent kinetic energy (TKE) decreases due to reduced TKE production. Stage 2 is characterized by TKE increasing with TKE shear production recovering and exceeding TKE dissipation. Transient TKE levels may exceed their stationary values due to inertial resonance and non-equilibrium turbulence. Turbulence relaxes to its equilibrium state at stage 3, but LT still adjusts due to slowly developing waves. During stages 1 and 2, greatly misaligned wind and waves lead to Eulerian TKE production exceeding Stokes TKE production. A Reynolds stress budget analysis and Reynolds-averaged Navier-Stokes equation models indicate that Stokes production furthermore drives the OSBL response. The Coriolis effects result in asymmetrical OSBL responses to wind turning directions. Our results suggest that transient wind conditions play a key role in understanding realistic OSBL dynamics.

scholarly journals Parameterization of Wave-Induced Mixing Using the Large Eddy Simulation (LES) (I)

Atmosphere ◽  
2020 ◽  
Vol 11 (2) ◽  
pp. 207 ◽  
Author(s):  
Haili Wang ◽  
Changming Dong ◽  
Yongzeng Yang ◽  
Xiaoqian Gao
Keyword(s):  
Boundary Layer ◽  
Large Eddy Simulation ◽  
Ocean Surface ◽  
Breaking Waves ◽  
Global Climate Models ◽  
Surface Boundary ◽  
Surface Boundary Layer ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Wave Induced

Turbulent motions in the thin ocean surface boundary layer control exchanges of momentum, heat and trace gases between the atmosphere and ocean. However, present parametric equations of turbulent motions that are applied to global climate models result in systematic or substantial errors in the ocean surface boundary layer. Significant mixing caused by surface wave processes is missed in most parametric equations. A Large Eddy Simulation model is applied to investigate the wave-induced mixed layer structure. In the wave-averaged equations, wave effects are calculated as Stokes forces and breaking waves. To examine the effects of wave parameters on mixing, a series of wave conditions with varying wavelengths and heights are used to drive the model, resulting in a variety of Langmuir turbulence and wave breaking outcomes. These experiments suggest that wave-induced mixing is more sensitive to wave heights than to the wavelength. A series of numerical experiments with different wind intensities-induced Stokes drifts are also conducted to investigate wave-induced mixing. As the wind speed increases, the influence depth of Langmuir circulation deepens. Additionally, it is observed that breaking waves could destroy Langmuir cells mainly at the sea surface, rather than at deeper layers.

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

2012 ◽  
Vol 39 (18) ◽  
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

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

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 “LESbrary” 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 “synthetic observations” 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>

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.

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