scholarly journals Initialization of an ice-sheet model for present-day Greenland

2015 ◽  
Vol 56 (70) ◽  
pp. 129-140 ◽  
Author(s):  
Victoria Lee ◽  
Stephen L. Cornford ◽  
Antony J. Payne
Keyword(s):  
Initial Conditions ◽  
Significant Proportion ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Adaptive Mesh ◽  
Flux Divergence ◽  
Traction Coefficient ◽  
Dependent Model ◽  
Time Dependent Model ◽  
Base Grid

AbstractWe construct initial conditions for an ice flow model of the Greenland ice sheet (GrIS). GrIS has been losing mass at an increasing rate over the past two decades, and a significant proportion of this loss is due to dynamic thinning of narrow outlet glaciers. We solve an inverse problem to estimate poorly known basal and englacial parameters given observed geometry and surface velocities. A weighted cost function, resolved to 4 km in the interior of the ice sheet and 1 km in regions of fast-flowing ice at the margin, is minimized to find two-dimensional fields for a stiffness factor, which is a coefficient of the effective viscosity, and basal traction coefficient. Using these fields, we run the model under present-day climate to damp large-amplitude, short-wavelength fluctuations in the flux divergence. The time-dependent model uses an adaptive mesh with resolution ranging from 8 km of the base grid to 500 m in areas of fast-flowing ice to capture the behaviour of the main outlet glaciers. The ice discharge calculated from the initial conditions for GrIS and individual glaciers compares well with values calculated from observations.

scholarly journals Quantifying supraglacial meltwater pathways in the Paakitsoq region, West Greenland

2017 ◽  
Vol 63 (239) ◽  
pp. 464-476 ◽  
Author(s):  
CONRAD KOZIOL ◽  
NEIL ARNOLD ◽  
ALLEN POPE ◽  
WILLIAM COLGAN
Keyword(s):  
Spatial Patterns ◽  
Surface Runoff ◽  
Significant Proportion ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Average Intensity ◽  
West Greenland ◽  
Surface Drainage ◽  
Subglacial Environment ◽  
Hydrology Model

ABSTRACTIncreased summer ice velocities on the Greenland ice sheet are driven by meltwater input to the subglacial environment. However, spatial patterns of surface input and partitioning of meltwater between different pathways to the base remain poorly understood. To further our understanding of surface drainage, we apply a supraglacial hydrology model to the Paakitsoq region, West Greenland for three contrasting melt seasons. During an average melt season, crevasses drain ~47% of surface runoff, lake hydrofracture drains ~3% during the hydrofracturing events themselves, while the subsequent surface-to-bed connections drain ~21% and moulins outside of lake basins drain ~15%. Lake hydrofracture forms the primary drainage pathway at higher elevations (above ~850 m) while crevasses drain a significant proportion of meltwater at lower elevations. During the two higher intensity melt seasons, model results show an increase (~5 and ~6% of total surface runoff) in the proportion of runoff drained above ~1300 m relative to the melt season of average intensity. The potential for interannual changes in meltwater partitioning could have implications for how the dynamics of the ice sheet respond to ongoing changes in meltwater production.

scholarly journals An ice sheet model validation framework for the Greenland ice sheet

2016 ◽  
Author(s):  
Stephen F. Price ◽  
Matthew J. Hoffman ◽  
Jennifer A. Bonin ◽  
Ian M. Howat ◽  
Thomas Neumann ◽  
...  
Keyword(s):  
Model Validation ◽  
Model Comparison ◽  
Dynamic Models ◽  
Initial Conditions ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Outlet Glacier ◽  
The Past ◽  
Validation Framework ◽  
Short Period

