scholarly journals Characterizing englacial drainage in the ablation zone of the Greenland ice sheet

2008 ◽  
Vol 54 (187) ◽  
pp. 567-578 ◽  
Author(s):  
Ginny A. Catania ◽  
Thomas A. Neumann ◽  
Stephen F. Price
Keyword(s):  
Simulation Model ◽  
Strong Correlation ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Surface Velocity ◽  
Ablation Zone ◽  
Moderate Correlation ◽  
Supraglacial Lakes ◽  
Infill Material ◽  
Englacial Drainage

AbstractRapid, local drainage of surface meltwater to the base of the Greenland ice sheet is thought to result in surface velocity variations as far inland as the equilibrium zone (Zwally and others, 2002). Ice-penetrating radar surveys throughout this region allow us to characterize englacial drainage features that appear as vertically stacked diffraction hyperbolae in common-offset profiles. These data are used with a radar-simulation model, which allows for variations in geometry, penetration depth and infill material, to understand the characteristics of these hyperbolae and the likelihood that they are produced by moulins. We find only a moderate correlation between the locations of these possible moulins and supraglacial lakes, indicating that many lakes drain over the surface of the ice sheet, or do not contain sufficient water to reach the bed through moulin formation. We find a strong correlation between moulin location in the ablation region and elevated along-flow tension (due to flow over rough bedrock), which generates surface crevassing and provides an entry point for meltwater. Although theory suggests that moulins may form anywhere on the ice sheet given sufficient meltwater input, our data suggest that they are far more common in the ablation zone than near, or inland from, the equilibrium line.

scholarly journals Sustained seismic tremors and icequakes detected in the ablation zone of the Greenland ice sheet

2014 ◽  
Vol 60 (221) ◽  
pp. 563-575 ◽  
Author(s):  
Claudia Röösli ◽  
Fabian Walter ◽  
Stephan Husen ◽  
Lauren C. Andrews ◽  
Martin P. Lüthi ◽  
...  
Keyword(s):  
Water Level ◽  
Strong Correlation ◽  
Seismic Activity ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Clear Correlation ◽  
Ablation Zone ◽  
Seismic Record ◽  
Short Term

AbstractDuring summer 2011, seismic activity in the ablation zone of the western Greenland ice sheet (GrIS) was monitored using a network of three-component seismometers. The seismic record includes a large variety of icequakes and seismic tremors that demonstrate a clear correlation with subglacial water flow. We verified the existence of well-known shallow icequakes (related to surface crevasse formation), deep icequakes (located at 100–160 m depth) and narrow-banded short-term seismic tremors (tens of seconds in duration). In addition, we present previously unreported long-term tremors lasting several hours. Using attenuation of the measured tremor amplitude, we locate the epicentre of this long-term tremor to a large moulin within our study area. Between 3 and 11 Hz, our continuous seismic record is dominated by this ‘moulin tremor’ and shows strong correlation with the water level of the generating moulin. We argue that monitoring of icequake and glacial tremor sources bears high potential for investigating glacier hydraulics and dynamics, and is thus an ideal supplement to traditional glaciological measurements.

scholarly journals Influence of supraglacial lakes and ice-sheet geometry on seasonal ice-flow variability

2013 ◽  
Vol 7 (2) ◽  
pp. 1101-1118 ◽  
Author(s):  
I. Joughin ◽  
S. B. Das ◽  
G. E. Flowers ◽  
M. D. Behn ◽  
R. B. Alley ◽  
...  
Keyword(s):  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Surface Velocity ◽  
Ice Flow ◽  
Bed Topography ◽  
Supraglacial Lakes ◽  
Melt Water ◽  
Flow Enhancement ◽  
Lake Drainage ◽  
Existing Data

