Browsing by Author "Dow, Christine"

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Examining the consistency between subglacial hydrology and basal friction inversion modeling for Slessor Glacier, East Antarctica.
(University of Waterloo, 2022-08-31) Ruth, Dylan; Dow, Christine
In light of rising global surface temperatures and sea-level rise, it is more important now than ever to understand what role the cryosphere will play in the Earth's evolution. The Antarctic ice sheet contains enough ice to raise the global sea-level by approximately 58 meters and for this reason alone, it is essential for scientists to be able to predict how the ice masses within will behave in the future. One way to study the future behaviour of glaciers and ice sheets is by applying geospatial data to mathematical models based on the relevant physics. In this thesis, the Glacier Drainage System model (GlaDS) and the Ice-sheet and Sea-level System Model (ISSM) are used to model subglacial hydrology and ice dynamics, respectively, for Slessor Glacier, East Antarctica. First, an in-depth description of the necessary physics and numerical framework governing the two models is presented. With the framework in place, a sensitivity test comprised of 48 transient runs is performed with the GlaDS model, and 13 inversion simulations are performed with ISSM. The sensitivity test consists of altering several poorly constrained parameters to understand their impact on the modeled hydrological network beneath Slessor Glacier. The results from the sensitivity test are then used to determine which model configuration is most appropriate based on the current understanding of subglacial networks beneath the Antarctic Ice Sheet, however, this is a limited method of model validation in the absence of observed data such as specularity content (data derived from geophysical radar surveys to determine locations of distributed subglacial water). To mediate this issue, the model outputs from the inversion simulations, observed ice sheet melt rates, and a hydrostatic inversion of high resolution surface data are used to validate the model outputs. This is followed by a suggested workflow that can be adopted by modelers to use inverse methods to validate subglacial hydrology model outputs. The model outputs from this study suggest an active subglacial hydrological network beneath Slessor Glacier and the surrounding area. There is good agreement between areas of fast observed ice flow and areas where the model predicts deep water, low effective pressure and an efficient drainage network. These results are consistent with areas of inferred low basal friction coefficient from the ice dynamics model, which also recovers observed velocities from a stress balance simulation. The results of this thesis demonstrate some control of basal hydrology on ice dynamics in the Slessor Glacier study area. Furthermore, the methods used here provide subglacial hydrology modelers an additional means of model validation, which is valuable where observed data is sparse or not available.
Ice/hydrology feedback in the Siple Coast, Antarctica, from two-way coupled modeling
(University of Waterloo, 2024-06-18) McArthur, Koi; Dow, Christine
Subglacial hydrological processes have long been understood to play a critical role in ice dynamics (Budd et al., 1979). Consequently, the recent emergence of complex two-dimensional subglacial hydrology models with both inefficient and efficient drainage components has led to two-way coupling of these complex hydrology models to ice flow models (Cook et al., 2022). Such two-way coupled models bring about questions regarding our implementation of friction in ice flow models and allow us to examine feedback mechanisms between the subglacial hydrological system and the ice sheet. This thesis investigates feedback mechanisms between the subglacial hydrological system and the ice sheet, and analyzes our current implementation of friction in ice flow models. This is accomplished through subglacial hydrology, ice flow, and coupled modeling of the Siple Coast of West Antarctica, which has a history of observable hydrology/ice flow feedback. We use the Glacier Drainage System (Werder et al., 2013, GlaDS) model as a subglacial hydrology model, and the Shallow Shelf Approximation (Larour et al., 2012, SSA) with a mass transport model as an ice flow model, both of which are implemented in the Ice-Sheet and Sea-Level Systems Model (Larour et al., 2012, ISSM). We model the steady state subglacial hydrology of the Ross Sea subglacial hydrologic catchment, along with ice flow and two-way coupled ice flow/hydrology from 2010-2100 using an SSP585 surface mass balance forcing scenario. We test three different friction laws – the Budd friction law, the Schoof friction law, and a version of the Schoof friction law that we modify to ensure the sliding regime is representative of the cavitation at the glacier bed. Additionally, we test coupling with variable melt from frictional heating of ice, coupling with subglacial lake geometry altering glacier driving stress, and coupling with a combination of the two. The effective pressure and the modeled sliding regime were found to be largely responsible for the evolution of fast flowing regions of the domain, highlighting the importance of two-way coupled models, which have a cavitation-dependent sliding regime. Feedback mechanisms between the subglacial hydrologic system and the ice sheet were identified, including a negative feedback mechanism that stabilized the basal shear stress and the effective pressure fields when variable melt was available to the subglacial hydrologic system. The inclusion of subglacial lake geometry on the glacier driving stress was found to have a large control on lake depth, with the potential for large speedup events corresponding to the fast filling of subglacial lakes. When all coupling components were active, a negative feedback mechanism between subglacial lake depth, glacier driving stress, and melt water production, which stabilized subglacial lake depth and ice motion was observed. The methods developed in this thesis and the limitations that we discovered for implementing subglacial processes in ice flow models will be highly valuable to the glaciological modeling community moving forwards.