Abstract. We propose a new ice sheet model validation framework – the Cryospheric Model Comparison Tool (CMCT) – that takes advantage of ice sheet altimetry and gravimetry observations collected over the past several decades and is applied here to modeling of the Greenland ice sheet. We use realistic simulations performed with the Community Ice Sheet Model (CISM) along with two idealized, non-dynamic models to demonstrate the framework and its use. Dynamic simulations with CISM are forced from 1991 to 2013 using combinations of reanalysis-based surface mass balance and observations of outlet glacier flux change. We propose and demonstrate qualitative and quantitative metrics for use in evaluating the different model simulations against the observations. We find that the altimetry observations used here are largely ambiguous in terms of their ability to distinguish one simulation from another. Based on basin- and whole-ice-sheet scale metrics, the model initial condition as well as output from idealized and dynamic models all provide an equally reasonable representation of the ice sheet surface (mean elevation differences of < 1 m). This is likely due to their short period of record, biases inherent to digital elevation models used for model initial conditions, and biases resulting from firn dynamics, which are not explicitly accounted for in the models or observations. On the other hand, we find that the gravimetry observations used here are able to unambiguously distinguish between simulations of varying complexity, and along with the CMCT, can provide a quantitative score for assessing a particular model and/or simulation. The new framework demonstrates that our proposed metrics can distinguish relatively better from relatively worse simulations and that dynamic ice sheet models, when appropriately initialized and forced with the right boundary conditions, demonstrate predictive skill with respect to observed dynamic changes occurring on Greenland over the past few decades. An extensible design will allow for continued use of the CMCT as future altimetry, gravimetry, and other remotely sensed data become available for use in ice sheet model validation.

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.

scholarly journals Greenland ice-sheet volume sensitivity to basal, surface and initial conditions derived from an adjoint model

2009 ◽  
Vol 50 (52) ◽  
pp. 67-80 ◽  
Author(s):  
Patrick Heimbach ◽  
Véronique Bugnion
Keyword(s):  
Code Generation ◽  
Automatic Differentiation ◽  
Initial Conditions ◽  
Three Dimensional ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Adjoint Model ◽  
Control Variables ◽  
Simulation Code ◽  
Basal Sliding

AbstractWe extend the application of control methods to a comprehensive three-dimensional thermomechanical ice-sheet model, SICOPOLIS (SImulation COde for POLythermal Ice Sheets). Lagrange multipliers, i.e. sensitivities, are computed with an exact, efficient adjoint model that has been generated from SICOPOLIS by rigorous application of automatic differentiation. The case study uses the adjoint model to determine the sensitivity of the total Greenland ice volume to various control variables over a 100 year period. The control space has of the order 1.2 × 106 elements, consisting of spatial fields of basal flow parameters, surface and basal forcings and initial conditions. Reliability of the adjoint model was tested through finite-difference perturbation calculations for various control variables and perturbation regions, ascertaining quantitative inferences of the adjoint model. As well as confirming qualitative aspects of ice-sheet sensitivities (e.g. expected regional variations), we detect regions where model sensitivities are seemingly unexpected or counter-intuitive, albeit ‘real’ in the sense of actual model behavior. An example is inferred regions where sensitivities of ice-sheet volume to basal sliding coefficient are positive, i.e. where a local increase in basal sliding parameter increases the ice-sheet volume. Similarly, positive (generally negative) ice temperature sensitivities in certain parts of the ice sheet are found, the detection of which seems highly unlikely if only conventional perturbation experiments had been used. The object of this paper is largely a proof of concept. Available adjoint-code generation tools now open up a variety of novel model applications, notably with regard to sensitivity and uncertainty analyses and ice-sheet state estimation or data assimilation.

scholarly journals A technique for generating consistent ice sheet initial conditions for coupled ice-sheet/climate models

2013 ◽  
Vol 6 (2) ◽  
pp. 2491-2516 ◽  
Author(s):  
J. G. Fyke ◽  
W. J. Sacks ◽  
W. H. Lipscomb
Keyword(s):  
Climate Models ◽  
Initial Conditions ◽  
Model Simulation ◽  
Ice Core ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Climate Projections ◽  
Past Climate ◽  
Surface Mass ◽  
Climate Conditions

