| 1. |
- Ahlkrona, Josefin, et al.
(author)
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Dynamically coupling the non-linear Stokes equations with the shallow ice approximation in glaciology : Description and first applications of the ISCAL method
- 2016
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In: Journal of Computational Physics. - : Elsevier BV. - 0021-9991 .- 1090-2716. ; 308, s. 1-19
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Journal article (peer-reviewed)abstract
- We propose and implement a new method, called the Ice Sheet Coupled Approximation Levels (ISCAL) method, for simulation of ice sheet flow in large domains during long time-intervals. The method couples the full Stokes (FS) equations with the Shallow Ice Approximation (SIA). The part of the domain where SIA is applied is determined automatically and dynamically based on estimates of the modeling error. For a three dimensional model problem, ISCAL computes the solution substantially faster with a low reduction in accuracy compared to a monolithic FS. Furthermore, ISCAL is shown to be able to detect rapid dynamic changes in the flow. Three different error estimations are applied and compared. Finally, ISCAL is applied to the Greenland Ice Sheet on a quasi-uniform grid, proving ISCAL to be a potential valuable tool for the ice sheet modeling community.
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| 2. |
- Löfgren, André, 1992-, et al.
(author)
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Increasing Numerical Stability of Mountain Valley Glacier Simulations : Implementation and Testing of Free-Surface Stabilization in Elmer/Ice
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Other publication (other academic/artistic)abstract
- This paper concerns a numerical stabilization method for free-surface ice flow called the free-surface stabilizationalgorithm (FSSA). In the current study, the FSSA is implemented into the numerical ice-flow software Elmer/Ice and tested onsynthetic two-dimensional (2D) glaciers, as well as on the real-world glacier of Midtre Lovénbreen, Svalbard. For the synthetic2D cases it is found that the FSSA method increases the largest stable time-step size at least by a factor of ten for the case of agently sloping ice surface ( 3◦), and by at least a factor of five for cases of moderately to steeply inclined surfaces ( 6◦ − 12◦) .5Furthermore, the FSSA method increases the overall accuracy for all surface slopes. The largest stable time-step size is foundto be smallest for the case of a low sloping surface, despite having overall smaller velocities. For Midtre Lovénbreen the FSSAmethod doubles the largest stable time-step size, however, the accuracy is in this case slightly lowered in the deeper parts ofthe glacier, while it increases near edges. The implication is that the non-FSSA method might be more accurate at predictingglacier thinning, while the FSSA method is more suitable for predicting future glacier extent. A possible application of the10larger time-step sizes allowed for by the FSSA is for spin-up simulations, where relatively fast changing climate data can beincorporated on short time scales, while the slowly changing velocity field is updated over larger time scales.
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| 3. |
- Memon, Shahbaz, et al.
(author)
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Scientific workflows applied to the coupling of a continuum (Elmer v8.3) and a discrete element (HiDEM v1.0) ice dynamic model
- 2019
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In: Geoscientific Model Development. - : COPERNICUS GESELLSCHAFT MBH. - 1991-959X .- 1991-9603. ; 12:7, s. 3001-3015
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Journal article (peer-reviewed)abstract
- 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.
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| 4. |
- Moore, John C., et al.
(author)
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Semiempirical and process-based global sea level projections
- 2013
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In: Reviews of geophysics. - : American Geophysical Union (AGU). - 8755-1209 .- 1944-9208. ; 51:3, s. 484-522
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Research review (peer-reviewed)abstract
- We review the two main approaches to estimating sea level rise over the coming century: physically plausible models of reduced complexity that exploit statistical relationships between sea level and climate forcing, and more complex physics-based models of the separate elements of the sea level budget. Previously, estimates of future sea level rise from semiempirical models were considerably larger than those from process-based models. However, we show that the most recent estimates of sea level rise by 2100 using both methods have converged, but largely through increased contributions and uncertainties in process-based model estimates of ice sheets mass loss. Hence, we focus in this paper on ice sheet flow as this has the largest potential to contribute to sea level rise. Progress has been made in ice dynamics, ice stream flow, grounding line migration, and integration of ice sheet models with high-resolution climate models. Calving physics remains an important and difficult modeling issue. Mountain glaciers, numbering hundreds of thousands, must be modeled by extensive statistical extrapolation from a much smaller calibration data set. Rugged topography creates problems in process-based mass balance simulations forced by regional climate models with resolutions 10-100 times larger than the glaciers. Semiempirical models balance increasing numbers of parameters with the choice of noise model for the observations to avoid overfitting the highly autocorrelated sea level data. All models face difficulty in separating out non-climate-driven sea level rise (e.g., groundwater extraction) and long-term disequilibria in the present-day cryosphere-sea level system.
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| 5. |
- Vallot, Dorothée, 1984-, et al.
(author)
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Basal dynamics of Kronebreen, a fast-flowing tidewater glacier in Svalbard : non-local spatio-temporal response to water input
- 2017
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In: Journal of Glaciology. - : Cambridge University Press (CUP). - 0022-1430 .- 1727-5652. ; 63:242, s. 1012-1024
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Journal article (peer-reviewed)abstract
- We evaluate the variability in basal friction for Kronebreen, Svalbard, a fast-flowing tidewater glacier. We invert 3 years (2013–15) of surface velocities at high temporal resolution (generally 11 days), to estimate the changing basal properties of the glacier. Our results suggest that sliding behaviour of Kronebreen within a year is primarily influenced by changes in water input patterns during the meltwater season and basal friction is highly variable from a year to another. At present, models usually employ parameterisations to encompass the complex physics of glacier sliding by mathematically simulate their net effect. For such ice masses with strong seasonal variations of surface melt, the spatio-temporal patterns of basal friction imply that it is neither possible nor appropriate to use a parameterisation for bed friction that is fixed in space and/or time, at least in a timescale of a few years. Basal sliding may not only be governed by local processes such as basal topography or summer melt, but also be mediated by factors that vary over a larger distance and over a longer time period such as subglacial hydrology organisation, ice-thickness changes or calving front geometry.
