Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session H20: Geophysical Fluid Dynamics Cryosphere |
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Chair: Claudia Cenedese, Woods Hole Oceanographic Institution Room: 602 |
Monday, November 25, 2019 8:00AM - 8:13AM |
H20.00001: Modeling the formation of wind-driven coastal polynyas Lailai Zhu, Bhargav Rallabandi, Michael Winton, Howard Stone Polynyas are persistent, recurrent regions of open sea water surrounded by sea ice and/or land in the polar zones. We conduct a combined theoretical and numerical study on the dynamics and thermodynamics of the formation of a wind-driven latent-heat polynya near a coastline with and without curvature. In the limit of weak ice internal pressure, we propose a one-dimensional, continuous, mass- and momentum-conserving theory characterizing the offshore distribution of the ice velocity and the spatial-temporal evolution of the ice concentration. Using an open-source solver based on finite-element method, we simulate this process considering the rheological properties of the ice. The theoretical and numerical results agree well with each other in most cases, and the obtained steady-state polynya width qualitatively matches the corresponding climate data. [Preview Abstract] |
Monday, November 25, 2019 8:13AM - 8:26AM |
H20.00002: Jumping frost Ranit Mukherjee, Farzad Ahmadi, Jonathan Boreyko Electrification of frost has been studied since the 1950s, mainly in the context of cloud physics. A few of these reports observed the ejection of ice particles from the growing frost, but only in the direction of a convective air flow. Moreover, the dynamics of the frost ejection were not modeled. Here, we show a phenomenon of micrometric frost dendrites jumping out-of-plane towards an opposing sheet of wetted paper, at characteristic velocities of about 0.1 m/s. This surprising jumping motion was in the absence of convective effects and in the opposite direction of the diffusive vapor flow growing the frost. Two different jumping regimes were observed: an initial constant acceleration regime followed by a constant velocity regime. The underlying mechanism for the frost jumping is a temperature-gradient-induced charge separation within the frost, resulting in an electrostatic attraction to the opposing surface. [Preview Abstract] |
Monday, November 25, 2019 8:26AM - 8:39AM |
H20.00003: Lubricated gravity currents of power--law fluids Ayala Gyllenberg, Roiy Sayag The discharge of ice from polar ice sheets into the ocean has the potential to induce climate change and a catastrophic rise in sea level. Significant ice discharge can occur via ice streams, bands of fast--moving ice lubricated by a mixture of water and clay. We present a fluid--dynamical model for such lubricated gravity currents that describes the axisymmetric spreading of a viscous, power--law fluid, such as ice, under its own weight on top of a viscous, Newtonian fluid. Both fluids are discharged at the origin at a time--dependent flux of a general power--law form. We investigate the model solutions by combined analytical and numerical methods. We find that the model admits self--similar solutions only in specific cases, such as when the top fluid is Newtonian or when the fluids are discharged at a certain time--dependent flux. When the flux is constant, similarity solutions are present when the viscosity of one fluid is much greater than that of the other fluid, as in the case of an ice sheet lubricated by water. In that case we find that the front of the lubricating fluid outstrips that of the power--law fluid, a phenomenon that has been observed in laboratory experiments. [Preview Abstract] |
Monday, November 25, 2019 8:39AM - 8:52AM |
H20.00004: Lubricated gravity currents of strain--rate softening fluids, and the formation of ice-streams Roiy Sayag, Pramoda Kumar, Shahar Zuri Ice--sheet instabilities are believed to be closely associated with the flow of ice streams, bands of fast--flowing ice that carry most of the mass flux towards the edge of the ice sheet, where it ultimately melts or calves. The formation and evolution of ice streams is believed to be controlled by a complex hydrological network under the ice that sets the dynamic boundary conditions on the base of the ice. Fluid mechanically, such a system can be modelled as a viscous gravity current of strain-rate softening fluid lubricated by a relatively inviscid and denser Newtonian fluid. We present an experimental study of such flows that were discharged axisymmetrically at a constant rate. We investigate the evolution of the fronts of the viscous and the lubricating fluids, and how the patterns they form depend on the main dimensionless groups of the flow. We find two distinguished classes of patterns. In the first class, both fronts remain axisymmetric throughout the flow. In the second class, the initially axisymmetric front of the lubricating fluid evolves into a set of localised fingers, which drive the initially axisymmetric flow of the viscous fluid to develop patterns that are reminiscent to ice streams. [Preview Abstract] |
