45. Anderson, LS; Geirsdottir, A; Flowers, GE; Wickert, AD; Adalgeirsdottir, G; Thorsteinsson, T.Controls on the lifespans of Icelandic ice caps.Earth Planet. Sci. Lett., 2019, 527 Controls on the lifespans of Icelandic ice caps
glacier; inception; ELA; Holocene; climate; Neoglaciation
The increasingly common disappearance of glaciers is striking, and it is often used to highlight the effects of anthropogenic climate change to the public. This approach is more effective when modern glacier and climate change is placed within a frame of reference. Here we present a new, efficient method to assess the timing of glacier birth and death. We model equilibrium line altitudes (ELAs) of 22 Icelandic glaciers through time, and in doing so produce the first regional estimates of glacier lifespans. We force the model with three Holocene temperature reconstructions and five future-climate projections, while also exploring the effects of glacial-isostatic adjustment and variable precipitation. Mass balance parameters are tuned for each ice cap using modern data. Geologically-constrained inception ages are used to validate individual simulations. The timing of Icelandic glacier inception likely ranged from before the Holocene to as late as the Little Ice Age. Anthropogenic warming is expected to cut short the lifespans of glaciers that have existed for thousands of years. By quantifying the controlling factors we show that topography and the rate of summer temperature change are the primary controls of glacier lifespans in Iceland. Topography, represented by the difference between the modern ELA and the highest local topography, is ten-times as important as ELA sensitivity to temperature. This easy-to-measure topographic metric exerts a first-order control on the timing of inception and the future loss of accumulation area and it can be used to contextualize ongoing glacier disappearance. (C) 2019 Elsevier B.V. All rights reserved. DOI
43. Pulwicki, A; Flowers, GE; Bingham, D.Pursuit of Optimal Design for Winter-Balance Surveys of Valley-Glacier Ablation Areas.Front. Earth Sci., 2019, 7 Pursuit of Optimal Design for Winter-Balance Surveys of Valley-Glacier Ablation Areas
glacier mass balance; winter balance; experimental design; snow survey; mountain glaciers; St. Elias Mountains; Yukon (Canada)
Efficient collection of snow depth and density data is important in field surveys used to estimate the winter surface mass balance of glaciers. Simultaneously extensive, high resolution, and accurate snow-depth measurements can be difficult to obtain, so optimisation of measurement configuration and spacing is valuable in any survey design. Using in-situ data from the ablation areas of three glaciers in the St. Elias Mountains of Yukon, Canada, we consider six possible survey designs for snow-depth sampling and N = 6-200+ sampling locations per glacier. For each design and number of sampling locations, we use a linear regression on topographic parameters to estimate winter balance at unsampled locations and compare these estimates with known values. Average errors decrease sharply with increasing sample size up to N approximate to 10-15, but reliable error reduction for any given sampling scheme requires significantly higher N. Lower errors are often, but not always, associated with sampling schemes that employ quasi-regular spacing. With both real- and synthetic data, the common centreline survey produces the poorest results overall. The optimal design often requires sampling near the glacier margin, even at low N. The unconventional "hourglass" design performed best of all designs tested when evaluated against known values of winter balance. DOI
42. Anderson, LS; Flowers, GE; Jarosch, AH; Adalgeirsdottir, GT; Geirsdottir, A; Miller, GH; Harning, DJ; Thorsteinsson, T; Magnusson, E; Palsson, F.Holocene glacier and climate variations in Vestfiroir, Iceland, from the modeling of Drangajokull ice cap.Quat. Sci. Rev., 2018, 190: 39-56 Holocene glacier and climate variations in Vestfiroir, Iceland, from the modeling of Drangajokull ice cap
Holocene; Glaciation; Glaciology; North Atlantic; Geomorphology; Glacial inception; Modelling; Little ice age; Holocene Thermal Maximum; Deglaciation
Drangajokull is a maritime ice cap located in northwest (Vestfiroir) Iceland. Drangajokull's evolution is therefore closely linked to atmospheric and ocean variability. In order to better constrain the Holocene climate and glacier history of Vestfiroir we model the past evolution of Drangajokull ice cap. Simulations from 10 ka to present are forced by general circulation model output, ice-core-based temperature reconstructions, and sea-surface temperature reconstructions. Based on these 10-thousand year simulations, Drangajokull did not persist through the Holocene. We estimate that air temperatures were 2.5-3.0 degrees C higher during the Holocene Thermal Maximum than the local 1960-1990 average. Simulations support Drangajokull's late Holocene inception between 2 and 1 ka, though intermittent ice likely occupied cirques as early as 2.6 ka. Drangajokull is primarily a Little Ice Age ice cap: it expanded between 1300 and 1750 CE, with the most rapid growth occurring between 1600 and 1750 CE. The maximum Holocene extent of Drangajokull occurred between 1700 and 1925 CE, despite the lowest late Holocene temperatures, occurring between 1650 and 1720 CE. Between 1700 and 1925 CE temperatures were likely 0.6-0.8 degrees C lower than the 1950-2015 reference temperature. The modern equilibrium line altitude (ELA) is bracketed by topographic thresholds: a 1 degrees C temperature increase from the modern ELA would eliminate the ice cap's accumulation area, while a reduction of 0.5 degrees C would lead to the rapid expansion of the ice cap across Vestfiroir. The proximity of Drangajokull to topographic thresholds may explain its late inception and rapid expansion during the Little Ice Age. (C) 2018 Elsevier Ltd. All rights reserved. DOI
41. Beaud, F; Flowers, GE; Venditti, JG.Modeling Sediment Transport in Ice-Walled Subglacial Channels and Its Implications for Esker Formation and Proglacial Sediment Yields.J. Geophys. Res.-Earth Surf., 2018, 123: 3206-3227 Modeling Sediment Transport in Ice-Walled Subglacial Channels and Its Implications for Esker Formation and Proglacial Sediment Yields
Sediment transport; Subglacial hydrology; Glacial erosion; Esker; Glacio-fluvial processes
Sediment yields from glacierized basins are used to quantify erosion rates on seasonal to decadal timescales as well as conditions at the glacier bed, and eskers hold valuable information about past subglacial hydraulic conditions in their spatial organization, geometry, and sedimentary structures. Ultimately, eskers are a record of past glacio-fluvial sediment transport, but there is currently no physical model for this process. We develop a 1-D model of morphodynamics in semicircular bedrock-floored subglacial channels. We adapt a sediment conservation law developed for mixed alluvial-bedrock conditions to subglacial channels. Channel evolution is a function of melt opening by viscous heat dissipation from flowing water and creep closure of the overlying ice, to which we add the closure or enlargement due to sediment deposition or removal, respectively. We apply the model to an idealized land-terminating glacier and find that temporary sediment accumulation in the vicinity of the terminus, or the formation of an incipient esker, is inherent to the dynamics of the channelized water flow. The alluviation of the bed combined with the pressurized channel flow produces unexpected patterns of sediment evacuation: We show that the direction of hysteresis between sediment and water discharge is not necessarily linked to a supply- or transport-limited system, as has been hypothesized for proglacial sediment yields. We also find that the deposition of an incipient esker is a function of a compromise between water discharge and sediment supply, but perhaps more importantly, ice-surface slope and the temporal pattern of water delivery to the bed. Plain Language Summary Glaciers and ice sheets are changing rapidly, impacting sea levels, landscapes, and ecosystems. These changes are tightly linked to the meltwater routing through glaciers' plumbing systems. If this plumbing is pressurized by water flowing into crevasses and moulin (which act like water wells), the ice base can move faster downstream, possibly leading to enhanced ice loss, or vice versa. As glaciers retreated at the end of the last glaciation, they left clues of their passage, including sediment casts of their plumbing system: eskers. Eskers are elongated ridges that snake across the landscape and can be hundreds of kilometers long. Although understanding their deposition can help us understand contemporary ice sheet plumbing systems, their origin has been puzzling for several decades. We build a numerical model tracking sediment as they move with the water under ice. Glaciers naturally produce a sediment bottleneck and tend to form such eskers, producing the first process-based model for their deposition. We identify ice geometry and temporal patterns of water input into the plumbing system as critical factors, when combined with sufficient sediment and water supplies. This model helps to reconcile contemporary glacier processes and sediment records, which is key to better understand glaciers' plumbing system. DOI
