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 R13: Geophysical Fluid Dynamics: Cryosphere |
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Sponsoring Units: DFD GPC Chair: Colin Meyer, Harvard University Room: C124 |
Tuesday, November 22, 2016 1:30PM - 1:43PM |
R13.00001: Meltwater percolation and refreezing in compacting snow Colin Meyer, Ian Hewitt Meltwater is produced on the surface of glaciers and ice sheets when the seasonal surface energy forcing warms the ice above its melting temperature. This meltwater percolates through the porous snow matrix and potentially refreezes, thereby warming the surrounding ice by the release of latent heat. Here we model this process from first principles using a continuum model. We determine the internal ice temperature and glacier surface height based on the surface forcing and the accumulation of snow. When the surface temperature exceeds the melting temperature, we compute the amount of meltwater produced and lower the glacier surface accordingly. As the meltwater is produced, we solve for its percolation through the snow. Our model results in traveling regions of meltwater with sharp fronts where refreezing occurs. We also allow the snow to compact mechanically and we analyze the interplay of compaction with meltwater percolation. We compare these models to observations of the temperature and porosity structure of the surface of glaciers and ice sheets and find excellent agreement. Our models help constrain the role that meltwater percolation and refreezing will have on ice-sheet mass balance and hence sea level. [Preview Abstract] |
Tuesday, November 22, 2016 1:43PM - 1:56PM |
R13.00002: Formation of wind-driven ice bridges in narrow straits Bhargav Rallabandi, Zhong Zheng, Michael Winton, Howard A. Stone An ice bridge is a static arch made of tightly packed ice that can be formed when sea ice flows through a narrow strait between landmasses. The formation of a stable ice arch prevents the further flow of sea ice into warmer oceans, and therefore plays an important role in the regulation of the local climate and ecology and to an extent, the mass balance of Arctic ice. While ice bridges are a seasonal phenomenon in many parts of the Canadian Archipelago, the process of their formation and breakup is poorly understood. Using thin-layer theory along with dynamic sea ice models widely used in climate modeling, we develop a reduced-order description of wind-driven ice bridge formation in long, narrow straits. Our theory predicts a critical static condition for arrested flow that involves the ice properties (thickness and compactness), the geometry of the channel, and the magnitude of the wind stress. Further, we show that in a channel of varying shape and under a constant wind stress, a spatially uniform ice field evolves towards a steady state with discontinuities in its properties, consistent with observed mechanisms of ice bridge formation. The reduced-order model thus provides a predictive tool for the flow and stoppage of sea ice in straits. [Preview Abstract] |
Tuesday, November 22, 2016 1:56PM - 2:09PM |
R13.00003: Snowflakes aggregation in turbulent flows: a case limit under dynamically critical Stokes conditions Michele Guala, Jiarong Hong A simple theory, based on observations of snowflake distribution in a turbulent flow, is proposed to model the growth of inertial particles as a result of dynamic clustering at scales larger than the Kolmogorov length scale. Particles able to stick or coalesce are expected to grow in size in flow regions where preferential concentration is predicted by a critical Stokes number $St=\tau_p/\tau_f \simeq 1 $. We postulate that, during growth, $St$ remains critical, with the particle response time $\tau_p$ evolving according to the specific flow time scale $\tau_f$ defined by the vortices around which progressively larger particles end up orbiting, colliding and aggregating. This mechanism leads to the prediction of the limiting size of droplets and snowflakes in a turbulent flow. Such limit, determined by the extent of the turbulent inertial range, can be formulated as a function of the r.m.s. velocity fluctuation and the integral length scale. The proposed dynamically critical Stokes growth provides a framework to interpret hydrometeor aggregation and, in general, particle size growth in geophysical multi-phase flows. [Preview Abstract] |
Tuesday, November 22, 2016 2:09PM - 2:22PM |