Mathematical modelling of supraglacial meltwater production and drainage
(University of Waterloo, 2021-08-30) Hill, Tim; Dow, Christine
Mountain glaciers and the polar ice sheets exert a critical control on water resource availability, drive sea level change, and impact global ocean circulation. These and other impacts are controlled by surface meltwater that flows through the glacier hydrologic system to the base of the ice and drives seasonal and long-term changes in ice flow velocity. This thesis presents numerical models for the production and transport of meltwater runoff across the surface of melting glaciers and ice sheet. First, a surface energy balance model is developed that improves on existing models by utilizing high resolution satellite data to capture spatial variations in surface melt. The model is applied to Kaskawulsh Glacier and Nàłùdäy (Lowell) Glacier in the St. Elias Mountains, Yukon, Canada using six years of in-situ meteorological data. By validating model outputs against in-situ measurements, it is shown that modelled seasonal melt agrees with observations within 9% across a range of elevations. In order to determine how surface meltwater is transported through moulins, we develop the Subaerial Drainage System (SaDS) model. SaDS is a physics-based, finite-volume numerical model that calculates supraglacial runoff in both a distributed sheet and through supraglacial channels. The benefit of this approach is that a connected network of supraglacial channels and lakes naturally emerges without using prior information about the channel network, for example from satellite-derived maps. In synthetic settings and when applied to the Greenland Ice Sheet, model outputs show realistic and varied moulin flux rates, and modelled supraglacial lake and channel locations match those mapped from satellite images. These results demonstrate that SaDS is a promising tool to provide moulin inputs for subglacial and ice dynamic studies. These models represent significant steps forward in their respective domains. Together, these tools will be valuable components of future modelling work, including for studies that aim to constrain how climatic variables control sea level contributions from glaciers and ice sheets.
Modelling Subglacial Hydrology under Future Climate Scenarios in Wilkes Subglacial Basin, Antarctica
(University of Waterloo, 2022-10-21) Siu, Kevin; Dow, Christine
The Greenland and Antarctic ice sheets have differing climates, which makes surface melt a significant hydrological source in Greenland but not currently in Antarctica. Due to a changing climate and warming air temperatures, Antarctica is predicted to experience more surface meltwater in the future. This will likely lead to surface features common in Greenland today, such as supraglacial lakes and moulins, to also form over grounded ice in Antarctica. Moulins in particular are important because they will route this surface melt into basal drainage networks. The resulting change in subglacial drainage characteristics and water volumes will potentially have far-reaching impacts on ice dynamics, ice shelf melt, grounding line stability, and ultimately global sea level rise. To examine this, we model the hydrological system in Wilkes Subglacial Basin, East Antarctica using estimations of the future climate to incorporate moulins and surface melt. We use predictive data generated by the Community Climate System Model 4 (CCSM4) for surface runoff in Antarctica for the year 2100 as inputs to the Glacier Drainage System (GlaDS) subglacial hydrology model. We compare the modelling results from two different Representative Concentration Pathway (RCP) scenarios, RCP 2.6 and RCP 8.5. Moulin locations are predicted using current strain rates along preferential surface hydrology flow pathways and we also compare modelling results with different numbers and locations of moulins. We find that an increase in surface water input from none to RCP 2.6 to RCP 8.5 has a larger impact on basal drainage rates, channel extent, and water pressure near the grounding line. However, compared to increasing surface water inputs, we also find that increasing the number and extent of moulins can have an even larger impact on the subglacial hydrology system. This shows that both moulin formation and the evolution of the climate will play a role in the development of the subglacial hydrology system, which will be important for future ice flow speeds and ice shelf melt rates near the grounding line.
