Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session D13: Geophysical: General II |
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Chair: Claudia Cenedese, Wood Hole Oceanographic Institution Room: 27A |
Sunday, November 18, 2012 2:15PM - 2:28PM |
D13.00001: Nonlinear Scale Interactions and Energy Pathways in the Ocean Matthew Hecht, Hussein Aluie, Geoffrey Vallis, Kirk Bryan, Mathew Maltrud, Robert Ecke, Beth Wingate Large-scale currents and eddies pervade the ocean and play a prime role in the general circulation and climate. The coupling between scales ranging from $O(10^4)$ km down to $O(1)$ mm presents a major difficulty in understanding, modeling, and predicting oceanic circulation and mixing, where the energy budget is uncertain within a factor possibly as large as ten. Identifying the energy sources and sinks at various scales can reduce such uncertainty and yield insight into new parameterizations. To this end, we refine a novel coarse-graining framework to directly analyze the coupling between scales. The approach is very general, allows for probing the dynamics simultaneously in scale and in space, and is not restricted by usual assumptions of homogeneity or isotropy. We apply these tools to study the energy pathways from high-resolution ocean simulations using LANL's Parallel Ocean Program. We examine the extent to which the traditional paradigm for such pathways is valid at various locations such as in western boundary currents, near the equator, and in the deep ocean. We investigate the contribution of various nonlinear mechanisms to the transfer of energy across scales such as baroclinic and barotropic instabilities, barotropization, and Rossby wave generation. [Preview Abstract] |
Sunday, November 18, 2012 2:28PM - 2:41PM |
D13.00002: How Do You Determine Whether The Earth Is Warming Up? Juan Restrepo, Darin Comeau, Hermann Flaschka How does one determine whether the high summer temperatures in Moscow of a few years ago was an extreme climatic fluctuation or the result of a systematic global warming trend? How does one perform an analysis of the causes of this summer's high temperatures in the US, if climate variability is poorly constrained? It is only under exceptional circumstances that one can determine whether a climate signal belongs to a particular statistical distribution. In fact, climate signals are rarely ``statistical;'' there is usually no way to obtain enough field data to produce a trend or tendency, based upon data alone. There are other challenges to obtaining a trend: inherent multi-scale manifestations, and nonlinearities and our incomplete knowledge of climate variability. We propose a trend or tendency methodology that does not make use of a parametric or a statistical assumption and it is capable of dealing with multi-scale time series. The most important feature of this trend strategy is that it is defined in very precise mathematical terms. [Preview Abstract] |
Sunday, November 18, 2012 2:41PM - 2:54PM |
D13.00003: Mechanical energy budget for horizontal convection Bishakhdatta Gayen, Ross W. Griffiths, Graham O. Hughes, Juan A. Saenz A three dimensional direct numerical simulation is performed to study horizontal convection in a long channel at a large Rayleigh number (of $O(10^{12})$). A different temperature is applied over each half of the channel base and the flow is allowed to reach a state of thermal equilibrium in which there is no net heat input. The circulation and temperature field accords with that observed in previous experiments and numerical simulations, and we focus on understanding horizontal convection from an energetics viewpoint. All terms in the mechanical energy budget can be evaluated explicitly, and we use this methodology to show how a strong circulation can be maintained despite the tight constraints on viscous dissipation in the flow established by previous work. An important conclusion is that horizontal convection represents a highly efficient mechanism of mixing in a stratified fluid. [Preview Abstract] |
Sunday, November 18, 2012 2:54PM - 3:07PM |
D13.00004: The Hydrodynamics of Iceberg Capsize Near a Glacier Terminus J.C. Burton, L.M. Cathles, D.R. MacAyeal, W.W. Zhang, J.M. Amundson, S. Correa-Legisos Marine-terminating glaciers lose most of their mass into the ocean by calving icebergs. The largest icebergs are frequently observed to capsize as they calve, releasing enormous amounts of gravitational potential energy. During this process they may collide with the glaciers' terminius, producing teleseismic ``glacial earthquakes'' which can be detected by the Global Seismic Network. We use a combination of laboratory wave-tank experiments and numerical modeling to show that the contact and pressure forces exerted on the glacier terminus are strongly influenced by the hydrodynamics of the capsize process. In particular, we find that hydrodynamics can significantly increase the magnitude and duration of the contact force with the terminus, and that the earthquake magnitude, expressed as a twice-integrated force history, is not simply proportional to iceberg size. Our results highlight the difficulty of interpreting seismograms due to iceberg collisions. [Preview Abstract] |
