scholarly journals Fracture-induced softening for large-scale ice dynamics

2014 ◽  
Vol 8 (2) ◽  
pp. 587-605 ◽  
Author(s):  
T. Albrecht ◽  
A. Levermann
Keyword(s):  
Large Scale ◽  
Ice Sheet ◽  
Fracture Initiation ◽  
Fracture Density ◽  
Ice Streams ◽  
Model Framework ◽  
Ice Dynamics ◽  
Ice Flow ◽  
Ice Shelves ◽  
Fracture Processes

Abstract. Floating ice shelves can exert a retentive and hence stabilizing force onto the inland ice sheet of Antarctica. However, this effect has been observed to diminish by the dynamic effects of fracture processes within the protective ice shelves, leading to accelerated ice flow and hence to a sea-level contribution. In order to account for the macroscopic effect of fracture processes on large-scale viscous ice dynamics (i.e., ice-shelf scale) we apply a continuum representation of fractures and related fracture growth into the prognostic Parallel Ice Sheet Model (PISM) and compare the results to observations. To this end we introduce a higher order accuracy advection scheme for the transport of the two-dimensional fracture density across the regular computational grid. Dynamic coupling of fractures and ice flow is attained by a reduction of effective ice viscosity proportional to the inferred fracture density. This formulation implies the possibility of non-linear threshold behavior due to self-amplified fracturing in shear regions triggered by small variations in the fracture-initiation threshold. As a result of prognostic flow simulations, sharp across-flow velocity gradients appear in fracture-weakened regions. These modeled gradients compare well in magnitude and location with those in observed flow patterns. This model framework is in principle expandable to grounded ice streams and provides simple means of investigating climate-induced effects on fracturing (e.g., hydro fracturing) and hence on the ice flow. It further constitutes a physically sound basis for an enhanced fracture-based calving parameterization.

scholarly journals Fracture-induced softening for large-scale ice dynamics

2013 ◽  
Vol 7 (5) ◽  
pp. 4501-4544 ◽  
Author(s):  
T. Albrecht ◽  
A. Levermann
Keyword(s):  
Large Scale ◽  
Ice Sheet ◽  
Damage Threshold ◽  
Fracture Density ◽  
Ice Streams ◽  
Model Framework ◽  
Ice Dynamics ◽  
Ice Flow ◽  
Ice Shelves ◽  
Linear Threshold

Abstract. Floating ice shelves can exert a retentive and hence stabilizing force onto the inland ice sheet of Antarctica. However, this effect has been observed to diminish by fracture-coupled dynamic processes within the protective ice shelves leading to accelerated ice flow and hence to a sea-level contribution. In order to better understand the role of fractures in ice dynamics we apply a large-scale continuum representation of fractures and related fracture growth into the prognostic Parallel Ice Sheet Model (PISM). To this end we introduce a higher-order accuracy advection scheme for the transport of the two-dimensional fracture density across the regular computational grid. Dynamic coupling of fractures and ice flow is attained by a reduction of effective ice viscosity proportional to the inferred fracture density. This formulation implies the possibility of a non-linear threshold behavior due to self-amplified fracturing in shear regions triggered by small variations in damage threshold. As a result of prognostic flow simulations, flow patterns with realistically large across-flow velocity gradients in fracture-weakened regions as seen in observations are reproduced. This model framework is expandable to grounded ice streams and accounts for climate-induced effects on fracturing and hence on the ice-flow dynamics. It further allows for an enhanced fracture-based calving parameterization.

scholarly journals Fracture field for large-scale ice dynamics

2012 ◽  
Vol 58 (207) ◽  
pp. 165-176 ◽  
Author(s):  
Torsten Albrecht ◽  
Anders Levermann
Keyword(s):  
Large Scale ◽  
Fracture Initiation ◽  
Spreading Rate ◽  
Density Field ◽  
Surface Structures ◽  
Fracture Density ◽  
Principal Stresses ◽  
Ice Dynamics ◽  
Ice Flow ◽  
Macroscopic Fracture

AbstractRecent observations and modeling studies emphasize the crucial role of fracture mechanics for the stability of ice shelves and thereby the evolution of ice sheets. Here we introduce a macroscopic fracture-density field into a prognostic continuum ice-flow model and compute its evolution incorporating the initiation and growth of fractures as well as their advection with two dimensional ice flow. To a first approximation, fracture growth is assumed to depend on the spreading rate only, while fracture initiation is defined in terms of principal stresses. The inferred fracture-density fields compare well with observed elongate surface structures. Since crevasses and other deep-reaching fracture structures have been shown to influence the overall ice-shelf dynamics, we propose the fracture- density field introduced here be used as a measure for ice softening and decoupling of the ice flow in fracture-weakened zones. This may yield more accurate and realistic velocity patterns in prognostic simulations. Additionally, the memory of past fracture events links the calving front to the upstream dynamics. Thus the fracture-density field proposed here may be employed in fracture-based calving parameterizations. The aim of this study is to introduce the field and investigate which of the observed surface structures can be reproduced by the simplest physically motivated fracture source terms.

