Browsing by Author "Dow, Christine F."

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A multi-resolution approach to point cloud registration without control points
(MDPI, 2023-02-20) Bash, Eleanor A.; Wecker, Lakin; Rahman, Mir Mustafizur; Dow, Christine F.; McDermid, Greg; Samavati, Faramarz F.; Whitehead, Ken; Moorman, Brian J.; Medrzycka, Dorota; Copland, Luke
Terrestrial photographic imagery combined with structure-from-motion (SfM) provides a relatively easy-to-implement method for monitoring environmental systems, even in remote and rough terrain. However, the collection of in-situ positioning data and the identification of control points required for georeferencing in SfM processing is the primary roadblock to using SfM in difficult-to-access locations; it is also the primary bottleneck for using SfM in a time series. We describe a novel, computationally efficient, and semi-automated approach for georeferencing unreferenced point clouds (UPC) derived from terrestrial overlapping photos to a reference dataset (e.g., DEM or aerial point cloud; hereafter RPC) in order to address this problem. The approach utilizes a Discrete Global Grid System (DGGS), which allows us to capitalize on easily collected rough information about camera deployment to coarsely register the UPC using the RPC. The DGGS also provides a hierarchical set of grids which supports a hierarchical modified iterative closest point algorithm with natural correspondence between the UPC and RPC. The approach requires minimal interaction in a user-friendly interface, while allowing for user adjustment of parameters and inspection of results. We illustrate the approach with two case studies: a close-range (<1 km) vertical glacier calving front reconstructed from two cameras at Fountain Glacier, Nunavut and a long-range (>3 km) scene of relatively flat glacier ice reconstructed from four cameras overlooking Nàłùdäy (Lowell Glacier), Yukon, Canada. We assessed the accuracy of the georeferencing by comparing the UPC to the RPC, as well as surveyed control points; the consistency of the registration was assessed using the difference between successive registered surfaces in the time series. The accuracy of the registration is roughly equal to the ground sampling distance and is consistent across time steps. These results demonstrate the promise of the approach for easy-to-implement georeferencing of point clouds from terrestrial imagery with acceptable accuracy, opening the door for new possibilities in remote monitoring for change-detection, such as monitoring calving rates, glacier surges, or other seasonal changes at remote field locations.
Antarctic basal environment shaped by high-pressure flow through a subglacial river system
(Springer Nature, 2022-10-27) Dow, Christine F.; Ross, Neil; Jeofry, Muhammad Hafeez; Siu, Kevin; Siegert, Martin J.
The stability of ice sheets and their contributions to sea level are modulated by high-pressure water that lubricates the base of the ice, facilitating rapid flow into the ocean. In Antarctica, subglacial processes are poorly characterized, limiting understanding of ice-sheet flow and its sensitivity to climate forcing. Here, using numerical modelling and geophysical data, we provide evidence of extensive, up to 460 km long, dendritically organized subglacial hydrological systems that stretch from the ice-sheet interior to the grounded margin. We show that these channels transport large fluxes (~24 m3 s−1) of freshwater at high pressure, potentially facilitating enhanced ice flow above. The water exits the ice sheet at specific locations, appearing to drive ice-shelf melting in these areas critical for ice-sheet stability. Changes in subglacial channel size can affect the water depth and pressure of the surrounding drainage system up to 100 km either side of the primary channel. Our results demonstrate the importance of incorporating catchment-scale basal hydrology in calculations of ice-sheet flow and in assessments of ice-shelf melt at grounding zones. Thus, understanding how marginal regions of Antarctica operate, and may change in the future, requires knowledge of processes acting within, and initiating from, the ice-sheet interior.
Basal channels drive active surface hydrology and transverse ice shelf fracture
(Science, 2018-06-13) Dow, Christine F.; Lee, Won Sang; Greenbaum, Jamin S.; Greene, Chad A.; Blankenship, Donald D.; Poinar, Kristin; Forrest, Alexander L.; Young, Duncan A.; Zappa, Christopher J.
