Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session A34: Geophysical Fluid Dynamics: CryosphereGeophysical
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Chair: Andrew Wells, University of Oxford Room: 102 |
Sunday, November 19, 2017 8:00AM - 8:13AM |
A34.00001: Onset and localisation of convection during transient growth of mushy sea ice Andrew Wells, Joe Hitchen More than 20 million square kilometres of the polar oceans freeze over each year to form sea ice. Sea ice is a mushy layer: a reactive, porous, multiphase material consisting of ice crystals bathed in liquid brine. Atmospheric cooling generates a density gradient in the interstitial brine, which can drive convection and rejection of brine from the sea ice to force ocean circulation and mixing. We use linear stability analysis and nonlinear numerical simulations to consider the convection in a transiently growing mushy layer. An initial salt water layer is cooled from above via a linearised thermal exchange with the atmosphere, and generates a growing mushy layer with the porosity varying in space and time. We determine how the critical porous-medium Rayleigh number for the onset of convection varies with the surface cooling rate, and the initial temperature and salinity of the solidifying salt water. Differences in the cooling conditions modify the structure of the ice and the resulting convection cells. Weak cooling leads to full-depth convection through ice with slowly varying porosity, whilst stronger cooling leads to localised convection confined to a highly permeable basal layer. These results provide insight into the onset of convective brine drainage from growing sea ice. [Preview Abstract] |
Sunday, November 19, 2017 8:13AM - 8:26AM |
A34.00002: Turbulent flow through channels in a viscously deforming matrix Colin Meyer, Ian Hewitt, Jerome Neufeld Channels of liquid melt form within a surrounding solid matrix in a variety of natural settings, for example, lava tubes and water flow through glaciers. Channels of water on the underside of glaciers, known as Rothlisberger (R-) channels, are essential components of subglacial hydrologic systems and can control the rate of glacier sliding. Water flow through these channels is turbulent, and dissipation melts open the channel while viscous creep of the surrounding closes the channel leading to the possibility of a steady state. Here we present an analogous laboratory experiment for R-channels. We pump warm water from the bottom into a tank of corn syrup and a channel forms. The pressure is lower in the water than in the corn syrup, therefore the syrup creeps inward. At the same time, the water ablates the corn syrup through dissolution and shear erosion, which we measure by determining the change in height of the syrup column over the course of the experiment. We find that the creep closure is much stronger than turbulent ablation which leads to traveling solitary waves along the water-syrup interface. These waves or `magmons' have been previously observed in experiments and theory for laminar magma melt conduits. We compliment our experiments with numerical simulations. [Preview Abstract] |
Sunday, November 19, 2017 8:26AM - 8:39AM |
A34.00003: A laboratory examination of the three-equation model of ice-ocean interactions Craig McConnochie, Ross Kerr Numerical models of ice-ocean interactions are typically unable to resolve the transport of heat and salt to the ice face. As such, models rely upon parameterizations that have not been properly validated by data. Recent laboratory experiments of ice-saltwater interactions allow us to test the standard parameterization of heat and salt transport to ice faces - the `three equation model'. We find a significant disagreement in the dependence of the melt rate on the fluid velocity. The three-equation model predicts that the melt rate is proportional to the fluid velocity while the experimental results typically show that the melt rate is independent of the fluid velocity. By considering a theoretical analysis of the boundary layer next to a melting ice face we suggest a resolution to this disagreement. We show that the three-equation model assumes that the thickness of the diffusive sublayer is set by a shear instability. However, at low flow velocities, the sublayer is instead set by a convective instability. This distinction leads to a threshold velocity of approximately 4 cm/s at geophysically relevant conditions, above which the form of the parameterization should be valid. In contrast, at flow speeds below 4 cm/s, the three-equation model will underestimate the melt rate. [Preview Abstract] |
Sunday, November 19, 2017 8:39AM - 8:52AM |
A34.00004: Large eddy simulation of heat entrainment under Arctic sea ice Eshwan Ramudu, Renske Gelderloos, Di Yang, Charles Meneveau, Anand Gnanadesikan Sea ice cover in the Arctic has declined rapidly in recent decades. To better understand ice loss through bottom melting, we choose to study the Canada Basin of the Arctic Ocean, which is characterized by a perennial anomalously warm Pacific Summer Water (PSW) layer residing at the base of the mixed layer and a summertime Near-Surface Temperature Maximum (NSTM) layer trapping heat from solar radiation. The interaction of these warm layers with a moving ice basal surface is investigated using large eddy simulation. We find that the presence of the NSTM enhances heat entrainment from the mixed layer. Another conclusion from our work is that there is no heat entrained from the PSW layer, even at the largest ice-drift velocity of 0.3 m s$^{-1}$ considered. We propose a scaling law for the heat flux at the ice basal surface which depends on the initial temperature anomaly in the NSTM layer and the ice-drift velocity. A case study of `The Great Arctic Cyclone of 2012' gives a turbulent heat flux from the mixed layer that is approximately 70\% of the total ocean-to-ice heat flux estimated from the PIOMAS model often used for short-term predictions. Present results highlight the need for large-scale climate models to account for the NSTM layer. [Preview Abstract] |
Sunday, November 19, 2017 8:52AM - 9:05AM |
