Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session H23: Geophysical Fluid Dynamics: Eddy/Wave-Mean Flow Interactions |
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Chair: Pascale Lelong, Northwest Research Associates Room: 2001 |
Monday, November 24, 2014 10:30AM - 10:43AM |
H23.00001: Near-inertial waves within an anticyclonic eddy in the Mediterranean Sea: Observations and numerical simulations Pascale Lelong, Pascale Bouruet-Aubertot, Yannis Cuypers One of the objectives of the BOUM field experiment, conducted in the Mediterranean Sea during the Summer of 2008, was to investigate the impact of submesoscale ocean dynamics on biogeochemical cycles. Analysis of data collected in the permanent, warm-core, anticyclonic Cyprus eddy provides a case study for near-inertial wave generation and turbulence in the presence of an eddy. Observations reveal the presence of near-inertial oscillations over the entire profile, from the mixed layer to below the base of the eddy. We present the results of a parallel LES numerical study with a Boussinesq pseudo-spectral code which was designed to explain the observed near-inertial signal. Two generation mechanisms are discussed: (i) inertial pumping at the base of the mixed layer following a wind event and (ii) adjustment of the eddy with possible trapping at the base of the eddy. Our numerical study confirms the role of anticyclonic eddies in influencing the propagation of wind-driven inertial oscillations into the thermocline. [Preview Abstract] |
Monday, November 24, 2014 10:43AM - 10:56AM |
H23.00002: Internal wave generation by a distributed vortex Surupa Shaw, John McHugh Internal wave generation in a continuously stratified fluid by a mature vortex pair and by a distributed line vortex is considered using direct numerical simulations. For large Froude number, the distributed vorticity for the line vortex quickly rolls up and forms a vortex pair, approximately matching the case that is initiated as a vortex pair. However for small Froude number, both cases disintegrate into internal waves. Recent results show that both cases exhibit a strong vertical oscillation with a frequency that depends on the buoyancy frequency $N$. The wave generation causes the initial energy to spread in the radial direction, however after several oscillations at the buoyancy frequency the spreading is slow and the overall size of the structures become approximately constant. The internal wave generation is identified by distinct radial structures in contours of energy flux. [Preview Abstract] |
Monday, November 24, 2014 10:56AM - 11:09AM |
H23.00003: Anisotropic mesoscale eddy transport in ocean general circulation models Scott Reckinger, Baylor Fox-Kemper, Scott Bachman, Frank Bryan, John Dennis, Gokhan Danabasoglu In modern climate models, the effects of oceanic mesoscale eddies are introduced by relating subgrid eddy fluxes to the resolved gradients of buoyancy or other tracers, where the proportionality is, in general, governed by an eddy transport tensor. The symmetric part of the tensor, which represents the diffusive effects of mesoscale eddies, is universally treated isotropically. However, the diffusive processes that the parameterization approximates, such as shear dispersion and potential vorticity barriers, typically have strongly anisotropic characteristics. Generalizing the eddy diffusivity tensor for anisotropy extends the number of parameters from one to three: major diffusivity, minor diffusivity, and alignment. The Community Earth System Model (CESM) with the anisotropic eddy parameterization is used to test various choices for the parameters, which are motivated by observations and the eddy transport tensor diagnosed from high resolution simulations. Simply setting the ratio of major to minor diffusivities to a value of five globally, while aligning the major axis along the flow direction, improves biogeochemical tracer ventilation and reduces temperature and salinity biases. These effects can be improved by parameterizing the oceanic anisotropic transport mechanisms. [Preview Abstract] |
Monday, November 24, 2014 11:09AM - 11:22AM |
