scholarly journals Comparison of adjoint and nudging methods to initialise ice sheet model basal conditions

2016 ◽  
Vol 9 (7) ◽  
pp. 2549-2562 ◽  
Author(s):  
Cyrille Mosbeux ◽  
Fabien Gillet-Chaulet ◽  
Olivier Gagliardini
Keyword(s):  
Sea Level ◽  
21St Century ◽  
Initial Conditions ◽  
Ice Sheets ◽  
Transient Effects ◽  
Flux Divergence ◽  
Short Term ◽  
Ice Flow ◽  
Flow Models ◽  
Model Response

Abstract. Ice flow models are now routinely used to forecast the ice sheets' contribution to 21st century sea-level rise. For such short term simulations, the model response is greatly affected by the initial conditions. Data assimilation algorithms have been developed to invert for the friction of the ice on its bedrock using observed surface velocities. A drawback of these methods is that remaining uncertainties, especially in the bedrock elevation, lead to non-physical ice flux divergence anomalies resulting in undesirable transient effects. In this study, we compare two different assimilation algorithms based on adjoints and nudging to constrain both bedrock friction and elevation. Using synthetic twin experiments with realistic observation errors, we show that the two algorithms lead to similar performances in reconstructing both variables and allow the flux divergence anomalies to be significantly reduced.

scholarly journals Towards a better ice sheet model initialisation and basal knowledge using data assimilation

2016 ◽  
Author(s):  
Cyrille Mosbeux ◽  
Fabien Gillet-Chaulet ◽  
Olivier Gagliardini
Keyword(s):  
Data Assimilation ◽  
Initial Conditions ◽  
Ice Sheets ◽  
Transient Effects ◽  
Flux Divergence ◽  
Short Term ◽  
Ice Flow ◽  
Flow Models ◽  
Model Response ◽  
Using Data

Abstract. Ice flow models are now routinely used to forecast the ice-sheets contribution to 21st century sea-level rise. For such short term simulations, the model response is greatly affected by the initial conditions. Data assimilation algorithms have been developed to invert for the friction of the ice on its bedrock using observed surface velocities. A drawback of these methods is that remaining uncertainties, especially in the bedrock elevation, lead to non-physical ice flux divergence anomalies resulting in undesirable transient effects. In this study, we compare two different assimilation algorithms based on adjoints and nudging to constrain both bedrock friction and elevation. Using synthetic twin experiments with realistic observation errors, we show that the two algorithms lead to similar performances in reconstructing both variables and allow the flux divergence anomalies to be significantly reduced.

scholarly journals Retreat of Thwaites Glacier, West Antarctica, over the next 100 years using various ice flow models, ice shelf melt scenarios and basal friction laws

2018 ◽  
Vol 12 (12) ◽  
pp. 3861-3876 ◽  
Author(s):  
Hongju Yu ◽  
Eric Rignot ◽  
Helene Seroussi ◽  
Mathieu Morlighem
Keyword(s):  
Sea Level ◽  
Initial Conditions ◽  
Numerical Models ◽  
West Antarctica ◽  
Ice Shelf ◽  
Flow Models ◽  
Friction Laws ◽  
Basal Friction ◽  
Grounding Line ◽  
Western Side

Abstract. Thwaites Glacier (TG), West Antarctica, has experienced rapid, potentially irreversible grounding line retreat and mass loss in response to enhanced ice shelf melting. Results from recent numerical models suggest a large spread in the evolution of the glacier in the coming decades to a century. It is therefore important to investigate how different approximations of the ice stress balance, parameterizations of basal friction and ice shelf melt parameterizations may affect projections. Here, we simulate the evolution of TG using ice sheet models of varying levels of complexity, different basal friction laws and ice shelf melt to quantify their effect on the projections. We find that the grounding line retreat and its sensitivity to ice shelf melt are enhanced when a full-Stokes model is used, a Budd friction is used and ice shelf melt is applied on partially floating elements. Initial conditions also impact the model results. Yet, all simulations suggest a rapid, sustained retreat of the glacier along the same preferred pathway. The fastest retreat rate occurs on the eastern side of the glacier, and the slowest retreat occurs across a subglacial ridge on the western side. All the simulations indicate that TG will undergo an accelerated retreat once the glacier retreats past the western subglacial ridge. Combining all the simulations, we find that the uncertainty of the projections is small in the first 30 years, with a cumulative contribution to sea level rise of 5 mm, similar to the current rate. After 30 years, the contribution to sea level depends on the model configurations, with differences up to 300 % over the next 100 years, ranging from 14 to 42 mm.