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| 6. |
- Vallot, Dorothée, et al.
(author)
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Effects of undercutting and sliding on calving : a global approach applied to Kronebreen, Svalbard
- 2018
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In: The Cryosphere. - : Copernicus GmbH. - 1994-0416 .- 1994-0424. ; 12, s. 609-625
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Journal article (peer-reviewed)abstract
- In this paper, we study the effects of basal friction, sub-aqueous undercutting and glacier geometry on the calving process by combining six different models in an offline-coupled workflow: a continuum-mechanical ice flow model (Elmer/Ice), a climatic mass balance model, a simple sub-glacial hydrology model, a plume model, an undercutting model and a discrete particle model to investigate fracture dynamics (Helsinki Discrete Element Model, HiDEM). We demonstrate the feasibility of reproducing the observed calving retreat at the front of Kronebreen, a tidewater glacier in Svalbard, during a melt season by using the output from the first five models as input to HiDEM. Basal sliding and glacier motion are addressed using Elmer/Ice, while calving is modelled by HiDEM. A hydrology model calculates subglacial drainage paths and indicates two main outlets with different discharges. Depending on the discharge, the plume model computes frontal melt rates, which are iteratively projected to the actual front of the glacier at subglacial discharge locations. This produces undercutting of different sizes, as melt is concentrated close to the surface for high discharge and is more diffuse for low discharge. By testing different configurations, we show that undercutting plays a key role in glacier retreat and is necessary to reproduce observed retreat in the vicinity of the discharge locations during the melting season. Calving rates are also influenced by basal friction, through its effects on near-terminus strain rates and ice velocity.
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| 7. |
- van Dongen, Eef C. H., et al.
(author)
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Dynamically coupling full Stokes and shallow shelf approximation for marine ice sheet flow using Elmer/Ice (v8.3)
- 2018
-
In: Geoscientific Model Development. - : Copernicus GmbH. - 1991-959X .- 1991-9603. ; 11:11, s. 4563-4576
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Journal article (peer-reviewed)abstract
- 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.
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| 8. |
- Zhao, Liyun, et al.
(author)
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Numerical simulations of Gurenhekou glacier on the Tibetan Plateau
- 2014
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In: Journal of Glaciology. - 0022-1430 .- 1727-5652. ; 60:219, s. 71-82
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Journal article (peer-reviewed)abstract
- We investigate the impact of climate change on Gurenhekou glacier, southern Tibetan Plateau, which is representative of the tens of thousands of mountain glaciers in the region. We apply a three-dimensional, thermomechanically coupled full-Stokes model to simulate the evolution of the glacier. The steep and rugged bedrock geometry requires use of such a flow model. We parameterize the temperature and surface mass-balance (SMB) uncertainties using nearby automatic weather and meteorological stations, 6 year measured SMB data and an energy-balance model for a nearby glacier. Summer air temperature increased at 0.02 K a(-1) over the past 50 years, and the glacier has retreated at an average rate of 8.3 m a(-1). Prognostic simulations suggest an accelerated annual average retreat rate of similar to 9.1 m a(-1) along the central flowline for the next 25 years under continued steady warming. However, regional climate models suggest a marked increase in warming rate over Tibet during the 21st century, and this rate causes about a 0.9 +/- 0.3% a(-1) loss of glaciated area and 1.1 +/- 0.6% a(-1) shrinkage of glacier volume. These results, the rather high warming rates predicted and the small sizes of most Tibetan glaciers, suggest that significant numbers of glaciers will be lost in the region during the 21st century.
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| 9. |
- Åström, Jan A., et al.
(author)
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Termini of calving glaciers as self-organized critical systems
- 2014
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In: Nature Geoscience. - : Springer Science and Business Media LLC. - 1752-0894 .- 1752-0908. ; 7:12, s. 874-878
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Journal article (peer-reviewed)abstract
- Over the next century, one of the largest contributions to sea level rise will come from ice sheets and glaciers calving ice into the ocean1. Factors controlling the rapid and nonlinear variations in calving fluxes are poorly understood, and therefore difficult to include in prognostic climate-forced land-ice models. Here we analyse globally distributed calving data sets from Svalbard, Alaska (USA), Greenland and Antarctica in combination with simulations from a first-principles, particle-based numerical calving model to investigate the size and inter-event time of calving events. We find that calving events triggered by the brittle fracture of glacier ice are governed by the same power-law distributions as avalanches in the canonical Abelian sandpile model2. This similarity suggests that calving termini behave as self-organized critical systems that readily flip between states of sub-critical advance and super-critical retreat in response to changes in climate and geometric conditions. Observations of sudden ice-shelf collapse and tidewater glacier retreat in response to gradual warming of their environment3 are consistent with a system fluctuating around its critical point in response to changing external forcing. We propose that self-organized criticality provides a yet unexplored framework for investigations into calving and projections of sea level rise.
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