We present steady state (diagnostic) and transient (prognostic) simulations of Midtre Lovénbreen, Svalbard performed with the thermo-mechanically coupled full-Stokes code Elmer. This glacier has an ...extensive data set of geophysical measurements available spanning several decades, that allow for constraints on model descriptions. Consistent with this data set, we included a simple model accounting for the formation of superimposed ice. Diagnostic results indicated that a dynamic adaptation of the free surface is necessary, to prevent non-physically high velocities in a region of under determined bedrock depths. Observations from ground penetrating radar of the basal thermal state agree very well with model predictions, while the dip angles of isochrones in radar data also match reasonably well with modelled isochrones, despite the numerical deficiencies of estimating ages with a steady state model. Prognostic runs for 53 years, using a constant accumulation/ablation pattern starting from the steady state solution obtained from the configuration of the 1977 DEM show that: 1 the unrealistic velocities in the under determined parts of the DEM quickly damp out; 2 the free surface evolution matches well measured elevation changes; 3 the retreat of the glacier under this scenario continues with the glacier tongue in a projection to 2030 being situated ≈500 m behind the position in 1977.
Scientific computing applications involving complex simulations and data-intensive processing are often composed of multiple tasks forming a workflow of computing jobs. Scientific communities running ...such applications on computing resources often find it cumbersome to manage and monitor the execution of these tasks and their associated data. These workflow implementations usually add overhead by introducing unnecessary input/output (I/O) for coupling the models and can lead to sub-optimal CPU utilization. Furthermore, running these workflow implementations in different environments requires significant adaptation efforts, which can hinder the reproducibility of the underlying science. High-level scientific workflow management systems (WMS) can be used to automate and simplify complex task structures by providing tooling for the composition and execution of workflows - even across distributed and heterogeneous computing environments. The WMS approach allows users to focus on the underlying high-level workflow and avoid low-level pitfalls that would lead to non-optimal resource usage while still allowing the workflow to remain portable between different computing environments. As a case study, we apply the UNICORE workflow management system to enable the coupling of a glacier flow model and calving model which contain many tasks and dependencies, ranging from pre-processing and data management to repetitive executions in heterogeneous high-performance computing (HPC) resource environments. Using the UNICORE workflow management system, the composition, management, and execution of the glacier modelling workflow becomes easier with respect to usage, monitoring, maintenance, reusability, portability, and reproducibility in different environments and by different user groups. Last but not least, the workflow helps to speed the runs up by reducing model coupling I/O overhead and it optimizes CPU utilization by avoiding idle CPU cores and running the models in a distributed way on the HPC cluster that best fits the characteristics of each model.
Dry snow avalanches consist of two distinct layers. A dense-flow layer is superposed by a powder-snow layer, a cloud of relatively small ice particles suspended in air. The density of this suspension ...is one order of magnitude smaller than that of the dense flow. A simulation model for dry avalanches has been developed, based on separate sub-models for the two layers. The sub-models are coupled by an additional transition model, describing the exchange of mass and momentum between the layers. The fundamentals of the two-dimensional granular flow model for the dense flow and of the three-dimensional turbulent mixture model for the powder flow are presented. Results of the complete coupled model, SAMOS (Snow Avalanche MOdelling and Simulation), applied to observed catastrophic avalanche events, are discussed, and the prediction of powder-snow pressures acting on a tunnel bridge is briefly described. SAMOS is used routinely for hazard zoning at the Austrian Federal Service for Torrent and Avalanche Control.
The Totten Ice Shelf (IS) has a large drainage basin,
much of which is grounded below sea level, leaving the glacier vulnerable to
retreat through the marine ice sheet instability mechanism. The ice ...shelf
has also been shown to be sensitive to changes in calving rate, as a very
small retreat of the calving front from its current position is predicted to
cause a change in flow at the grounding line. Therefore understanding the
processes behind calving on the Totten IS is key to predicting its future
sea level rise contribution. Here we use the Helsinki Discrete Element Model (HiDEM)
to show that not all of the fractures visible at the front of the
Totten IS are produced locally, but that the across-flow basal crevasses,
which are part of the distinctive cross-cutting fracture pattern, are
advected into the calving front area from upstream. A separate simulation of
the grounding line shows that re-grounding points may be key areas of basal
crevasse production, and can produce basal crevasses in both an along- and
across-flow orientation. The along-flow basal crevasses at the grounding
line may be a possible precursor to basal channels, while we suggest the
across-flow grounding-line fractures are the source of the across-flow
features observed at the calving front. We use two additional models to
simulate the evolution of basal fractures as they advect downstream,
demonstrating that both strain and ocean melt have the potential to deform
narrow fractures into the broad basal features observed near the calving
front. The wide range of factors which influence fracture patterns and
calving on this glacier will be a challenge for predicting its future mass loss.