Abstract. Supraglacial lakes play an important role in establishing hydrological connections that allow lubricating seasonal melt water to reach the base of the Greenland Ice Sheet. Here we use new surface velocity observations to examine the influence of supraglacial lake drainages and surface melt rate on ice flow. We find large, spatially extensive speedups concurrent with times of lake drainage, showing that lakes play a key role in modulating regional ice flow. While surface meltwater is supplied to the bed via a geographically sparse network of moulins, the observed ice-flow enhancement suggests that this meltwater spreads widely over the ice-sheet bed. We also find that the complex spatial pattern of speedup is strongly determined by the combined influence of bed and surface topography on subglacial water flow. Thus, modeling of ice-sheet basal hydrology likely will require knowledge of bed topography resolved at scales (sub-kilometer) far finer than existing data (several km).

scholarly journals Influence of ice-sheet geometry and supraglacial lakes on seasonal ice-flow variability

2013 ◽  
Vol 7 (4) ◽  
pp. 1185-1192 ◽  
Author(s):  
I. Joughin ◽  
S. B. Das ◽  
G. E. Flowers ◽  
M. D. Behn ◽  
R. B. Alley ◽  
...  
Keyword(s):  
Spatial Pattern ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Surface Velocity ◽  
Ice Flow ◽  
Bed Topography ◽  
Supraglacial Lakes ◽  
Flow Enhancement ◽  
Lake Drainage ◽  
Existing Data

Abstract. Supraglacial lakes play an important role in establishing hydrological connections that allow lubricating seasonal meltwater to reach the base of the Greenland Ice Sheet. Here we use new surface velocity observations to examine the influence of supraglacial lake drainages and surface melt rate on ice flow. We find large, spatially extensive speedups concurrent with times of lake drainage, showing that lakes play a key role in modulating regional ice flow. While surface meltwater is supplied to the bed via a geographically sparse network of moulins, the observed ice-flow enhancement suggests that this meltwater spreads widely over the ice-sheet bed. We also find that the complex spatial pattern of speedup is strongly determined by the combined influence of bed and surface topography on subglacial water flow. Thus, modeling of ice-sheet basal hydrology likely will require knowledge of bed topography resolved at scales (sub-kilometer) far finer than existing data (several km).

scholarly journals Investigating Controls on the Formation and Distribution of Wintertime Storage of Water in Supraglacial Lakes

2020 ◽  
Vol 8 ◽  
Author(s):  
Derrick Julius Lampkin ◽  
Lora Koenig ◽  
Casey Joseph ◽  
Jason Eric Box
Keyword(s):  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Lake Area ◽  
Channel Networks ◽  
Supraglacial Lakes ◽  
Lake Model ◽  
Temporal Occurrence ◽  
Atmospheric Factors ◽  
And Behavior ◽  
Percolation Zone

Supraglacial lakes over the Greenland Ice Sheet can demonstrate multi-model drainage states. Lakes can demonstrate incomplete drainage, where residual melt can become buried under ice and snow and survive throughout the winter. We evaluate atmospheric factors that influence the propensity for the formation of buried lakes over the ice sheet. We examine the spatial and temporal occurrence and behavior of buried lakes over the Jakobshavn Isbrae and Zachariae Isstrøm outlet basins and assess the magnitude of insolation necessary to preserve melt water using a numerical lake model from 2009 to 2012. Buried lakes tend to occur at higher elevations within the ablation zone and those present at elevations > 1000 m tend to reoccur over several seasons. Lakes without buried water are relatively small (∼1 km2), whereas lakes with buried water are larger (∼6–10 km2). Lake area is correlated with the number of seasons sub-surface water persists. Buried lakes are relatively deep and associated with complex supraglacial channel networks. Winter stored water could be a precursor to the formation of supraglacial channels. Simulations of the insulation potential of accumulated snow and ice on the surface of lakes indicate substantial regional differences and inter-annual variability. With the possibility of inland migration of supraglacial lakes, buried lakes could be important in the evolution of ablation/percolation zone hydrology.

scholarly journals A comparison of supraglacial lake observations derived from MODIS imagery at the western margin of the Greenland ice sheet

