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  • Articles  (9)
  • 2015-2019  (9)
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  • 1
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    Surface energy and mass balance at Purogangri ice cap, central Tibetan Plateau, 2001–2011 (2015)
    Cambridge University Press
    Details
    Publication Date: 2015-12-21
    Print ISSN: 0022-1430
    Electronic ISSN: 1727-5652
    Topics: Geography , Geosciences
    Published by Cambridge University Press on behalf of The International Glaciological Society (IGS).
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  • 2
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    Recent slowdown and thinning of debris-covered glaciers in south-eastern Tibet (2017)
    Elsevier
    In:  EPIC3Earth and Planetary Science Letters, Elsevier, 464, pp. 95-102
    Details
    Publication Date: 2017-04-18
    Description: Recent large-scale remote sensing studies have shown that glacier mass loss in south-eastern Tibet, specifically in the eastern Nyainqêntanglha Range exceeds the average in High Asia. However, detailed studies at individual glaciers are scarce and the drivers behind the observed changes are poorly constrained to date. Employing feature tracking techniques on TerraSAR-X data for the periods 2008/2009, 2012/2013 and 2013/2014 we found measurable surface velocities through to the glacier terminus positions of five debris-covered glacier tongues. This is contrary to debris-covered glaciers in other parts of High Asia, where stagnant glacier tongues are common. Our feature tracking results for the 2013/2014 period suggest an average deceleration of 51% when compared with published Landsat velocities for the period 1999/2003. Further, we estimated surface elevation changes for the five glaciers from recently released one arc second resolution elevation data obtained during the Shuttle Radar Topography Mission in 2000 and an interferometrical derived TanDEM-X elevation model for the year 2014. With an average rate of −0.83 ± 0.57 m a^-1 we confirm strong surface lowering in the region, despite the widely discussed insulation effect of debris cover. Beside the influence of thermokarst processes and delayed response times of debris-covered glaciers, we highlight that abundant monsoonal summer rainfall might contribute significantly to the pronounced negative mass balances in the study region.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , isiRev
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  • 3
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    Detecting high spatial variability of ice-shelf basal mass balance (2018)
    In:  EPIC3EGU General Assembly, Vienna, 2018-04-08-2018-04-13
    Details
    Publication Date: 2019-08-21
    Description: Ice shelves control the dynamic mass loss of ice sheets through buttressing and their integrity depends on the spatial variability of their basal mass balance (BMB), i.e. the difference between refreezing and melting. Here, we present an improved technique – based on satellite observations – to capture the small-scale variability in the BMB of ice shelves. We use mass conservation in a Lagrangian framework based on high-resolution surface velocities, atmospheric-model surface mass balance and hydrostatic ice-thickness fields (derived from TanDEM-X surface elevation). Spatial derivatives are implemented using the total-variation differentiation, which preserves abrupt changes inflow velocities and their spatial gradients. Such changes may reflect a dynamic response to localized basal melting and should be included in the mass budget. After testing our technique on the Roi Baudouin Ice Shelf, East Antarctica, we test our methodology on other ice shelves, with different flow regimes. Whereas the detected large-scale pattern in the BMB is very similar to previous and coarser studies, we are nevertheless able detect small-scale features in the BMB with unprecedented detail (10 m gridding). Examples include elevated melting at an ice-shelf channel’s flank and surface lowering of an elliptical surface depression. Although the absolute, satellite-based BMB values remain uncertain, we have high confidence in the spatial variability on sub-kilometre scales. This work highlights expected challenges for a full coupling between ice and ocean models.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , notRev
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  • 4
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    Near real time surface velocity measurements of North-East Greenland Ice Stream outlet glaciers from Sentinel-1A data (2016)
    In:  EPIC3ESA Living Planet Symposium, Prague, Czech Republic, 2016-05-09-2016-05-13
    Details
    Publication Date: 2016-05-24
    Description: Compared to the western and southern parts of the Greenland ice sheet, where narrow tidewater and land terminating glaciers dominate, the north-eastern part is characterized by a major ice stream, the North-East Greenland Ice Stream (NEGIS), which drains about 8% of the ice sheet via three outlet glaciers. While the mass loss of the ice sheet was most prominent in the southern and western areas in the past decade, within the past few years also the north-eastern part showed changes. Therefore, a continuous monitoring of surface velocities of the large outlet glaciers Nioghalvfjerdsfjorden (79 NG) and Zachariae Isstrøm (ZIS) is intended. For this purpose we make use of Sentinel-1a data, aiming at near real time velocity measurements. Surface velocities are calculated for every 12-day and 24-day repeat pass by means of offset intensity tracking. Here we make use of Terrain Observation with Progressive Scans SAR (TOPSAR) Single Look Complex (SLC) data and precise orbit information. In order to achieve robust velocity fields and to close data gaps, offset tracking results are stacked on a monthly basis resulting in a continuous time series of velocity measurements since December 2014.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , notRev
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  • 5
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    A Combined Approach for Filtering Ice Surface Velocity Fields Derived from Remote Sensing Methods (2017)
    MDPI AG
    In:  EPIC3Remote Sensing, MDPI AG, 9(10), pp. 1062, ISSN: 2072-4292
    Details
    Publication Date: 2017-10-23