Modelling the Subglacial Hydrology of Trinity-Wykeham Glaciers of the Northern Canadian Arctic
(University of Waterloo, 2023-11-09) Willette, Michael; Dow, Christine
In Canada’s High Arctic region, the melting of glacial ice contributes substantially to the world’s increasing sea levels (Harig and Simons, 2016). Of the icebergs in the region, approximately 62% are discharged by Trinity-Wykeham Glaciers. In the last twenty years, these glaciers have retreated approximately 5 km, also doubling in speed and tripling in iceberg production during that period (Van Wychen et al., 2016). It is argued that warming air temperatures are the primary cause of glacial retreat and consequently sea level rise in the Canadian High Arctic region (Cook et al., 2019). This is because the warmer air causes ice on the surface of glaciers to melt at a higher rate. This meltwater is routed to the base of the ice, where it can directly impact the glacier’s velocity. If the water spreads into high-pressure cavities over a large enough area, the ice can often accelerate. However, if the water input is great enough to form large, low-pressure channels at the base of the glacier, water will be drawn from the higher-pressure regions and the ice will often decelerate (Iken and Bindschadler, 1986). The Glacier Drainage System (GlaDS) subglacial hydrology model (Werder et al., 2013) is used to examine the development of hydrological networks at the base of Trinity-Wykeham Glaciers in response to variable surface melt rates between 2016 and 2019. GlaDS couples distributed and channelized subglacial drainage, allowing the mathematical model to capture the spatiotemporal evolution of the subglacial drainage system. The interplay between these two modes of drainage is highly influential on glacier dynamics. Satellite-derived datasets are used as inputs to the model, including basal and surface topography, surface velocities, and daily ice surface runoff products. Model outputs including subglacial water sheet thickness, water pressure, and channel discharge are compared to satellite-derived glacier surface velocities to determine how subglacial hydrology affects Trinity-Wykeham Glaciers. Nine model runs are completed to gauge the sensitivity of the GlaDS model to variations in two poorly constrained parameters that control the ease of subglacial water flow and determine a practical range of values for these parameters in future studies. The results of this project suggest that Trinity-Wykeham Glaciers’ velocities are directly influenced by surface melting rates and the configuration of the subglacial hydrology networks. Model outputs indicate that high subglacial water pressures cause acceleration at Trinity-Wykeham Glaciers and that with a sufficiently high influx of water to the bed, an efficient channelized drainage network develops that reduces local water pressures and causes a drop in flow velocity. The year 2018 is identified as a year in which channelized drainage is minimal, resulting in comparatively high water pressures and velocities after the melt season.
Radarsat constellation mission derived winter glacier velocities for the St. Elias Icefield, Yukon/Alaska: 2022 and 2023
(Taylor & Francis, 2023-10-10) Van Wychen, W.; Bayer, Courtney; Copland, Luke; Brummel, Erika; Dow, Christine
Here we use high resolution (5 m) Radarsat Constellation Mission (RCM) imagery acquired in winters 2022 and 2023 to determine motion across glaciers of the St. Elias Icefield in Yukon/Alaska. Our regional velocity mapping largely conforms with previous studies, with faster motion (>600 m/yr) for the glaciers originating in the Yukon that drain southward and westward to the coast of Alaska and relatively slower motion (100–400 m/yr) for the land terminating glaciers that drain eastward and northeastward and stay within the Yukon. We also identify two new glacier surges within the icefields: the surge of Nàłùdäy (Lowell) Glacier in Winter 2022, and Chitina Glacier in Winter 2023, and track the progression of each surge from January to March utilizing ∼4-day repeat RCM imagery. To evaluate the quality of RCM-derived velocities, we compare our results with 50 simultaneous measurements at three on-ice dGPS stations located on two Yukon glaciers and find the average absolute difference between measurements to be 6.6 m/yr. Our results demonstrate the utility of RCM data to determine glacier motion across large regions with complex topography, to support process-based studies of fast flowing and surge-type glaciers and continue the legacy of velocity products derived from the Radarsat-2 mission.
A Spatial Analysis of the Nansen Ice Shelf Basal Channel, Using Ice Penetrating Radar
(University of Waterloo, 2019-01-21) Wray, Peter; Dow, Christine
The stability of floating ice shelves in the Antarctic is of great concern as their current thinning and future collapse will release mass from grounded ice sheets into the ocean, contributing to sea level rise. The study of sub-ice shelf environments is essential for understanding ice-ocean interactions, where warming ocean temperatures have already begun to reduce the mass of Antarctic ice shelves through enhanced melt. Obtaining direct measurements of the sub-ice shelf cavity remains challenging. This thesis demonstrates that ground-based and airborne Ice Penetrating Radar (IPR) can deliver high resolution geospatial data of sub-ice shelf features, which can be used to observe: ice draft and surface morphology, vertical melt rates, and hydrostatic balance of the ice shelf. In November 2016 and January/February 2017, IPR surveys were completed over the Nansen Ice Shelf in Terra Nova Bay, Antarctica. Surveys examined an ocean-sourced basal channel incised into the bottom of the ice shelf. Results reveal a 6 - 10 km wide, 90 m high basal channel, with 20 m high sub-channels. Data from 2011 and 2014 airborne IPR surveys are compared to the November 2016 ground based IPR to calculate a dozen direct measurements of vertical melt rate values within the channel. Melt was focused more on one channel flank, away from the channel center in local apexes. Many of the detected features were not in hydrostatic equilibrium as calculated from surface elevations, indicating the need for more radar determined ice thickness measurements to fully characterize basal channel morphology and monitor future melt.