Sunday, November 18, 2012 3:07PM - 3:20PM |
D13.00005: Laboratory experiments investigating the influence of fjord circulation on submarine melting of Greenland's Glaciers Claudia Cenedese A set of idealized laboratory experiments investigates the ice-ocean boundary dynamics near a vertical ``glacier'' (i.e. no floating ice tongue) in a two-layer stratified fluid, representative of Sermilik Fjord where Helheim Glacier terminates. Two fjord circulations are compared to a control experiment with no forced flow. The estuarine circulation is generated by introducing fresh water at melting temperatures from a source at the water free surface near the ice block representing the glacier. The wind driven circulation is generated by vertically displacing a solid block at the end of the tank opposite the ice block, mimicking the observed fjord circulation driven by wind events. The magnitude of both circulations can be systematically varied. The circulation pattern observed in the control and estuarine experiments is similar to those observed in previous studies. A thin boundary layer of cold melt water mixes with ambient waters and rises until it finds either the interface between the two layers, if in the bottom layer, or the free surface, if in the top layer. The results suggest that the melt water mainly deposits within the interior of the water column and not entirely at the free surface, as confirmed by field observations. In the wind driven experiments, the submarine melting of the glacier is enhanced and it increases with increasing wind frequency, suggesting that this circulation is more efficient in transporting heat to the glacier. [Preview Abstract] |
Sunday, November 18, 2012 3:20PM - 3:33PM |
D13.00006: 3D Baroclinic Vortices in Rotating Stratified Shear: from an Orange Great Red Spot to Planet Formation Pedram Hassanzadeh, Philip Marcus The presence of horizontal shear strongly influences the dynamics of vortices in rotating stratified flows. Examples of such vortices are the Jovian vortices in zonal shear, and the vortices of the protoplanetary disks in strong Keplerian shear. Studying the physics of these vortices and their interaction with the environment requires high-resolution 3D simulations: ignoring the vertical direction or lack of enough resolution eliminates or changes important physical processes such as the secondary circulation. This ageostrophic flow might be essential in dust accretion and planet formation in protoplanetary disks, and a key in longevity, color, and color-change of Jovian vortices. For example, the very recent (July 2012) color change of the Great Red Spot to pale orange is most likely created by such secondary circulation. We have used high-resolution 3D numerical simulations of the Boussinesq equations to study 3D baroclinic vortices embedded in rotating stratified shear. We discuss the physics of their secondary circulation its ability to transport red chromphores and dust particles. We also present preliminary results on the interaction of vortices with shear, and show how this interaction affects their longevity. [Preview Abstract] |
Sunday, November 18, 2012 3:33PM - 3:46PM |
D13.00007: Zonal turbulence driven by baroclinic instabilities in the thermal quasigeostrophic equations Emma Warneford, Paul Dellar We present a mass- and momentum-conserving single layer model for the atmospheres of gas giant planets that produces both sub- and super-rotating equatorial jets. The thermal shallow water equations support horizontal temperature variations by treating the reduced gravity as an advected scalar. They were originally proposed for terrestrial lakes and tropical oceans. We add a radiative coupling term to create a model for the atmospheres of gas giant planets. Reducing the radiative relaxation time produces transitions from sub- to super-rotating equatorial jets. The radiative coupling also enhances the rate at which eddy kinetic energy supplied by small-scale random forcing is absorbed into the mean zonal flow. The quasigeostrophic limit of our model supports an instability whose dispersion relation coincides, up to an O(1) numerical factor, with the dispersion relation for baroclinic instability in continuously stratified fluids. This instability can drive sustained turbulence from imposed large-scale variations in the background temperature from pole to equator. Running our spectral numerical simulations on graphical processing units (GPUs) yields substantial (factors of ten) performance increases with little additional programming effort. [Preview Abstract] |