Monday, November 25, 2019 8:52AM - 9:05AM |
H20.00005: Ice-Shelf Rippling From Temporally Varying Melt and Ice Flow Andrew Wells, Chris MacMackin The Antarctic and Greenland ice sheets often discharge into the ocean through floating ice shelves. Ice-shelf geometry is controlled by the coupling of a viscous ice flow with basal melt driven by buoyant meltwater plumes rising through the warm and salty ocean along the sloping ice-shelf base. Motivated by recent observations of the spatial patterns of ice-shelf thickness, we assess the response of an ice shelf to a range of temporally-periodic forcing conditions. We model the depth-integrated viscous extensional flow of an ice shelf downstream of the grounding line, where the grounded ice transitions to the floating shelf. Melting is determined from a meltwater plume model with imposed subglacial discharge. We use a linear perturbation analysis to explore the sensitivity of the ice shelf geometry to periodic variation of the freshwater source flux for the meltwater plume, ice flux across the grounding line, and coupled variation of ice flux and grounding line position. Such periodic variation leads to propagating ripple-like features on the ice shelf base, with greatest sensitivity to varying ice influx with coincident grounding line motion. We analyse the mechanisms controlling ripple amplitude, and discuss potential implications for formation of ice-shelf basal terraces. [Preview Abstract] |
Monday, November 25, 2019 9:05AM - 9:18AM |
H20.00006: Relaxation of ice-sheet uplift on a porous bed Ching-Yao Lai, Danielle L. Chase, Laura A. Stevens, Timothy T. Creyts, Howard A. Stone When surface meltwater reaches the base of an ice-sheet with high flow rate, it peels apart the interface between the ice and bed, forming a water-filled blister and inducing ice-sheet uplift. The uplift relaxes after water injection, with a wide range of relaxation times (Geodetic observations show relaxation occurs over hours to days). Improved characterization of water flow at the base of ice sheets is vital for predicting ice-sheet dynamics and mass loss when the bed is lubricated. Although properties of the lubricated porous sheet beneath the ice impact ice sliding, the permeability of the porous sheet has not been measured directly and remains unknown. Here, we show that the elastic relaxation of a water-filled blister above a porous substrate can be used to probe permeability. Combining field data, a mathematical model, and laboratory experiments, we show that the blister height decreases exponentially with time as the water in the blister permeates through the porous sheet. We find that the relaxation dynamics obeys a universal law with a time scale involving the thickness and permeability of the porous sheet, water viscosity, and the Young's modulus of ice. We show that the range of observed relaxation times can be explained by the evolving permeability of the porous sheet. Our reduced-order model better characterizes the evolution of bed permeability based on surface observations. [Preview Abstract] |
(Author Not Attending)
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H20.00007: Direct numerical simulations of ice melting in a turbulent ocean Louis-Alexandre Couston In this talk I will present preliminary results of direct numerical simulations of ice melting in a stratified turbulent ocean. The model solves the evolution of the turbulent fluid phase and of the diffusive solid ice phase, due to melting and freezing, in a fully coupled way. This is done by combining a highly-efficient fully-spectral Direct Numerical Simulation (DNS) code with a novel formulation of the equations for the solid and liquid phases of water based on the phase-field method, which is routinely used in metallurgy. DNS enables turbulent motions to be simulated without approximation, i.e. solving Navier Stokes equations, while the phase-field method allows the ice-ocean interface to be rough and evolve in response to melting. I will present results on the turbulent boundary layer and on the self-generated roughness at the ice-ocean interface for fresh water. The ultimate goal of this work is to propose a new DNS-based parameterization of the melting process at rough ice-ocean boundaries that takes into account the effects of temperature and salt stratification, and flow velocities.~ [Preview Abstract] |