scholarly journals Increased West Antarctic ice discharge and East Antarctic stability over the last seven years

2017 ◽  
Author(s):  
Alex S. Gardner ◽  
Geir Moholdt ◽  
Ted Scambos ◽  
Mark Fahnstock ◽  
Stefan Ligtenberg ◽  
...  
Keyword(s):  
Synthetic Aperture Radar ◽  
Sea Level Rise ◽  
Sea Level ◽  
Ice Sheets ◽  
Synthetic Aperture ◽  
Direct Role ◽  
Ice Flow ◽  
West Antarctic ◽  
Landsat 7 ◽  
Ice Velocity

Abstract. Ice discharge from large ice sheets plays a direct role in determining rates of sea level rise. We map present-day Antarctic-wide surface velocities using Landsat 7 & 8 imagery spanning 2013–2015 and compare to earlier estimates derived from synthetic aperture radar, revealing heterogeneous changes in ice flow since ~ 2008. The new mapping provides complete coastal and inland coverage of ice velocity with a mean error of

Antarctic blue ice areas - towards extracting palaeoclimate information

2010 ◽  
Vol 22 (2) ◽  
pp. 99-115 ◽  
Author(s):  
Anna Sinisalo ◽  
John C. Moore
Keyword(s):  
Ice Sheets ◽  
Ice Sheet ◽  
Thermal Transition ◽  
Gas Composition ◽  
Accumulation Rates ◽  
Ice Flow ◽  
Flow Models ◽  
Flow Modelling ◽  
Substantial Progress ◽  
The Last Glacial

AbstractWe review the current scientific knowledge about Antarctic Blue Ice Areas (BIAs) with emphasis on their application for palaeoclimate studies. Substantial progress has been made since the review by Bintanja (1999), in particular dating the archive of ancient ice found on the surface of BIAs has progressed with advances in 14C measurements, tephrachronology, and geomorphological evidence giving better constraints to more sophisticated ice flow models. Flow modelling also provides information about past changes in ice flow velocities, accumulation rates and ice sheet elevation. The availability of gas composition in vertical cores from BIAs allows matching to well-dated global records of greenhouse gas variability over the last glacial-interglacial cycle and longer. It is clear from the limited number of studies to date that BIAs from different regions have quite different histories of formation and preservation, and that they are intimately linked to the response of their surrounding ice sheets to climate variability on glacial-interglacial time-scales. Looking to the future, climate records from BIAs are expected to provide information on variations in Southern Ocean processes as well as ice sheet evolution within the East Antarctic ice sheet at the thermal transition from cold based to warm based ice.

scholarly journals Sensitivity of Greenland ice sheet projections to spatial resolution in higher-order simulations: the AWI contribution to ISMIP6-Greenland using ISSM

2020 ◽  
Author(s):  
Martin Rückamp ◽  
Heiko Goelzer ◽  
Angelika Humbert
Keyword(s):  
Sea Level Rise ◽  
Sea Level ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Higher Order ◽  
Initial State ◽  
Ice Flow ◽  
Outlet Glacier ◽  
Flow Models ◽  
High Emission

Abstract. Projections of the contribution of the Greenland ice sheet to future sea-level rise include uncertainties primarily due to the imposed climate forcing and the initial state of the ice sheet model. Several state-of-the-art ice flow models are currently being employed on various grid resolutions to estimate future mass changes in the framework of the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6). Here we investigate the sensitivity to grid resolution on centennial sea-level contributions from the Greenland ice sheet and study the mechanism at play. To this end, we employ the finite-element higher-order ice flow model ISSM and conduct experiments with four different horizontal resolutions, namely 4, 2, 1 and 0.75 km. We run the simulation based on the ISMIP6 core GCM MIROC5 under the high emission scenario RCP8.5 and consider both atmospheric and oceanic forcing in full and separate scenarios. Under the full scenarios, finer simulations unveil up to ~5 % more sea-level rise compared to the coarser resolution. The sensitivity depends on the magnitude of outlet glacier retreat, which is implemented as a series of retreat masks following the ISMIP6 protocol. Without imposed retreat under atmosphere-only forcing, the resolution dependency exhibits an opposite behaviour with about ~ 5 % more sea-level contribution in the coarser resolution. The sea-level contribution indicates a converging behaviour

scholarly journals Glacier hydromechanics: early insights and the lasting legacy of three works by Iken and colleagues