scholarly journals Ice shelf fracture parameterization in an ice sheet model

2017 ◽  
Vol 11 (6) ◽  
pp. 2543-2554 ◽  
Author(s):  
Sainan Sun ◽  
Stephen L. Cornford ◽  
John C. Moore ◽  
Rupert Gladstone ◽  
Liyun Zhao
Keyword(s):  
Enhancement Factor ◽  
Large Scale ◽  
Damage Model ◽  
Ice Sheet ◽  
Ice Shelf ◽  
Ice Flow ◽  
Ice Shelves ◽  
Order Of Magnitude ◽  
Eventual Loss ◽  
Spatially Varying

Abstract. Floating ice shelves exert a stabilizing force onto the inland ice sheet. However, this buttressing effect is diminished by the fracture process, which on large scales effectively softens the ice, accelerating its flow, increasing calving, and potentially leading to ice shelf breakup. We add a continuum damage model (CDM) to the BISICLES ice sheet model, which is intended to model the localized opening of crevasses under stress, the transport of those crevasses through the ice sheet, and the coupling between crevasse depth and the ice flow field and to carry out idealized numerical experiments examining the broad impact on large-scale ice sheet and shelf dynamics. In each case we see a complex pattern of damage evolve over time, with an eventual loss of buttressing approximately equivalent to halving the thickness of the ice shelf. We find that it is possible to achieve a similar ice flow pattern using a simple rule of thumb: introducing an enhancement factor ∼ 10 everywhere in the model domain. However, spatially varying damage (or equivalently, enhancement factor) fields set at the start of prognostic calculations to match velocity observations, as is widely done in ice sheet simulations, ought to evolve in time, or grounding line retreat can be slowed by an order of magnitude.

scholarly journals The Dynamics of Ice-Sheet Outlets

1985 ◽  
Vol 31 (108) ◽  
pp. 99-107 ◽  
Author(s):  
N. F. Mcintyre
Keyword(s):  
Landsat Imagery ◽  
Ice Sheet ◽  
Ice Streams ◽  
Ice Flow ◽  
Ice Shelves ◽  
Surface Slopes ◽  
Basal Sliding ◽  
Subglacial Topography ◽  
Flow Through ◽  
Pressure Melting

AbstractA comparison of data from aircraft altimetry, Landsat imagery, and radia echo-sounding has shown characteristic surface topographies associated with sheet and stream flow. The transition between the two is abrupt and occurs at a step in the subglacial topography. This marks the onset of basal sliding and high velocities caused by subglacial water; it results in crevassed amphitheatre-like basins round the head of outlet glaciers. It is also the zone of maximum driving stress beyond which values decline rapidly as velocities increase. This abrupt transition appears to be topographically controlled since basal temperatures are at the pressure-melting point well inland of the change in regime. The Marie Byrd Land ice streams exhibit qualitative differences from other ice-sheet outlets, however; the change to lower driving stresses is much more gradual and occurs several hundred kilometres inland. Such ice streams have particularly low surface slopes and appear in form and flow regime to resemble confined ice shelves rather than grounded ice. The repeated association of the transition to rapid sliding with a distinct subglacial feature implies a stabilizing effect on discharge through outlet glaciers. Acceleration of the ice is pinned to a subglacial step and propagation of high velocities inland of this feature seems improbable. Rapid ice flow through subglacial trenches may also ensure a relatively permanent trough through accentuation of the feature by erosion. This is concentrated towards the heads of outlet glaciers up-stream of the region where significant basal decoupling occurs. This may be a mechanism for the overdeepening of fjords at their inland ends and the development of very steep fjord headwalls.

scholarly journals A balanced water layer concept for subglacial hydrology in large scale ice sheet models

2012 ◽  
Vol 6 (6) ◽  
pp. 5225-5253 ◽  
Author(s):  
S. Goeller ◽  
M. Thoma ◽  
K. Grosfeld ◽  
H. Miller
Keyword(s):  
Large Scale ◽  
Water Flux ◽  
Ice Sheet ◽  
Water Layer ◽  
Ice Streams ◽  
Ice Dynamics ◽  
Continental Scale ◽  
Basal Sliding ◽  
Subglacial Lakes ◽  
Layer Concept

