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 M29: Geophysical Fluid Dynamics: Cryosphere and Ice-Ocean Interactions |
Hide Abstracts |
Sponsoring Units: DFD GPC Chair: Colin Meyer, Harvard University Room: 310 |
Tuesday, November 24, 2015 8:00AM - 8:13AM |
M29.00001: Moffatt eddies at the base of ice sheets Colin R. Meyer, Timothy T. Creyts, James R. Rice Despite extensive radar surveys of the deep ice, the conditions at the base of ice sheets remain uncertain. Complex structures that include large stratigraphic folds and basal freeze-on ice appear to be related to topography. In many locations beneath both Greenland and Antarctica, ice flows across deep valleys, potentially forming viscous eddies that can confound the interpretation of ice-bed processes. To understand the formation of these eddies, we use a set up analogous to that of Moffatt (1964), where our domain is a subglacial valley. We numerically solve the non-Newtonian Stokes equations with a shear-thinning power-law rheology to determine the critical valley angle for the eddies to form. The shear-thinning nature of ice allows for greater shear localization and, therefore, ice requires smaller valley angles (steeper slopes) to form eddies than a Newtonian fluid. Due to the significant variation of temperature from the warm base to the cold surface of the ice sheet, we analyze eddy formation when the rheology is temperature dependent. The warmer basal ice is less viscous and eddies form in larger valley angles (shallower slopes). Finally, we solve for the ice flow over topography from the Gamburtsev subglacial mountains and show Moffatt eddies in the subglacial valleys. [Preview Abstract] |
Tuesday, November 24, 2015 8:13AM - 8:26AM |
M29.00002: Forced convective melting at an evolving ice-water interface Eshwan Ramudu, Benjamin Hirsh, Peter Olson, Anand Gnanadesikan The intrusion of warm Circumpolar Deep Water into the ocean cavity between the base of ice shelves and the sea bed in Antarctica causes melting at the ice shelves' basal surface, producing a turbulent melt plume. We conduct a series of laboratory experiments to investigate how the presence of forced convection (turbulent mixing) changes the delivery of heat to the ice-water interface. We also develop a theoretical model for the heat balance of the system that can be used to predict the change in ice thickness with time. In cases of turbulent mixing, the heat balance includes a term for turbulent heat transfer that depends on the friction velocity and an empirical coefficient. We obtain a new value for this coefficient by comparing the modeled ice thickness against measurements from a set of nine experiments covering one order of magnitude of Reynolds numbers. Our results are consistent with the altimetry-inferred melting rate under Antarctic ice shelves and can be used in climate models to predict their disintegration. [Preview Abstract] |
Tuesday, November 24, 2015 8:26AM - 8:39AM |
M29.00003: Direct numerical simulation of convection and dissolution at a vertical ice-seawater interface Bishakhdatta Gayen, Ross W. Griffiths, Ross C. Kerr Direct numerical simulations are performed to investigate the convection generated when a wall of ice dissolves into seawater under Antarctic ocean conditions. The ambient water temperatures are kept between $-1\:^{\circ}$C and $6\:^{\circ}$C and salinities around $35$ ppm, where diffusion of salt to the ice-water interface depresses the freezing point and further enhances heat diffusion to the ice. We use three coupled interface equations, along with the Boussinesq approximation and the equation of state for seawater, to solve for interface temperature, salinity and melt rate. Fluxes of both heat and salt to the interface play a significant role in governing the rate of dissolution of ice. At the presently achievable Grashof numbers turbulence is equally produced from both buoyancy and velocity shear, which indicates the importance of shear production at geophysical scales. [Preview Abstract] |
Tuesday, November 24, 2015 8:39AM - 8:52AM |