2010 ◽  
Vol 56 (200) ◽  
pp. 1069-1078 ◽  
Author(s):  
Gwenn E. Flowers
Keyword(s):  
Shear Stress ◽  
Vertical Velocity ◽  
Water Pressure ◽  
Ice Sheets ◽  
Strong Correlations ◽  
Velocity Measurements ◽  
Short Term ◽  
Ice Flow ◽  
Alpine Glaciers ◽  
Basal Shear

AbstractThe association between basal hydrology and glacier sliding has become nearly synonymous with the early work of Almut Iken and colleagues. Their research published in theJournal of Glaciologyfrom 1981 to 1986 made an indelible impact on the study of glacier hydromechanics by documenting strong correlations between basal water pressure and short-term ice-flow variations. With a passion for elucidating the physics of glacier-bed processes, Iken herself made fundamental contributions to our theoretical and empirical understanding of the sliding process. From the theoretical bound on basal shear stress, to the inferences drawn from detailed horizontal and vertical velocity measurements, the work of Iken and colleagues continues to inform the interpretation of data from alpine glaciers and has found increasing relevance to observations from the ice sheets.

scholarly journals Sensitivity of grounding line dynamics to the choice of the friction law

2017 ◽  
Vol 63 (241) ◽  
pp. 854-866 ◽  
Author(s):  
JULIEN BRONDEX ◽  
OLIVIER GAGLIARDINI ◽  
FABIEN GILLET-CHAULET ◽  
GAËL DURAND
Keyword(s):  
Sea Level Rise ◽  
Sea Level ◽  
Basal Slip ◽  
Ice Streams ◽  
Ice Flow ◽  
Friction Law ◽  
Flow Models ◽  
Friction Laws ◽  
Grounding Line ◽  
Physically Based

ABSTRACTBasal slip accounts for a large part of the flow of ice streams draining ice from Antarctica and Greenland into the ocean. Therefore, an appropriate representation of basal slip in ice flow models is a prerequisite for accurate sea level rise projections. Various friction laws have been proposed to describe basal slip in models. Here, we compare the influence on grounding line (GL) dynamics of four friction laws: the traditional Weertman law and three effective pressure-dependent laws, namely the Schoof, Tsai and Budd laws. It turns out that, even when they are tuned to a common initial reference state, the Weertman, Budd and Schoof laws lead to thoroughly different steady-state positions, although the Schoof and Tsai laws lead to much the same result. In particular, under certain circumstances, it is possible to obtain a steady GL located on a reverse slope area using the Weertman law. Furthermore, the predicted transient evolution of the GL as well as the projected contributions to sea level rise over a 100-year time horizon vary significantly depending on the friction law. We conclude on the importance of choosing an appropriate law for reliable sea level rise projections and emphasise the need for a coupling between ice flow models and physically based subglacial hydrological models.

scholarly journals A rapidly converging spin-up method for the present-day Greenland ice sheet using the GRISLI ice-sheet model

2018 ◽  
Author(s):  
Sébastien Le clec'h ◽  
Aurélien Quiquet ◽  
Sylvie Charbit ◽  
Christophe Dumas ◽  
Masa Kageyama ◽  
...  
Keyword(s):  
Sea Level ◽  
Initial Conditions ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Flow Dynamics ◽  
Ice Thickness ◽  
Rapid Convergence ◽  
Ice Flow ◽  
Ice Volume ◽  
Basal Shear

