scholarly journals The cooling signature of basal crevasses in a hard-bedded region of the Greenland Ice Sheet

2020 ◽  
Author(s):  
Ian E. McDowell ◽  
Neil F. Humphrey ◽  
Joel T. Harper ◽  
Toby W. Meierbachtol
Keyword(s):  
Latent Heat ◽  
Thermal Structure ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Heat Sinks ◽  
Temperature Structure ◽  
Conductive Heat ◽  
Ablation Zone ◽  
Geothermal Heat ◽  
Basal Ice

Abstract. Temperature sensors installed in a grid of 9 full-depth boreholes drilled in the southwestern ablation zone of the Greenland Ice Sheet consistently record cooling over time within the lowest third of the ice column. Rates of temperature change outpace cooling expected from vertical conduction alone. Additionally, observed static temperature profiles deviate significantly from modeled purely diffusional thermal profiles, implying strong non-conductive heat transfer processes within the lowest portion of the ice column. Although numerous heat sources exist to add energy and warm ice as it moves from the central divide towards the margin such as strain heat from internal deformation, latent heat from refreezing meltwater, and the conduction of geothermal heat across the ice-bedrock interface, identifying heat sinks proves more difficult. After eliminating possible mechanisms that could cause cooling, we find that the observed cooling is a manifestation of previous warming in basal ice. Thermal decay after latent heat is released from freezing water in basal crevasses is the most likely mechanism resulting in the temporal evolution of temperature and the vertical thermal structure observed at our site. Basal crevasses are a viable englacial heat source in the basal ice of Greenland's ablation zone and may have a controlling influence on the temperature structure of the near basal ice.

scholarly journals The cooling signature of basal crevasses in a hard-bedded region of the Greenland Ice Sheet

2021 ◽  
Vol 15 (2) ◽  
pp. 897-907
Author(s):  
Ian E. McDowell ◽  
Neil F. Humphrey ◽  
Joel T. Harper ◽  
Toby W. Meierbachtol
Keyword(s):  
Latent Heat ◽  
Thermal State ◽  
Thermal Structure ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Temperature Structure ◽  
Ablation Zone ◽  
Geothermal Heat ◽  
Basal Ice ◽  
Thermomechanical Processes

Abstract. Temperature sensors installed in a grid of nine full-depth boreholes drilled in the southwestern ablation zone of the Greenland Ice Sheet recorded cooling in discrete sections of ice over time within the lowest third of the ice column in most boreholes. Rates of temperature change outpace cooling expected from vertical conduction alone. Additionally, observed temperature profiles deviate significantly from the site-average thermal profile that is shaped by all thermomechanical processes upstream. These deviations imply recent, localized changes to the basal thermal state in the boreholes. Although numerous heat sources exist to add energy and warm ice as it moves from the central divide towards the margin such as strain heat from internal deformation, latent heat from refreezing meltwater, and the conduction of geothermal heat across the ice–bedrock interface, identifying heat sinks proves more difficult. After eliminating possible mechanisms that could cause cooling, we find that the observed cooling is a manifestation of previous warming in near-basal ice. Thermal decay after latent heat is released from freezing water in basal crevasses is the most likely mechanism resulting in the transient evolution of temperature and the vertical thermal structure observed at our site. We argue basal crevasses are a viable englacial heat source in the basal ice of Greenland's ablation zone and may have a controlling influence on the temperature structure of the near-basal ice.

scholarly journals Englacial latent-heat transfer has limited influence on seaward ice flux in western Greenland

2016 ◽  
Vol 63 (237) ◽  
pp. 1-16 ◽  
Author(s):  
KRISTIN POINAR ◽  
IAN JOUGHIN ◽  
JAN T. M. LENAERTS ◽  
MICHIEL R. VAN DEN BROEKE
Keyword(s):  
Heat Transfer ◽  
Latent Heat ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Dynamic Evolution ◽  
Ablation Zone ◽  
Slow Movement ◽  
Limited Effect ◽  
Ice Flow ◽  
Numeric Model

ABSTRACTSurface meltwater can refreeze within firn layers and crevasses to warm ice through latent-heat transfer on decadal to millennial timescales. Earlier work posited that the consequent softening of the ice might accelerate ice flow, potentially increasing ice-sheet mass loss. Here, we calculate the effect of meltwater refreezing on ice temperature and softness in the Pâkitsoq (near Swiss Camp) and Jakobshavn Isbræ regions of western Greenland using a numeric model and existing borehole measurements. We show that in the Jakobshavn catchment, meltwater percolation within the firn warms the ice at depth by 3–5°C. By contrast, meltwater refreezing in crevasses (cryo-hydrologic warming) at depths of ~300 m warms the ice in Pâkitsoq by up to 10°C, but this causes minimal increase in ice motion (<10 m a−1). Pâkitsoq is representative of western Greenland's land-terminating ice, where the slow movement of ice through a wide ablation zone provides ideal conditions for cryo-hydrologic warming to occur. We find that only ~37% of the western Greenland ice flux, however, travels through such areas. Overall, our findings suggest that cryo-hydrologic warming will likely have only a limited effect on the dynamic evolution of the Greenland ice sheet.

