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  • 1
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    Massive subsurface ice formed by refreezing of ice-shelf melt ponds (2016)
    Macmillan Publishers Limited
    In:  EPIC3Nature Communications, Macmillan Publishers Limited, 7, pp. 11897, ISSN: 2041-1723
    Details
    Publication Date: 2016-06-13
    Description: Surface melt ponds form intermittently on several Antarctic ice shelves. Although implicated in ice-shelf break up, the consequences of such ponding for ice formation and ice-shelf structure have not been evaluated. Here we report the discovery of a massive subsurface ice layer, at least 16 km across, several kilometres long and tens of metres deep, located in an area of intense melting and intermittent ponding on Larsen C Ice Shelf, Antarctica. We combine borehole optical televiewer logging and radar measurements with remote sensing and firn modelling to investigate the layer, found to be ~10 °C warmer and ~170 kg/m3 denser than anticipated in the absence of ponding and hitherto used in models of ice-shelf fracture and flow. Surface ponding and ice layers such as the one we report are likely to form on a wider range of Antarctic ice shelves in response to climatic warming in forthcoming decades.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , isiRev
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  • 2
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    Observed propagation of a large rift in the Larsen C ice shelf: rift development and possible consequences for the stability of the ice shelf (2015)
    In:  EPIC3International Symposium on contemporary ice sheet dynamics, Cambridge, UK, 2015-08-16-2015-08-21
    Details
    Publication Date: 2015-10-05
    Description: The Larsen C ice shelf is the most northerly of the remaining major Antarctic Peninsula ice shelves and is vulnerable to changes in both ocean and atmospheric forcing. It is the largest ice shelf in the region and its loss would lead to a significant drawdown of ice from the Antarctic Peninsula ice sheet. There have been observations of widespread thinning, melt ponding in the northern inlets, and a speed-up in ice flow, processes that have all been linked to former ice-shelf collapses. Previous studies have also highlighted the vulnerability of the Larsen C ice shelf to specific potential changes in its geometry, including a retreat from the Bawden and Gipps Ice Rise. Rift tips in the vicinity of Gipps Ice Rise have been observed to align at a suture zone between two flow units within the shelf. Several studies have provided evidence for marine ice in these suture zones, which has been found to act as a weak coupling between flow units with different flow velocities. It has been concluded that this ice inhibits the propagation of rifts because it can accommodate strain in the ice without fracturing further. In a change from the usual pattern, a northwards-propagating rift from Gipps Ice Rise has recently penetrated through the suture zone and is now more than halfway towards calving a large section of the ice shelf. The rate of propagation of this rift accelerated during 2014. When the next major calving event occurs, the Larsen C ice shelf is likely to lose around 10% of its area to reach a new minimum area for the ice shelf. We followed the rift propagation on MODIS and Landsat imagery and used a numerical model to investigate the influence of the future calving event on ice-shelf stability. We find that the ice front is at risk of becoming unstable when the anticipated calving event occurs.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , notRev
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  • 3
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    Föhn winds on Larsen C ice shelf generate an unusually massive ice layer (2015)
    In:  EPIC3International Symposium on contemporary ice sheet dynamics, Cambridge, UK, 2015-08-16-2015-08-21
    Details
    Publication Date: 2015-10-05