The Wordie Ice Shelf–Fleming Glacier system in the southern Antarctic
Peninsula has experienced a long-term retreat and disintegration of its ice
shelf in the past 50 years. Increases in the glacier ...velocity and dynamic
thinning have been observed over the past two decades, especially after 2008
when only a small ice shelf remained at the Fleming Glacier front. It is
important to know whether the substantial further speed-up and greater
surface draw-down of the glacier since 2008 is a direct response to ocean
forcing, or driven by feedbacks within the grounded marine-based glacier
system, or both. Recent observational studies have suggested the
2008–2015 velocity change was due to the ungrounding of the Fleming Glacier
front. To explore the mechanisms underlying the recent changes, we use a
full-Stokes ice sheet model to simulate the basal shear stress distribution
of the Fleming system in 2008 and 2015. This study is part of the first high
resolution modelling campaign of this system. Comparison of inversions for
basal shear stresses for 2008 and 2015 suggests the migration of the
grounding line ∼9 km upstream by 2015 from the 2008 ice front/grounding
line positions, which virtually coincided with the 1996 grounding line
position. This migration is consistent with the change in floating area
deduced from the calculated height above buoyancy in 2015. The retrograde
submarine bed underneath the lowest part of the Fleming Glacier may have
promoted retreat of the grounding line. Grounding line retreat may also be
enhanced by a feedback mechanism upstream of the grounding line by which
increased basal lubrication due to increasing frictional heating enhances
sliding and thinning. Improved knowledge of bed topography near the grounding
line and further transient simulations with oceanic forcing are required to
accurately predict the future movement of the Fleming Glacier system
grounding line and better understand its ice dynamics and future contribution
to sea level.
Ice flow forced by gravity is governed by the full Stokes (FS) equations, which are computationally expensive to solve due to the nonlinearity introduced by the rheology. Therefore, approximations to ...the FS equations are commonly used, especially when modeling a marine ice sheet (ice sheet, ice shelf, and/or ice stream) for 103 years or longer. The shallow ice approximation (SIA) and shallow shelf approximation (SSA) are commonly used but are accurate only for certain parts of an ice sheet. Here, we report a novel way of iteratively coupling FS and SSA that has been implemented in Elmer/Ice and applied to conceptual marine ice sheets. The FS–SSA coupling appears to be very accurate; the relative error in velocity compared to FS is below 0.5 % for diagnostic runs and below 5 % for prognostic runs. Results for grounding line dynamics obtained with the FS–SSA coupling are similar to those obtained from an FS model in an experiment with a periodical temperature forcing over 3000 years that induces grounding line advance and retreat. The rapid convergence of the FS–SSA coupling shows a large potential for reducing computation time, such that modeling a marine ice sheet for thousands of years should become feasible in the near future. Despite inefficient matrix assembly in the current implementation, computation time is reduced by 32 %, when the coupling is applied to a 3-D ice shelf.
As fibers or other crystalline materials exhibiting hexagonal symmetry, the crystal of ice can be orientated by using only one single vector, i.e. its
c-axis. Such a characteristic allows to apply ...specific methods to deal with the properties of the polycrystalline aggregate. Among others, the fabric (texture) of the ice polycrystal can be described by an ODF, i.e. a scalar function of two angles that gives the distribution of the orientation of all the constituents (grains).
This paper presents a strain-induced anisotropic flow law for polycrystalline ice and the associated equations describing the evolution of its fabric. This constitutive law is formulated at the polycrystal scale and tabulated using a micro–macro model. The fabric is defined by the second- and fourth-order orientation tensors for the
c-axes, assuming the so-called “invariant-based optimal fitting closure approximation”. Both the anisotropic constitutive law and the fabric evolution equations have been implemented in a finite element code in order to solve large scale ice flow problem. As an application, the flow of an idealized ice sheet over a bumpy bed is studied.
PDF