R13.00004: Snowflake Impact on the Air-Sea Interface David Murphy The air-sea interface is the site of globally important exchanges of mass, momentum, and heat between the sea and atmosphere. These climate-driving exchanges occur through small-scale processes such as bubble entrainment and bursting, raindrop impact, and wind-wave creation. The physics of snowflakes falling on the sea surface has not been previously considered. High speed imaging of natural snowflakes of characteristic size up to 6.5 mm falling at a mean speed of 1 m/s into an aquarium of chilled seawater reveals a complex multiphase flow. Snowflakes impacting and crossing the air-seawater interface appear to entrain a thin air film which forms micro-bubbles as the snowflake melts. Large, morphologically complex snowflakes may entrain hundreds of micro-bubbles which are up to 0.15 mm in diameter. Large snowflakes melt milliseconds after entry and subsequently form a downward-moving vortex ring of freshwater, evident from the motion of the bubbles it contains, which may penetrate up to 16 mm below the surface. Buoyant freshwater and bubbles then rise, with larger bubbles escaping from the downward flow more quickly than the smaller bubbles. The dissolution and popping of these bubbles represent previously unrecognized sources of air-sea gas transfer and marine aerosol droplet creation, respectively. [Preview Abstract] |
Tuesday, November 22, 2016 2:22PM - 2:35PM |
R13.00005: The formation of grounding zone wedges Katarzyna Kowal, Grae Worster Ice sheets are generally lubricated by a layer of sub-glacial sediment, or till, which plays a central role in determining their large-scale dynamics. Sub-glacial till has been found to accumulate into distinctive sedimentary wedges at ice-sheet grounding zones, separating floating ice shelves from grounded ice sheets. These grounding-zone wedges have important implications for stabilizing ice sheets against grounding-zone retreat in response to rising sea levels. We develop a theoretical model of wedge formation in which we treat both ice and till as viscous fluids spreading under gravity into an inviscid ocean and present a fluid-mechanical explanation of the formation of these wedges in terms of the jump in hydrostatic loading and unloading of till across the grounding zone. We also conduct a series of fluid-mechanical experiments in a confined setting in which we find that the underlying layer of less viscous fluid accumulates spontaneously in a similar wedge-shaped region at the experimental grounding line. We also extend our theory to more natural, unconfined settings in two dynamical regimes in which the overlying ice is resisted dominantly either by vertical shear or by extensional stresses and compare our findings with available geophysical data. [Preview Abstract] |
Tuesday, November 22, 2016 2:35PM - 2:48PM |
R13.00006: Sidewall-driven convection in a thermally and compositionally stratified fluid. Keaton Burns, Glenn Flierl, Andrew Wells We present direct numerical simulations of incompressible turbulent convection along a heated sidewall in a thermally and compositionally stratified fluid, as a simplified model of meltwater flows along marine-terminating glaciers. Our model considers a 2D domain that is horizontally bounded and vertically periodic, with constant background thermal and compositional buoyancy gradients. We apply a fixed thermal perturbation along one sidewall, driving upward convective plumes and horizontally spreading layers with compensating thermal and compositional buoyancy perturbations. We examine the formation and structure of these layers as the background stratification is varied from thermally to compositionally dominated, and as the sidewall is tilted away from vertical. We also examine the variations in heat flux along the sidewall that arise with the layers. [Preview Abstract] |
Tuesday, November 22, 2016 2:48PM - 3:01PM |
R13.00007: Ice sheets on plastically-yielding beds Ian Hewitt Many fast flowing regions of ice sheets are underlain by a layer of water-saturated sediments, or till. The rheology of the till has been the subject of some controversy, with laboratory tests suggesting almost perfectly plastic behaviour (stress independent of strain rate), but many models adopting a pseudo-viscous description. In this work, we consider the behaviour of glaciers underlain by a plastic bed. The ice is treated as a viscous gravity current, on a bed that allows unconstrained slip above a critical yield stress. This simplified description allows rapid sliding, and aims to investigate 'worst-case' scenarios of possible ice-sheet disintegration. The plastic bed results in an approximate ice-sheet geometry that is primarily controlled by force balance, whilst ice velocity is determined from mass conservation (rather than the other way around, as standard models would hold). The stability of various states is considered, and particular attention is given to the pace at which transitions between unstable states can occur. Finally, we observe that the strength of basal tills depends strongly on pore pressure, and combine the model with a description of subglacial hydrology. Implications for the present-day ice sheets in Greenland and Antarctica will be discussed. [Preview Abstract] |