2013 ◽  
Vol 59 (218) ◽  
pp. 1179-1188 ◽  
Author(s):  
Amber A. Leeson ◽  
Andrew Shepherd ◽  
Aud V. Sundal ◽  
A. Malin Johansson ◽  
Nick Selmes ◽  
...  
Keyword(s):  
Satellite Imagery ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Automated Classification ◽  
Temporal Sampling ◽  
Supraglacial Lakes ◽  
Modis Imagery ◽  
Single Dataset ◽  
Average Size

AbstractSupraglacial lakes (SGLs) affect the dynamics of the Greenland ice sheet by storing runoff and draining episodically. We investigate the evolution of SGLs as reported in three datasets, each based on automated classification of satellite imagery. Although the datasets span the period 2001–10, there are differences in temporal sampling, and only the years 2005–07 are common. By subsampling the most populous dataset, we recommend a sampling frequency of one image per 6.5 days in order to minimize uncertainty associated with poor temporal sampling. When compared with manual classification of satellite imagery, all three datasets are found to omit a sizeable (29, 48 and 41 %) fraction of lakes and are estimated to document the average size of SGLs to within 0.78, 0.48 and 0.95 km2. We combine the datasets using a hierarchical scheme, producing a single, optimized, dataset. This combined record reports up to 67% more lakes than a single dataset. During 2005–07, the rate of SGL growth tends to follow the rate at which runoff increases in each year. In 2007, lakes drain earlier than in 2005 and 2006 and remain absent despite continued runoff. This suggests that lakes continue to act as open surface–bed conduits following drainage.

scholarly journals Characteristics of the lower ablation zone of the West Greenland ice sheet for energy-balance modelling

1996 ◽  
Vol 23 ◽  
pp. 160-166 ◽  
Author(s):  
Michiel van den Broeke
Keyword(s):  
Heat Flux ◽  
Energy Balance ◽  
Global Radiation ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Longwave Radiation ◽  
Turbulent Fluxes ◽  
Ablation Zone ◽  
The West ◽  
West Greenland

In this paper, we present the summer-time energy balance for a site in the lower ablation zone of the West Greenland ice sheet. The summer climate of this part of Greenland is sunny and dry. The energy that is available for melting (on average 174 W m−2or 4.5 cm w.e.d−1) is mainly provided by net global radiation two-thirds and sensible-heat flux (one-third). The contribution of the sub-surface heat flux, the latent-heat flux and the net longwave radiation to the energy balance are small. We tested some parameterizations to calculate energy-balance components that are currently used in general circulation models, energy-balance models and mesoscale meteorological models. For the area and time period under consideration, parameterizations that use screen-level temperature for the calculation of incoming longwave radiation systematically underestimate this quantity by 10 W m−2owing to the proximity of the melting-ice surface that restricts temperature increase of the lowest air layers. The incoming global radiation was predicted correctly. Simple explicit schemes that calculate the stability corrections for turbulent fluxes as a function of the bulk Richardson number tend to underestimate the turbulent fluxes by 15 W m−2. The aerodynamic roughness lengthz0derived from wind-speed profiles appears to be erroneously small, leading to underestimation of the fluxes by 30 W m−2. Probably, the wind profile is distorted by the rough terrain. An estimate ofz0biased on microtopographical survey yielded a more realistic result. Because all errors work in the same direction, the use of some of the parameterizations can cause serious underestimation of the melting energy.

scholarly journals Evaluation of the updated regional climate model RACMO2.3: summer snowfall impact on the Greenland Ice Sheet

2015 ◽  
Vol 9 (5) ◽  
pp. 1831-1844 ◽  
Author(s):  
B. Noël ◽  
W. J. van de Berg ◽  
E. van Meijgaard ◽  
P. Kuipers Munneke ◽  
R. S. W. van de Wal ◽  
...  
Keyword(s):  
Large Scale ◽  
Climate Model ◽  
Regional Climate ◽  
Elevation Gradient ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Ice Cores ◽  
Detailed Comparison ◽  
Ablation Zone ◽  
Surface Mass