40. Beaud, F; Venditti, JG; Flowers, GE; Koppes, M.Excavation of subglacial bedrock channels by seasonal meltwater flow.Earth Surf. Process. Landf., 2018, 43: 1960-1972 Excavation of subglacial bedrock channels by seasonal meltwater flow
glacial erosion; subglacial hydrology; subglacial meltwater erosion; tunnel valley; inner gorge
Subglacial water flow drives the excavation of a variety of bedrock channels including tunnel valleys and inner gorges. Subglacial floods of various magnitudes - events occurring once per year or less frequently with discharges larger than a few hundred cubic metres per second - are often invoked to explain the erosive power of subglacial water flow. In this study we examine whether subglacial floods are necessary to carve bedrock channels, or if more frequent melt season events (e.g. daily production of meltwater) can explain the formation of substantial bedrock channels over a glacial cycle. We use a one-dimensional numerical model of bedrock erosion by subglacial meltwater, where water flows through interacting distributed and channelized drainage systems. The shear stresses produced drive bedrock erosion by bed- and suspended-load abrasion. We show that seasonal meltwater discharge can incise an incipient bedrock channel a few tens of centimetres deep and several metres wide, assuming abrasion is the only mechanism of erosion, a particle size of D=256 mm and a prescribed sediment supply per unit width. Using the same sediment characteristics, flood flows yield wider but significantly shallower bedrock channels than seasonal meltwater flows. Furthermore, the smaller the shear stresses produced by a flood, the deeper the bedrock channel. Shear stresses produced by seasonal meltwater are sufficient to readily transport boulders as bedload. Larger flows produce greater shear stresses and the sediment is carried in suspension, which produces fewer contacts with the bed and less erosion. We demonstrate that seasonal meltwater discharge can excavate bedrock volumes commensurate with channels several tens of metres to a few hundred metres wide and several tens of metres deep over several thousand years. Such simulated channels are commensurate with published observations of tunnel valleys and inner gorges. Copyright (c) 2018 John Wiley & Sons, Ltd. DOI
39. Crompton, JW; Flowers, GE; Stead, D.Bedrock Fracture Characteristics as a Possible Control on the Distribution of Surge-Type Glaciers.J. Geophys. Res.-Earth Surf., 2018, 123: 853-873 Bedrock Fracture Characteristics as a Possible Control on the Distribution of Surge-Type Glaciers
glacier surges; basal processes; bedrock fracture; St; Elias Mountains; glacier dynamics; discontinuity properties
Glacier surging has been studied extensively and is understood as a dynamic instability at the glacier bed. Yet an explanation for the heterogeneous distribution of surge-type glaciers at the scale of a mountain range remains elusive. Here we investigate bedrock discontinuity properties in the basins of 16 surge-type and nonsurge-type glaciers in the St. Elias Mountains of Yukon, Canada. Using scaled photographs of bedrock outcrops at the margins of each glacier, we digitize traces of the bedrock discontinuities and with automated purpose-built software, quantify discontinuity properties that are a function of length, orientation, and spacing of bedrock fractures. We obtain an unexpected result: outcrops in the basins of surge-type glaciers are less fractured than those in the basins of nonsurge-type glaciers. We hypothesize that the degree of bedrock fracture may control the extent and location of a clast-rich till transition zone at the glacier bed. This zone would provide flow resistance conducive to the development of an ice reservoir and thus to surging behavior. To reconcile our observations with the global distribution of surge-type glaciers, we speculate that surge-type glaciers may occur in geological settings characterized by an intermediate range of bedrock fracture. Plain Language Summary Surging is a perplexing form of glacier flow in which ice moves slowly for decades and then suddenly speeds up by a factor of 10 to 1,000. Surging gives us a window into some of the most dramatic forms of mass movement and sheds light on the general mechanics of glacier flow. Here we revisit an early hypothesis that glacier surging is related to fractures in the underlying bedrock. To test this hypothesis, we studied the bedrock characteristics around 16 glaciersnine of which are known to surgein the St. Elias Mountains of Yukon, Canada. We quantified the extent of bedrock fracture in each of the glacier basins from photographs, using software developed specifically for this project. Contrary to expectation, we find that surging glaciers in our study area inhabit basins with less fractured bedrock than the basins with nonsurging glaciers. To explain our results, we propose a hypothesis for surging that links bedrock fracture to the nature and distribution of rocky debris under the glacier, creating friction between the ice and bed. To reconcile our results with previous studies, we propose that surging glaciers worldwide may be found on bedrock that is not too fractured, but just fractured enough. DOI
38.Flowers, GE.Hydrology and the future of the Greenland Ice Sheet.Nat. Commun., 2018, 9 Hydrology and the future of the Greenland Ice Sheet
Detection, attribution and projection of mass loss from the Greenland Ice Sheet has been a central focus of the glaciological community, with surface meltwater thought to play a key role in feedbacks that could accelerate sea-level rise. While the prospect of runaway sliding has faded, much remains uncertain when it comes to the role of surface runoff and subglacial discharge in Greenland's future. DOI PubMed
37. Pulwicki, A; Flowers, GE; Radic, V; Bingham, D.Estimating winter balance and its uncertainty from direct measurements of snow depth and density on alpine glaciers.J. Glaciol., 2018, 64: 781-795 Estimating winter balance and its uncertainty from direct measurements of snow depth and density on alpine glaciers
winter balance; accumulation; glacier mass balance; snow; St. Elias Mountains; Yukon
Accurately estimating winter surface mass balance on glaciers is central to assessing glacier health and predicting glacier run-off. However, measuring and modelling snow distribution is inherently difficult in mountainous terrain. Here, we explore rigorous statistical methods of estimating winter balance and its uncertainty from multiscale measurements of snow depth and density. In May 2016, we collected over 9000 manual measurements of snow depth across three glaciers in the St. Elias Mountains, Yukon, Canada. Linear regression, combined with cross-validation and Bayesian model averaging, as well as ordinary kriging are used to interpolate point-scale values to glacier-wide estimates of winter balance. Elevation and a wind-redistribution parameter exhibit the highest correlations with winter balance, but the relationship varies considerably between glaciers. A Monte Carlo analysis reveals that the interpolation itself introduces more uncertainty than the assignment of snow density or the representation of grid-scale variability. For our study glaciers, the winter balance uncertainty from all assessed sources ranges from 0.03 to 0.15 m w.e. (5-39%). Despite the challenges associated with estimating winter balance, our results are consistent with a regional-scale winter-balance gradient. DOI
36. Aso, N; Tsai, VC; Schoof, C; Flowers, GE; Whiteford, A; Rada, C.Seismologically Observed Spatiotemporal Drainage Activity at Moulins.J. Geophys. Res.-Solid Earth, 2017, 122: 9095-9108 Seismologically Observed Spatiotemporal Drainage Activity at Moulins
cryoseismology; ambient noise; backprojection; glacier hydrology; moulins