M29.00004: Subglacial hydrology as a control on ice stream shear margin locations Thibaut Perol, James R. Rice, John D. Platt, Jenny Suckale Ice streams are fast-flowing bands of ice separated from the nearly stagnant ice in the adjacent ridge by zones of highly localized deformation known as shear margins. However, it is presently unclear what mechanisms can control the location of shear margins. Within the shear margin, the transition from a slipping bed beneath the ice stream to a locked bed beneath the ridge concentrates stresses. We show that subglacial hydrology can select the shear margin location by strengthening the till within the margin. Our study uses a two-dimensional thermo-mechanical model in a cross-section perpendicular to the direction of flow. We show that the intense straining at the shear margins can generate large temperate regions within the deforming ice. Assuming that the melt generated in the temperate ice collects in a drainage channel at the base, we show that the channel locally decreases the pore pressure in the till. For a Coulomb-plastic rheology, this depressed pore pressure leads to a basal strength substantially higher than that inferred under the majority of the stream. Our results show that the additional basal resistance produced by the channel can reduce the stresses concentrated on the locked bed. Matching the model to surface velocity data at Whillans ice stream margin, we show that a stable shear margin occurs when the slipping-to-locked bed transition is less than 500 m away from a channel operating at an effective pressure of 200 kPa if the basal hydraulic transmissivity is equivalent to that of a water-film 0.2 mm thick. [Preview Abstract] |
Tuesday, November 24, 2015 8:52AM - 9:05AM |
M29.00005: A Theoretical and Experimental Investigation of Ice-Shelf Flow Dynamics Martin Wearing, Grae Worster, Richard Hindmarsh Ice-shelf buttressing is a major control on the rate of ice discharged from fast-flowing ice streams that drain the Antarctic Ice Sheet. The magnitude of the buttressing force depends on the shelf geometry and confinement. This geometry is determined by the ice-shelf extent, resulting from retreat due to iceberg calving and shelf advance due to flow. In contrast to large-scale ice-sheet models, which require high resolution datasets, we aim to gain insight using simple idealized models, focusing on the transition from lateral confinement to non-confinement. By considering a confined shelf with lateral shear stresses controlling the flow, steady-state analytical solutions can be calculated. These solutions are then compared to a numerical model for a confined flow, which incorporates both shear and extensional stresses. A boundary layer close to the calving front is identified, where both extensional and shear stresses control the dynamics. We test these idealized models against fluid-mechanical laboratory experiments, designed to simulate the flow of an ice shelf in a narrow channel. From these experiments velocity fields and altimetry for the ice-shelf are collected, allowing for comparison with the theoretical models and geophysical data. [Preview Abstract] |
Tuesday, November 24, 2015 9:05AM - 9:18AM |
M29.00006: Stability of lubricated ice sheets. Katarzyna N. Kowal, M. Grae Worster A significant amount of the Antarctic ice sheet drains towards the ocean through a network of ice streams, fast-flowing regions of ice that are generally well lubricated at their base by a layer of water-saturated, sub-glacial sediment known as till. Although till has a complex, nonlinear rheology, it deforms viscously over large spatial scales with an effective viscosity much lower than that of ice. Its dynamical interaction with the overlying ice can initiate a spontaneous instability of ice flow resulting in the formation of ice streams. We examine this interaction both mathematically and experimentally by considering the viscous coupling between two layers of fluid spreading under gravity. A series of our recent fluid-mechanical experiments reveal a novel cross-flow fingering instability if the lower layer is less viscous. We perform a linear stability analysis and explain the instability mechanism in terms of a jump in hydrostatic pressure gradient, stabilised by horizontal shear at large wave numbers, and assess the possibility of this mechanism leading to ice-stream formation. [Preview Abstract] |
Tuesday, November 24, 2015 9:18AM - 9:31AM |
M29.00007: Free fingering at the contact between spreading viscous fluids Jerome Neufeld, Laura Gell, Finn Box The spreading of viscous fluids is an everyday phenomena with large-scale applications to the flow of glaciers and the dynamics of mountain formation in continental collisions. When viscous fluids spread on an undeformable base the contact line is stable to perturbations. In contrast, when less viscous fluids displace more viscous fluids, as in a Hele-Shaw cell or porous matrix, the contact line is unstable to a fingering phenomena. Here we show, experimentally and theoretically, that when a viscous fluid spreads on a pre-existing layer of fixed depth and differing viscosity the geometry of the contact line depends sensitively on the ratio of fluid viscosities, the input flux and the initial layer depth. When the injected fluid is less viscous the contact line may become unstable to a fingering pattern reminiscent of Saffman-Taylor fingering. We explore the parameter space of this new instability, and highlight its applicability to understanding mountain formation and glacial ice streams. [Preview Abstract] |
