Modelling seasonal meltwater forcing of the velocity of land-terminating margins of the Greenland Ice Sheet

Koziol, Conrad P.; Arnold, Neil

doi:10.5194/tc-12-971-2018

Cited by 35 publications

(51 citation statements)

References 71 publications

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“…Code availability. The SHAKTI model is freely available as part of the open source Ice Sheet System Model (ISSM), which is hosted in a subversion repository at https://issm.jpl.nasa.gov/download/ (Larour et al, 2012; last access: 14 July 2018).…”

Section: Discussionmentioning

confidence: 99%

“…4. (Larour et al, 2012; http://issm.jpl.nasa.gov, last access: 14 July 2018). ISSM uses finite-element methods and parallel computing technologies, and includes sophisticated data assimilation and sensitivity analysis tools, to support numerous capabilities for ice sheet modeling applications on a variety of scales.…”

Section: Computational Strategy and Implementation In The Ice Sheet Smentioning

confidence: 99%

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SHAKTI: Subglacial Hydrology and Kinetic, Transient Interactions v1.0

2018

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Abstract. Subglacial hydrology has a strong influence on glacier and ice sheet dynamics, particularly through the dependence of sliding velocity on subglacial water pressure. Significant challenges are involved in modeling subglacial hydrology, as the drainage geometry and flow mechanics are constantly changing, with complex feedbacks that play out between water and ice. A clear tradition has been established in the subglacial hydrology modeling literature of distinguishing between channelized (efficient) and sheetlike (inefficient or distributed) drainage systems or components and using slightly different forms of the governing equations in each subsystem to represent the dominant physics. Specifically, many previous subglacial hydrology models disregard opening by melt in the sheetlike system or redistribute it to adjacent channel elements in order to avoid runaway growth that occurs when it is included in the sheetlike system. We present a new subglacial hydrology model, SHAKTI (Subglacial Hydrology and Kinetic, Transient Interactions), in which a single set of governing equations is used everywhere, including opening by melt in the entire domain. SHAKTI employs a generalized relationship between the subglacial water flux and the hydraulic gradient that allows for the representation of laminar, turbulent, and transitional regimes depending on the local Reynolds number. This formulation allows for the coexistence of these flow regimes in different regions, and the configuration and geometry of the subglacial system evolves naturally to represent sheetlike drainage as well as systematic channelized drainage under appropriate conditions. We present steady and transient example simulations to illustrate the features and capabilities of the model and to examine sensitivity to mesh size and time step size. The model is implemented as part of the Ice Sheet System Model (ISSM).

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Section: Discussionmentioning

confidence: 99%

Section: Computational Strategy and Implementation In The Ice Sheet Smentioning

confidence: 99%

SHAKTI: Subglacial Hydrology and Kinetic, Transient Interactions v1.0

2018

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“…Numerical modelling investigations generally support these inferences. R-channel formation has been modelled up to ∼50 km inland [nearly 1,200 m above sea level (a.s.l), beneath ∼900 m thick ice; (de Fleurian et al, 2016;Koziol and Arnold, 2018)], but not in a setup representative of conditions ∼70 km inland and ∼1,400 m a.s.l (1,200 m thick ice; Dow et al, 2014). This would suggest that there is a transition in potential drainage system efficiency between areas overlaid by ∼900 and ∼1,200 m thick ice.…”

Section: Efficient Channel Formationmentioning

confidence: 99%

“…The mechanisms by which variations in runoff supply produce these velocity fluctuations have recently been the subject of considerable debate (e.g., Phillips et al, 2013;Bougamont et al, 2014;Tedstone et al, 2015;Hoffman et al, 2016;Christoffersen et al, 2018). Seemingly conflicting lines of evidence have indicated different mechanisms, with contrasting implications for the future stability of the ice sheet in a changing climate (e.g., Shannon et al, 2013;Koziol and Arnold, 2018). This uncertainty hinders our ability to accurately predict the future dynamic behaviour of land-terminating sectors of the ice sheet.…”

Section: Introductionmentioning

confidence: 99%

The Influence of Hydrology on the Dynamics of Land-Terminating Sectors of the Greenland Ice Sheet

