Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session G30: Geophysical Fluid Dynamics: Mesoscale and Submesoscale |
Hide Abstracts |
Chair: Baylor Fox-Kemper, Brown University Room: 311 |
Monday, November 23, 2015 8:00AM - 8:13AM |
G30.00001: Numerical simulations of turbulence and mixing induced by submesoscale instabilities Megan Stamper, John Taylor Submesoscale features in the upper ocean with horizontal scales between 1-10km have received significant attention in the oceanography community in recent years. Previous work has found that submesoscales play an important role in setting the stratification of the upper ocean, and these scales are associated with large vertical velocities that modify biological productivity. Submesoscales bridge the dynamical gap between the mesoscale ($\sim$100km) where the earth's rotation plays a major role, and turbulent overturning scales ($\sim$1-10m) where the earth's rotation is not directly felt. Here, we use very high resolution direct numerical simulations (DNS) to explore the interaction and feedbacks between submesoscales and small scale turbulence. In simulations with submesoscale motions generated via symmetric and baroclinic instability, we find that the emergence of secondary instabilities leads to significant small-scale turbulence and mixing, even in the absence of wind and convective forcing. From the DNS results, we quantify the additional mixing, dissipation, and vertical fluxes induced by small scale turbulence, and its feedback on the primary submesoscale instabilities. [Preview Abstract] |
Monday, November 23, 2015 8:13AM - 8:26AM |
G30.00002: Nonlinear evolution of a baroclinic wave and imbalanced dissipation Balu Nadiga The question of how ocean circulation equilibrates in the presence of continuous large-scale forcing and a tendency of geostrophic turbulence to confine energy to large and intermediate scales is considered. By considering the nonlinear evolution of an unstable baroclinic wave at small Rossby and Froude numbers (small aspect ratio domain) at high resolutions, it is shown that submesoscale instabilities provide an {\em interior} pathway between the energetic oceanic mesoscales and smaller unbalanced scales. An estimate of the magnitude of this pathway is presented. Phenomenology-wise, mesoscale shear and strain resulting from the primary baroclinic instability drive frontogenesis; fronts in turn support ageostrophic secondary circulation and instabilities. These two processes together lead to a quick rise in dissipation rate which then reaches a peak and begins to fall as frontogenesis slows down; eventually balanced and imbalanced modes decouple. Dissipation of balanced energy by imbalanced processes is shown to scale exponentially with Rossby number of the base flow. Further, a break is seen in the total energy (TE) spectrum at small scales with a transition from $k^{-3}$ to $k^{-5/3}$ reminiscent of the atmospheric spectra of Nastrom \& Gage. For details see JFM 756, 965-1006. [Preview Abstract] |
Monday, November 23, 2015 8:26AM - 8:39AM |
G30.00003: Submesoscale baroclinic instability and the Balance Equations Ian Grooms Ocean submesoscale baroclinic instability is studied in the framework of the Balance Equations. The Balance Equations are an intermediate model that includes balanced ageostrophic effects with higher accuracy than the quasigeostrophic approximation, but rules out unbalanced wave motions; as such, they are particularly suited to the study of baroclinic instability in submesoscale ocean dynamics. The linear baroclinic instability problem is developed in generality and then specialized to the case of constant vertical shear. The primary finding is that at low Richardson numbers the growth rate of some instability modes is increased compared to larger-scale quasigeostrophic dynamics, and that the increase can be attributed to both ageostrophic baroclinic production and shear production of perturbation energy. This suggests that the nonlinear development of submesoscale baroclinic instability will proceed more vigorously than mesoscale/quasigeostrophic, and may include a downscale/forward transfer of kinetic energy. [Preview Abstract] |
Monday, November 23, 2015 8:39AM - 8:52AM |
