Determination of the Surface and Bed Topography in Central Greenland

AbstractThe surface and bottom topography of the central Greenland ice sheet was determined from airborne ice-radar soundings over a 180 km by 180 km grid centered on the 1974 “Summit” site (lat. 72°18′N., long. 37°55′W.), using the Technical University of Denmark 60 MHz ice radar. Over 6100 km of high-quality radar data were obtained, covering over 99'% of the grid, along lines spaced 12.5 km apart in both north-south and east-west directions. Aircraft location was done with an inertial navigation system (INS) and a pressure altimeter, with control provided by periodically flying over a known point at the center of the grid. The ice radar was used to determine ice thickness; the surface topography was determined independently using height-above-terrain measurements from the aircraft’s radar altimeter. The calculated surface topography is accurate to about ±6 m, with this error arising mostly from radar-altimeter errors. The ice thickness and bottom topography are accurate to about ±50 m, with this error dominated by the horizontal navigation uncertainties due to INS drift; this error increases to about ±125 m in areas of rough bottom relief (about 12% of the grid).The highest point on Greenland is at lat. 72°34′ N., long. 37°38′W., at an altitude of 3233 ± 6 m a.s.l. The ice surface at this point divides into three sectors, one facing north, one east-south-east, and one west-south-west, with each having a roughly uniform slope. The ice divide between the last two sectors is a well-defined ridge running almost due south. The ice is about 3025 m thick at the summit. Excluding the mountainous north-east corner of the grid, where the ice locally reaches a thickness of about 3470 m and the bed dips to about 370 m below sea-level, the maximum ice thickness, approximately 3375 m, occurs about 97 km south-south-west of the summit. The average bed altitude over the entire grid is 180 m and the average ice thickness is 2975 ± 235 m. The ice in most of the south-west quadrant of the grid is over 3200 m thick, and overlies a relatively smooth, flat basin with altitudes mostly below sea-level. There is no predominant direction to the basal topography over most of the grid; it appears to be undulating, rolling terrain with no obvious ridge/valley structure. The summit of the ice sheet is above the eastern end of a relatively large, smooth, flat plateau, about 10–15 km wide and extending about 50 km to the west. If the basal topography were the sole criterion, then a site somewhere on this plateau or in the south-west basin would be suitable for the drilling of a new deep ice core.

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Determination of the Surface and Bed Topography in Central Greenland

Journal of Glaciology ◽

10.3189/s0022143000005505 ◽

1990 ◽

Vol 36 (122) ◽

pp. 17-30 ◽

Cited By ~ 1

Author(s):

Steven M. Hodge ◽

David L. Wright ◽

Jerry A. Bradley ◽

Robert W. Jacobel ◽

Neils Skou ◽

...

Keyword(s):

Surface Topography ◽

Sea Level ◽

Bottom Topography ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Ice Thickness ◽

Radar Altimeter ◽

The South ◽

South West ◽

Ice Radar

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High-resolution ice thickness and bed topography of a land-terminating section of the Greenland Ice Sheet

Earth System Science Data Discussions ◽

10.5194/essdd-7-129-2014 ◽

2014 ◽

Vol 7 (1) ◽

pp. 129-148 ◽

Cited By ~ 4

Author(s):

K. Lindbäck ◽

R. Pettersson ◽

S. H. Doyle ◽

C. Helanow ◽

P. Jansson ◽

...

Keyword(s):

Mass Balance ◽

Sea Level ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Airborne Radar ◽

Ice Thickness ◽

Equilibrium Line Altitude ◽

Bed Topography ◽

Ice Surface ◽

Ice Sheet Dynamics

Abstract. We present ice thickness and bed topography maps with high spatial resolution (250 to 500 m) of a and-terminating section of the Greenland Ice Sheet derived from combined ground-based and airborne radar surveys. The data have a total area of ~12000 km2 and cover the whole ablation area of the outlet glaciers of Isunnguata Sermia, Russell, Leverett, Ørkendalen and Isorlersuup up to the long-term mass balance equilibrium line altitude at ~1600 m above sea level. The bed topography shows highly variable subglacial trough systems, and the trough of the Isunnguata Sermia Glacier is over-deepened and reaches an elevation of several hundreds of meters below sea level. The ice surface is smooth and only reflects the bedrock topography in a subtle way, resulting in a highly variable ice thickness. The southern part of our study area consists of higher bed elevations compared to the northern part. The covered area is one of the most studied regions of the Greenland Ice Sheet with studies of mass balance, dynamics, and supraglacial lakes, and our combined dataset can be valuable for detailed studies of ice sheet dynamics and hydrology. The compiled datasets of ground-based and airborne radar surveys are accessible for reviewers (password protected) at doi.pangaea.de/10.1594/pangaea.830314 and will be freely available in the final revised paper.

