Constraining variable density of ice shelves using wide-angle radar measurements

Drews, Reinhard; Brown, Jacqueline A.; Matsuoka, Kenichi; Witrant, Emmanuel; Philippe, Morgane; Hubbard, Bryn; Pattyn, Frank

doi:10.5194/tc-10-811-2016

Cited by 17 publications

(14 citation statements)

References 54 publications

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“…4). Almost all ice shelf channels at RBIS are connected to the grounding line and may arise from water-filled subglacial conduits injecting subglacial melt water into the ice shelf cavity, driving a spatially localized buoyant melt water plume (Jenkins, 2011;Le Brocq et al, 2013;Drews et al, 2017;Sergienko, 2013). Such localized melting near the grounding zone has been previously observed on Pine Island Ice Shelf using similar methods as done here (Dutrieux et al, 2013).…”

Section: Discussionsupporting

confidence: 61%

Detailed response to the reviewes, modifications made on manuscript tc-2017-41 and new supplementary

Berger¹

2017

Preprint

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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. As a case study, we apply the methodology to the Roi Baudouin Ice Shelf, Dronning Maud Land, East Antarctica, and derive its yearly averaged BMB at 10 m horizontal gridding. We use mass conservation in a Lagrangian framework based on high-resolution surface velocities, atmosphericmodel 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. Our BMB field exhibits much spatial detail and ranges from −14.7 to 8.6 m a −1 ice equivalent. Highest melt rates are found close to the grounding line where the pressure melting point is high, and the ice shelf slope is steep. The BMB field agrees well with on-site measurements from phase-sensitive radar, although independent radar profiling indicates unresolved spatial variations in firn density. We show that an elliptical surface depression (10 m deep and with an extent of 0.7 km × 1.3 km) lowers by 0.5 to 1.4 m a −1 , which we tentatively attribute to a transient adaptation to hydrostatic equilibrium. We find evidence for elevated melting beneath ice shelf channels (with melting being concentrated on the channel's flanks). However, farther downstream from the grounding line, the majority of ice shelf channels advect passively (i.e. no melting nor refreezing) toward the ice shelf front. Although the absolute, satellite-based BMB values remain uncertain, we have high confidence in the spatial variability on sub-kilometre scales. This study highlights expected challenges for a full coupling between ice and ocean models.

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Section: Discussionsupporting

confidence: 61%

Detailed response to the reviewes, modifications made on manuscript tc-2017-41 and new supplementary

Berger¹

2017

Preprint

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“…Consequently, we do not capture their spatial (and temporal) variations on the length scales associated with ice shelf channels. Both Drews et al (2016) and Langley et al (2014) found evidence in the shallow radar stratigraphy that the SMB may be locally elevated in those areas, potentially reflecting the deposition of drifting snow at the bottom of surface slopes (Frezzotti et al, 2007). If this holds true, then the systematic underestimation of the SMB would result in a positive bias of the LBMB in those areas.…”

Section: Surface Mass Balancementioning

confidence: 99%

“…The impact of the firn air content misestimation on the derivation of the hydrostatic ice thickness is further discussed in Lenaerts et al (2017). Moreover, Drews et al (2016) used wide-angle radar measurements in conjunction with ice coring and found that firn density varies spatially over tens of kilometres scales, in particular across ice shelf channels, where surface melt water collects in the corresponding surface depressions and locally refreezes. Therefore, we anticipate that at least some of the variability seen in the LBMB field is due to unresolved variations in firn density.…”

Section: Hydrostatic Inversionmentioning

confidence: 99%

Detecting high spatial variability of ice shelf basal mass balance, Roi Baudouin Ice Shelf, Antarctica

et al. 2017

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Abstract. 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. As a case study, we apply the methodology to the Roi Baudouin Ice Shelf, Dronning Maud Land, East Antarctica, and derive its yearly averaged BMB at 10 m horizontal gridding. We use mass conservation in a Lagrangian framework based on high-resolution surface velocities, atmosphericmodel 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. Our BMB field exhibits much spatial detail and ranges from −14.7 to 8.6 m a −1 ice equivalent. Highest melt rates are found close to the grounding line where the pressure melting point is high, and the ice shelf slope is steep. The BMB field agrees well with on-site measurements from phase-sensitive radar, although independent radar profiling indicates unresolved spatial variations in firn density. We show that an elliptical surface depression (10 m deep and with an extent of 0.7 km × 1.3 km) lowers by 0.5 to 1.4 m a −1 , which we tentatively attribute to a transient adaptation to hydrostatic equilibrium. We find evidence for elevated melting beneath ice shelf channels (with melting being concentrated on the channel's flanks). However, farther downstream from the grounding line, the majority of ice shelf channels advect passively (i.e. no melting nor refreezing) toward the ice shelf front. Although the absolute, satellite-based BMB values remain uncertain, we have high confidence in the spatial variability on sub-kilometre scales. This study highlights expected challenges for a full coupling between ice and ocean models.

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“…The largest and most ill-constrained error source presumably lies in the NMO correction, which presupposes horizontal IRHs and oblique (but straight) raypaths. In Antarctica, dips of IRHs are typically small and even if the density varies strongly with depth, effects of ray bending are equally small (Barrett and others, 2007;Drews and others, 2016). However, the positions of the sleds carrying both receiver and transmitters were not well defined: depending on the surface slope, the towing rope was not always fully tensioned and sleds also drifted into an off-axis position compared with the heading of the snowmobile.…”

Section: Radar Data Acquisition and Processingmentioning

confidence: 99%

Temporally stable surface mass balance asymmetry across an ice rise derived from radar internal reflection horizons through inverse modeling

et al. 2016

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ABSTRACT. Ice rises are locally grounded parts of Antarctic ice shelves that play an important role in regulating ice flow from the continent towards the ocean. Because they protrude out of the otherwise horizontal ice shelves, ice rises induce an orographic uplift of the atmospheric flow, resulting in an asymmetric distribution of the surface mass balance (SMB). Here, we combine younger and older internal reflection horizons (IRHs) from radar to quantify this distribution in time and space across Derwael Ice Rise (DIR), Dronning Maud Land, Antarctica. We employ two methods depending on the age of the IRHs, i.e. the shallow layer approximation for the younger IRHs near the surface and an optimization technique based on an ice flow model for the older IRHs. We identify an SMB ratio of 2.5 between the flanks and the ice divide with the SMB ranging between 300 and 750 kg m −2 a −1. The SMB maximum is located on the upwind side, ∼4 km offset to today's topographic divide. The large-scale asymmetry is consistently observed in time until 1966. The SMB from older IRHs is less-well constrained, but the asymmetry has likely persisted for >ka, indicating that DIR has been a stable features over long time spans.

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Constraining variable density of ice shelves using wide-angle radar measurements

Cited by 17 publications

References 54 publications

Detailed response to the reviewes, modifications made on manuscript tc-2017-41 and new supplementary

Detailed response to the reviewes, modifications made on manuscript tc-2017-41 and new supplementary

Detecting high spatial variability of ice shelf basal mass balance, Roi Baudouin Ice Shelf, Antarctica

Temporally stable surface mass balance asymmetry across an ice rise derived from radar internal reflection horizons through inverse modeling

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