G30.00004: Coupled evolution of near-inertial waves and quasigeostrophic flow Gregory Wagner, William Young We derive a model describing the coupled nonlinear evolution of three fields: near-inertial wave (NIW) amplitude, quasigoestrophic potential vorticity, and the NIW second harmonic. The model is derived by asymptotic reduction of the Boussinesq equations using the method of multiple scales. The model conserves two distinct quantities: wave action, and coupled energy. Wave action conservation implies energy exchange between NIW kinetic energy and energy in the NIW second harmonic. Coupled energy conservation implies energy exchange between NIW potential energy and quasigeostrophic flow. We explore the implications of the model with two-dimensional numerical solutions meant to approximate NIW evolution in non-uniform quasigeostrophic flow following storm-driven excitation. For this scenario we find good agreement between the model and solutions of the full Boussinesq equations. Preliminary results show the initial transient evolution of the NIW field extracts energy from the quasigeostrophic flow. Further, the quasigeostrophic flow catalyzes an interaction between the NIW and the NIW second harmonic which ultimately leads to the generation of small NIW vertical scales. [Preview Abstract] |
Monday, November 23, 2015 8:52AM - 9:05AM |
G30.00005: Lagrangian and Eulerian Statistics of Vorticity Dynamics in Turbulent Stratified Shear Flows Frank Jacobitz, Kai Schneider, Marie Farge The Lagrangian and Eulerian time-rate of change statistics of vorticity in homogeneous turbulence with shear and stable stratification are studied. Direct numerical simulations are performed, in which the Richardson number is varied from Ri=0, corresponding to unstratified shear flow, to Ri=1, corresponding to strongly stratified shear flow. The probability density functions (pdfs) of both Lagrangian and Eulerian time-rates of change show a strong influence on the Richardson number. The Lagrangian time-rate of change pdf has a stretched-exponential shape due to the vortex stretching and tilting term in the equation for fluctuating vorticity. The shape of the Eulerian time-rate of change pdf was also observed to be stretched-exponential and the extreme values for the Eulerian time-rate of change are larger than those observed for the Lagrangian counterpart due to the nonlinear term in the vorticity equation. The Lagrangian and Eulerian acceleration pdfs are mainly determined by the pressure-gradient and nonlinear terms in the Navier-Stokes equation, respectively. The Lagrangian time-rate of change pdf of fluctuating density does not show a stretched exponential shape, while its Eulerian counterpart does due to the nonlinear term in the in the density advection-diffusion equation. [Preview Abstract] |
Monday, November 23, 2015 9:05AM - 9:18AM |
G30.00006: Flow dynamics at a river confluence on Mississippi River: field measurement and large eddy simulation Trung Le, Ali Khosronejad, Nicole Bartelt, Solomon Woldeamlak, Bonnie Peterson, Petronella DeWall, Fotis Sotiropoulos We study the dynamics of a river confluence on Mississippi River branch in the city of Minneapolis, Minnesota, United States. Field measurements by Acoustic Doppler Current Profiler using on-board GPS tracking were carried out for five campaigns in the summer of 2014 and 2015 to collect both river bed elevation data and flow fields. Large Eddy Simulation is carried out to simulate the flow field with the total of 100 million grid points for the domain length of 3.2 km. The simulation results agree well with field measurements at measured cross-sections. The results show the existence of wake mode on the mixing interface of two branches near the upstream junction corner. The mutual interaction between the shear layers emanating from the river banks leading to the formation of large scale energetic structures that leads to ``switching'' side of the flow coherent structures. Our result here is a feasibility study for the use of eddy-resolving simulations in predicting complex flow dynamics in medium-size natural rivers. [Preview Abstract] |
Monday, November 23, 2015 9:18AM - 9:31AM |