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Glacier algae accelerate melt rates on the south-western Greenland Ice Sheet

The Cryosphere ◽

10.5194/tc-14-309-2020 ◽

2020 ◽

Vol 14 (1) ◽

pp. 309-330 ◽

Cited By ~ 10

Author(s):

Joseph M. Cook ◽

Andrew J. Tedstone ◽

Christopher Williamson ◽

Jenine McCutcheon ◽

Andrew J. Hodson ◽

...

Keyword(s):

Sea Level ◽

Algal Blooms ◽

Ice Sheet ◽

Algal Growth ◽

Greenland Ice Sheet ◽

Radiation Absorption ◽

Transfer Model ◽

High Biomass ◽

The South ◽

Ice Surface

Abstract. Melting of the Greenland Ice Sheet (GrIS) is the largest single contributor to eustatic sea level and is amplified by the growth of pigmented algae on the ice surface, which increases solar radiation absorption. This biological albedo-reducing effect and its impact upon sea level rise has not previously been quantified. Here, we combine field spectroscopy with a radiative-transfer model, supervised classification of unmanned aerial vehicle (UAV) and satellite remote-sensing data, and runoff modelling to calculate biologically driven ice surface ablation. We demonstrate that algal growth led to an additional 4.4–6.0 Gt of runoff from bare ice in the south-western sector of the GrIS in summer 2017, representing 10 %–13 % of the total. In localized patches with high biomass accumulation, algae accelerated melting by up to 26.15±3.77 % (standard error, SE). The year 2017 was a high-albedo year, so we also extended our analysis to the particularly low-albedo 2016 melt season. The runoff from the south-western bare-ice zone attributed to algae was much higher in 2016 at 8.8–12.2 Gt, although the proportion of the total runoff contributed by algae was similar at 9 %–13 %. Across a 10 000 km2 area around our field site, algae covered similar proportions of the exposed bare ice zone in both years (57.99 % in 2016 and 58.89 % in 2017), but more of the algal ice was classed as “high biomass” in 2016 (8.35 %) than 2017 (2.54 %). This interannual comparison demonstrates a positive feedback where more widespread, higher-biomass algal blooms are expected to form in high-melt years where the winter snowpack retreats further and earlier, providing a larger area for bloom development and also enhancing the provision of nutrients and liquid water liberated from melting ice. Our analysis confirms the importance of this biological albedo feedback and that its omission from predictive models leads to the systematic underestimation of Greenland's future sea level contribution, especially because both the bare-ice zones available for algal colonization and the length of the biological growth season are set to expand in the future.

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Melting of Northern Greenland during the last interglacial

The Cryosphere Discussions ◽

10.5194/tcd-5-3517-2011 ◽

2011 ◽

Vol 5 (6) ◽

pp. 3517-3539 ◽

Cited By ~ 5

Author(s):

A. Born ◽

K. H. Nisancioglu

Keyword(s):

Sea Level Rise ◽

Sea Level ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Last Interglacial ◽

The South ◽

High Accumulation ◽

Positive Feedbacks ◽

Geological Past

Abstract. The Greenland ice sheet (GrIS) is losing mass at an increasing rate, making it the primary contributor to global eustatic sea level rise. Large melting areas and rapid thinning at its margins has raised concerns about its stability. However, it is conceivable that these observations represent the transient adjustment of the fastest reacting parts of the ice sheet, masking slower processes that dominate the long term fate of the GrIS and its contribution to sea level rise. Studies of the geological past provide valuable information on the long term response of the GrIS to warm periods. We simulate the GrIS during the Eemian interglacial, a period 126 000 yr before present (126 ka) with Arctic temperatures comparable to projections for the end of this century. The northeastern part of the GrIS is unstable and retreats significantly, despite moderate melt rates. Unlike the south and west, strong melting in the northeast is not compensated by high accumulation, or fast ice flow. The analogy with the present warming suggests that in coming decades, positive feedbacks could increase the rate of mass loss of the northeastern GrIS, exceeding the currently observed melting in the south.