    Description: Surface melt and ponding has been observed on many Antarctic ice shelves and is implicated in ice-shelf collapse through firn compaction and hydrofracture (van den Broeke, 2005). Ice-shelf surface meltwater can percolate into the firn, transferring heat to deeper layers by refreezing (Vaughan, 2008). This can lead to warming and densification to the point where firn air content approaches zero (Holland and others, 2011), potentially impacting ice dynamics and fracture toughness. Surface processes also contribute to the recent thinning observed on Larsen C ice shelf (LCIS; Pritchard and others, 2012; Holland and others, 2015). In northern LCIS, föhn winds provide extra sensible heat to drive surface melt (Luckman and others, 2014). The NERC MIDAS Project (Impact of Melt on Ice Dynamics and Stability: 2014–2017) aims to investigate the mechanisms of melt and ponding, and test their impact on the stability of the LCIS. In November 2014 we visited Cabinet Inlet in northern LCIS to install an ARGOS-enabled automatic weather station, drill a 100 m borehole, and determine ice layering and temperature using an optical televiewer (OPTV) and logged thermistor string. The OPTV log shows a ~40 m layer of refrozen meltwater perched above glacier ice presumably advected from beyond the grounding line 40 km away. The thermistor data show the ice to be many degrees warmer than mean atmospheric temperatures would lead us to expect. We infer the source of this unusual ice configuration and temperature profile as föhn-driven surface melt. In April 2015, well after normal mean surface temperature had fallen below freezing, several föhn wind events lasting a few days raised the temperature well above 0°C and, in MODIS satellite data, melt ponds were seen to form. We continue to investigate using numerical modelling the potential impact on ice-shelf stability of melt, warming and massive ice layers.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , notRev
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  • 4
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    Observationally constrained surface mass balance of Larsen C ice shelf, Antarctica (2017)
    COPERNICUS GESELLSCHAFT MBH
    In:  EPIC3The Cryosphere, COPERNICUS GESELLSCHAFT MBH, 11, pp. 2411-2426, ISSN: 1994-0416
    Details
    Publication Date: 2017-11-13
    Description: The surface mass balance (SMB) of the Larsen C ice shelf (LCIS), Antarctica, is poorly constrained due to a dearth of in situ observations. Combining several geophysical techniques, we reconstruct spatial and temporal patterns of SMB over the LCIS. Continuous time series of snow height (2.5–6 years) at five locations allow for multi-year estimates of seasonal and annual SMB over the LCIS. There is high interannual variability in SMB as well as spatial variability: in the north, SMB is 0.40+/-0.06 to 0.41+/-0.04mw.e. per year, while farther south, SMB is up to 0.50+/-0.05mw.e. per year. This difference between north and south is corroborated by winter snow accumulation derived from an airborne radar survey from 2009, which showed an average snow thickness of 0.34mw.e. north of 66° S, and 0.40mw.e. south of 68° S. Analysis of ground-penetrating radar from several field campaigns allows for a longer-term perspective of spatial variations in SMB: a particularly strong and coherent reflection horizon below 25–44m of waterequivalent ice and firn is observed in radargrams collected across the shelf. We propose that this horizon was formed synchronously across the ice shelf. Combining snow height observations, ground and airborne radar, and SMB output from a regional climate model yields a gridded estimate of SMB over the LCIS. It confirms that SMB increases from north to south, overprinted by a gradient of increasing SMB to the west, modulated in the west by föhn-induced sublimation. Previous observations show a strong decrease in firn air content toward the west, which we attribute to spatial patterns of melt, refreezing, and densification rather than SMB.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , isiRev
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  • 5
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    Brief Communication: Newly developing rift in Larsen C Ice Shelf presents significant risk to stability (2015)
    COPERNICUS GESELLSCHAFT MBH
    In:  EPIC3The Cryosphere, COPERNICUS GESELLSCHAFT MBH, 9, pp. 1223-1227, ISSN: 1994-0416
    Details
    Publication Date: 2015-06-18
