Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session H13: Geophysical Fluid Dynamics: Oceans II General |
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Sponsoring Units: DFD GPC Chair: Chris Vogl, University of Washington Room: C124 |
Monday, November 21, 2016 10:40AM - 10:53AM |
H13.00001: ABSTRACT WITHDRAWN |
Monday, November 21, 2016 10:53AM - 11:06AM |
H13.00002: Simulation of Earthquake-Generated Sea-Surface Deformation Chris Vogl, Randy LeVeque Earthquake-generated tsunamis can carry with them a powerful, destructive force. One of the most well-known, recent examples is the tsunami generated by the Tohoku earthquake, which was responsible for the nuclear disaster in Fukushima. Tsunami simulation and forecasting, a necessary element of emergency procedure planning and execution, is typically done using the shallow-water equations. A typical initial condition is that using the Okada solution for a homogeneous, elastic half-space. This work focuses on simulating earthquake-generated sea-surface deformations that are more true to the physics of the materials involved. In particular, a water layer is added on top of the half-space that models the seabed. Sea-surface deformations are then simulated using the Clawpack hyperbolic PDE package. Results from considering the water layer both as linearly elastic and as “nearly incompressible” are compared to that of the Okada solution. [Preview Abstract] |
Monday, November 21, 2016 11:06AM - 11:19AM |
H13.00003: Airborne infrared remote sensing characterization of submesoscale eddies Geoffrey Smith, George Marmorino, W. David Miller, Ryan North, Ingrid Angel-Benavides, Burckard Baschek Airborne remote sensing surveys off Santa Catalina Island, CA ($33^{\circ} 30'N \; 118^{\circ} 31' W$) were conducted as part of a larger study of the occurrence and behavior of submesoscale phenomena. This builds upon previous work by DiGiacomo and Holt, who utilized SAR imagery to characterize the size and distribution of predominately cyclonic 'spiral eddies' in the Southern California Bight. In the present work the thermal surface expression of a single cyclonic eddy captured in February 2013 will be investigated. Advances made in methods to estimate eddy circulation and vorticity directly from the thermal imagery will be discussed and compared with \textit{in situ} measurements. Inferences about localized mixing and flow instabilities can also be drawn from the imagery, and these too will be discussed in the context of \textit{in situ} data. A simple model will be offered describing the three dimensional flow in the core of the eddy and how that can be used to explain the surface imagery. Connections between the signatures surrounding the eddy and the core itself will also be discussed in the context of the model. [Preview Abstract] |
Monday, November 21, 2016 11:19AM - 11:32AM |
H13.00004: Predictability of the Dynamic Mode Decomposition in Coastal Processes Ruo-Qian Wang, Liv Herdman, Mark Stacey, Patrick Barnard Dynamic Mode Decomposition (DMD) is a model order reduction technique that helps reduce the complexity of computational models. DMD is frequently easier to interpret physically than the Proper Orthogonal Decomposition. The DMD can also produce the eigenvalues of each mode to show the trend of the mode, establishing the rate of growth or decay, but the original DMD cannot produce the contributing weights of the modes. The challenge is selecting the important modes to build a reduced order model. DMD variants have been developed to estimate the weights of each mode. One of the popular methods is called Optimal Mode Decomposition (OMD). This method decomposes the data matrix into a product of the DMD modes, the diagonal weight matrix, and the Vandermonde matrix. The weight matrix can be used to rank the importance of the mode contributions and ultimately leads to the reduced order model for prediction and controlling purpose. We are currently applying DMD to a numerical simulation of the San Francisco Bay, which features complicated coastal geometry, multiple frequency components, and high periodicity. Since DMD defines modes with specific frequencies, we expect DMD would produce a good approximation, but the preliminary results show that the predictability of the DMD is poor if unimportant modes are dropped according to the OMD. We are currently testing other DMD variants and will report our findings in the presentation. [Preview Abstract] |