Ice shelves control sea-level rise through frictional resistance, which slows the seaward flow of grounded glacial ice. Evidence from around Antarctica indicates that ice shelves are thinning and weakening, primarily driven by warm ocean water entering into the shelf cavities. We have identified a mechanism for ice shelf destabilization where basal channels underneath the shelves cause ice thinning that drives fracture perpendicular to flow. These channels also result in ice surface deformation, which diverts supraglacial rivers into the transverse fractures. We report direct evidence that a major 2016 calving event at Nansen Ice Shelf in the Ross Sea was the result of fracture driven by such channelized thinning and demonstrate that similar basal channel-driven transverse fractures occur elsewhere in Greenland and Antarctica. In the event of increased basal and surface melt resulting from rising ocean and air temperatures, ice shelves will become increasingly vulnerable to these tandem effects of basal channel destabilitization.
Characterizing subglacial hydrology within the Amery Ice Shelf catchment using numerical modeling and satellite altimetry
(Wiley, 2024-04-20) Wearing, Martin G.; Dow, Christine F.; Goldberg, Daniel N.; Gourmelen, Noel; Hogg, Anna E.; Jakob, Livia
Meltwater forms at the base of the Antarctic Ice Sheet due to geothermal heat flux (GHF) and basal frictional dissipation. Despite the relatively small volume, this water has a profound effect on ice-sheet dynamics. However, subglacial melting and hydrology in Antarctica remain highly uncertain, limiting our ability to assess their impact on ice-sheet dynamics. Here we examine subglacial hydrology within the Amery Ice Shelf catchment, East Antarctica, using the subglacial hydrology model GlaDS. We calculate subglacial melt rates using a higher-order ice-flow model and two GHF estimates. We find a catchment-wide melt rate of 7.03 Gt year−1 (standard deviation = 1.94 Gt year−1), which is ≥50% greater than previous estimates. The contribution from basal dissipation is approximately 40% of that from GHF. However, beneath fast-flowing ice streams, basal dissipation is an order of magnitude larger than GHF, leading to a significant increase in channelized subglacial flux upstream of the grounding line. We validate GlaDS using high-resolution interferometric-swath radar altimetry, with which we detect active subglacial lakes and fine-scale ice-shelf basal melting. We find a network of subglacial channels that connects areas of deep subglacial water coincident with active subglacial lakes, and channelized discharge at the grounding line coinciding with enhanced ice-shelf basal melting. The concentrated discharge of meltwater provides 36% of the freshwater released into the ice-shelf cavity, in addition to ice-shelf basal melting. This suggests that ice-shelf basal melting is strongly influenced by subglacial hydrology and could be affected by future changes in subglacial discharge, such as lake drainage or channel rerouting.
Migration of the shear margins at Thwaites Glacier: Dependence on basal conditions and testablities against field data.
(AGU, 2023-03-07) Summers, Paul T.; Elsworth, Cooper W.; Dow, Christine F.; Suckale, Jenny
Projections of global sea level depend sensitively on whether Thwaites Glacier, Antarctica, will continue to lose ice rapidly. Prior studies have focused primarily on understanding the evolution of ice velocity and whether the reverse-sloping bed at Thwaites Glacier could drive irreversible retreat. However, the overall ice flux to the ocean and the possibility of irreversible retreat depend not only on the ice speed but also on the width of the main ice trunk. Here, we complement prior work by focusing specifically on understanding whether the lateral boundaries of the main ice trunk, termed shear margins, might migrate over time. We hypothesize that the shear margins at Thwaites Glacier will migrate on a decadal timescale in response to continued ice thinning and surface steepening. We test this hypothesis by developing a depth-averaged, thermomechanical free-boundary model that captures the complex topography underneath the glacier and solves for both the ice velocity and for the position of the shear margins. We find that both shear margins are prone to migration in response to ice thinning with basal strength and surface slope steepening determining their relative motion. We construct four end-member cases of basal strength that represent different physical properties governing friction at the glacier bed and present two cases of ice thinning to contrast the effects of surface steepening and ice thinning. We test our model by hindcasting historic data and discuss how data from ongoing field campaigns could further be used to test our model.