    Description: An established rift in the Larsen C Ice Shelf, formerly constrained by a suture zone containing marine ice, grew rapidly during 2014 and is likely in the near future to generate the largest calving event since the 1980s and result in a new minimum area for the ice shelf. Here we investigate the recent development of the rift, quantify the projected calving event and, using a numerical model, assess its likely impact on ice shelf stability. We find that the ice front is at risk of becoming unstable when the anticipated calving event occurs.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , isiRev
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  • 6
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    Observed rift propagation in the Larsen C Ice Shelf and future calving front stability (2015)
    In:  EPIC3Twenty-Second Annual WAIS Workshop, Sylvan Dale Ranch, Loveland, CO, USA, 2015-09-16-2015-09-19
    Details
    Publication Date: 2015-10-05
    Description: The Larsen C Ice Shelf is the most northerly of the remaining major Antarctic Peninsula ice shelves and is vulnerable to changes in both to ocean and atmospheric forcing. It is the largest ice shelf in the region and its loss would lead to a significant drawdown of ice from the Antarctic Peninsula Ice Sheet. There have been observations of widespread thinning, melt ponding in the northern inlets, and, in the northern part, a speed-up in ice flow, all processes which have been linked to former ice shelf collapses. Previous studies have also highlighted the vulnerability of Larsen C Ice Shelf to specific potential changes in its geometry including a retreat from the Bawden and Gipps Ice Rise. In a change from the usual pattern, a northwards-propagating rift from Gipps Ice Rise has recently advanced towards the center of the ice shelf. It is now more than halfway towards calving a large section of the ice shelf and continues to widen. We followed the rift propagation on MODIS and Landsat imagery and, during the austral winter 2015, on Sentinel 1 radar data. Sentinel 1 data was also used to calculate flow velocity fields for the ice shelf. We used a numerical model to investigate the influence of the future calving event on ice shelf stability. To investigate a range of possible outcomes from a future calving event, we assumed two scenarios for the rift trajectory based on its current orientation and direction of propagation. To assess the stability of the future calving front we analyzed the difference between the predicted directions of ice flow and of first principal stress (stress-flow angle). Regions of the shelf exhibiting low stress-flow angles are likely to be more affected by small-scale calving because stresses act to open existing weaknesses. We find that the ice front is at risk of becoming unstable when the anticipated calving event occurs.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , notRev
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  • 7
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    Massive Ice Layer Formed by Refreezing of Ice-shelf Surface Melt Ponds: Larsen C Ice Shelf, Antarctica (2016)
    In:  EPIC3EGU General Assembly 2016, Vienna, Austria, 2016-04-17-2016-04-22
    Details
    Publication Date: 2016-05-02
    Description: Surface melt ponds now form frequently on ice shelves across the northern sector of the Antarctic Peninsula in response to regional warming and local föhn winds. A potentially important, but hitherto unknown, consequence of this surface melting and ponding is the formation of high-density near-surface ice from the refreezing of that water. We report the discovery and physical character of a massive subsurface ice layer located in an area of intense melting and intermittent ponding on Larsen C Ice Shelf, Antarctica. We combine borehole optical televiewer logging and ground-based radar measurements with remote sensing and firn modelling to investigate the formation and spatial extent of this layer, found to be tens of kilometres across and tens of metres deep. The presence of this ice layer has the effect of raising local ice shelf density by about 190 kgm-3 and temperature by 5 - 10 degrees C above values found in areas unaffected by ponding and hitherto used in models of ice-shelf fracture and flow.