Abstract. Providing reliable projections of the ice-sheet contribution to future sea-level rise has become one of the main challenges of the ice-sheet modelling community. To increase confidence in future projections, a good knowledge of the present-day state of the ice flow dynamics, which is critically dependent on basal conditions, is strongly needed. The main difficulty is tied to the scarcity of observations at the ice-bed interface at the scale of the whole ice sheet, resulting in poorly constrained parameterisations in ice-sheet models. To circumvent this drawback, inverse modelling approaches can be developed and validated against available data to infer reliable initial conditions of the ice sheet. Here, we present a spin-up method for the Greenland ice sheet using the thermo-mechanical hybrid GRISLI ice-sheet model. Our approach is based on the adjustment of the basal drag coefficient that relates the sliding velocities at the ice-bed interface to basal shear stress in unfrozen bed areas. This method relies on an iterative process in which the basal drag is periodically adjusted in such as way that the simulated ice thickness matches the observed one. The process depends on three parameters controlling the duration and the number of iterations. The best spin-up parameters are chosen according to two criteria to minimize errors in sea-level projections: the final difference between the simulated and the observed Greenland ice volume as well as the final ice volume trend which must both be as low as possible. To increase confidence in the inferred parameters, we also make sure that the final ice thickness root mean square error from the observations is not greater than a few tens of meters. Our best results are obtained after only 420 years of simulation, highlighting a rapid convergence and demonstrating that our method can be used for computationally expensive ice sheet models.

Growth rate of the Laurentide Ice Sheet and sea Level Lowering (with Emphasis on the 115,000 BP Sea Level Low)

1976 ◽  
Vol 6 (2) ◽  
pp. 167-183 ◽  
Author(s):  
J.T. Andrews ◽  
M.A.W. Mahaffy
Keyword(s):  
Sea Level ◽  
Three Dimensional ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Laurentide Ice Sheet ◽  
Baffin Island ◽  
Optimum Conditions ◽  
Ice Flow ◽  
Rapid Fall ◽  
Level Lowering

A physically plausible three-dimensional numerical ice flow model is used to examine the rate at which the Laurentide Ice Sheet could spread and thicken using as input likely values for the rate of fall of snowline and the amount of net mass balance over the growing ice sheet. This provides then both a test of the hypothesis of “instantaneous glacierization” and of the suggested rapid fall of world sea level to between −20 and −70 m below present at 115,000 BP. Two experiments are described: The first terminated after 10,050 years of model run with ice sheets centered over Labrador-Ungava and Baffin Island with a total volume of 3.0 × 106 km3 of ice, whereas the second was completed after 10,000 years and resulted in a significantly larger ice sheet (still with two main centers) with a volume of 7.78 × 106 km3 of ice. This latter figure is equivalent to the mass required to lower world sea level by 19.4 m. Our results indicate that large ice sheets can develop in about 10,000 years under optimum conditions.

scholarly journals Deep learning speeds up ice flow modelling by several orders of magnitude

2021 ◽  
pp. 1-14
Author(s):  
Guillaume Jouvet ◽  
Guillaume Cordonnier ◽  
Byungsoo Kim ◽  
Martin Lüthi ◽  
Andreas Vieli ◽  
...  
Keyword(s):  
High Spatial Resolution ◽  
State Of The Art ◽  
Ice Sheets ◽  
Ice Dynamics ◽  
Ice Flow ◽  
Flow Models ◽  
Mountain Glaciers ◽  
Central Processing ◽  
Model Component ◽  
User Friendly

Abstract This paper introduces the Instructed Glacier Model (IGM) – a model that simulates ice dynamics, mass balance and its coupling to predict the evolution of glaciers, icefields or ice sheets. The novelty of IGM is that it models the ice flow by a Convolutional Neural Network, which is trained from data generated with hybrid SIA + SSA or Stokes ice flow models. By doing so, the most computationally demanding model component is substituted by a cheap emulator. Once trained with representative data, we demonstrate that IGM permits to model mountain glaciers up to 1000 × faster than Stokes ones on Central Processing Units (CPU) with fidelity levels above 90% in terms of ice flow solutions leading to nearly identical transient thickness evolution. Switching to the GPU often permits additional significant speed-ups, especially when emulating Stokes dynamics or/and modelling at high spatial resolution. IGM is an open-source Python code which deals with two-dimensional (2-D) gridded input and output data. Together with a companion library of trained ice flow emulators, IGM permits user-friendly, highly efficient and mechanically state-of-the-art glacier and icefields simulations.

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