Modeling the Deformation Regime of Thwaites Glacier, West Antarctica, Using a Simple Flow Reaction for Ice Anisotropy (ESTAR)
(Wiley; American Geophysical Union (AGU), 2022-02-21) McCormack, Felicity S.; Warner, Roland C.; Seroussi, Helene; Dow, Christine F.; Roberts, Jason L.; Treverrow, Adam
ce deformation dominates the evolution of ice shelf flow and the slow-moving regions in the interior of ice sheets. However, deformation may be poorly represented in large-scale ice sheet models that use the Glen flow relation, due to its questionable applicability to the steady-state flow of anisotropic ice that prevails in ice sheets, having been derived from secondary creep rates of isotropic ice. We assess the deformation regimes of Thwaites Glacier, West Antarctica, using the Glen and “Empirical Scalar Tertiary Anisotropy Regime”, (ESTAR) flow relations, the latter being derived from steady-state deformation rates of anisotropic ice. For grounded ice, the character of the flow relation determines the contribution of deformation to overall flow, with ESTAR producing greater bed-parallel shear deformation than the standard Glen flow relation. The ESTAR experiments show larger basal shear stress maxima than the standard Glen experiment because ESTAR treats the responses to simple shear stresses and compression stresses differently, reducing the role of lateral and longitudinal stresses in momentum balance. On the Thwaites Glacier Tongue, ESTAR provides the best match to observed speeds by accounting for the differing effects of stresses on ice flow. Our results highlight the importance of the numerical description of anisotropy, particularly: In regions of transition from deformation-dominated to sliding-dominated flow; in the approach to the grounding line, and across ice shelves. Given the importance of these locations in determining mass flux into the ocean, our results have implications for projections of sea level change from Antarctic ice loss.
Observations of submesoscale eddy-driven heat transport at an ice shelf calving front
(Springer Nature, 2022-06-22) Friedrichs, Drew M.; McInerney, Jasmin B. T.; Oldroyd, Holly J.; Lee, Won Sang; Yun, Sukyoung; Yoon, Seung-Tae; Stevens, Craig L.; Zappa, Christopher J.; Dow, Christine F.; Mueller, Derek; Steiner, Oscar Sepulveda; Forrest, Alexander L.
Antarctica’s ice shelves buttress the continent’s terrestrial ice, helping slow the loss of grounded ice into the ocean and limiting sea level rise. Ice-ocean interaction plays a critical role in ice shelf stability by driving basal melt rates. Consequently, improved prediction of the future state of ice shelves lies in understanding the coastal ocean mechanics that deliver heat to their cavities. Here, we present autonomous glider-based observations of a coherent structure at the calving front of a cold-water cavity ice shelf (Nansen Ice Shelf, East Antarctica). This ~10 km-wide eddy dominated the local ocean circulation in the austral summer of 2018/2019, promoting an upwelling of cold ice shelf water and a deepening of warm surface water. Microstructure turbulence measurements show a resulting maximum vertical heat transport of 10 W m−2 at depths equivalent to the ice shelf draft. Similar eddy-driven heat transport further into the ice shelf cavity would support enhanced summertime melt in regions of shallower ice draft.
Seismic evidence for complex sedimentary control of Greenland Ice Sheet flow
(Science, 2021-08-16) Kulessa, Bernd; Hubbard, Alun L.; Booth, Adam D.; Bougamont, Marion; Dow, Christine F.; Doyle, Samuel H.; Christoffersen, Poul; Lindback, Katrin; Pettersson, Rickard; Fitzpatrick, Andrew A. W.; Jones, Glenn A.