Tuesday, November 22, 2016 3:01PM - 3:14PM |
R13.00008: Turbulent convection and dissolution under sloping Ice-shelves in Saline water Mainak Mondal, Bishakhdatta Gayen, Ross Griffiths, Ross kerr We have carried out numerical experiments with geophysicallyrelevant slope angles (5\textordmasculine -20\textordmasculine ) over domains of O(1 m) -O(10 m) bracketingAntarctic conditions. Contact of the ice with cold and saltywater drives a turbulentbuoyant flow predominantly due to freshening from melting ice-interface in the upslope direction. The dissolution rate decreases with shallower angles and is predicted by our theory. Thickness of the thermal and salinity boundary layer increases with decreasing slope angle whereas the interface conditions remains insensitive to the inclination. The dissolution rate is independent of slope length when boundary layer is turbulent. For steeper angles turbulent kinetic energy is mainly produced by the buoyancy flux but it's contribution rapidly decreases with shallower angles. For shallower slopes turbulence is sustained by shear production due to decrease of velocity boundary layer. Our results can be used to explain many dynamical processes under inclined ice shelves around Antarctica. [Preview Abstract] |
Tuesday, November 22, 2016 3:14PM - 3:27PM |
R13.00009: Glacial uplift: fluid injection beneath an elastic sheet on a poroelastic substrate Jerome Neufeld, Duncan Hewitt, Greg Chini Supraglacial lakes can drain to the base of glaciers extremely rapidly, causing localised uplift of the surrounding glacier and affecting its sliding velocity. The means by which large volumes of drained water interact with and leak into the subglacial hydrological system is unclear, as is the role of the basal till. A theoretical study of the spread of fluid injected below an elastic sheet (the ice) is presented, where the ice lies above, and initially compresses, a deformable poroelastic layer. As pressurized fluid is injected, the deformable layer swells to accommodate more fluid. If sufficient fluid is injected, a `blister’ of fluid forms above the layer, causing the overburden to lift off the base. The flow is controlled by the local pressure drop across the tip of this blister, which depends subtly on both the flow of fluid through the porous layer below the tip, and on poroelastic deformation in the till ahead of the tip. The spreading behaviour and dependence on key parameters is analysed. Predictions of the model are compared to field measurements of uplift from draining glacial lakes in Greenland. [Preview Abstract] |
Tuesday, November 22, 2016 3:27PM - 3:40PM |
R13.00010: An Experimental Investigation of Ice-melting and heat transfer rates from submerged warm water jets upward impinging into ice-blocks as analogous for water-filled cavities formed during subglacial eruptions.. Hamidreza Jamshidnia, Magnus Tumi Gudmundsson Rates of energy transfer in water-filled cavities formed under glaciers by geothermal and volcanic activity are investigated by conducting experiments in which hot water jets (10\textdegree - 90\textdegree C) impinging into an ice block for jet Reynolds numbers in turbulent regime of 10000 -70000. It is found that heat flux is linearly dependent on jet flow temperature. Water jet melts a cavity into an ice block. Cavities had steep to vertical sides with a doming roof. Some of ice blocks used had trapped air bubbles. In these cases that melting of the ice could have led to trapping of air at the top of cavity, partially insulating the roof from hot water jet. The overall heat transfer rate in cavity formation varied with jet temperature from \textless 100 kW m$^{\mathrm{-2}}$ to \textasciitilde 900 kW m$^{\mathrm{-2}}$ while melting rates in the vertical direction yield heat transfer rates of 200-1200 kW m$^{\mathrm{-2}}$. Experimental heat transfer rates can be compared to data on subglacial melting observed for ice cauldrons in Iceland. For lowest temperatures the numbers are comparable to those for geothermal water in cool, subglacial water bodies and above subglacial flowpaths of j\"{o}kulhlaups. Highest experimental rates for 80-90\textdegree C jets are 3-10 times less than inferred from observations of recent subglacial eruptions (2000-4000 kW m$^{\mathrm{-2}})$. This can indicate that single phase liquid water convection alone may not be sufficient to explain the rates seen in recent subglacial eruptions, suggesting that forced 2 or 3 phase convection can be common. [Preview Abstract] |
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