Hydrology is important for glacier dynamics, but it is difficult to monitor the subsurface drainage systems of glaciers by direct observations. Since meltwater drainage generates seismic signals, passive seismic analysis has the potential to be used to monitor these processes. To study continuous seismic radiation from the drainage, we analyze geophone data from six stations deployed at the Kaskawulsh Glacier in Yukon, Canada, during the summer of 2014 using ambient noise cross-correlation techniques. We locate the noise sources by backprojecting the amplitude of the cross correlation to the glacier surface. Most of the ambient noise sequences are found in two clusters, with each cluster located in the vicinity of a moulin identified at the surface. Stronger seismic radiation is observed during the day, consistent with expected variability in melt rates. We demonstrate that the sparse seismic network array with 2km station separation has the ability to detect moulins within the array with a precision of 50m. We confirm that seismic activity is correlated with air temperature, and thus, melt, on a diurnal timescale, and precipitation correlates with the activity at longer timescales. Our results highlight the potential of passive seismic observations for monitoring water flow into subglacial channels through moulins with an affordable number of seismic stations, but quantification of water flow rates still remains a challenge. The cross-correlation backprojection technique described here can also potentially be applied to any localized source of ambient noise such as ocean noise, tectonic tremor, and volcanic tremor. DOI
35. Gilbert, A; Flowers, GE; Miller, GH; Refsnider, KA; Young, NE; Radic, V.The projected demise of Barnes Ice Cap: Evidence of an unusually warm 21st century Arctic.Geophys. Res. Lett., 2017, 44: 2810-2816 The projected demise of Barnes Ice Cap: Evidence of an unusually warm 21st century Arctic
Barnes Ice Cap; Arctic climate; climate change; ice flow modeling; paleoglaciations
As a remnant of the Laurentide Ice Sheet, Barnes Ice Cap owes its existence and present form in part to the climate of the last glacial period. The ice cap has been sustained in the present interglacial climate by its own topography through the mass balance-elevation feedback. A coupled mass balance and ice-flow model, forced by Coupled Model Intercomparison Project Phase 5 climate model output, projects that the current ice cap will likely disappear in the next 300years. For greenhouse gas Representative Concentration Pathways of +2.6 to +8.5Wm(-2), the projected ice-cap survival times range from 150 to 530years. Measured concentrations of cosmogenic radionuclides Be-10, Al-26, and C-14 at sites exposed near the ice-cap margin suggest the pending disappearance of Barnes Ice Cap is very unusual in the last million years. The data and models together point to an exceptionally warm 21st century Arctic climate. DOI
34. Pimentel, S; Flowers, GE; Sharp, MJ; Danielson, B; Copland, L; Van Wychen, W; Duncan, A; Kavanaugh, JL.Modelling intra-annual dynamics of a major marine-terminating Arctic glacier.Ann. Glaciol., 2017, 58: 118-130 Modelling intra-annual dynamics of a major marine-terminating Arctic glacier
glacier hydrology; glacier modelling; ice dynamics; ice/ocean interactions
Significant intra-annual variability in flow rates of tidewater-terminating Arctic glaciers has been observed in recent years. These changes may result from oceanic and/or atmospheric forcing through (1) perturbations at the terminus, such as enhanced submarine melt and changes in sea-ice buttressing, or (2) increased surface melt, in response to atmospheric warming, reaching the bed and promoting glacier slip. We examine the influence of these processes on Belcher Glacier, a large fast-flowing tidewater outlet of the Devon Island ice cap in the Canadian Arctic. A hydrologically-coupled high-erorder ice flow model is used to estimate changes in glacier flow speed as a result of changes in seaice buttressing and hydrologically-driven melt-season dynamics. Daily run-off from five sub-catchments over the 2008 and 2009 melt seasons provides meltwater forcing for the model simulations. Model results are compared with remotely-sensed and in situ ice-surface velocity measurements. Sea-ice effects are found to have a minor influence on glacier flow speed relative to that of meltwater drainage, which is clearly implicated in short-term velocity variations during the melt season. We find that threshold drainage is essential in determining the timing of these short-lived accelerations. DOI
33. Pratola, MT; Harari, O; Bingham, D; Flowers, GE.RETRACTED: Design and Analysis of Experiments on Nonconvex Regions (Retracted article. See vol. 60, pg. 270, 2018).Technometrics, 2017, 59: 36-47 RETRACTED: Design and Analysis of Experiments on Nonconvex Regions (Retracted article. See vol. 60, pg. 270, 2018)
Gaussian Process; Geodesic distance; Glacier stake network; ISOMAP; Multidimensional scaling
Modeling a response over a nonconvex design region is a common problem in diverse areas such as engineering and geophysics. The tools available to model and design for such responses are limited and have received little attention. We propose a new method for selecting design points over nonconvex regions that is based on the application of multidimensional scaling to the geodesic distance. Optimal designs for prediction are described, with special emphasis on Gaussian process models, followed by a simulation study and an application in glaciology. Supplementary materials for this article are available online. DOI
32. Beaud, F; Flowers, GE; Venditti, JG.Efficacy of bedrock erosion by subglacial water flow.Earth Surf. Dyn., 2016, 4: 125-145 Efficacy of bedrock erosion by subglacial water flow
Bedrock erosion by sediment-bearing subglacial water remains little-studied; however, the process is thought to contribute to bedrock erosion rates in glaciated landscapes and is implicated in the excavation of tunnel valleys and the incision of inner gorges. We adapt physics-based models of fluvial abrasion to the subglacial environment, assembling the first model designed to quantify bedrock erosion caused by transient subglacial water flow. The subglacial drainage model consists of a one-dimensional network of cavities dynamically coupled to one or several Rothlisberger channels (R-channels). The bedrock erosion model is based on the tools and cover effect, whereby particles entrained by the flow impact exposed bedrock. We explore the dependency of glacial meltwater erosion on the structure and magnitude of water input to the system, the ice geometry, and the sediment supply. We find that erosion is not a function of water discharge alone, but also depends on channel size, water pressure, and sediment supply, as in fluvial systems. Modelled glacial meltwater erosion rates are 1 to 2 orders of magnitude lower than the expected rates of total glacial erosion required to produce the sediment supply rates we impose, suggesting that glacial meltwater erosion is negligible at the basin scale. Nevertheless, due to the extreme localization of glacial meltwater erosion (at the base of R-channels), this process can carve bedrock (Nye) channels. In fact, our simulations suggest that the incision of bedrock channels several centimetres deep and a few metres wide can occur in a single year. Modelled incision rates indicate that subglacial water flow can gradually carve a tunnel valley and enhance the relief or even initiate the carving of an inner gorge. DOI
31. Crompton, JW; Flowers, GE.Correlations of suspended sediment size with bedrock lithology and glacier dynamics.Ann. Glaciol., 2016, 57: 142-150 Correlations of suspended sediment size with bedrock lithology and glacier dynamics
glacier surges; subglacial processes; subglacial sediment; grain size distribution; St. Elias Mountains
The hypothesized link between glacier surging and bedrock geology motivates this study of the suspended sediment size distributions (SSSD) from surge-type and non-surge-type glaciers. We analyze SSSDs from proglacial streams in 20 individual basins comprising various fractions of metasedimentary (MS) and felsic plutonic rocks. We compare the size distributions by performing tests of significance on the distribution statistics, and a principal component analysis on discrete grain sizes. We find that surge-type and non-surge-type glaciers underlain solely by MS rocks have significantly different SSSDs, while surge-type glaciers as a whole have remarkably similar SSSDs, regardless of the underlying bedrock geology. These observations hint at a relationship between sediment characteristics and glacier surging, though causation in either direction cannot be established without additional data. DOI