Abstract. A new technique for generating ice sheet preindustrial 1850 initial conditions for coupled ice-sheet/climate models is developed and demonstrated over the Greenland Ice Sheet using the Community Earth System Model (CESM). Paleoclimate end-member simulations and ice core data are used to derive continuous surface mass balance fields which are used to force a long transient ice sheet model simulation. The procedure accounts for the evolution of climate through the last glacial period and converges to a simulated preindustrial 1850 ice sheet that is geometrically and thermodynamically consistent with the 1850 preindustrial simulated CESM state, yet contains a transient memory of past climate that compares well to observations and independent model studies. This allows future coupled ice-sheet/climate projections of climate change that include ice sheets to integrate the effect of past climate conditions on the state of the Greenland Ice Sheet, while maintaining system-wide continuity between past and future climate simulations.

scholarly journals Linear response of east Greenland’s tidewater glaciers to ocean/atmosphere warming

2018 ◽  
Vol 115 (31) ◽  
pp. 7907-7912 ◽  
Author(s):  
T. R. Cowton ◽  
A. J. Sole ◽  
P. W. Nienow ◽  
D. A. Slater ◽  
P. Christoffersen
Keyword(s):  
East Coast ◽  
Significant Proportion ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Ocean Temperature ◽  
Position Change ◽  
Tidewater Glaciers ◽  
Meltwater Runoff ◽  
Tidewater Glacier ◽  
Ocean Atmosphere

Predicting the retreat of tidewater outlet glaciers forms a major obstacle to forecasting the rate of mass loss from the Greenland Ice Sheet. This reflects the challenges of modeling the highly dynamic, topographically complex, and data-poor environment of the glacier–fjord systems that link the ice sheet to the ocean. To avoid these difficulties, we investigate the extent to which tidewater glacier retreat can be explained by simple variables: air temperature, meltwater runoff, ocean temperature, and two simple parameterizations of “ocean/atmosphere” forcing based on the combined influence of runoff and ocean temperature. Over a 20-y period at 10 large tidewater outlet glaciers along the east coast of Greenland, we find that ocean/atmosphere forcing can explain up to 76% of the variability in terminus position at individual glaciers and 54% of variation in terminus position across all 10 glaciers. Our findings indicate that (i) the retreat of east Greenland’s tidewater glaciers is best explained as a product of both oceanic and atmospheric warming and (ii) despite the complexity of tidewater glacier behavior, over multiyear timescales a significant proportion of terminus position change can be explained as a simple function of this forcing. These findings thus demonstrate that simple parameterizations can play an important role in predicting the response of the ice sheet to future climate warming.

scholarly journals Dependence of century-scale projections of the Greenland ice sheet on its thermal regime

2013 ◽  
Vol 59 (218) ◽  
pp. 1024-1034 ◽  
Author(s):  
H. Seroussi ◽  
M. Morlighem ◽  
E. Rignot ◽  
A. Khazendar ◽  
E. Larour ◽  
...  
Keyword(s):  
Steady State ◽  
Thermal Regime ◽  
Adaptive Mesh Refinement ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Adaptive Mesh ◽  
Atmospheric Conditions ◽  
Geothermal Heat ◽  
Ice Flow ◽  
Basal Sliding

AbstractObservations show that the Greenland ice sheet has been losing mass at an increasing rate over the past few decades, which makes it a major contributor to sea-level rise. Here we use a three-dimensional higher-order ice-flow model, adaptive mesh refinement and inverse methods to accurately reproduce the present-day ice flow of the Greenland ice sheet. We investigate the effect of the ice thermal regime on (1) basal sliding inversion and (2) projections over the next 100 years. We show that steady-state temperatures based on present-day conditions allow a reasonable representation of the thermal regime and that both basal conditions and century-scale projections are weakly sensitive to small changes in the initial temperature field, compared with changes in atmospheric conditions or basal sliding. We conclude that although more englacial temperature measurements should be acquired to validate the models, and a better estimation of geothermal heat flux is needed, it is reasonable to use steady-state temperature profiles for short-term projections, as external forcings remain the main drivers of the changes occurring in Greenland.