    Description: Various glaciological topics require observations of horizontal velocities over vast areas, e.g., detecting acceleration of glaciers, as well as for estimating basal parameters of ice sheets using inverse modelling approaches. The quality of the velocity is of high importance; hence, methods to remove noisy points in remote sensing derived data are required. We present a three-step filtering process and assess its performance for velocity fields in Greenland and Antarctica. The filtering uses the detection of smooth segments, removal of outliers using the median and constraints on the variability of the flow direction over short distances. The applied filter preserves the structures in the velocity fields well (e.g., shear margins) and removes noisy data points successfully, while keeping 72–96% of the data. In slow flowing regions, which are particularly challenging, the standard deviation is reduced by up to 96%, an improvement that affects vast areas of the ice sheets.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , isiRev
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  • 6
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    Calving Induced Speedup of Petermann Glacier (2019)
    Wiley
    In:  EPIC3Journal of Geophysical Research-Earth Surface, Wiley, 124, pp. 216-228, ISSN: 0148-0227
    Details
    Publication Date: 2019-08-14
    Description: This study assesses the response on ice dynamics of Petermann Glacier, a major outlet glacier in northern Greenland, to the 2012 and a possible future calving event. So far Petermann Glacier has been believed to be dynamically stable as another large calving event in 2010 had no significant impact on flow velocity or grounding line retreat. By analyzing a time series of remotely sensed surface velocities, we find an average acceleration of 10% between winter 2011/2012 and winter 2016/2017. This increase in surface velocity is not linear but can be separated into two parts, starting in 2012 and 2016 respectively. By conducting modeling experiments, we show that the first speedup can be directly connected to the 2012 calving event, while the second speedup is not captured. However, on recent remote sensing imagery newly developing fractures are clearly visible ∼12 km upstream from the terminus, propagating from the eastern fjord wall to the center of the ice tongue, indicating a possible future calving event. By including these fracture zones as a new terminus position in the modeling domain, we are able to reproduce the second speedup, suggesting that surface velocities remain on the 2016/2017 level after the anticipated calving event. This indicates that, from a dynamical point of view, the terminus region has already detached from the main ice tongue.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , isiRev , info:eu-repo/semantics/article
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  • 7
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    Detecting and monitoring ice-shelf basal mass balance in Dronning Maud Land, East Antarctica (2019)
    In:  EPIC3EGU General Assembly, Vienna, 2019-04-07-2019-04-12
    Details
    Publication Date: 2019-08-21
    Description: Ice shelves control the dynamic mass loss of ice sheets through buttressing. Their integrity also depends on their total mass balance, with the the spatial variability of their basal mass balance (BMB), i.e. the difference between basal refreezing and melting, being an important component. Here, we present an improved technique – based on satellite observations – to capture the small-scale variability in the BMB of ice shelves. We use mass conservation in a Lagrangian framework based on high-resolution horizontal surface velocities, atmospheric-model surface mass balance and hydrostatic ice-thickness fields (derived from TanDEM-X surface elevation). Spatial derivatives are implemented using the total-variation differentiation, which preserves abrupt changes in flow velocities and their spatial gradients. Such changes may reflect a dynamic response to localized basal melting and should be included in the mass budget. After successfully developing the technique with TanDEM-X elevations from 2013-2014 for the Roi Baudouin Ice Shelf, Dronning Maud Land, East Antarctica (Fig. 1), we upscaled our results spatially to all ice shelves in Dronning Maud Land that are located between Fimbul and Roi Baudouin ice shelves. The BMB field we produce shows a large-scale pattern in close agreement with previous and studies in coarser resolution. However,our results also indicate that we are in addition able to detect small-scale features in the BMB with unprecedented detail (at a gridding of 〈50 m). Beyond the static field of BMB we also investigate temporal changes in the BMB by combining our BMB basedon TanDEM-X elevations with coarser BMB based on Cryosat-2 data.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , notRev
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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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    Signature of NEGIS shear margins in radar stratigraphy (2019)
    In:  EPIC3IGS Symposium Five Decades of Radioglaciology, Stanford, USA, 2019-07-08-2019-07-12
    Details
    Publication Date: 2020-03-09
    Description: The North East Greenland Ice Stream (NEGIS) is an essential part of the Greenland mass balance, reaching up to the central divide of the ice sheet. Its shape is dominated by very distinct shear margins that appear not to be linked to bedrock topography in the upper part of the ice stream. To understand the effective mass transport within an ice stream it is necessary to investigate the nature of the shear margins, in which very localized deformation decouples the inner ice stream from the slower-flowing surrounding ice sheet. We present results from an airborne radar survey with the Alfred Wegener Institute Ultra Wide Band Radar system in the vicinity of the EGRIP ice core location, focusing on the signatures of the shear margins in the radar data. In regions of localized shear, the internal reflections show disturbances in the form of steep undulations, which are intensified with ongoing shear. We show the evolution of these characteristic signatures over the survey area. Our data covers the main trunk and the shear margins over 200 km along the ice stream, in which the flow velocity in the center increases from 12 m a–1 to 75 m a–1. We analyze the change in the shape of the internal reflections in the shear zones in combination with a strain rate field calculated from high-resolution flow velocities derived from TerraSAR-X SAR interferometry. In the vicinity of the EGRIP drilling site, 240 km downstream of the divide, the NEGIS widens and the shear margin in the northwest steps 6 km outward, which is visible in optical satellite imagery as well as in the satellite-derived velocity field. A special focus of our analysis lies on the step in the shear margin whose clearly visible radar-backscatter signatures indicate a newly forming vertical disturbance caused by a draw-down of the layers.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , notRev
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