30.Flowers, GE; Jarosch, AH; Belliveau, PTAP; Fuhrman, LA.Short-term velocity variations and sliding sensitivity of a slowly surging glacier.Ann. Glaciol., 2016, 57: 71-83 Short-term velocity variations and sliding sensitivity of a slowly surging glacier
glacier flow; glacier mechanics; glacier surges; mountain glaciers; subglacial processes
We use daily surface velocities measured over several weeks in 2007 and 2008 on a slowly surging glacier in Yukon, Canada, to examine the ordinary melt-season dynamics in the context of the ongoing surge. Horizontal velocities within and just below the similar to 1.5 km-long zone of fastest flow, where the surge is occurring, are often correlated during intervals of low melt. This correlation breaks down during melt events, with the lower reaches of the fast-flow zone responding first. Velocity variability in this lower reach is most highly correlated with melt; velocities above and below appear to respond at least as strongly to the velocity variations of this reach as to local melt. GPS height records are suggestive of ice/bed separation occurring in the fast-flow zone but not below it, pointing to a hydrological cause for the short-term flow variability in the surging region. Independent velocity measurements over 6 years show a maximum July flow anomaly coincident with the location most responsive to melt. Results from a simple model of dashpots and frictional elements lend support to the hypothesis that this zone partly drives the dynamics of the ice above and below it. We speculate that the slow surge may enhance glacier sensitivity to melt-season processes, including short-term summer sliding events. DOI
29. Gilbert, A; Flowers, GE; Miller, GH; Rabus, BT; Van Wychen, W; Gardner, AS; Copland, L.Sensitivity of Barnes Ice Cap, Baffin Island, Canada, to climate state and internal dynamics.J. Geophys. Res.-Earth Surf., 2016, 121: 1516-1539 Sensitivity of Barnes Ice Cap, Baffin Island, Canada, to climate state and internal dynamics
ice cap stability; climate; Canadian Arctic
Barnes Ice Cap is a remnant of the Laurentide Ice Sheet, which covered much of northern North America during the Last Glacial Maximum. Barnes reached a quasi-equilibrium state similar to 2000years ago and has remained similar in size since then, with a small increase during the Little Ice Age. In this study, we combine historical observations (1960-1980) with more recent satellite and airborne data (1995-2010) to drive a mass balance model coupled to a transient thermomechanical model with an adaptive mesh geometry. The model is used to characterize the current state of the ice cap and to investigate its stability as a function of climate and its own internal dynamics. On millennial time scales we show that ice flow is influenced by adjustment of an unsteady shape, by gently sloping bedrock, and by contrasting viscosities between the Pleistocene and Holocene ice. On shorter time scales, Barnes is affected by surge activity. Sensitivity tests reveal that Barnes experienced climate conditions which enabled its stability 2000 to 3000years ago but will disappear under current climate conditions in the next millennium. DOI
28. King, L; Hassan, MA; Yang, K; Flowers, G.Flow Routing for Delineating Supraglacial Meltwater Channel Networks.Remote Sens., 2016, 8 Flow Routing for Delineating Supraglacial Meltwater Channel Networks
supraglacial channels; flow routing; glacial hydrology; surface hydrology; meltwater; Greenland Ice Sheet
Growing interest in supraglacial channels, coupled with the increasing availability of high-resolution remotely sensed imagery of glacier surfaces, motivates the development and testing of new approaches to delineating surface meltwater channels. We utilized a high-resolution (2 m) digital elevation model of parts of the western margin of the Greenland Ice Sheet (GrIS) and retention of visually identified sinks (i.e., moulins) to investigate the ability of a standard D8 flow routing algorithm to delineate supraglacial channels. We compared these delineated channels to manually digitized channels and to channels extracted from multispectral imagery. We delineated GrIS supraglacial channel networks in six high-elevation (above 1000 m) and one low-elevation (below 1000 m) catchments during and shortly after peak melt (July and August 2012), and investigated the effect of contributing area threshold on flow routing performance. We found that, although flow routing is sensitive to data quality and moulin identification, it can identify 75% to 99% of channels observed with multispectral analysis, as well as low-order, high-density channels (up to 15.7 km/km(2) with a 0.01 km(2) contributing area threshold) in greater detail than multispectral methods. Additionally, we found that flow routing can delineate supraglacial channel networks on rough ice surfaces with widespread crevassing. Our results suggest that supraglacial channel density is sufficiently high during peak melt that low contributing area thresholds can be employed with little risk of overestimating the channel network extent. DOI
27. Crompton, JW; Flowers, GE; Kirste, D; Hagedorn, B; Sharp, MJ.Clay mineral precipitation and low silica in glacier meltwaters explored through reaction-path modelling.J. Glaciol., 2015, 61: 1061-1078 Clay mineral precipitation and low silica in glacier meltwaters explored through reaction-path modelling
glacier hydrology; meltwater chemistry; subglacial precipitates and ice regelation; subglacial processes; subglacial sediments
The subglacial chemical weathering environment is largely controlled by low temperatures and the presence of freshly comminuted minerals with a high surface area. These characteristics are believed to promote dissolution processes that give rise to low silica and high Ca2+ fluxes emanating from glacierized basins. We test an alternative hypothesis, that mineral precipitation reactions in the subglacial environment play an equally important role in controlling the water chemistry in glacierized basins. We analyze borehole and proglacial water chemistry from a subarctic polythermal glacier, complemented by mineral XRD analysis of suspended sediment, till and bedrock samples. In conjunction with a thermodynamic analysis of the water and mineral chemistry, we use reaction path modelling to study the chemical enrichment of water through the glacier system. We find that the high pH of the subglacial environment is conducive to secondary mineral precipitation, and that it is not possible to balance the water chemistry using dissolution reactions alone. We show that low silica can be explained by standard weathering reactions without having to invoke mineral-leaching reactions. Our results suggest that subglacial weathering intensity may be significantly underestimated if the production of secondary minerals is not considered. DOI
26.Flowers, GE.Modelling water flow under glaciers and ice sheets.Proc. R. Soc. A-Math. Phys. Eng. Sci., 2015, 471 Modelling water flow under glaciers and ice sheets
cryosphere; subglacial hydrology; basal processes; hydraulics; hydrological models; glacier and ice-sheet dynamics
Recent observations of dynamic water systems beneath the Greenland and Antarctic ice sheets have sparked renewed interest in modelling subglacial drainage. The foundations of today's models were laid decades ago, inspired by measurements from mountain glaciers, discovery of the modern ice streams and the study of landscapes evacuated by former ice sheets. Models have progressed from strict adherence to the principles of groundwater flow, to the incorporation of flow 'elements' specific to the subglacial environment, to sophisticated two-dimensional representations of interacting distributed and channelized drainage. Although presently in a state of rapid development, subglacial drainage models, when coupled to models of ice flow, are now able to reproduce many of the canonical phenomena that characterize this coupled system. Model calibration remains generally out of reach, whereas widespread application of these models to large problems and real geometries awaits the next level of development. DOI PubMed
25. Beaud, F; Flowers, GE; Pimentel, S.Seasonal-scale abrasion and quarrying patterns from a two-dimensional ice-flow model coupled to distributed and channelized subglacial drainage.Geomorphology, 2014, 219: 176-191 Seasonal-scale abrasion and quarrying patterns from a two-dimensional ice-flow model coupled to distributed and channelized subglacial drainage
Glacial erosion; Subgjacial hydrology; Abrasion; Quarrying; Ice-flow modeling; Sliding law
