Using Field Data to Constrain Ice-Flow Models: A Study of A Small Alpine Glacier

Abstract. The method of elevation classes, in which the ice surface model is run at multiple elevations within each grid cell, has proven to be a useful way for a low-resolution atmosphere inside a general circulation model (GCM) to produce high-resolution downscaled surface mass balance fields for use in one-way studies coupling atmospheres and ice flow models. Past uses of elevation classes have failed to conserve mass and energy because the transformation used to regrid to the atmosphere was inconsistent with the transformation used to downscale to the ice model. This would cause problems for two-way coupling. A strategy that resolves this conservation issue has been designed and is presented here. The approach identifies three grids between which data must be regridded and five transformations between those grids required by a typical coupled atmosphere–ice flow model. This paper develops a theoretical framework for the problem and shows how each of these transformations may be achieved in a consistent, conservative manner. These transformations are implemented in Glint2, a library used to couple atmosphere models with ice models. Source code and documentation are available for download. Confounding real-world issues are discussed, including the use of projections for ice modeling, how to handle dynamically changing ice geometry, and modifications required for finite element ice models.

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A system of conservative regridding for ice/atmosphere coupling in a GCM

Geoscientific Model Development Discussions ◽

10.5194/gmdd-6-6493-2013 ◽

2013 ◽

Vol 6 (4) ◽

pp. 6493-6568 ◽

Cited By ~ 4

Author(s):

R. Fischer ◽

S. Nowicki ◽

M. Kelley ◽

G. A. Schmidt

Keyword(s):

Finite Element ◽

General Circulation ◽

Flow Model ◽

Source Code ◽

Circulation Model ◽

Surface Mass Balance ◽

Ice Flow ◽

Surface Mass ◽

Flow Models ◽

Ice Model

Abstract. The method of elevation classes has proven to be a useful way for a low-resolution general circulation model (GCM) to produce high-resolution downscaled surface mass balance fields, for use in one-way studies coupling GCMs and ice flow models. Past uses of elevation classes have been a cause of non-conservation of mass and energy, caused by inconsistency in regridding schemes chosen to regrid to the atmosphere vs. downscaling to the ice model. This causes problems for two-way coupling. A strategy that resolves this conservation issue has been designed and is presented here. The approach identifies three grids between which data must be regridded, and five transformations between those grids required by a typical coupled GCM–ice flow model. This paper shows how each of those transformations may be achieved in a consistent, conservative manner. These transformations are implemented in GLINT2, a library used to couple GCMs with ice models. Source code and documentation are available for download. Confounding real-world issues are discussed, including the use of projections for ice modeling, how to handle dynamically changing ice geometry, and modifications required for finite element ice models.

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Enthalpy benchmark experiments for numerical ice sheet models

The Cryosphere ◽

10.5194/tc-9-217-2015 ◽

2015 ◽

Vol 9 (1) ◽

pp. 217-228 ◽

Cited By ~ 20

Author(s):

T. Kleiner ◽

M. Rückamp ◽

J. H. Bondzio ◽

A. Humbert

Keyword(s):

Analytical Solutions ◽

Ice Sheet ◽

Jump Conditions ◽

Cold Side ◽

Ice Flow ◽

Flow Models ◽

Transient Simulations ◽

Rate Calculation ◽

Transition Surface ◽

Simulation Results

Abstract. We present benchmark experiments to test the implementation of enthalpy and the corresponding boundary conditions in numerical ice sheet models. Since we impose several assumptions on the experiment design, analytical solutions can be formulated for the proposed numerical experiments. The first experiment tests the functionality of the boundary condition scheme and the basal melt rate calculation during transient simulations. The second experiment addresses the steady-state enthalpy profile and the resulting position of the cold–temperate transition surface (CTS). For both experiments we assume ice flow in a parallel-sided slab decoupled from the thermal regime. We compare simulation results achieved by three different ice flow-models with these analytical solutions. The models agree well to the analytical solutions, if the change in conductivity between cold and temperate ice is properly considered in the model. In particular, the enthalpy gradient on the cold side of the CTS goes to zero in the limit of vanishing temperate-ice conductivity, as required from the physical jump conditions at the CTS.