H23.00004: Effects of Submesoscale Turbulence on Tracer Evolution in the Oceanic Mixed Layer Katherine Smith, Spencer Alexander, Luke Van Roekel, Baylor Fox-Kemper, Peter Hamlington Ocean tracers such as CO$_2$, nutrients, and plankton evolve mainly in the mixed layer where light and air-sea gas exchange occur. It is known from prior studies there can be substantial heterogeneity in tracer distributions due to vertical and horizontal turbulent mixing across a range of scales. The contribution of submesoscale turbulence to these distributions is not entirely understood, particularly in the sub-kilometer range where both large-scale, nearly 2D and small-scale, 3D turbulence are active, resulting in dynamical complexity from which heterogeneity can arise. In this talk, results from large eddy simulations of a large temperature front evolving are used to examine effects of multi-scale turbulence on idealized tracer distributions from scales 20km to 5m. Simulations include the effect of Langmuir turbulence by solving the wave-averaged Boussinesq equations with an imposed Stokes drift velocity. Tracers with different source and boundary conditions are examined to understand the role of both small-scale, near-surface vertical mixing and larger-scale upwelling motions typically associated with submesoscale eddies. Tracer evolution is characterized using spectra, multi-scale fluxes, and probability distribution functions, and implications of the results are outlined. [Preview Abstract] |
Monday, November 24, 2014 11:22AM - 11:35AM |
H23.00005: Estimates of Lagrangian particle transport by wave groups: forward transport by Stokes drift and backward transport by the return flow Ton S. van den Bremer, Paul H. Taylor Although the literature has examined Stokes drift, the net Lagrangian transport by particles due to of surface gravity waves, in great detail, the motion of fluid particles transported by surface gravity wave groups has received considerably less attention. In practice nevertheless, the wave field on the open sea often has a group-like structure. The motion of particles is different, as particles at sufficient depth are transported backwards by the Eulerian return current that was first described by Longuet-Higgins \& Stewart (1962) and forms an inseparable counterpart of Stokes drift for wave groups ensuring the (irrotational) mass balance holds. We use WKB theory to study the variation of the Lagrangian transport by the return current with depth distinguishing two-dimensional seas, three-dimensional seas, infinite depth and finite depth. We then provide dimensional estimates of the net horizontal Lagrangian transport by the Stokes drift on the one hand and the return flow on the other hand for realistic sea states in all four cases. Finally we propose a simple scaling relationship for the transition depth: the depth above which Lagrangian particles are transported forwards by the Stokes drift and below which such particles are transported backwards by the return current. [Preview Abstract] |
Monday, November 24, 2014 11:35AM - 11:48AM |
H23.00006: Effects of swell on dispersion of oil plumes within the ocean mixed layer Bicheng Chen, Di Yang, Marcelo Chamecki, Charles Meneveau Oil plumes from deep-water blowouts rise through the ocean and reach the ocean mixed layer (OML), where dispersion is strongly affected by Langmuir turbulence generated by interactions between the wind forcing and the wave regime. The wind-driven wave field is approximately aligned with wind direction. However, the swell wave can have an arbitrary orientation relative to the local wind. We used large-eddy simulation (LES) to study the influences of the misalignment between wind and wave field on the transport and dispersion of oil plumes in the OML. Results show that the plume response to these forcing is strongly dependent on the size of the oil droplets. For the large oil droplets, the center line of the time-averaged surface plume tends to follow the mean surface current direction; for small droplets, the change of orientation of center line with wave direction is smaller than that of large droplets. Vertical eddy diffusivity calculated from LES data is compared to closures currently used in ocean models (such as the KPP model employed in HYCOM). The magnitude of the eddy diffusivity changes by a factor of two as the misalignment between swell and wind changes, and it is typically much larger than predicted by KPP. [Preview Abstract] |
Monday, November 24, 2014 11:48AM - 12:01PM |
H23.00007: Oil droplet plume evolution in Langmuir turbulence: a Large Eddy Simulation study Di Yang, Bicheng Chen, Marcelo Chamecki, Charles Meneveau When the oil plumes from deep water blowouts reach the ocean mixed layer (OML), their fates on the sea surface are highly affected by the interactions with wind and wave-generated Langmuir turbulence in the OML. In this study, we use large eddy simulations (LES) to quantify the complex oil dispersion phenomena. We find that although the instantaneous surface oil slick patterns are very complex, the time-averaged surface oil plume can be parameterized as a Gaussian-type plume. The centerline of the surface plume is inclined clockwise (in the Northern Hemisphere) with respect to the wind and wave direction due to Ekman transport. The initial width of the mean surface plume and the inclination angle increase as the droplet size decreases. The surface plume width grows downstream, with a growth rate that varies non-monotonically with oil droplet size. Using LES data, we evaluate the eddy viscosity and eddy diffusivity following the K-profile parameterization (KPP) framework. We also evaluate stress-strain misalignments caused by Stokes drift and evaluate means of parameterizing these effects. Improvements to the KPP model will be discussed. [Preview Abstract] |