Monday, November 21, 2016 11:32AM - 11:45AM |
H13.00005: The response of the Ocean Surface Boundary Layer and Langmuir turbulence to tropical cyclones Dong Wang, Tobias Kukulka, Brandon Reichl, Tetsu Hara, Isaac Ginis The interaction of turbulent ocean surface boundary layer (OSBL) currents and the surface waves' Stokes drift generates Langmuir turbulence (LT), which enhances OSBL mixing. This study investigates the response of LT to extreme wind and complex wave forcing under tropical cyclones (TCs), using a large eddy simulation (LES) approach based on the wave-averaged Navier-Stokes equations. We simulate the OSBL response to TC systems by imposing the wind forcing of an idealized TC storm model, covering the entire horizontal extent of the storm systems. The Stokes drift vector that drives the wave forcing in the LES is determined from realistic spectral wave simulations forced by the same wind fields. We find that the orientations of Langmuir cells are vertically uniform and aligned with the wind in most regions despite substantial wind-wave misalignment in TC conditions. LT's penetration depth is related to Stokes drift depth and limited by OSBL depth. A wind-projected surface layer Langmuir number is proposed and successfully applied to scale turbulent vertical velocity variance in extreme TC conditions. [Preview Abstract] |
Monday, November 21, 2016 11:45AM - 11:58AM |
H13.00006: Laboratory experiments with a buoyancy forced circulation in a rotating basin Catherine Vreugdenhil, Ross Griffiths, Bishakhdatta Gayen We consider the relative influence of buoyancy forcing and Coriolis effects on convection forced by a differential in heating at a horizontal surface in a rectangular basin. Laboratory experiments with water are reported for a rotating $f$-plane basin and a range of Ekman number $E=2\times10^{-7}-1\times10^{-5}$. Heating is applied over half of the base as a uniform flux and cooling applied over the other half as a uniform temperature, resulting in a flux Rayleigh number $Ra_F=O(10^{14})$ large enough to ensure turbulent convection, where $Ra_F$ defined in terms of domain length $L$. Compared to the non-rotating circulation where Nusselt number (a measure of the convective to conductive heat transfer) scales as $Nu\sim Ra_F^{1/6}$, the strongly rotating regime is determined by a geostrophic balance of the larger scales of horizontal flow in the inviscid thermal boundary with $Nu\sim Ro^{1/6}$, where $Ro=B^{1/2}/(f^{3/2}L)$ is the natural Rossby number ($B$ is buoyancy flux per unit area and $f$ is Coriolis parameter). We also find evidence for a further transition into a regime where the circulation is dominated by deep `chimney' convection in a field of small vortical plumes and $Nu$ is more weakly dependent on rotation. [Preview Abstract] |
Monday, November 21, 2016 11:58AM - 12:11PM |
H13.00007: Turbulent mixing at high Schmidt number: new results from a hybrid spectral compact finite difference and dual grid resolution approach M.P. Clay, P.K. Yeung, T. Gotoh Turbulent mixing at high Schmidt number ($Sc$) (low molecular diffusivity) is characterized by fluctuations that arise at sub-Kolmogorov scales and are hence difficult to resolve or measure. Simulations in the recent past have provided some basic results but were still limited in either the Reynolds number or the Schmidt number. We have developed a massively parallel implementation of a hybrid pseudo-spectral and combined compact finite difference technique [Gotoh {\em et al.} {\em J. Comput Phys.} {\bf 231}, 7398-7414 (2012)] where the velocity and scalar fields are computed at different grid resolutions (the latter up to $8192^3$). A specific target is the scalar field maintained by a uniform mean gradient at Taylor-scale Reynolds number 140 and $Sc=512$, which is comparable to the value (700) for salinity in the ocean. Preliminary results at moderately high $Sc$ are in support of Batchelor ($k^{-1}$) scaling for the spectrum in the viscous-convective range, followed by exponential fall-off in the viscous-diffusive range. Data over a wide range of Reynolds and Schmidt numbers are used to examine the approach to local isotropy and a saturation of intermittency suggested by previous work. [Preview Abstract] |
Monday, November 21, 2016 12:11PM - 12:24PM |