scholarly journals Initial results of the SeaRISE numerical experiments with the models SICOPOLIS and IcIES for the Greenland ice sheet

2011 ◽  
Vol 52 (58) ◽  
pp. 23-30 ◽  
Author(s):  
Ralf Greve ◽  
Fuyuki Saito ◽  
Ayako Abe-Ouchi
Keyword(s):  
Mass Loss ◽  
Sea Level ◽  
Climate Warming ◽  
Large Scale ◽  
Initial Conditions ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Surface Mass ◽  
Temperature And Precipitation ◽  
Basal Sliding

AbstarctSeaRISE (Sea-level Response to Ice Sheet Evolution) is a US-led multi-model community effort to predict the likely range of the contribution of the Greenland and Antarctic ice sheets to sea-level rise over the next few hundred years under global warming conditions. The Japanese ice-sheet modelling community is contributing to SeaRISE with two large-scale, dynamic/thermodynamic models: SICOPOLIS and IcIES. Here we discuss results for the Greenland ice sheet, obtained using both models under the forcings (surface temperature and precipitation scenarios) defined by the SeaRISE effort. A crucial point for meaningful simulations into the future is to obtain initial conditions that are close to the observed state of the present-day ice sheet. This is achieved by proper tuning during model spin-up from the last glacial/interglacial cycle to today. Experiments over 500 years indicate that both models are more sensitive (exhibit a larger rate of ice-sheet mass loss) to future climate warming (based on the A1B emission scenario) than to a doubling in the basal sliding speed. Ice-sheet mass loss varies between the two models by a factor of ~2 for sliding experiments and a factor of ~3 for climate-warming experiments, highlighting the importance of improved constraints on the parameterization of basal sliding and surface mass balance in ice-sheet models.

scholarly journals Design and results of the ice sheet model initialisation experiments initMIP-Greenland: an ISMIP6 intercomparison

2017 ◽  
Author(s):  
Heiko Goelzer ◽  
Sophie Nowicki ◽  
Tamsin Edwards ◽  
Matthew Beckley ◽  
Ayako Abe-Ouchi ◽  
...  
Keyword(s):  
Sea Level ◽  
Large Scale ◽  
Initial Conditions ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Data Sets ◽  
Model Intercomparison ◽  
Surface Mass ◽  
Large Perturbation ◽  
Intercomparison Exercises

Abstract. Earlier large-scale Greenland ice sheet sea-level projections (e.g., those run during the ice2sea and SeaRISE initiatives) have shown that ice sheet initial conditions can have a large effect on the projections and give rise to important uncertainties. The goal of the initMIP-Greenland intercomparison exercise is to compare, evaluate and improve the initialisation techniques used in the ice sheet modelling community and to estimate the associated uncertainties. initMIP-Greenland is the first in a series of ice sheet model intercomparison activities within ISMIP6 (the Ice Sheet Model Intercomparison Project for CMIP6). Two experiments for the large-scale Greenland ice sheet have been designed to allow intercomparison between participating models of 1) the initial present-day state of the ice sheet and 2) the response in two schematic forward experiments. The forward experiments serve to evaluate the initialisation in terms of model drift (forward run without additional forcing) and in response to a large perturbation (prescribed surface mass balance anomaly), and should not be interpreted as sea-level projections. We present and discuss results that highlight the wide diversity of data sets, boundary conditions and initialisation techniques used in the community to generate initial states of the Greenland ice sheet. We find good agreement across the ensemble for the dynamic response to SMB changes in areas where the simulated ice sheets overlap, but in general differences arise due to the initial size of the ice sheet. The spread in model drift is reduced compared to earlier intercomparison exercises.

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