Monday, November 24, 2014 12:01PM - 12:14PM |
H23.00008: Langmuir circulation in shallow water waves W.R.C. Phillips, Albert Dai The instability of shallow water waves on a moderate shear to Langmuir circulation (LC) is considered. In such instances the shear can significantly affect the drift giving rise to profiles markedly different from the simple Stokes drift. Since drift and shear are instrumental in the instability to LC, of key interest is how that variation in turn affects onset to LC. The initial value problem describing the wave-mean flow interaction is crafted from scratch and includes a consistent set of free-surface boundary conditions. The problem necessitates that Navier Stokes be employed side by side with a set of mean-field equations; these are seen to reduce to the well known CL-equations, albeit with different time and velocity scales. Typical shear driven and pressure driven flows are considered. Shear driven flow is found to be destabilizing while pressure driven are stabilizing to LC. It is further found that multiple layers, as opposed to a single layer, of LC can form, with the most intense circulations at the ocean floor. LC can also extend into a region of flow beyond which instability applies thus deepening the mixed layer. Two preferred spacings occur, one closely in accord with observation for small aspect ratio LC. [Preview Abstract] |
Monday, November 24, 2014 12:14PM - 12:27PM |
H23.00009: LES of full-depth Langmuir circulation with surface cooling Rachel Walker, Andres E. Tejada-Martinez, Chester E. Grosch Results are presented from large-eddy simulations (LES) of full-depth Langmuir circulation (LC) in the presence of surface cooling in a domain representative of the shallow coastal ocean. LC consists of counter-rotating vortices aligned roughly in the direction of the wind and generated by the interaction of the wind-driven shear with the Stokes drift velocity induced by surface gravity waves. In LES of open channel flow (without LC), surface cooling has been found to lead to the development of full-depth convection cells similar in structure to LC. As such, in the current simulations unstable stratification is imposed by a constant surface cooling flux and an adiabatic bottom wall to assess the impact of cooling-induced buoyancy on the strength of the wind and wave-driven LC. The surface cooling flux will be quantified by the value of the Rayleigh number, representative of surface buoyancy forcing relative to wind shear forcing. The impact of the convection on LC will be assessed by analysis of mean velocity, root mean square of velocity, and budgets of Reynolds stress components. It is intended that results may assist in determining the dominant mechanism in large-scale cell structure development when both LC and surface cooling are present. [Preview Abstract] |
Monday, November 24, 2014 12:27PM - 12:40PM |
H23.00010: DNS of scalar transport across a wind-driven air-water interface Amine Hafsi, Andres Tejada-Martinez, Fabrice Veron When wind blows over an initially quiescent air-sea interface, it first generates short capillary waves which in time coexist with longer waves as part of a broad spectrum of waves. The interaction between the wind-driven waves and shear current on the waterside leads to Langmuir turbulence characterized by Langmuir circulation (LC) consisting of counter rotating vortices roughly aligned in the direction of the wind. The typical length scale of LC ranges from several centimeters when short capillary waves first appear up to tens of meters when the spectrum of waves broadens. Results are presented from direct numerical simulation (DNS) of a coupled air-water interface driven by an air flow with free stream speed of 5 m/s. The evolution of the air-water interface starting from rest and the accompanying development of centimeter-scale Langmuir turbulence on the waterside during the first 20 seconds of simulation are investigated. Emphasis is placed on the impact of the Langmuir turbulence on scalar transfer from the airside to the waterside, in particular the transfer velocity which is a measure of scalar transfer efficiency. Simulations are made with a finite volume discretization employing the volume of fluid method for interface tracking. [Preview Abstract] |
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