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Migration of the Siple Dome ice divide, West Antarctica

Journal of Glaciology ◽

10.3189/s0022143000002148 ◽

1998 ◽

Vol 44 (148) ◽

pp. 643-652 ◽

Cited By ~ 7

Author(s):

N. A. Nereson ◽

C. F. Raymond ◽

E. D. Waddington ◽

R. W. Jacobel

Keyword(s):

Migration Rate ◽

Internal Layer ◽

Internal Layers ◽

West Antarctica ◽

Ice Streams ◽

Ice Flow ◽

Flow Models ◽

The Past ◽

Siple Dome ◽

Local Accumulation

AbstractThe non-linearity of the ice-flow law or a local accumulation low over an ice divide can cause isochrones (internal layers) to be shallower under the divide relative to the flanks, forming a “divide bump” in the internal layer pattern. This divide signature is analyzed using ice-flow models and inverse techniques to detect and quantify motion of the Siple Dome ice divide, West Antarctica. The principal feature indicating that migration has occurred is a distinct tilt of the axis of the peaks of the warped internal layers beneath the divide. The calculated migration rate is 0.05-0.50 m a−1 toward Ice Stream D and depends slightly on whether the divide bump is caused by the non-linearity of ice flow or by a local accumulation low. Our calculations also suggest a strong south-north accumulation gradient of 5-10 x 10−6 a−1 in a narrow zone north of the divide. A consequence of divide migration is that pre-Holocene ice is thickest about 0.5 km south of the present divide position. Divide motion indicates that non-steady processes, possibly associated with activity of the bounding ice streams, are affecting the geometry of Siple Dome. The migration rate is sufficiently slow that the divide bump is maintained in the internal layer pattern at all observable depths. This suggests that major asynchronous changes in the elevation or position of the bounding ice streams are unlikely over at least the past 103-104 years.

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Precise altimetric topography in ice-sheet flow studies

Annals of Glaciology ◽

10.1017/s0260305500012830 ◽

1993 ◽

Vol 17 ◽

pp. 195-200 ◽

Cited By ~ 9

Author(s):

F. Remy ◽

J.F. Minster

Keyword(s):

Activation Energy ◽

Flow Direction ◽

Ice Sheet ◽

Rheological Parameters ◽

Least Squares Regression ◽

Sheet Flow ◽

Ice Flow ◽

Flow Models ◽

Altimetric Data ◽

Basal Shear

The precision of radar altimetry above an ice sheet can improve glaciological studies such as mass balance surveys or ice-sheet flow models, the first by comparing altimetric data at different times (see this issue), the second by testing or constraining models with data. This paper is a first step towards the latter. From a precise topography deduced by inversion of altimetric data (Remy and others, 1989), we calculate ice-flow direction, balance velocity and basal shear stress. The rheological parameters involved in the relation linking velocity, stress and temperature are then derived by least-squares regression. Ice flow is well represented by setting the Glen parameter,nto 1 ± 0.25 and the activation energy as 70 ± 10 kJ mol−1.

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Restoring mass conservation to shallow ice flow models over complex terrain

The Cryosphere ◽

10.5194/tc-7-229-2013 ◽

2013 ◽

Vol 7 (1) ◽

pp. 229-240 ◽

Cited By ~ 21

Author(s):

A. H. Jarosch ◽

C. G. Schoof ◽

F. S. Anslow

Keyword(s):

Complex Terrain ◽

Model Performance ◽

Grid Cell ◽

Mass Conservation ◽

Ice Flow ◽

Flow Models ◽

Mountainous Regions ◽

Steep Terrain ◽

Flux Limiting ◽

Ice Model

Abstract. Numerical simulation of glacier dynamics in mountainous regions using zero-order, shallow ice models is desirable for computational efficiency so as to allow broad coverage. However, these models present several difficulties when applied to complex terrain. One such problem arises where steep terrain can spuriously lead to large ice fluxes that remove more mass from a grid cell than it originally contains, leading to mass conservation being violated. This paper describes a vertically integrated, shallow ice model using a second-order flux-limiting spatial discretization scheme that enforces mass conservation. An exact solution to ice flow over a bedrock step is derived for a given mass balance forcing as a benchmark to evaluate the model performance in such a difficult setting. This benchmark should serve as a useful test for modellers interested in simulating glaciers over complex terrain.