A34.00005: Effect of turbulence and convection on melting of the ice shelves in stratified environment Bishakdatta Gayen, Mainak Mondal, Ross Griffiths We have performed high-resolution simulations to investigate the convective boundary layer when a wall of ice dissolves into stratified seawater under polar ocean conditions. Under the stratified ambient condition, melt water spreads out into the interior in a series of nearly horizontal layers due to double diffusive convection. The layer thickness depends on the ambient density gradient and the difference in density between the freezing point (interface temperature) and the ambient water temperature. For a small O(1) m hight box the layers are laminar and results for layer depth are in agreement with the experimental results. However, for significantly higher ice walls the layer scaling differs as a result of turbulent mixing. Stratification has a significant effect on melt rate which further helps in the shaping of ice-wall. The temperature and density structures found under Pine Island Glacier show several layers having a vertical scale that can also be explained by this study. [Preview Abstract] |
Sunday, November 19, 2017 9:05AM - 9:18AM |
A34.00006: Icebergs Melting in Uniform and Vertically Sheared Flows. Claudia Cenedese, Anna FitzMaurice, Fiammetta Straneo Icebergs calving into Greenlandic Fjords frequently experience strongly sheared flows over their draft, but the impact of this flow past the iceberg on the melt plumes generated along the iceberg sides is not fully captured by existing melt parameterizations. A series of novel laboratory experiments showed that side melting of icebergs subject to relative velocities is controlled by two distinct regimes, which depend on the melt plume behavior (side-attached or side-detached). These two regimes produce a nonlinear dependence of melt rate on velocity, and different distributions of meltwater in the water column. Iceberg meltwater may either be confined to a thin surface layer, when the melt plumes are side-attached, or mixed down to the iceberg draft, when the melt plumes are side-detached. In a two-layer vertically sheared flow, the average flow speed in existing melt parameterizations gives an underestimate of the submarine melt rate, in part due to the nonlinearity of the dependence of melt rate on flow speed, but also because vertical shear in the velocity profile fundamentally changes the flow splitting around the ice block and consequently the velocity felt by the ice surface. Including this nonlinear velocity dependence in melting parameterizations applied to observed icebergs increases iceberg side melt in the side-attached regime, improving agreement with observations of iceberg submarine melt rates. [Preview Abstract] |
Sunday, November 19, 2017 9:18AM - 9:31AM |
A34.00007: Turbulent properties under sloping Ice-wall in polar water Mainak Mondal, Bishakhdatta Gayen, Ross W. Griffiths, Ross C. Kerr Ice-shelves around West Antarctic basins are the most vulnerable to melting in the presence of warmer continental shelf water. A large extent of slope exists under these ice-shelves, where turbulent transport of salt and heat into the ice wall drives a convective melt-water plume against it. Large scale ice-ocean models neglect the effect of convection which can lead to a wrong estimation of melt rate. We perform direct numerical simulations under sloping ice-shelves with realistic ambient conditions. We estimated the melt rates, boundary layer thicknesses and entrainment coefficients as a function of slope angle. The numerical results are further supported by theoretical predictions. Over the range of slope angles, different mechanisms are active for sustaining turbulence. For near vertical case, buoyancy production is the primary source of turbulent kinetic energy whereas for shallower angles turbulence is produced by velocity shear in the meltwater plume. [Preview Abstract] |
Sunday, November 19, 2017 9:31AM - 9:44AM |
A34.00008: On ice rifts and the stability of non-Newtonian extensional flows on a sphere Roiy Sayag Rifts that form at the fronts of floating ice shelves that spread into the ocean can trigger major calving events in the ice. The deformation of ice can be modeled as a thin viscous film driven by buoyancy. The front of such a viscous film that propagates over a flat surface with no-slip basal conditions is known to have stable axisymmetric solutions. In contrast, when the fluid propagates under free-slip conditions at the substrate, the front can become unstable to small perturbations if the fluid is sufficiently strain-rate softening. Consequently, the front will develop tongues with a characteristic wavelength that coarsens over time, a pattern that is reminiscent of ice rifts. Here we investigate the stability of a spherical sheet of power-law fluids under free-slip basal conditions. The fluid is discharged at constant flux and axisymmetrically with respect to the pole, and propagates towards the equator. The propagating front in such a situation may become unstable due to its failure to sustain large extensional forces, resulting in the formation of rifts. This study has implications to understanding the cause of patterns that are observed on shells of floating ice in a range of planetary objects, and whether open rifts that sustain life were feasible in snowball earth. [Preview Abstract] |
Sunday, November 19, 2017 9:44AM - 9:57AM |
A34.00009: Hydraulically-driven cavitation beneath ice sheets Ian Hewitt Fluctuations in water pressure beneath glaciers and ice sheets influence the effective slip that facilitates rapid ice motion. This is especially the case for the Greenland ice sheet, where large amounts of summer meltwater are injected through and under the ice. Since the rate of ice motion is a primary control on sea-level contribution, it is of interest to understand the mechanism controlling this slip. The ice-sheet bed is rough on a small (meter) scale, and subglacial water collects in cavities whose size is controlled by the local bed topography, the water pressure and overburden ice pressure, and the viscous and elastic properties of the ice. Existing theories have established a steady-state relationship for the size of the cavities under conditions of constant water pressure, but this theory is inadequate to describe the rapid fluctuations that have been observed through bore-hole measurements. Here, I examine theoretically the dynamics of the cavities under conditions of varying water pressure and/or volume. Each cavity can be treated as a hydraulic fracture, and neighboring cavities are coupled to each other through the elastic or viscous stresses in the ice as well as the assumed hydraulic connections. I examine how the areal extent of the cavities responds to rapidly varying forcing, and establish the impact on the effective slip length for ice flow. [Preview Abstract] |
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