scholarly journals Variations of ablation, albedo and energy balance at the margin of the Greenland ice sheet, Kronprins Christian Land, eastern north Greenland

1995 ◽  
Vol 41 (137) ◽  
pp. 174-182 ◽  
Author(s):  
Thomas Konzelmann ◽  
Roger J. Braithwaite
Keyword(s):  
Heat Flux ◽  
Energy Balance ◽  
Latent Heat ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Small Scale ◽  
Conductive Heat ◽  
Net Radiation ◽  
Conductive Heat Flux ◽  
North Greenland

AbstractA meteorological and glaciological experiment was carried out in July 1993 at the margin of the Greenland ice sheet in Kronprins Christian Land, eastern north Greenland. Within a small area (about 100 m2) daily measurements were made on ten ablation stakes fixed in “light” and “dark” ice and were compared to each other. Simultaneously, the components of the energy balance, including net radiation, sensible-heat flux, latent-heat flux and conductive-heat flux in the ice were determined. Global radiation, longwave incoming radiation and albedo were measured, and longwave outgoing radiation was calculated by assuming that the glacier surface was melting. Sensible-and latent-heat fluxes were calculated from air temperature, humidity and wind speed. Conductive-heat flux in the ice was estimated by temperature-profile measurements in the uppermost ice layer. Net radiation is the major source of ablation energy, and turbulent fluxes are smaller energy sources by about three times, while heat flux into the ice is a substantial heat sink, reducing energy available for ice melt. Albedo varies from 0.42 to 0.56 within the experimental site and causes relatively large differences in ablation at stakes close to each other. Small-scale albedo variations should therefore be carefully sampled for large-scale energy-balance calculations.

scholarly journals Variations of ablation, albedo and energy balance at the margin of the Greenland ice sheet, Kronprins Christian Land, eastern north Greenland

1995 ◽  
Vol 41 (137) ◽  
pp. 174-182 ◽  
Author(s):  
Thomas Konzelmann ◽  
Roger J. Braithwaite
Keyword(s):  
Heat Flux ◽  
Energy Balance ◽  
Latent Heat ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Small Scale ◽  
Conductive Heat ◽  
Net Radiation ◽  
Conductive Heat Flux ◽  
North Greenland

AbstractA meteorological and glaciological experiment was carried out in July 1993 at the margin of the Greenland ice sheet in Kronprins Christian Land, eastern north Greenland. Within a small area (about 100 m2) daily measurements were made on ten ablation stakes fixed in “light” and “dark” ice and were compared to each other. Simultaneously, the components of the energy balance, including net radiation, sensible-heat flux, latent-heat flux and conductive-heat flux in the ice were determined. Global radiation, longwave incoming radiation and albedo were measured, and longwave outgoing radiation was calculated by assuming that the glacier surface was melting. Sensible-and latent-heat fluxes were calculated from air temperature, humidity and wind speed. Conductive-heat flux in the ice was estimated by temperature-profile measurements in the uppermost ice layer. Net radiation is the major source of ablation energy, and turbulent fluxes are smaller energy sources by about three times, while heat flux into the ice is a substantial heat sink, reducing energy available for ice melt. Albedo varies from 0.42 to 0.56 within the experimental site and causes relatively large differences in ablation at stakes close to each other. Small-scale albedo variations should therefore be carefully sampled for large-scale energy-balance calculations.

scholarly journals Uppermost crustal structure regulates the flow of the Greenland Ice Sheet

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
G. A. Jones ◽  
A. M. G. Ferreira ◽  
B. Kulessa ◽  
M. Schimmel ◽  
A. Berbellini ◽  
...  
Keyword(s):  
Basal Slip ◽  
Seismic Velocity ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Velocity Model ◽  
Upper Crust ◽  
Geothermal Heat ◽  
Ice Flow ◽  
The Past ◽  
Basal Ice

AbstractThe flow of the Greenland Ice Sheet is controlled by subglacial processes and conditions that depend on the geological provenance and temperature of the crust beneath it, neither of which are adequately known. Here we present a seismic velocity model of the uppermost 5 km of the Greenlandic crust. We show that slow velocities in the upper crust tend to be associated with major outlet glaciers along the ice-sheet margin, and elevated geothermal heat flux along the Iceland hotspot track inland. Outlet glaciers particularly susceptible to basal slip over deformable subglacial sediments include Jakobshavn, Helheim and Kangerdlussuaq, while geothermal warming and softening of basal ice may affect the onset of faster ice flow at Petermann Glacier and the Northeast Greenland Ice Stream. Interactions with the solid earth therefore control the past, present and future dynamics of the Greenland Ice Sheet and must be adequately explored and implemented in ice sheet models.

scholarly journals Discharge of debris from ice at the margin of the Greenland ice sheet

2002 ◽  
Vol 48 (161) ◽  
pp. 192-198 ◽  
Author(s):  
Peter G. Knight ◽  
Richard I. Waller ◽  
Carrie J. Patterson ◽  
Alison P. Jones ◽  
Zoe P. Robinson
Keyword(s):  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
High Rate ◽  
Outlet Glacier ◽  
Tidewater Glaciers ◽  
Sediment Production ◽  
Basal Ice ◽  
Order Of Magnitude ◽  
Sediment Transfer ◽  
A Minor