Field data and numerical modeling show that glaciations have the potential either to enhance relief or to dampen topography. We aim to model the effect of the subglacial hydraulic system on spatiotemporal patterns of glacial erosion by abrasion and quarrying on time scales commensurate with drainage system fluctuations (e.g., seasonal to annual). We use a numerical model that incorporates a dual-morphology subglacial drainage system coupled to a higher-order ice-flow model and process-specific erosion laws. The subglacial drainage system allows for a dynamic transition between two morphologies: the distributed system, characterized by an increase in basal water pressure with discharge, and the channelized system, which exhibits a decrease in equilibrium water pressure with increasing discharge. We apply the model to a simple synthetic glacier geometry, drive it with prescribed meltwater input variations, and compute sliding and erosion rates over a seasonal cycle. When both distributed and channelized systems are included, abrasion and sliding maxima migrate similar to 20% upglacier compared to simulations with distributed drainage only. Power-law sliding generally yields to a broader response of abrasion to water pressure changes along the flowline compared to Coulomb-friction sliding. Multiday variations in meltwater input elicit a stronger abrasion response than either diurnal- or seasonal variations alone for the same total input volume. An increase in water input volume leads to increased abrasion. We find that ice thickness commensurate with ice sheet outlet glaciers can hinder the up-glacier migration of abrasion. Quarrying patterns computed with a recently published law differ markedly from calculated abrasion patterns, with effective pressure being a stronger determinant than sliding speeds of quarrying rates. These variations in calculated patterns of instantaneous erosion as a function of hydrology-, sliding-, and erosion-model formulation, as well as model forcing, may lead to significant differences in predicted topographic profiles on long time scales. (C) 2014 Elsevier B.V. All rights reserved. DOI
24.Flowers, GE; Copland, L; Schoof, CG.Contemporary Glacier Processes and Global Change: Recent Observations from Kaskawulsh Glacier and the Donjek Range, St. Elias Mountains.Arctic, 2014, 67: 22-34 Contemporary Glacier Processes and Global Change: Recent Observations from Kaskawulsh Glacier and the Donjek Range, St. Elias Mountains
Kluane Lake Research Station; St. Elias Mountains; glaciology; Kaskawulsh Glacier; Donjek Range; glacier mass balance; glacier change; glacier dynamics; glacier surges; glacier-climate interactions; subglacial processes
With an extensive ice cover and rich display of glacier behaviour, the St. Elias Mountains continue to be an enviable natural laboratory for glaciological research. Recent work has been motivated in part by the magnitude and pace of observed glacier change in this area, which is so ice-rich that ice loss has a measurable impact on global sea level. Both detection and attribution of these changes, as well as investigations into fundamental glacier processes, have been central themes in projects initiated within the last decade and based at the Kluane Lake Research Station. The scientific objectives of these projects are (1) to quantify recent area and volume changes of Kaskawulsh Glacier and place them in historical perspective, (2) to investigate the regional variability of glacier response to climate and the modulating influence of ice dynamics, and (3) to characterize the hydromechanical controls on glacier sliding. A wide range of methods is being used, from ground-based manual measurements to space-based remote sensing. The observations to date show glaciers out of equilibrium, with significant ongoing changes to glacier area, volume, and dynamics. Computer models are being used to generalize these results, and to identify the processes most critical to our understanding of the coupled glacier-climate system. DOI
23. Schoof, C; Rada, CA; Wilson, NJ; Flowers, GE; Haseloff, M.Oscillatory subglacial drainage in the absence of surface melt.Cryosphere, 2014, 8: 959-976 Oscillatory subglacial drainage in the absence of surface melt
The presence of strong diurnal cycling in basal water pressure records obtained during the melt season is well established for many glaciers. The behaviour of the drainage system outside the melt season is less well understood. Here we present borehole observations from a surge-type valley glacier in the St Elias Mountains, Yukon Territory, Canada. Our data indicate the onset of strongly correlated multi-day oscillations in water pressure in multiple boreholes straddling a main drainage axis, starting several weeks after the disappearance of a dominant diurnal mode in August 2011 and persisting until at least January 2012, when multiple data loggers suffered power failure. Jokulhlaups provide a template for understanding spontaneous water pressure oscillations not driven by external supply variability. Using a subglacial drainage model, we show that water pressure oscillations can also be driven on a much smaller scale by the interaction between conduit growth and distributed water storage in smaller water pockets, basal crevasses and moulins, and that oscillations can be triggered when water supply drops below a critical value. We suggest this in combination with a steady background supply of water from ground water or englacial drainage as a possible explanation for the observed wintertime pressure oscillations. DOI
22. Wheler, BA; MacDougall, AH; Flowers, GE; Petersen, EI; Whiffield, PH; Kohfeld, KE.Effects of Temperature Forcing Provenance and Extrapolation on the Performance of an Empirical Glacier-Melt Model.Arct. Antarct. Alp. Res., 2014, 46: 379-393 Effects of Temperature Forcing Provenance and Extrapolation on the Performance of an Empirical Glacier-Melt Model
Temperature-index models are popular tools for glacier melt-modeling due to their minimal data requirements and generally favorable performance. We examine the effects of temperature forcing provenance and extrapolation on the performance of one such model applied to a small glacier in the Saint Elias Mountains of northwestern Canada. The model is forced with air temperatures recorded (a) on two glaciers, (b) at two nearby ice-free locations, and (c) by two low-elevation valley stations. We extrapolate these temperatures using constant lapse rates and assess model performance by comparing measured and modeled cumulative summer ablation at a network of stakes over five melt seasons. When the model is calibrated individually for each temperature forcing and lapse rate, the variation in model performance is modest relative to inter-annual variations associated with melt-season conditions and calibration data quality. Despite <30% variation in estimated summer ablation arising from the combined influences of temperature forcing and lapse rate, the resulting variations in estimated annual mass balance can be significant (>100% in some cases). While model parameters calibrated in this way suffer from error compensation and exhibit equifinality, the lapse rates associated with minimum model error exhibit inter-annual variation that can be related to prevailing meteorological conditions. When the model is instead calibrated at the point scale without employing a lapse rate, and the resulting parameters are paired with an arbitrary temperature forcing, lapse rates associated with minimum model error vary widely between forcing types and years. Low-elevation stations distal from the study site sometimes outperform the calibration station, but the prescribed lapse rate becomes critical in this case. With either calibration method, lapse rates that minimize model error for the valley stations are generally steeper than the measured environmental lapse rates. DOI
21. Williamson, SN; Hik, DS; Gamon, JA; Kavanaugh, JL; Flowers, GE.Estimating Temperature Fields from MODIS Land Surface Temperature and Air Temperature Observations in a Sub-Arctic Alpine Environment.Remote Sens., 2014, 6: 946-963 Estimating Temperature Fields from MODIS Land Surface Temperature and Air Temperature Observations in a Sub-Arctic Alpine Environment
mean daily surface temperature; land surface temperature; air temperature; MODIS; meteorological station; tundra; Yukon Canada: alpine