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

10.5194/gmd-2017-322 ◽

2018 ◽

Cited By ~ 3

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.

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Dark ice dynamics of the south-west Greenland Ice Sheet

10.5194/tc-2017-79 ◽

2017 ◽

Cited By ~ 1

Author(s):

Andrew J. Tedstone ◽

Jonathan L. Bamber ◽

Joseph M. Cook ◽

Christopher J. Williamson ◽

Xavier Fettweis ◽

...

Keyword(s):

Climate Model ◽

Regional Climate ◽

Sensible Heat Flux ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Ice Algae ◽

The South ◽

Ice Dynamics ◽

South West ◽

Algae Growth

Abstract. Runoff from the Greenland Ice Sheet (GrIS) has increased in recent years due largely to declining albedo and enhanced surface melting. Some of the largest declines in GrIS albedo have occurred in the ablation zone of the south-west sector and are associated with the development of 'dark' ice surfaces. Field observations at local scales reveal that a variety of light-absorbing impurities (LAIs) can be present on the surface, ranging from inorganic particulates, to cryoconite materials and ice algae. Meanwhile, satellite observations show that the areal extent of dark ice has varied significantly between recent successive melt seasons. However, the processes that drive such large inter-annual variability in dark ice extent remain essentially unconstrained. At present we are therefore unable to project how the albedo of bare-ice sectors of the GrIS will evolve, causing uncertainty in the projected sea level contribution from the GrIS over the coming decades. Here we use MODIS satellite imagery to examine dark ice dynamics on the south-west GrIS each year from 2000 to 2016. We quantify dark ice in terms of its annual extent, duration, intensity and timing of first appearance. Not only does dark ice extent vary significantly between years, but so too does its duration (from 0 % to > 80 % of June–July–August, JJA), intensity and the timing of its first appearance. Comparison of dark ice dynamics with potential meteorological drivers from the regional climate model MAR reveals that the JJA sensible heat flux, the number of positive minimum-air-temperature days and the timing of bare ice appearance are significant inter-annual synoptic controls. We use these findings to identify the surface processes which are most likely to explain recent dark ice dynamics.We suggest that whilst the spatial distribution of dark ice is best explained by outcropping of particulates from ablating ice, these particulates alone do not drive dark ice dynamics. Instead, they may enable the growth of pigmented ice algal assemblages which cause visible surface darkening, but only when the climatological pre-requisites of liquid meltwater presence and sufficient photosynthetically-active radiation fluxes are met. Further field studies are required to fully constrain the processes by which ice algae growth proceeds and the apparent dependency of algae growth on melt-out particulates.

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Dark ice dynamics of the south-west Greenland Ice Sheet

The Cryosphere ◽

10.5194/tc-11-2491-2017 ◽

2017 ◽

Vol 11 (6) ◽

pp. 2491-2506 ◽

Cited By ~ 31

Author(s):

Andrew J. Tedstone ◽

Jonathan L. Bamber ◽

Joseph M. Cook ◽

Christopher J. Williamson ◽

Xavier Fettweis ◽

...

Keyword(s):

Climate Model ◽

Regional Climate ◽

Sensible Heat Flux ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Ice Algae ◽

The South ◽

Ice Dynamics ◽

South West ◽

Algae Growth

Abstract. Runoff from the Greenland Ice Sheet (GrIS) has increased in recent years due largely to changes in atmospheric circulation and atmospheric warming. Albedo reductions resulting from these changes have amplified surface melting. Some of the largest declines in GrIS albedo have occurred in the ablation zone of the south-west sector and are associated with the development of dark ice surfaces. Field observations at local scales reveal that a variety of light-absorbing impurities (LAIs) can be present on the surface, ranging from inorganic particulates to cryoconite materials and ice algae. Meanwhile, satellite observations show that the areal extent of dark ice has varied significantly between recent successive melt seasons. However, the processes that drive such large interannual variability in dark ice extent remain essentially unconstrained. At present we are therefore unable to project how the albedo of bare ice sectors of the GrIS will evolve in the future, causing uncertainty in the projected sea level contribution from the GrIS over the coming decades. Here we use MODIS satellite imagery to examine dark ice dynamics on the south-west GrIS each year from 2000 to 2016. We quantify dark ice in terms of its annual extent, duration, intensity and timing of first appearance. Not only does dark ice extent vary significantly between years but so too does its duration (from 0 to > 80 % of June–July–August, JJA), intensity and the timing of its first appearance. Comparison of dark ice dynamics with potential meteorological drivers from the regional climate model MAR reveals that the JJA sensible heat flux, the number of positive minimum-air-temperature days and the timing of bare ice appearance are significant interannual synoptic controls. We use these findings to identify the surface processes which are most likely to explain recent dark ice dynamics. We suggest that whilst the spatial distribution of dark ice is best explained by outcropping of particulates from ablating ice, these particulates alone do not drive dark ice dynamics. Instead, they may enable the growth of pigmented ice algal assemblages which cause visible surface darkening, but only when the climatological prerequisites of liquid meltwater presence and sufficient photosynthetically active radiation fluxes are met. Further field studies are required to fully constrain the processes by which ice algae growth proceeds and the apparent dependency of algae growth on melt-out particulates.