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Calving cycle of the Brunt Ice Shelf, Antarctica, driven by changes in ice shelf geometry

The Cryosphere ◽

10.5194/tc-13-2771-2019 ◽

2019 ◽

Vol 13 (10) ◽

pp. 2771-2787 ◽

Cited By ~ 1

Author(s):

Jan De Rydt ◽

Gudmundur Hilmar Gudmundsson ◽

Thomas Nagler ◽

Jan Wuite

Keyword(s):

Large Scale ◽

Ice Shelf ◽

Ice Flow ◽

Ice Shelves ◽

Flow Models ◽

Speed Up ◽

The Antarctic ◽

Detrimental Impact ◽

Natural Laboratory

Abstract. Despite the potentially detrimental impact of large-scale calving events on the geometry and ice flow of the Antarctic Ice Sheet, little is known about the processes that drive rift formation prior to calving, or what controls the timing of these events. The Brunt Ice Shelf in East Antarctica presents a rare natural laboratory to study these processes, following the recent formation of two rifts, each now exceeding 50 km in length. Here we use 2 decades of in situ and remote sensing observations, together with numerical modelling, to reveal how slow changes in ice shelf geometry over time caused build-up of mechanical tension far upstream of the ice front, and culminated in rift formation and a significant speed-up of the ice shelf. These internal feedbacks, whereby ice shelves generate the very conditions that lead to their own (partial) disintegration, are currently missing from ice flow models, which severely limits their ability to accurately predict future sea level rise.

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Assessment of heat sources on the control of fast flow of Vestfonna ice cap, Svalbard

The Cryosphere ◽

10.5194/tc-8-1951-2014 ◽

2014 ◽

Vol 8 (5) ◽

pp. 1951-1973 ◽

Cited By ~ 11

Author(s):

M. Schäfer ◽

F. Gillet-Chaulet ◽

R. Gladstone ◽

R. Pettersson ◽

V. A. Pohjola ◽

...

Keyword(s):

Latent Heat ◽

Temperature Record ◽

Heat Sources ◽

Ice Flow ◽

Outlet Glacier ◽

Flow Models ◽

Ice Cap ◽

Melt Water ◽

Friction Parameters ◽

Fast Flow

Abstract. Understanding the response of fast flowing ice streams or outlet glaciers to changing climate is crucial in order to make reliable projections of sea level change over the coming decades. Motion of fast outlet glaciers occurs largely through basal motion governed by physical processes at the glacier bed, which are not yet fully understood. Various subglacial mechanisms have been suggested for fast flow but common to most of the suggested processes is the requirement of presence of liquid water, and thus temperate conditions. We use a combination of modelling, field, and remote observations in order to study links between different heat sources, the thermal regime and basal sliding in fast flowing areas on Vestfonna ice cap. A special emphasis lies on Franklinbreen, a fast flowing outlet glacier which has been observed to accelerate recently. We use the ice flow model Elmer/Ice including a Weertman type sliding law and a Robin inverse method to infer basal friction parameters from observed surface velocities. Firn heating, i.e. latent heat release through percolation of melt water, is included in our model; its parameterisation is calibrated with the temperature record of a deep borehole. We found that strain heating is negligible, whereas friction heating is identified as one possible trigger for the onset of fast flow. Firn heating is a significant heat source in the central thick and slow flowing area of the ice cap and the essential driver behind the ongoing fast flow in all outlets. Our findings suggest a possible scenario of the onset and maintenance of fast flow on the Vestfonna ice cap based on thermal processes and emphasise the role of latent heat released through refreezing of percolating melt water for fast flow. However, these processes cannot yet be captured in a temporally evolving sliding law. In order to simulate correctly fast flowing outlet glaciers, ice flow models not only need to account fully for all heat sources, but also need to incorporate a sliding law that is not solely based on the basal temperature, but also on hydrology and/or sediment physics.