H13.00008: The role of turbulence driven by tidal and librational forcing in planetary fluid layers. Alexander Grannan, Benjamin Favier, Bruce Bills, Michael Le Bars, Jonathan Aurnou The turbulence generated in the liquid metal cores and oceans of planetary bodies can have profound effects on energy dissipation and magnetic field generation. An important driver of such turbulence is mechanical forcing from precession, libration, and tidal forcing. On Earth, such forcing mechanisms in the oceans are crucial but the role that such forcings play for other planetary bodies also possessing oceans and liquid metal cores are not generally considered. Recent laboratory experimental and numerical studies of Grannan et al. Phys. Fluids 2014, Favier et al. Phys. Fluids 2015, and Grannan et al. Geophys. J. Int. 2016 have shown that turbulent flow is driven by an elliptic instability which is a triadic resonance between two inertial modes and the base flow. Based on the most recent work, a generalized scaling law for the saturated r.m.s. velocity is found, $U\sim\beta$, where $\beta$ is the dimensionless equatorial ellipticity of the body. Using planetary values for tidal and librational forcing parameters, we argue that mechanically forced turbulent flows can play a significant role in dissipative processes, mixing, and magnetic field generation. [Preview Abstract] |
Monday, November 21, 2016 12:24PM - 12:37PM |
H13.00009: Dimensions of continents and oceans -- water has carved a perfect cistern John A Whitehead The ocean basins have almost exactly the correct surface area and average depth to hold Earth's water. Two processes are responsible for this. First, Earth's continental crust is thinned by erosion so that average elevation is a few hundred meters above sea level. Second, the crust is thickened by lateral compression from mountain formation and sediments and water lost in subduction is resupplied at least in part by voclanics. The resulting continents are approximately tabular in cross section, resulting in the well-known double hypsometric curve for Earth's elevation. Therefore, erosion and mountain building have enabled water to carve its own cistern in the form of all the ocean basins. A theoretical fluid model, suggested partly by laboratory experiments, produces such a tabular continent with a surface above sea level. A simple hydrostatic balance gives a first approximation for the average depth and area of oceans and continents for present Earth as a function of material volumes and densities. Using a wide range of possible crust volumes with the present water volume, the average continental crust thickness exceeds 22 km and ocean area exceeds 25{\%} of the globe. Other volumes of water produce a wide range of areas and depths of oceans and crust. [Preview Abstract] |
Monday, November 21, 2016 12:37PM - 12:50PM |
H13.00010: Ocean acidification: Towards a better understanding of calcite dissolution Monica M. Wilhelmus, Jess Adkins, Dimitris Menemenlis The drastic increase of anthropogenic CO$_{\mathrm{2}}$~emissions over the past two centuries has altered the chemical structure of the ocean, acidifying upper ocean waters. The net impact of this pH decrease on marine ecosystems is still unclear, given the unprecedented rate at which CO$_{\mathrm{2}}$~is being released into the atmosphere.~ As part of the carbon cycle, calcium carbonate dissolution in sediments neutralizes CO$_{\mathrm{2}}$: phytoplankton at the surface produce carbonate minerals, which sink and reach the seafloor after the organisms die. On time scales of thousands of years, the calcium carbonate in these shells ultimately reacts with CO$_{\mathrm{2}}$~in seawater. Research in this field has been extensive; nevertheless, the dissolution rate law, the impact of boundary layer transport, and the feedback with the global ocean carbon cycle remain controversial. Here, we (i) develop a comprehensive numerical framework via 1D modeling of carbonate dissolution in sediments, (ii) approximate its impact on water column properties by implementing a polynomial approximation to the system's response into a global ocean biogeochemistry general circulation model (OBGCM), and (iii) examine the OBGCM sensitivity response to different formulations of sediment boundary layer properties. We find that, even though the burial equilibration time scales of calcium carbonate are in the order of thousands of years, the formulation of a bottom sediment model along with an improved description of the dissolution rate law can have consequences on multi-year to decadal time scales. [Preview Abstract] |
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