Abstract. We discuss Greenland Ice Sheet (GrIS) surface mass balance (SMB) differences between the updated polar version of the RACMO climate model (RACMO2.3) and the previous version (RACMO2.1). Among other revisions, the updated model includes an adjusted rainfall-to-snowfall conversion that produces exclusively snowfall under freezing conditions; this especially favours snowfall in summer. Summer snowfall in the ablation zone of the GrIS has a pronounced effect on melt rates, affecting modelled GrIS SMB in two ways. By covering relatively dark ice with highly reflective fresh snow, these summer snowfalls have the potential to locally reduce melt rates in the ablation zone of the GrIS through the snow-albedo-melt feedback. At larger scales, SMB changes are driven by differences in orographic precipitation following a shift in large-scale circulation, in combination with enhanced moisture to precipitation conversion for warm to moderately cold conditions. A detailed comparison of model output with observations from automatic weather stations, ice cores and ablation stakes shows that the model update generally improves the simulated SMB-elevation gradient as well as the representation of the surface energy balance, although significant biases remain.

scholarly journals Seasonal changes of ice surface characteristics and productivity in the ablation zone of the Greenland Ice Sheet

2015 ◽  
Vol 9 (2) ◽  
pp. 487-504 ◽  
Author(s):  
D. M. Chandler ◽  
J. D. Alcock ◽  
J. L. Wadham ◽  
S. L. Mackie ◽  
J. Telling
Keyword(s):  
Remote Sensing ◽  
Surface Albedo ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Surface Characteristics ◽  
Biological Processes ◽  
Ablation Zone ◽  
Ice Surface ◽  
Surface Productivity ◽  
Cryoconite Hole

Abstract. Field and remote sensing observations in the ablation zone of the Greenland Ice Sheet have revealed a diverse range of ice surface characteristics, primarily reflecting the variable distribution of fine debris (cryoconite). This debris reduces the surface albedo and is therefore an important control on melt rates and ice sheet mass balance. Meanwhile, studies of ice sheet surface biological processes have found active microbial communities associated with the cryoconite debris, which may themselves modify the cryoconite distribution. Due to the considerable difficulties involved with collecting ground-based observations of the ice surface, our knowledge of the physical and biological surface processes, and their links, remains very limited. Here we present data collected at a field camp established in the ice sheet ablation zone at 67° N, occupied for almost the entire melt season (26 May–10 August 2012), with the aim of gaining a much more detailed understanding of the physical and biological processes occurring on the ice surface. These data sets include quadrat surveys of surface type, measurements of ice surface ablation, and in situ biological oxygen demand incubations to quantify microbial activity. In addition, albedo at the site was retrieved from AVHRR (Advanced Very High Resolution Radiometer) remote sensing data. Observations of the areal coverage of different surface types revealed a rapid change from complete snow cover to the "summer" (summer study period) ice surface of patchy debris ("dirty ice") and cryoconite holes. There was significant correlation between surface albedo, cryoconite hole coverage and surface productivity during the melt season, but microbial activity in "dirty ice" was not correlated with albedo and varied widely throughout the season. While this link suggests the potential for a remote-sensing approach to monitoring cryoconite hole biological processes, very wide seasonal and spatial variability in net surface productivity demonstrates the need for caution when extrapolating point measurements of biological processes to larger temporal or spatial scales.

scholarly journals The ablation zone in northeast Greenland: ice types, albedos and impurities

2010 ◽  
Vol 56 (195) ◽  
pp. 101-113 ◽  
Author(s):  
Carl Egede Bøggild ◽  
Richard E. Brandt ◽  
Kendrick J. Brown ◽  
Stephen G. Warren
Keyword(s):  
Ice Sheet ◽  
Impurity Content ◽  
Greenland Ice Sheet ◽  
Mineralogical Composition ◽  
Dust Particles ◽  
Ablation Zone ◽  
Ice Surface ◽  
Debris Cover ◽  
Fine Mode ◽  
Major Surface