Abstract. There is currently no doubt about the existence of a wide-spread hydrological network under the Antarctic ice sheet, which lubricates the ice base and thus leads to increased ice velocities. Consequently, ice models should incorporate basal hydrology to obtain meaningful results for future ice dynamics and their contribution to global sea level rise. Here, we introduce the balanced water layer concept, covering two prominent subglacial hydrological features for ice sheet modeling on a continental scale: the evolution of subglacial lakes and balance water fluxes. We couple it to the thermomechanical ice-flow model RIMBAY and apply it to a synthetic model domain inspired by the Gamburtsev Mountains, Antarctica. In our experiments we demonstrate the dynamic generation of subglacial lakes and their impact on the velocity field of the overlaying ice sheet, resulting in a negative ice mass balance. Furthermore, we introduce an elementary parametrization of the water flux–basal sliding coupling and reveal the predominance of the ice loss through the resulting ice streams against the stabilizing influence of less hydrologically active areas. We point out, that established balance flux schemes quantify these effects only partially as their ability to store subglacial water is lacking.

scholarly journals Ice shelf fracture parameterization in an ice sheet model

2017 ◽  
Author(s):  
Sainan Sun ◽  
Stephen Cornford ◽  
Rupert Gladstone ◽  
Liyun Zhao ◽  
John Moore
Keyword(s):  
Fracture Process ◽  
Large Scale ◽  
Damage Model ◽  
Ice Sheet ◽  
Continuum Damage ◽  
Ice Shelf ◽  
Ice Dynamics ◽  
Continuum Damage Model ◽  
Grounding Line ◽  
Fracture Processes

Abstract. Floating ice shelves exert a stabilizing force onto the inland ice sheet. However, this buttressing effect is diminished by the fracture process, which on large scales effectively softens the ice, accelerating its flow, increasing calving, and potentially leading to ice shelf breakup. Here, we explore how the application of a continuum damage model (CDM) to the prognostic ice sheet model BISICLES can account for the effects of fracture processes on viscous ice dynamics. Damage is created by the local stress field and advects downstream. This continuum damage model is coupled to the dynamical ice flow model by decreasing the effective viscosity proportional to the damage field. To evaluate the physical role of the fracture process on large-scale ice sheet dynamics and also discern the relative importance of the parameters used in the damage model, we carry out a suite of numerical experiments based on the MISMIP+ (Marine Ice Sheet Model Intercomparison Project) marine ice sheet geometry. We find that behavior of the simulated marine ice sheet is sensitive to fracture processes on the ice shelf. In the case of a geometry that produces strong lateral stress, the stiffness of ice around the grounding line is essential to ice sheet evolution, with softer or more damaged ice leading to thinning and grounding line retreat.

scholarly journals The Dynamics of Ice-Sheet Outlets

1985 ◽  
Vol 31 (108) ◽  
pp. 99-107 ◽  
Author(s):  
N. F. Mcintyre
Keyword(s):  
Landsat Imagery ◽  
Ice Sheet ◽  
Ice Streams ◽  
Ice Flow ◽  
Ice Shelves ◽  
Surface Slopes ◽  
Basal Sliding ◽  
Subglacial Topography ◽  
Flow Through ◽  
Pressure Melting

AbstractA comparison of data from aircraft altimetry, Landsat imagery, and radia echo-sounding has shown characteristic surface topographies associated with sheet and stream flow. The transition between the two is abrupt and occurs at a step in the subglacial topography. This marks the onset of basal sliding and high velocities caused by subglacial water; it results in crevassed amphitheatre-like basins round the head of outlet glaciers. It is also the zone of maximum driving stress beyond which values decline rapidly as velocities increase. This abrupt transition appears to be topographically controlled since basal temperatures are at the pressure-melting point well inland of the change in regime. The Marie Byrd Land ice streams exhibit qualitative differences from other ice-sheet outlets, however; the change to lower driving stresses is much more gradual and occurs several hundred kilometres inland. Such ice streams have particularly low surface slopes and appear in form and flow regime to resemble confined ice shelves rather than grounded ice. The repeated association of the transition to rapid sliding with a distinct subglacial feature implies a stabilizing effect on discharge through outlet glaciers. Acceleration of the ice is pinned to a subglacial step and propagation of high velocities inland of this feature seems improbable. Rapid ice flow through subglacial trenches may also ensure a relatively permanent trough through accentuation of the feature by erosion. This is concentrated towards the heads of outlet glaciers up-stream of the region where significant basal decoupling occurs. This may be a mechanism for the overdeepening of fjords at their inland ends and the development of very steep fjord headwalls.