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Surface lakes on Greenland will spread further inland as the climate warms

Climanosco Research Articles - https://www.climanosco.org/collection/climate-change-and-its-impacts/ ◽

10.37207/cra.1.9 ◽

2016 ◽

Author(s):

Amber A. Leeson

Keyword(s):

Thin Film ◽

Computer Simulation ◽

Sea Level ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Supraglacial Lakes ◽

South West ◽

Excess Water ◽

Speed Up ◽

Global Sea Level

Each summer, a rash of lakes forms from ponded meltwater on top of the Greenland ice sheet. These 'supraglacial' (on top of ice) lakes can drain through the ice sheet, delivering their contents to its base. The ice sheet slides on a thin film of water, and when extra water is added to this film (for example from a draining supraglacial lake) the sliding (‘flow’) happens a bit faster. At present, under-ice pipe-like features enable excess water to drain out from under the ice sheet quickly and efficiently. However, in recent years supraglacial lakes have begun to form further inland, potentially supplying water to the base beyond the reach of these features. Here, we use a computer simulation of lake initiation and growth to show that the inland spread of supraglacial lakes will continue as the climate warms; by 2060, 100% more of South West Greenland and 50% more of the whole ice sheet will be populated by supraglacial lakes. Of these ‘new’ lakes, up to half will be large enough to drain, delivering the water they contain to the base of the ice sheet and all of these new lakes will form at locations where we would expect to see an ice sheet speed-up with the addition of more water at the base. If the ice sheet flows faster, it can thin out and melt quicker, thus contributing to global sea level rise. Supraglacial lakes and their impacts are not currently considered in our best predictions of future ice sheet change. Given that they possess significant leverage to affect ice sheet flow, and that they are likely to form (and drain) at locations more sensitive to their impact in future years, it is clear that they need to be accounted for in these predictions as a matter of priority.

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High-resolution ice thickness and bed topography of a land-terminating section of the Greenland Ice Sheet

Earth System Science Data ◽

10.5194/essd-6-331-2014 ◽

2014 ◽

Vol 6 (2) ◽

pp. 331-338 ◽

Cited By ~ 27

Author(s):

K. Lindbäck ◽

R. Pettersson ◽

S. H. Doyle ◽

C. Helanow ◽

P. Jansson ◽

...

Keyword(s):

Sea Level ◽

Ice Sheet ◽

Greenland Ice Sheet ◽

Airborne Radar ◽

Ice Thickness ◽

Data Sets ◽

Equilibrium Line Altitude ◽

Data Set ◽

Bed Topography ◽

Ice Sheet Dynamics

Abstract. We present ice thickness and bed topography maps with a high spatial resolution (250–500 m) of a land-terminating section of the Greenland Ice Sheet derived from ground-based and airborne radar surveys. The data have a total area of ~12 000 km2 and cover the whole ablation area of the outlet glaciers of Isunnguata Sermia, Russell, Leverett, Ørkendalen and Isorlersuup up to the long-term mass balance equilibrium line altitude at ~1600 m above sea level. The bed topography shows highly variable subglacial trough systems, and the trough of Isunnguata Sermia Glacier is overdeepened and reaches an elevation of ~500 m below sea level. The ice surface is smooth and only reflects the bedrock topography in a subtle way, resulting in a highly variable ice thickness. The southern part of our study area consists of higher bed elevations compared to the northern part. The compiled data sets of ground-based and airborne radar surveys cover one of the most studied regions of the Greenland Ice Sheet and can be valuable for detailed studies of ice sheet dynamics and hydrology. The combined data set is freely available at doi:10.1594/pangaea.830314.

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