AbstractIce types, albedos and impurity content are characterized for the ablation zone of the Greenland ice sheet in Kronprinz Christians Land (80° N, 24° W). Along this ice margin the width of the ablation zone is only about 8 km. The emergence and melting of old ice in the ablation zone creates a surface layer of dust that was originally deposited with snowfall high on the ice sheet. This debris cover is augmented by locally derived wind-blown sediment. Subsequently, the surface dust particles often aggregate together to form centimetre-scale clumps that melt into the ice, creating cryoconite holes. The debris in the cryoconite holes becomes hidden from sunlight, raising the area-averaged albedo relative to surfaces with uniform debris cover. Spectral and broadband albedos were obtained for snow, ice hummocks, debris-covered ice, cryoconite-studded ice and barren tundra surfaces. Broadband ice albedos varied from 0.2 (for ice with heavy loading of uniform debris) to 0.6 (for ice hummocks with cryoconite holes). The cryoconite material itself has albedo 0.1 when wet. Areal distribution of the major surface types was estimated visually from a transect video as a function of distance from the ice edge (330 m a.s.l.). Ablation rates were measured along a transect from the ice margin to the slush zone 8 km from the margin (550 m a.s.l.), traversing both Pleistocene and Holocene ice. Ablation rates in early August averaged 2 cm d−1. Impurity concentrations were typically 4.3 mg L−1 in the subsurface ice. Surface concentrations were about 16 g m−2 on surfaces with low impurity loading, and heavily loaded surfaces had concentrations as high as 1.4 kg m−2. The mineralogical composition of the cryoconite material is comparable with that of the surrounding soils and with dust on a snowdrift in front of the ice margin, implying that much of the material is derived from local sources. A fine mode (clay) is present in the oldest ice but not in the nearby soil, suggesting that its origin is from wind deposition during Pleistocene glaciation.

scholarly journals Dust from the dark region in the western ablation zone of the Greenland ice sheet

2011 ◽  
Vol 5 (3) ◽  
pp. 589-601 ◽  
Author(s):  
I. G. M. Wientjes ◽  
R. S. W. Van de Wal ◽  
G. J. Reichart ◽  
A. Sluijs ◽  
J. Oerlemans
Keyword(s):  
Organic Material ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Ice Cores ◽  
Mineralogical Composition ◽  
Anthropogenic Activity ◽  
Ablation Zone ◽  
Elemental Ratios ◽  
Dark Region ◽  
Volcanic Material

Abstract. A dark region tens of kilometres wide is located in the western ablation zone of the Greenland ice sheet. The dark appearance is caused by higher amounts of dust relative to the brighter surroundings. This dust has either been deposited recently or was brought to the surface by melting of outcropping ice. Because the resulting lower albedos may have a significant effect on melt rates, we analysed surface dust on the ice, also called cryoconite, from locations in the dark region as well as locations from the brighter surrounding reference ice with microscopic and geochemical techniques to unravel its composition and origin. We find that (part of) the material is derived from the outcropping ice, and that there is little difference between dust from the dark region and from the reference ice. The dust from the dark region seems enriched in trace and minor elements that are mainly present in the current atmosphere because of anthropogenic activity. This enrichment is probably caused by higher precipitation and lower melt rates in the dark region relative to the ice marginal zone. The rare earth elemental ratios of the investigated material are approximately the same for all sites and resemble Earth's average crust composition. Therefore, the cryoconite probably does not contain volcanic material. The mineralogical composition of the dust excludes Asian deserts, which are often found as provenance for glacial dust in ice cores, as source regions. Consequently, the outcropping dust likely has a more local origin. Finally, we find cyanobacteria and algae in the cryoconite. Total Organic Carbon accounts for up to 5 weight per cent of the cryoconite from the dark region, whereas dust samples from the reference ice contain only 1 % or less. This organic material is likely formed in situ. Because of their high light absorbency, cyanobacteria and the organic material they produce contribute significantly to the low albedo of the dark region.

Sign in / Sign up

Export Citation Format

Share Document