Spatially continuous satellite infrared temperature measurements are essential for understanding the consequences and drivers of change, at local and regional scales, especially in northern and alpine environments dominated by a complex cryosphere where in situ observations are scarce. We describe two methods for producing daily temperature fields using MODIS. clear-sky. day-time Land Surface Temperatures (LST). The Interpolated Curve Mean Daily Surface Temperature (ICM) method, interpolates single daytime Terra LST values to daily means using the coincident diurnal air temperature curves. The second method calculates daily mean LST from daily maximum and minimum LST (MMM) values from MODIS Aqua and Terra. These ICM and MMM models were compared to daily mean air temperatures recorded between April and October at seven locations in southwest Yukon, Canada, covering characteristic alpine land cover types (tundra, barren, glacier) at elevations between 1,408 m and 2,319 m. Both methods for producing mean daily surface temperatures have advantages and disadvantages. ICM signals are strongly correlated with air temperature (R-2 = 0.72 to 0.86), but have relatively large variability (RMSE = 4.09 to 4.90 K), while MMM values had a stronger correlation to air temperature (R-2 = 0.90) and smaller variability (RMSE = 2.67 K). Finally, when comparing 8-day LST averages, aggregated from the MMM method, to air temperature, we found a high correlation (R-2 = 0.84) with less variability (RMSE = 1.54 K). Where the trend was less steep and the y-intercept increased by 1.6 degrees C compared to the daily correlations. This effect is likely a consequence of LST temperature averages being differentially affected by cloud cover over warm and cold surfaces. We conclude that satellite infrared skin temperature (e.g., MODIS LST), which is often aggregated into multi-day composites to mitigate data reductions caused by cloud cover, changes in its relationship to air temperature depending on the period of aggregation. DOI
20. Wilson, NJ; Flowers, GE; Mingo, L.Mapping and interpretation of bed-reflection power from a surge-type polythermal glacier, Yukon, Canada.Ann. Glaciol., 2014, 55: 1-8 Mapping and interpretation of bed-reflection power from a surge-type polythermal glacier, Yukon, Canada
glacier hydrology; glacier surges; mountain glaciers; radio-echo sounding; subglacial processes
Bed-reflection power (BRP) from ice-penetrating radar has been used to make inferences about subglacial conditions and processes, yet is subject to confounding influences, including englacial attenuation and bed geometry. We use radar data collected in 2008-11 from a polythermal glacier to compute BRP with the aim of relating BRP to basal conditions. We examine the relationship between raw BRP and ice thickness, apparent bed slope and thickness of the englacial scattering layer as a proxy for internal reflection power. We then analyze a corrected form of the BRP with a graph-segmentation algorithm to delineate areas of high and low reflection power. Low corrected BRP values are found near the glacier terminus where the bed is most likely to be cold, while high corrected BRP is found in the region thought to be undergoing a slow surge. We find a spatial correlation between high BRP and high values of subglacial hydraulic upstream area, suggestive of a hydrological control on BRP. Whereas in dominantly cold glaciers BRP seems to distinguish cold from temperate regions of the bed, BRP in a polythermal glacier with a substantial volume of temperate ice may be a more complex product of thermal and hydrological conditions. DOI
19. Joughin, I; Das, SB; Flowers, GE; Behn, MD; Alley, RB; King, MA; Smith, BE; Bamber, JL; van den Broeke, MR; van Angelen, JH.Influence of ice-sheet geometry and supraglacial lakes on seasonal ice-flow variability.Cryosphere, 2013, 7: 1185-1192 Influence of ice-sheet geometry and supraglacial lakes on seasonal ice-flow variability
Supraglacial lakes play an important role in establishing hydrological connections that allow lubricating seasonal meltwater to reach the base of the Greenland Ice Sheet. Here we use new surface velocity observations to examine the influence of supraglacial lake drainages and surface melt rate on ice flow. We find large, spatially extensive speedups concurrent with times of lake drainage, showing that lakes play a key role in modulating regional ice flow. While surface meltwater is supplied to the bed via a geographically sparse network of moulins, the observed ice-flow enhancement suggests that this meltwater spreads widely over the ice-sheet bed. We also find that the complex spatial pattern of speedup is strongly determined by the combined influence of bed and surface topography on subglacial water flow. Thus, modeling of ice-sheet basal hydrology likely will require knowledge of bed topography resolved at scales (sub-kilometer) far finer than existing data (several km). DOI
18. Werder, MA; Hewitt, IJ; Schoof, CG; Flowers, GE.Modeling channelized and distributed subglacial drainage in two dimensions.J. Geophys. Res.-Earth Surf., 2013, 118: 2140-2158 Modeling channelized and distributed subglacial drainage in two dimensions
glacier drainage system; subglacial drainage; drainage network; self organized; glaciology; modeling
We present a two-dimensional Glacier Drainage System model (GlaDS) that couples distributed and channelized subglacial water flow. Distributed flow occurs through linked cavities that are represented as a continuous water sheet of variable thickness. Channelized flow occurs through Rothlisberger channels that can form on any of the edges of a prescribed, unstructured network of potential channels. Water storage is accounted for in an englacial aquifer and in moulins, which also act as point sources of water to the subglacial system. Solutions are presented for a synthetic topography designed to mimic an ice sheet margin. For low discharge, all the flow is accommodated in the sheet, whereas for sufficiently high discharge, the model exhibits a channelization instability which leads to the formation of a self-organized channel system. The random orientation of the network edges allows the channel system geometry to be relatively unbiased, in contrast to previous structured grid-based models. Under steady conditions, the model supports the classical view of the subglacial drainage system, with low pressure regions forming around the channels. Under diurnally varying input, water flows in and out of the channels, and a rather complex spatiotemporal pattern of water pressures is predicted. We explore the effects of parameter variations on the channel system topology and mean effective pressure. The model is then applied to a mountain glacier and forced with meltwater calculated by a temperature index model. The results are broadly consistent with our current understanding of the glacier drainage system and demonstrate the applicability of the model to real settings. DOI
17. Wilson, NJ; Flowers, GE.Environmental controls on the thermal structure of alpine glaciers.Cryosphere, 2013, 7: 167-182 Environmental controls on the thermal structure of alpine glaciers
Water entrapped in glacier accumulation zones represents a significant latent heat contribution to the development of thermal structure. It also provides a direct link between glacier environments and thermal regimes. We apply a two-dimensional mechanically-coupled model of heat flow to synthetic glacier geometries in order to explore the environmental controls on flowband thermal structure. We use this model to test the sensitivity of thermal structure to physical and environmental variables and to explore glacier thermal response to environmental changes. In different conditions consistent with a warming climate, mean glacier temperature and the volume of temperate ice may either increase or decrease, depending on the competing effects of elevated meltwater production, reduced accumulation zone extent and thinning firn. For two model reference states that exhibit commonly-observed thermal structures, the fraction of temperate ice is shown to decline with warming air temperatures. Mass balance and aquifer sensitivities play an important role in determining how the englacial thermal regimes of alpine glaciers will adjust in the future. DOI
16. Wilson, NJ; Flowers, GE; Mingo, L.Comparison of thermal structure and evolution between neighboring subarctic glaciers.J. Geophys. Res.-Earth Surf., 2013, 118: 1443-1459 Comparison of thermal structure and evolution between neighboring subarctic glaciers
GPR; model; thermal; Yukon; dynamics; glaciers