    Repository Name: EPIC Alfred Wegener Institut
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  • 8
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    Observed rift propagation in the Larsen C Ice Shelf from Sentinel 1-A radar data (2016)
    In:  EPIC3EGU General Assembly 2016, Vienna, Austria, 2016-04-17-2016-04-22
    Details
    Publication Date: 2016-05-02
    Description: The Larsen C Ice Shelf is the most northerly of the remaining major Antarctic Peninsula ice shelves and is vulnerable to changes in both to ocean and atmospheric forcing. It is the largest ice shelf in the region and its loss would lead to a significant drawdown of ice from the Antarctic Peninsula Ice Sheet. There have been observations of widespread thinning, melt ponding in the northern inlets, and, in the northern part, a speed-up in ice flow, all processes which have been linked to former ice shelf collapses. Previous studies have also highlighted the vulnerability of Larsen C Ice Shelf to specific potential changes in its geometry including a retreat from the Bawden and Gipps Ice Rise. In a change from the usual pattern, a northwards-propagating rift from Gipps Ice Rise has recently advanced towards the center of the ice shelf. It is now more than halfway towards calving a large section of the ice shelf and continues to widen. We followed the rift propagation on MODIS and Landsat imagery and,during the austral winter 2015, on Sentinel-1A radar data. Due to the very cloudy weather conditions during the austral Summer 2015 / 2016 the Sentinel data became an essential part of the monitoring. By calculating differential interferograms it was possible to clearly identify the active tip of the rift, which was not always obvious on the Landsat images. Further, surface velocities were derived from recent Sentinel-1A acquisitions by means of offset intensity tracking. In order to investigate a possible speed-up of the ice shelf we extended the study area to the north including Bawden ice rise.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , notRev
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  • 9
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    Intercomparison and Validation of SAR-Based Ice Velocity Measurement Techniques within the Greenland Ice Sheet CCI Project (2019)
    Details
    Publication Date: 2022-05-06
    Description: ce velocity is one of the products associated with the Ice Sheets Essential Climate Variable. This paper describes the intercomparison and validation of ice-velocity measurements carried out by several international research groups within the European Space Agency Greenland Ice Sheet Climate Change Initiative project, based on space-borne Synthetic Aperture Radar (SAR) data. The goal of this activity was to survey the best SAR-based measurement and error characterization approaches currently in practice. To this end, four experiments were carried out, related to different processing techniques and scenarios, namely differential SAR interferometry, multi aperture SAR interferometry and offset-tracking of incoherent as well as of partially-coherent data. For each task, participants were provided with common datasets covering areas located on the Greenland ice-sheet margin and asked to provide mean velocity maps, quality characterization and a description of processing algorithms and parameters. The results were then intercompared and validated against GPS data, revealing in several cases significant differences in terms of coverage and accuracy. The algorithmic steps and parameters influencing the coverage, accuracy and spatial resolution of the measurements are discussed in detail for each technique, as well as the consistency between quality parameters and validation results. This allows several recommendations to be formulated, in particular concerning procedures which can reduce the impact of analyst decisions, and which are often found to be the cause of sub-optimal algorithm performance.
    Description: Published
    Description: id 929
    Description: 4A. Oceanografia e clima
    Description: JCR Journal
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Type: article
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  • 10
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    Monitoring Greenland ice sheet buoyancy-driven calving discharge using glacial earthquakes (2019)
    Cambridge University Press
    Details
    Publication Date: 2019
    Description: 〈div data-abstract-type="normal"〉〈p〉Since the 2000s, Greenland ice sheet mass loss has been accelerating, followed by increasing numbers of glacial earthquakes (GEs) at near-grounded glaciers. GEs are caused by calving of km-scale icebergs which capsize against the terminus. Seismic record inversion allows a reconstruction of the history of GE sources which captures capsize dynamics through iceberg-to-terminus contact. When compared with a catalog of contact forces from an iceberg capsize model, seismic force history accurately computes calving volumes while the earthquake magnitude fails to uniquely characterize iceberg size, giving errors up to 1 km〈span〉3〈/span〉. Calving determined from GEs recorded ateight glaciers in 1993–2013 accounts for up to 21% of the associated discharge and 6% of the Greenland mass loss. The proportion of discharge attributed to capsizing calving may be underestimated by at least 10% as numerous events could not be identified by standard seismic detections (Olsen and Nettles, 2018). While calving production tends to stabilize in East Greenland, Western glaciers have released more and larger icebergs since 2010 and have become major contributors to Greenland dynamic discharge. Production of GEs and calving behavior are controlled by glacier geometry with bigger icebergs being produced when the terminus advances in deepening water. We illustrate how GEs can help in partitioning and monitoring Greenland mass loss and characterizing capsize dynamics.〈/p〉〈/div〉
    Print ISSN: 0260-3055
    Electronic ISSN: 1727-5644
    Topics: Geography , Geosciences
    Published by Cambridge University Press
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