Monday, November 25, 2019 9:31AM - 9:44AM |
H20.00008: Glacial Squeegee: Elastic Landau-Levich and the Tidal Modulation of Ice Streams Katarzyna Warburton, Duncan Hewitt, Jerome Neufeld Glacier speed is sensitive to fluctuations in subglacial water pressure. For marine ice sheets, the tidal cycle is linked to the upstream pressure fluctuations as water enters and exits the subglacial environment across the grounding line. We analyse the elastic analogue of the Landau-Levich dip-coating problem, in which a plate (here the earth) is withdrawn from a bath of fluid (the ocean) on whose surface lies a thin elastic sheet (the ice), for arbitrary angle of withdrawal $\theta$ (basal slope). The flow is controlled by the elasticity number, $El$, which is a measure of the relative importance of viscous and bending stresses, and $\theta$. The leading order solution for small $El$ is a steady profile in which the thickness of the film deposited on the plate is found. The breakdown of this asymptotic regime in the limit of large $El$ is also discussed. These solutions are interpreted in the context of the tidal grounding line problem, and related to observations from the Rutford Ice Stream. [Preview Abstract] |
Monday, November 25, 2019 9:44AM - 9:57AM |
H20.00009: Ice floe dispersion from moderate remote sensing imagery Rosalinda Lopez, Monica M. Wilhelmus Sea ice transport directly affects the heat budget and freshwater flux in the Arctic. To understand the Arctic climate system, it is thus important to quantify the dispersion regime of free-drifting sea ice. In this study, we employed an in-house algorithm to automatically process Moderate Resolution Imaging Spectroradiometer satellite images and track ice floes (8 to 65 km) along the eastern coast of Greenland during the spring of 2017. By quantifying the drift fields, the dispersion regime of the ice floes was analyzed to understand the drivers of sea ice transport. Preliminary results show that, as expected, in the presence of a strong shear flow as the East Greenland Current, the absolute dispersion of ice floes grows quadratically in time. Further analysis of the fluctuating component of sea ice velocities also yields an absolute dispersion that grows quadratically in time at short time scales (few days). It was observed, however, that this behavior changes when considering longer time scales as the influence of the underlying eddy field becomes more prominent. We examine the effect of small scale oceanic turbulence on sea ice drift and discuss the feasibility of extending our study to improve our understanding of sea-ice ocean interactions. [Preview Abstract] |
Monday, November 25, 2019 9:57AM - 10:10AM |
H20.00010: Enhanced submarine melting of glaciers due to the effect of sediments on subglacial discharge plumes Claudia Cenedese, Craig McConnochie, Jim McElwaine Recent studies suggest that marine terminating glacier faces melt at particularly high rates where subglacial discharge plumes are present. These subglacial discharge plumes have high sediment concentrations, as revealed by the surface expression of some of them, yet the impact of suspended sediment on the plume dynamics and the melting has, until now, been overlooked. Recent laboratory experiments and Direct Numerical Simulations (DNS) suggest that the sediment load could have an important and complex effect on the plume dynamics and thus the melting of the ice face. In particular, dilute concentrations of small and dense particles can increase the entrainment of ambient waters into the plume as much as 50{\%}, leading to a 50{\%} increase in the melt rate. In these experiments and simulations, the settling velocity of the particles is smaller than and in the opposite direction to the plume velocity at all heights and particle inertial clustering, leading to convective instability in the fluid, is thought to be the mechanism responsible for the enhanced entrainment. [Preview Abstract] |
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