scholarly journals A balanced water layer concept for subglacial hydrology in large-scale ice sheet models

2013 ◽  
Vol 7 (4) ◽  
pp. 1095-1106 ◽  
Author(s):  
S. Goeller ◽  
M. Thoma ◽  
K. Grosfeld ◽  
H. Miller
Keyword(s):  
Large Scale ◽  
Water Flux ◽  
Ice Sheet ◽  
Water Layer ◽  
Ice Streams ◽  
Ice Dynamics ◽  
Continental Scale ◽  
Basal Sliding ◽  
Subglacial Lakes ◽  
Layer Concept

Abstract. There is currently no doubt about the existence of a widespread hydrological network under the Antarctic Ice Sheet, which lubricates the ice base and thus leads to increased ice velocities. Consequently, ice models should incorporate basal hydrology to obtain meaningful results for future ice dynamics and their contribution to global sea level rise. Here, we introduce the balanced water layer concept, covering two prominent subglacial hydrological features for ice sheet modeling on a continental scale: the evolution of subglacial lakes and balance water fluxes. We couple it to the thermomechanical ice-flow model RIMBAY and apply it to a synthetic model domain. In our experiments we demonstrate the dynamic generation of subglacial lakes and their impact on the velocity field of the overlaying ice sheet, resulting in a negative ice mass balance. Furthermore, we introduce an elementary parametrization of the water flux–basal sliding coupling and reveal the predominance of the ice loss through the resulting ice streams against the stabilizing influence of less hydrologically active areas. We point out that established balance flux schemes quantify these effects only partially as their ability to store subglacial water is lacking.

scholarly journals Distribution of glacial geomorphic features on the Antarctic continental shelf and correlation with substrate: implications for ice behavior

2001 ◽  
Vol 47 (158) ◽  
pp. 397-411 ◽  
Author(s):  
J. S. Wellner ◽  
A. L. Lowe ◽  
S. S. Shipp ◽  
J. B. Anderson
Keyword(s):  
Large Scale ◽  
Ice Sheet ◽  
Ice Streams ◽  
Ice Flow ◽  
Crystalline Bedrock ◽  
Marguerite Bay ◽  
Crystalline Substrate ◽  
Geomorphic Features ◽  
Geological Substrates ◽  
The Antarctic

AbstractSurveys were conducted seaward of all the major drainage outlets of the Antarctic ice sheet from the Pennell Coast, north Victoria Land, to Marguerite Bay, Antarctic Peninsula. The results show that the ice sheet extended onto the outer shelf. Glacial troughs occur offshore of all major glacial outlets. Where the substrate is crystalline bedrock, ice flow tended to follow the structural grain of the bedrock, deposited little sediment and eroded the underlying bedrock. Where ice flowed over relatively soft, more easily eroded, sedimentary strata, the direction of ice flow was more directly offshore, and depositional features characterize the sea-floor. In these areas the signature of the grounded ice consists of till deposits and large-scale geomorphic features. Drumlins occur within the region of contact between crystalline and sedimentary substrates. The different geological substrates are interpreted to have exerted a fundamental control on the behavior of past ice sheets. The troughs in the areas of bedrock composed of sedimentary substrate are interpreted to have been occupied by relatively fast-flowing ice, ice streams, and the troughs in the areas of crystalline substrate are interpreted to have been occupied by slower-moving ice. The area between these two zones was characterized by ice acceleration and is marked by drumlins.

scholarly journals Ice-sheet surging and ice-stream formation

1996 ◽  
Vol 23 ◽  
pp. 68-73 ◽  
Author(s):  
A. C. Fowler ◽  
C. Johnson
Keyword(s):  
Large Scale ◽  
Ice Sheet ◽  
Type Equation ◽  
Laurentide Ice Sheet ◽  
Spontaneous Generation ◽  
Ice Streams ◽  
Diffusion Type ◽  
Ice Flow ◽  
Linear Diffusion ◽  
Sliding Dynamics

A simplified model of ice-sheet behaviour is described. It combines the assumptions of rapid ice flow, high viscous activation energy and realistic sediment-based sliding dynamics to form a non-linear diffusion-type equation which can display relaxation oscillations analogous to those of surging glaciers, and which may be relevant to large-scale surges of the Hudson Strait and Cabot Strait ice streams of the Laurentide ice sheet. When the physics of this model is applied to a laterally extensive unidirectional ice flow, such as that in the Siple Coast of Antarctica, an appropriate mechanism may exist for the spontaneous generation of ice streams.

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