The distribution of cold and temperate ice within glaciers and ice sheets affects processes relevant to englacial and basal hydrology, sliding, and material rheology. Thermal regimes, in turn, are shaped by glacier and ice sheet dynamics, as well as environmental setting. We investigate the thermal structures of two small (<7 km(2)) neighboring glaciers in the St. Elias Mountains of southwestern Yukon, Canada, using ice-penetrating radar and borehole temperature measurements. Our data reveal polythermal regimes in both glaciers that are strongly influenced by accumulation zone meltwater entrapment, suggesting a climatic control on thermal structure. Differences in hypsometry and glacier dynamics nevertheless result in observed variations in the distribution of temperate ice between the two sites. Experiments with a thermomechanically coupled flow band model corroborate the strong control of meltwater entrapment on thermal structure and suggest a generally minimal role for strain heating. An exception to this occurs where localized basal sliding produces lateral shearing and thus enhanced heat generation. Time-dependent model simulations suggest that the future thermal evolution of the two glaciers may differ, and therefore simple parameterizations of thermal response based on regional climate may not capture realistic variability between individual glaciers. Despite these differences, both glaciers are ultimately expected to become fully cold prior to disappearing under negative mass balance conditions. DOI
15.Flowers, GE; Roux, N; Pimentel, S; Schoof, CG.Present dynamics and future prognosis of a slowly surging glacier.Cryosphere, 2011, 5: 299-313 Present dynamics and future prognosis of a slowly surging glacier
Glacier surges are a well-known example of an internal dynamic oscillation whose occurrence is not a direct response to the external climate forcing, but whose character (i.e. period, amplitude, mechanism) may depend on the glacier's environmental or climate setting. We examine the dynamics of a small (similar to 5 km(2)) valley glacier in Yukon, Canada, where two previous surges have been photographically documented and an unusually slow surge is currently underway. To characterize the dynamics of the present surge, and to speculate on the future of this glacier, we employ a higher-order flowband model of ice dynamics with a regularized Coulomb-friction sliding law in both diagnostic and prognostic simulations. Diagnostic (force balance) calculations capture the measured ice-surface velocity profile only when non-zero basal water pressures are prescribed over the central region of the glacier, coincident with where evidence of the surge has been identified. This leads to sliding accounting for 50-100% of the total surface motion in this region. Prognostic simulations, where the glacier geometry evolves in response to a prescribed surface mass balance, reveal a significant role played by a bedrock ridge beneath the current equilibrium line of the glacier. Ice thickening occurs above the ridge in our simulations, until the net mass balance reaches sufficiently negative values. We suggest that the bedrock ridge may contribute to the propensity for surges in this glacier by promoting the development of the reservoir area during quiescence, and may permit surges to occur under more negative balance conditions than would otherwise be possible. Collectively, these results corroborate our interpretation of the current glacier flow regime as indicative of a slow surge that has been ongoing for some time, and support a relationship between surge incidence or character and the net mass balance. Our results also highlight the importance of glacier bed topography in controlling ice dynamics, as observed in many other glacier systems. DOI
14. MacDougall, AH; Flowers, GE.Spatial and Temporal Transferability of a Distributed Energy-Balance Glacier Melt Model.J. Clim., 2011, 24: 1480-1498 Spatial and Temporal Transferability of a Distributed Energy-Balance Glacier Melt Model
Modeling melt from glaciers is crucial to assessing regional hydrology and eustatic sea level rise. The transferability of such models in space and time has been widely assumed but rarely tested. To investigate melt model transferability, a distributed energy-balance melt model (DEBM) is applied to two small glaciers of opposing aspects that are 10 km apart in the Donjek Range of the St. Elias Mountains, Yukon Territory, Canada. An analysis is conducted in four stages to assess the transferability of the DEBM in space and time: 1) locally derived model parameter values and meteorological forcing variables are used to assess model skill; 2) model parameter values are transferred between glacier sites and between years of study; 3) measured meteorological forcing variables are transferred between glaciers using locally derived parameter values; 4) both model parameter values and measured meteorological forcing variables are transferred from one glacier site to the other, treating the second glacier site as an extension of the first. The model parameters are transferable in time to within a <10% uncertainty in the calculated surface ablation over most or all of a melt season. Transferring model parameters or meteorological forcing variables in space creates large errors in modeled ablation. If select quantities (ice albedo, initial snow depth. and summer snowfall) are retained at their locally measured values, model transferability can be improved to achieve <= 15% uncertainty in the calculated surface ablation. DOI
13. MacDougall, AH; Wheler, BA; Flowers, GE.A preliminary assessment of glacier melt-model parameter sensitivity and transferability in a dry subarctic environment.Cryosphere, 2011, 5: 1011-1028 A preliminary assessment of glacier melt-model parameter sensitivity and transferability in a dry subarctic environment
Efforts to project the long-term melt of mountain glaciers and ice-caps require that melt models developed and calibrated for well studied locations be transferable over large regions. Here we assess the sensitivity and transferability of parameters within several commonly used melt models for two proximal sites in a dry subarctic environment of northwestern Canada. The models range in complexity from a classical degree-day model to a simplified energy-balance model. Parameter sensitivity is first evaluated by tuning the melt models to the output of an energy balance model forced with idealized inputs. This exercise allows us to explore parameter sensitivity both to glacier geometric attributes and surface characteristics, as well as to meteorological conditions. We then investigate the effect of model tuning with different statistics, including a weighted coefficient of determination (omega R-2), the Nash-Sutcliffe efficiency criterion (E), mean absolute error (MAE) and root mean squared error (RMSE). Finally we examine model parameter transferability between two neighbouring glaciers over two melt seasons using mass balance data collected in the St. Elias Mountains of the southwest Yukon. The temperature-index model parameters appear generally sensitive to glacier aspect, mean surface elevation, albedo, wind speed, mean annual temperature and temperature lapse rate. The simplified energy balance model parameters are sensitive primarily to snow albedo. Model tuning with E, MAE and RMSE produces similar, or in some cases identical, parameter values. In twelve tests of spatial and/or temporal parameter transferability, the results with the lowest RMSE values with respect to ablation stake measurements were achieved twice with a classical temperature-index (degree-day) model, three times with a temperature-index model in which the melt parameter is a function of potential radiation, and seven times with a simplified energy-balance model. A full energy-balance model produced better results than the other models in nine of twelve cases, though the tuning of this model differs from that of the others. DOI
12. Pimentel, S; Flowers, GE.A numerical study of hydrologically driven glacier dynamics and subglacial flooding.Proc. R. Soc. A-Math. Phys. Eng. Sci., 2011, 467: 537-558 A numerical study of hydrologically driven glacier dynamics and subglacial flooding
ice sheets; subglacial hydrology; glacier sliding; glaciology
A hydrologically coupled flowband model of 'higher order' ice dynamics is used to explore perturbations in response to supraglacial water drainage and subglacial flooding. The subglacial drainage system includes interacting 'fast' and 'slow' drainage elements. The fast drainage system is assumed to be composed of ice-walled conduits and the slow system of a macroporous water sheet. Under high subglacial water pressures, flexure of the overlying ice is modelled using elastic beam theory. A regularized Coulomb friction law describes basal boundary conditions that enable hydrologically driven acceleration. We demonstrate the modelled interactions between hydrology and ice dynamics by means of three observationally inspired examples: (i) simulations of meltwater drainage at an Alpine-type glacier produce seasonal and diurnal variability, and exhibit drainage evolution characteristic of the so-called 'spring transition'; (ii) horizontal and vertical diurnal accelerations are modelled in response to summer meltwater input at a Greenland-type outlet glacier; and (iii) short-lived perturbations to basal water pressure and ice-flow speed are modelled in response to the prescribed drainage of a supraglacial lake. Our model supports the suggestion that a channelized drainage system can form beneath the margins of the Greenland ice sheet, and may contribute to reducing the dynamic impact of floods derived from supraglacial lakes. DOI