AbstractSediment production at a terrestrial section of the ice-sheet margin in West Greenland is dominated by debris released through the basal ice layer. The debris flux through the basal ice at the margin is estimated to be 12–45 m3 m−1 a−1. This is three orders of magnitude higher than that previously reported for East Antarctica, an order of magnitude higher than sites reported from in Norway, Iceland and Switzerland, but an order of magnitude lower than values previously reported from tidewater glaciers in Alaska and other high-rate environments such as surging glaciers. At our site, only negligible amounts of debris are released through englacial, supraglacial or subglacial sediment transfer. Glaciofluvial sediment production is highly localized, and long sections of the ice-sheet margin receive no sediment from glaciofluvial sources. These findings differ from those of studies at more temperate glacial settings where glaciofluvial routes are dominant and basal ice contributes only a minor percentage of the debris released at the margin. These data on debris flux through the terrestrial margin of an outlet glacier contribute to our limited knowledge of debris production from the Greenland ice sheet.

The effect of geothermal heat flux on the deglacial evolution of the Greenland ice sheet

2021 ◽  
Author(s):  
Parviz Ajourlou ◽  
François PH Lapointe ◽  
Glenn A Milne ◽  
Yasmina Martos
Keyword(s):  
Boundary Condition ◽  
Heat Flux ◽  
Thermal State ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Sampling Procedure ◽  
Geophysical Research ◽  
Geothermal Heat ◽  
Input Field ◽  
Uncertainty Estimates

&lt;p&gt;Geothermal heat flux (GHF) is known to be an important control on the basal thermal state of an ice sheet which, in turn, is a key factor in governing how the ice sheet will evolve in response to a given climate forcing. In recent years, several studies have estimated GHF beneath the Greenland ice sheet using different approaches (e.g. Rezvanbehbahani et al., Geophysical Research Letters, 2017; Martos et al., Geophysical Research Letters, 2018; Greve, Polar Data Journal, 2019). Comparing these different estimates indicates poor agreement and thus large uncertainty in our knowledge of this important boundary condition for modelling the ice sheet. The primary aim of this study is to quantify the influence of this uncertainty on modelling the past evolution of the ice sheet with a focus on the most recent deglaciation. We build on past work that considered three GHF models (Rogozhina et al., 2011) by considering over 100 different realizations of this input field. We use the uncertainty estimates from Martos et al. (Geophysical Research Letters, 2018) to generate GHF realisations via a statistical sampling procedure. A sensitivity analysis using these realisations and the Parallel Ice Sheet Model (PISM, Bueler and Brown, Journal of Geophysical Research, 2009) indicates that uncertainty in GHF has a dramatic impact on both the volume and spatial distribution of ice since the last glacial maximum, indicating that more precise constraints on this boundary condition are required to improve our understanding of past ice sheet evolution and, consequently, reduce uncertainty in future projections.&lt;/p&gt;

Implications for the lithospheric structure of Greenland by applying different heat flow models

2021 ◽  
Author(s):  
Agnes Wansing ◽  
Jörg Ebbing ◽  
Mareen Lösing ◽  
Sergei Lebedev ◽  
Nicolas Celli ◽  
...  
Keyword(s):  
Heat Flow ◽  
Heat Dissipation ◽  
Ice Sheet ◽  
Magnetic Data ◽  
Similarity Function ◽  
Lithospheric Structure ◽  
Temperature Structure ◽  
Geothermal Heat ◽  
Flow Models ◽  
Ice Sheet Dynamics

&lt;p&gt;The lithospheric structure of Greenland is still poorly known due to its thick ice sheet, the sparseness of seismological stations, and the limitation of geological outcrops near coastal areas. As only a few geothermal measurements are available for Greenland, one must rely on geophysical models. Such models of Moho and LAB depths and sub-ice geothermal heat-flow vary largely.&lt;/p&gt;&lt;p&gt;Our approach is to model the lithospheric architecture by geophysical-petrological modelling with LitMod3D. The model is built to reproduce gravity observations, the observed elevation with isostasy assumptions and the velocities from a tomography model. Furthermore, we adjust the thermal parameters and the temperature structure of the model to agree with different geothermal heat flow models. We use three different heat flow models, one from machine learning, one from a spectral analysis of magnetic data and another one which is compiled from a similarity study with tomography data.&lt;/p&gt;&lt;p&gt;For the latter, a new shear wave tomography model of Greenland is used. Vs-depth profiles from Greenland are compared with velocity profiles from the US Array, where a statistical link between Vs profiles and surface heat flow has been established. A similarity function determines the most similar areas in the U.S. and assigns the mean heat-flow from these areas to the corresponding area in Greenland.&lt;/p&gt;&lt;p&gt;The geothermal heat flow models will be further used to discuss the influence on ice sheet dynamics by comparison to friction heat and viscous heat dissipation from surface meltwater.&lt;/p&gt;

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 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.

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