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Coupling ice flow models of varying orders of complexity with the Tiling method

Journal of Glaciology ◽

10.3189/2012jog11j195 ◽

2012 ◽

Vol 58 (210) ◽

pp. 776-786 ◽

Cited By ~ 17

Author(s):

Helene Seroussi ◽

Hachmi Ben Dhia ◽

Mathieu Morlighem ◽

Eric Larour ◽

Eric Rignot ◽

...

Keyword(s):

Numerical Models ◽

Computational Cost ◽

Higher Order ◽

Ice Flow ◽

Flow Models ◽

Continental Scale ◽

Critical Areas ◽

Warming Climate ◽

Computationally Intensive ◽

A New Technique

AbstractIce flow numerical models are essential for predicting the evolution of ice sheets in a warming climate. Recent research emphasizes the need for higher-order and even full-Stokes flow models, instead of the traditional shallow-ice approximation, whose assumptions are not valid in certain critical areas. These higher-order models are, however, computationally intensive and difficult to use at the continental scale. Here we present a new technique, the Tiling method, to couple ice flow models of varying orders of complexity. The goal of the method is to limit the spatial extent of where higherorder models are applied to reduce the computational cost, while maintaining the model precision. We apply this method on synthetic geometries to demonstrate its practical use. We first use a geometry for which all models yield the same results to check the consistency of the method. Then we apply our method to a geometry for which a full-Stokes model is required in the vicinity of the ice front. Our results show that the hybrid models present significant improvements over mono-model approaches and reduce computational times.

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Relating polarimetric radar measurements of ice fabric to ice-flow enhancement of Rutford Ice Stream

10.5194/egusphere-egu2020-7294 ◽

2020 ◽

Author(s):

Thomas Jordan ◽

Alex Brisbourne ◽

Carlos Martin ◽

Rebecca Schlegel ◽

Dustin Schroeder ◽

...

Keyword(s):

Polarimetric Radar ◽

Ice Streams ◽

Ice Flow ◽

Near Surface ◽

Flow Models ◽

Relative Contribution ◽

Ice Stream ◽

Glacier Ice ◽

Flow Enhancement ◽

The Impact

Lateral shear margins provide resistance to ice flow within ice streams and play an important role in the overall dynamics of ice sheets. The strength and location of shear margins are known to be influenced by both subglacial factors (e.g. bed roughness, meltwater availability) and ice rheology (ice temperature, ice fabric, and damage). Assessing the relative contribution of these factors upon ice-stream flow is complex but can be aided by geophysical measurements (e.g. radar-sounding and seismic imaging) of the ice-stream subsurface. There are, however, ongoing challenges in obtaining geophysical information in an appropriate form to be incorporated into ice-flow models. This is true of ice fabric, and its direction-dependent effect upon ice viscosity is typically neglected in models of ice streams.Here we develop a framework to relate ice fabric measurements from polarimetric radar sounding to ice-flow enhancement within ice streams. First, we extend a `polarimetric coherence&#8217; radar method to automate the extraction of ice fabric using quad-polarized data.&#160; Second, using a previously developed anisotropic flow-law formulation, we relate the radar fabric measurements to direction-dependent enhancement factors of glacier ice. We demonstrate the approach using a radar ground survey, collected by the British Antarctic Survey, which traverses between the centre and shear margin of Rutford Ice Stream. The data indicate that a vertical girdle fabric is present in the near-surface of the ice stream (approximately the top 300 m) which azimuthally rotates and strengthens toward the shear margin. We then assess the effect that the girdle fabric has upon shear and compression and the impact upon ice-flow models of Rutford Ice Stream.&#160;&#160;&#160;

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