G30.00007: Anisotropic shear dispersion parameterization for ocean eddy transport Scott Reckinger, Baylor Fox-Kemper The effects of mesoscale eddies are universally treated isotropically in global ocean general circulation models. However, observations and simulations demonstrate that the mesoscale processes that the parameterization is intended to represent, such as shear dispersion, are typified by strong anisotropy. We extend the Gent-McWilliams/Redi mesoscale eddy parameterization to include anisotropy and test the effects of varying levels of anisotropy in 1-degree Community Earth System Model (CESM) simulations. Anisotropy has many effects on the simulated climate, including a reduction of temperature and salinity biases, a deepening of the southern ocean mixed-layer depth, impacts on the meridional overturning circulation and ocean energy and tracer uptake, and improved ventilation of biogeochemical tracers, particularly in oxygen minimum zones. A process-based parameterization to approximate the effects of unresolved shear dispersion is also used to set the strength and direction of anisotropy. The shear dispersion parameterization is similar to drifter observations in spatial distribution of diffusivity and high-resolution model diagnosis in the distribution of eddy flux orientation. [Preview Abstract] |
Monday, November 23, 2015 9:31AM - 9:44AM |
G30.00008: Mesoscale Ocean Large Eddy Simulations Brodie Pearson, Baylor Fox-Kemper, Scott Bachman, Frank Bryan The highest resolution global climate models (GCMs) can now resolve the largest scales of mesoscale dynamics in the ocean. This has the potential to increase the fidelity of GCMs. However, the effects of the smallest, unresolved, scales of mesoscale dynamics must still be parametrized. One such family of parametrizations are mesoscale ocean large eddy simulations (MOLES), but the effects of including MOLES in a GCM are not well understood. In this presentation, several MOLES schemes are implemented in a mesoscale-resolving GCM (CESM), and the resulting flow is compared with that produced by more traditional sub-grid parametrizations. Large eddy simulation (LES) is used to simulate flows where the largest scales of turbulent motion are resolved, but the smallest scales are not resolved. LES has traditionally been used to study 3D turbulence, but recently it has also been applied to idealized 2D and quasi-geostrophic (QG) turbulence. The MOLES presented here are based on 2D and QG LES schemes. [Preview Abstract] |
Monday, November 23, 2015 9:44AM - 9:57AM |
G30.00009: SOMAR-LES for multiscale modeling of internal tide generation Vamsi Krishna Chalamalla, Edward Santilli, Masoud Jalali, Alberto Scotti, Sutanu Sarkar A novel modeling technique is developed to study baroclinic energy conversion when the barotropic tide oscillates over underwater topography. In SOMAR-LES, a Large Eddy Simulation (LES) model that resolves turbulence scales is coupled with a large-scale model, Stratified Ocean Model with Adaptive Refinement (SOMAR). Thus, we overcome the constraints posed by the wide range of temporal and spatial scales during tide and topographic interaction. Two-way coupling is developed: LES is driven with large scale forcing, and SOMAR receives feedback in the form of eddy viscosity and diffusivity. Numerical simulations are performed with SOMAR-LES for supercritical and subcritical ridges with ridge length scales of $ \mathcal{O}$(10 km) and barotropic forcing that corresponds to the regime of low outer excursion number, $Ex = U_0/\omega l \simeq 0.1$. Results from the coupled model are compared against ongoing high-resolution LES to ascertain the accuracy of this technique. The simulation data is analyzed to quantify baroclinic energy conversion and the change in modal composition from near to far field. [Preview Abstract] |
Monday, November 23, 2015 9:57AM - 10:10AM |
G30.00010: Baroclinic mixed layer instability in the presence of convection Joern Callies, Raffaele Ferrari It has recently been discovered that geostrophic turbulence in the upper ocean undergoes a seasonal cycle at submesoscales, the scales smaller than the most energetic mesoscale eddies. Observations and theory suggest that baroclinic mixed layer instabilities release potential energy stored in deep mixed layers, energizing the submesoscales in winter. In shallow summer mixed layers, there is no such energization. The oceanic mixed layer, besides being prone to baroclinic instabilities, is subject to atmospheric forcing, which drives convective overturns. We here study how this forced convection interacts with baroclinic instabilities in a set of idealized numerical simulations resolving both processes. A major question is whether baroclinic instabilities can be damped out by convection. Implications for the seasonal cycle in submesoscale turbulence will be discussed. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700