Tuesday, November 24, 2015 9:31AM - 9:44AM |
M29.00008: Formation of snow penitentes by radiative instabilities Wilko Rohlfs Penitents are large scale snow or ice structures covering snow and glacier fields in the tropics or subtropics at high altitudes as for instance in the Andes. The characteristic structure of penitentes is their blade-like shape with a size ranging from a few centimeters up to 30m, with their walls preferentially orientated from east to west. Although the distribution of penitents on a snow or ice field appears chaotic, they exhibit a characteristic pattern with a distinct separation distance, for which their formation can be associated to some kind of instability. The instability results from the process of radiative trapping, which is caused by higher absorbed solar radiation in local cavities and troughs. As a consequence, a variation in the ablation forms the penitentes. A simple two-dimensional numerical model is developed to investigate the ablation process of penitentes. Special focus is given to the wavelength of the arising instability and its selection mechanism. The results of the numerical simulation indicate that the wavelength increases during ablation season and is directly tied to the inclination angle between snow surface and sun. [Preview Abstract] |
Tuesday, November 24, 2015 9:44AM - 9:57AM |
M29.00009: The dynamics of a suspension of solidifying, buoyant ice crystals David Rees Jones, Andrew Wells In a wide range of geophysical and industrial situations, the solidification of a liquid melt occurs through the growth of solid crystals suspended in the melt. For example, so-called frazil ice crystals form by freezing of the polar oceans, and crystals also form in the interior of solidifying magma chambers. The growth of these crystals is dynamically coupled to the fluid flow: advection enhances the transport and removal of latent heat that controls crystal growth, whilst the particles provide hydrodynamic feedbacks on the flow. The crystal density is typically different to the liquid density, which induces relative motion, and crystals may also induce density gradients within the liquid itself through the temperature field. We develop scaling arguments for the relative importance of crystal growth, agglomeration, nucleation and transport as a function of particle size and properties of the fluid flow. We introduce a new framework for the direct numerical simulation of the coupling of solidifying, buoyant particles to the fluid flow using a Lattice Boltzmann Method and present results for idealized test cases motivated by our scaling analysis. [Preview Abstract] |
Tuesday, November 24, 2015 9:57AM - 10:10AM |
M29.00010: Turbulent plumes from ice melting into a linearly stratified ocean Andrew Wells, Samuel Magorrian The melting of submerged marine glacier termini and ice shelves floating atop the ocean has important implications for ice sheet dynamics and sea level rise. When vertical or inclined ice faces melt into a warm salty ocean, the fresh meltwater rises in a buoyant plume along the ice-ocean interface and the resulting turbulent heat transfer provides a feedback on melting rates. We apply a turbulent plume model to consider the dynamics of well-mixed meltwater plumes rising along planar ice faces through a linearly stratified ocean, with vertical gradients of background ocean temperature and salinity. When the driving buoyancy force is dominated by salinity differences, the flow develops in a repeating series of layers, with the meltwater plume accelerating along the slope, rising past its neutral density level, and then separating from the ice face and intruding into the background ocean. We determine approximate scaling laws for the layer heights, melting rates and flow properties as a function of the background ocean temperature and salinity. These scaling laws provide a good collapse across a range of numerical solutions of the plume model, and may prove useful as a simple parameterisation of glacial melting in stratified Greenland fjords. [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