The land-terminating margin of the Greenland Ice Sheet has slowed down in recent decades, although the causes and implications for future ice flow are unclear. Explained originally by a self-regulating mechanism where basal slip reduces as drainage evolves from low to high efficiency, recent numerical modeling invokes a sedimentary control of ice sheet flow as an alternative hypothesis. Although both hypotheses can explain the recent slowdown, their respective forecasts of a long-term deceleration versus an acceleration of ice flow are contradictory. We present amplitude-versus-angle seismic data as the first observational test of the alternative hypothesis. We document transient modifications of basal sediment strengths by rapid subglacial drainages of supraglacial lakes, the primary current control on summer ice sheet flow according to our numerical model. Our observations agree with simulations of initial postdrainage sediment weakening and ice flow accelerations, and subsequent sediment restrengthening and ice flow decelerations, and thus confirm the alternative hypothesis. Although simulated melt season acceleration of ice flow due to weakening of subglacial sediments does not currently outweigh winter slowdown forced by self-regulation, they could dominate over the longer term. Subglacial sediments beneath the Greenland Ice Sheet must therefore be mapped and characterized, and a sedimentary control of ice flow must be evaluated against competing self-regulation mechanisms.
Subglacial discharge accelerates future retreat of Denman and Scott Glaciers, East Antarctica
(American Association for the Advancement of Science, 2023-10-27) Pelle, Tyler; Greenbaum, Jamin S.; Dow, Christine F.; Jenkins, Adrian; Morlighem, Mathieu
Ice shelf basal melting is the primary mechanism driving mass loss from the Antarctic Ice Sheet, yet it is unknown how the localized melt enhancement from subglacial discharge will affect future Antarctic glacial retreat. We develop a parameterization of ice shelf basal melt that accounts for both ocean and subglacial discharge forcing and apply it in future projections of Denman and Scott Glaciers, East Antarctica, through 2300. In forward simulations, subglacial discharge accelerates the onset of retreat of these systems into the deepest continental trench on Earth by 25 years. During this retreat, Denman Glacier alone contributes 0.33 millimeters per year to global sea level rise, comparable to half of the contemporary sea level contribution of the entire Antarctic Ice Sheet. Our results stress the importance of resolving complex interactions between the ice, ocean, and subglacial environments in future Antarctic Ice Sheet projections.
Totten Glacier subglacial hydrology determined from geophysics and modeling
(Elsevier, 2020-02-01) Dow, Christine F.; McCormack, Felicity S.; Young, Duncan A.; Greenbaum, Jamin S.; Roberts, Jason L.; Blankenship, Donald D.
Aurora Subglacial Basin (ASB), which feeds Totten Glacier, is a marine basin lying below sea level and contains up to 3.5 m of global sea level equivalent. Rates of future sea level rise from this area are primarily dependent on the stability of Totten Ice Shelf and the controls on ice flow dynamics upstream of the grounding line, both of which may be influenced by subglacial hydrology. We apply the GlaDS subglacial hydrology model to ASB to examine whether the spatial patterns of distributed and efficient drainage systems impact the dynamics of Totten Glacier. We determine the most appropriate model configuration from our series of sensitivity tests by comparing the modeled basal water pressure and water depth results with specularity content data. Those data are derived from ICECAP radar surveys over the same region and represent regions of basal water accumulation. The best match between simulated basal hydrology properties and specularity content shows a strong correspondence in regions of distributed water in the ASB troughs for both water depth and water pressure, but weak correspondence between water depth and specularity content near the grounding line. This may be due to the presence of several large channels draining over the grounding line into the head of Totten Ice Shelf, which are likely not as well represented in the specularity content data as distributed systems. These channels may have a significant impact on melt, and therefore the stability, of Totten Ice Shelf. Within ASB, regions of high water pressure and greater water accumulation correspond well with regions of faster ice flow, suggesting some control of basal hydrology on ice dynamics in this region.