et al. 2019

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Coupling between runoff, hydrology, basal motion, and mass loss ("hydrology-dynamics") is a critical component of the Greenland Ice Sheet system. Despite considerable research effort, the mechanisms by which runoff influences ice dynamics and the net long-term (decadal and longer) dynamical effect of variations in the timing and magnitude of runoff delivery to the bed remain a subject of debate. We synthesise key research into land-terminating ice sheet hydrology-dynamics, in order to reconcile several apparent contradictions that have recently arisen as understanding of the topic has developed. We suggest that meltwater interaction with subglacial channels, cavities, and deforming subglacial sediment modulates ice flow variability. Increasing surface runoff supply to the bed induces cavity expansion and sediment deformation, leading to early-melt season ice flow acceleration. In the ablation area, drainage of water at times of low runoff from high-pressure subglacial environments toward more efficient drainage pathways is thought to result in reductions in water pressure, ice-bed separation and sediment deformation, causing net slowdown on annual to decadal timescales (ice flow self-regulation), despite increasing surface melt. Further inland, thicker ice, small surface gradients and reduced runoff suppress efficient drainage development, and a small net increase in both summer and winter ice flow is observed. Predicting ice motion across land-terminating sectors of the ice sheet over the twenty-first century is confounded by inadequate understanding of the processes and feedbacks between runoff and subglacial motion. However, if runoff supply increases, we suggest that ice flow in marginal regions will continue to decrease on annual and longer timescales, principally due to (i) increasing drainage system efficiency in marginal areas, (ii) progressive depression of basal water pressure, and (iii) thinning-induced lowering of driving stresses. At higher elevations, we suggest that minor year-on-year ice flow acceleration will continue and extend further into the interior where self-regulation mechanisms cannot operate and if surface-to-bed meltwater connections form. Based on current understanding, we expect that ice flow deceleration due to the seasonal development of efficient drainage beneath the land-terminating margins of the Greenland Ice Sheet will continue to regulate its future mass loss.

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“…These unknowns inland add to the uncertainty in predicting future mass loss from the GrIS. There is a need, therefore, to study the seasonal filling and drainage of lakes on the GrIS, and to understand its spatial distribution and inter-annual variation, in order to inform the boundary conditions for GrIS hydrology and ice-dynamic models (Banwell et al, , 2016Leeson et al, 2012;Arnold et al, 2014;Koziol et al, 2017).…”

mentioning

confidence: 99%

Author response to reviewer comments on manuscript tc-2018-56

Williamson¹

2018

Preprint

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Remote sensing is commonly used to monitor supraglacial lakes on the Greenland Ice Sheet (GrIS); however, most satellite records must trade off higher spatial resolution for higher temporal resolution (e.g. MODIS) or vice versa (e.g. Landsat). Here, we overcome this issue by developing and applying a dual-sensor method that can monitor changes to lake areas and volumes at high spatial resolution (10-30 m) with a frequent revisit time ( ∼ 3 days). We achieve this by mosaicking imagery from the Landsat 8 Operational Land Imager (OLI) with imagery from the recently launched Sentinel-2 Multispectral Instrument (MSI) for a ∼ 12 000 km 2 area of West Greenland in the 2016 melt season. First, we validate a physically based method for calculating lake depths with Sentinel-2 by comparing measurements against those derived from the available contemporaneous Landsat 8 imagery; we find close correspondence between the two sets of values (R 2 = 0.841; RMSE = 0.555 m). This provides us with the methodological basis for automatically calculating lake areas, depths, and volumes from all available Landsat 8 and Sentinel-2 images. These automatic methods are incorporated into an algorithm for Fully Automated Supraglacial lake Tracking at Enhanced Resolution (FASTER). The FASTER algorithm produces time series showing lake evolution during the 2016 melt season, including automated rapid (≤ 4 day) lake-drainage identification. With the dual Sentinel-2-Landsat 8 record, we identify 184 rapidly draining lakes, many more than identified with either imagery collection alone (93 with Sentinel-2; 66 with Landsat 8), due to their inferior temporal resolution, or would be possible with MODIS, due to its omission of small lakes < 0.125 km 2 . Finally, we estimate the water volumes drained into the GrIS during rapid-lake-drainage events and, by analysing downscaled regional climate-model (RACMO2.3p2) run-off data, the water quantity that enters the GrIS via the moulins opened by such events. We find that during the lake-drainage events alone, the water drained by small lakes (< 0.125 km 2 ) is only 5.1 % of the total water volume drained by all lakes. However, considering the total water volume entering the GrIS after lake drainage, the moulins opened by small lakes deliver 61.5 % of the total water volume delivered via the moulins opened by large and small lakes; this is because there are more small lakes, allowing more moulins to open, and because small lakes are found at lower elevations than large lakes, where run-off is higher. These findings suggest that small lakes should be included in future remote-sensing and modelling work.

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Modelling seasonal meltwater forcing of the velocity of land-terminating margins of the Greenland Ice Sheet

Cited by 35 publications

References 71 publications

SHAKTI: Subglacial Hydrology and Kinetic, Transient Interactions v1.0

SHAKTI: Subglacial Hydrology and Kinetic, Transient Interactions v1.0

The Influence of Hydrology on the Dynamics of Land-Terminating Sectors of the Greenland Ice Sheet

Author response to reviewer comments on manuscript tc-2018-56

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