Sunday, November 18, 2012 3:46PM - 3:59PM |
D13.00008: Zonal winds and flows generated by harmonic forcing in planetary atmospheres, subsurface oceans and cores Alban Sauret, David Cebron, Michael Le Bars, Stephane Le Dizes, Patrice Le Gal A huge amount of energy is stored in the spin and orbital motions of any planet, and under certain circumstances, harmonic forcings such as libration, precession and tides are capable of conveying a portion of this energy to drive intense three-dimensional flows in its liquid layers. These mechanisms are studied here by combining theoretical, experimental and numerical approaches. At first, we focus on the effect of longitudinal librations, corresponding to oscillations of the rotation rate of a planet. This boundary forcing systematically leads to a correction to the mean solid body rotation through non-linear interactions in the Ekman layers. This geostrophic zonal wind is well described by an analytical approach. Additionally, at sufficiently large libration amplitude or small Ekman number, the oscillating flow is periodically unstable with respect to centrifugal instability. The resulting Taylor-G\"{o}rtler vortices generated in the Ekman layers then generate inertial waves in the bulk with well-defined characteristics and temporal signatures. Inertial waves can also be resonantly excited by any harmonic forcing, when the forcing frequency ranges between plus and minus twice the rotation rate. In any case, the nonlinear self-interaction of excited inertial waves may drive an intense and localised axisymmetric jet, which becomes unstable at low Ekman number following a shear instability, generating space-filling turbulence. This generic mechanism is illustrated here by an experimental study of tidal forcing in a spherical shell. [Preview Abstract] |
Sunday, November 18, 2012 3:59PM - 4:12PM |
D13.00009: Coherent structures and warm-core rings in the Gulf of Mexico Doug Lipinski, Kamran Mohseni We use Lagrangian coherent structures (LCS) to investigate the three-dimensional structure of warm core rings shed from the loop current in the Gulf of Mexico. Using LCS allows for a precise computation of the eddies' depth that closely matches a model to predict eddy depth based on the geostrophic balance. Additionally, the LCS reveal a checkerboard pattern and interesting flow dynamics in the near surface boundary layer that causes fluid to be stretched and wrapped around the eddy. The flow behavior in this region is analyzed and compared to an analytically defined flow model where many properties may be proved directly. Notably, the relative strength of hyperbolic stretching and shear influences the transport and mixing properties of the flow. [Preview Abstract] |
Sunday, November 18, 2012 4:12PM - 4:25PM |
D13.00010: An experimental and numerical study of cyclones produced by suction in rotating stratified flows Patrice Le Gal, Pedram Hassanzadeh, Oriane Aubert, Michael Le Bars, Philip Marcus Rotating and stratified flow motions are well-described by the gradient-wind equation, from which Hassanzadeh \textit{et al.} (2012) and Aubert \textit{et al.} (2012) derived a new law for the shape of the 3D vortices. The new equation was confirmed experimentally and numerically, and using the measurement data of the Atlantic Meddies and Jovian vortices. One consequence of this equation is that the interior of cyclones (anticyclones) must be more (less) stratified than the background flow. This means that to generate a cyclone (anticyclone) in nature, a process must produce both cyclonic (anticyclonic) vorticity and local-superstratification (a locally mixed patch of density). We have used laboratory experiments and 3D numerical simulations to study cyclones produced by localized suction, and we show that this process in fact produces both cyclonic vorticity and super-stratification. The physics of super-stratification and decaying cyclones in rotating stratified flows is studied. This brings a new understanding of the asymmetry between cyclones and anticyclones in nature, as it appears to be easier to locally mix some fluid to create a patch with a weaker stratification than the background as inside anticyclones, rather than to locally super-stratify it as inside cyclones. [Preview Abstract] |
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