11. Wheler, BA; Flowers, GE.Glacier subsurface heat-flux characterizations for energy-balance modelling in the Donjek Range, southwest Yukon, Canada.J. Glaciol., 2011, 57: 121-133 Glacier subsurface heat-flux characterizations for energy-balance modelling in the Donjek Range, southwest Yukon, Canada
We apply a point-scale energy-balance model to a small polythermal glacier in the St Elias Mountains of Canada in order to investigate the applicability and limitations of different treatments of the glacier surface temperature and subsurface heat flux. These treatments range in complexity from a multilayer subsurface model that simulates snowpack evolution, to the assumption of a constant glacier surface temperature equal to 0 degrees C. The most sophisticated model includes dry densification of the snowpack, penetration of shortwave radiation into the subsurface, internal melting, refreezing of percolating meltwater and generation of slush layers. Measurements of subsurface temperature and surface lowering are used for model validation, and highlight the importance of including subsurface penetration of shortwave radiation in the model. Using an iterative scheme to solve for the subsurface heat flux as the residual of the energy-balance equation results in an overestimation of total ablation by 18%, while the multilayer subsurface model underestimates ablation by 6%. By comparison, the 0 degrees C surface assumption leads to an overestimation of ablation of 29% in this study where the mean annual air temperature is about -8 degrees C. DOI
10.Flowers, GE.Glacier hydromechanics: early insights and the lasting legacy of three works by Iken and colleagues.J. Glaciol., 2010, 56: 1069-1078 Glacier hydromechanics: early insights and the lasting legacy of three works by Iken and colleagues
The association between basal hydrology and glacier sliding has become nearly synonymous with the early work of Almut Iken and colleagues. Their research published in the journal of Glaciology from 1981 to 1986 made an indelible impact on the study of glacier hydromechanics by documenting strong correlations between basal water pressure and short-term ice-flow variations. With a passion for elucidating the physics of glacier-bed processes, Iken herself made fundamental contributions to our theoretical and empirical understanding of the sliding process. From the theoretical bound on basal shear stress, to the inferences drawn from detailed horizontal and vertical velocity measurements, the work of Iken and colleagues continues to inform the interpretation of data from alpine glaciers and has found increasing relevance to observations from the ice sheets. DOI
9. Mingo, L; Flowers, GE.An integrated lightweight ice-penetrating radar system.J. Glaciol., 2010, 56: 709-714 An integrated lightweight ice-penetrating radar system
We describe a portable low-frequency impulse radar system intended for ground-based surveys that employs off-the-shelf hardware integrated with custom-designed software. The hardware comprises a 1-200 MHz transmitter, digitizer, computer and GPS receiver, which together weigh similar to 1.5 kg. The entire system, including waterproof enclosures and batteries suited for >8 hours of continuous operation, weighs <10 kg plus the weight of the antenna housing. The system design is flexible, permitting hardware components such as the digitizer or navigation device to be exchanged. The software includes acquisition parameter control, real-time visual ice-depth rendering and data management capabilities using a hierarchical data format. The system described here has been successfully used to sound polythermal ice up to similar to 220 m thick in ski-based surveys in the Yukon, Canada, and temperate ice up to similar to 550m thick in machine-based surveys in Iceland. DOI
8. Pimentel, S; Flowers, GE; Schoof, CG.A hydrologically coupled higher-order flow-band model of ice dynamics with a Coulomb friction sliding law.J. Geophys. Res.-Earth Surf., 2010, 115 A hydrologically coupled higher-order flow-band model of ice dynamics with a Coulomb friction sliding law
The influence of hydrologic transience and heterogeneity on basal motion is an often-neglected aspect of numerical ice-flow models. We present a flow-band model of glacier dynamics with a Coulomb friction sliding law that is coupled to a model of the basal drainage system by means of subglacial water pressure. The ice-flow model contains "higher-order" stress gradients from the Stokes flow approximation originally conceived by Blatter (1995). The resulting system of nonlinear equations is solved using a modified Picard iteration that is shown to improve the rate of convergence. A parameterization of lateral shearing is included to account for the effects of three-dimensional geometry. We find that lateral drag has a discernible effect on glacier speed even when glacier width exceeds glacier length. Variations in flow-band width are shown to have a greater influence on flow line speed than either different valley cross-sectional shapes or the presence or absence of glacier sliding along valley walls. Modeled profiles of subglacial water pressure depart significantly from pressures prescribed as a uniform fraction of overburden, thus producing profiles of glacier sliding that are distinctly different from those that would be described by a sliding law controlled by overburden pressures. Simulations of hydraulically driven glacier acceleration highlight the value of including a representation of basal hydrology in models aiming for improved predictive capability of glacier dynamics. DOI
7. Boon, S; Flowers, GE; Munro, DS.Canadian Glacier Hydrology, 2003-2007.Can. Water Resour. J., 2009, 34: 195-203 Canadian Glacier Hydrology, 2003-2007
Glacier hydrological research in Canada from 2002-2007 continues to advance, driven by new observations of glacier retreat in all regions of the country. New observation networks have been formed to study various aspects of glacier change and linkages with the hydrological system. Small-scale studies of accumulation and melt processes on glacier surfaces continue, and are being used to parameterize spatially distributed models of glacier mass balance and melt. Increasing emphasis has been placed on downscaling of regional and global climate model output to use as input to mass balance models. Advances in our understanding of water movement from the glacier surface to the bed has led to increased interest in runoff from glacierized catchments, which has significant policy implications for downstream water users. Continuing research includes maintenance and enhancement of field monitoring capabilities, improved algorithms to downscale climate model output, and adjustments to hydrological models to more accurately represent glacier cover for streamflow prediction. DOI
6. De Paoli, L; Flowers, GE.Dynamics of a small surge-type glacier using one-dimensional geophysical inversion.J. Glaciol., 2009, 55: 1101-1112 Dynamics of a small surge-type glacier using one-dimensional geophysical inversion
We investigate the dynamics of a small surge-type valley glacier as part of a study to characterize glacier response to climate in the Donjek Range, southwest Yukon, Canada. Pole displacements were measured using kinematic GPS techniques during three consecutive summer field seasons. Measured surface velocities range from <10 m a(-1) over the lower 1500 m of the 5 km long glacier to a maximum of similar to 25-35 m a(-1) over the upper 3500 m. Basal velocities along an approximate flowline are reconstructed from the measured surface velocities using inverse methods. Control tests are used to validate the inversion scheme, and sensitivity tests are performed to evaluate the influence of the flow-law coefficient, shape factor and longitudinal averaging length. Inversion of the real data shows that basal motion accounts for 50-100% of the total surface motion along the flowline. Based on these results, and several other lines of evidence, we suggest this glacier may be undergoing a slow surge. DOI
5.Flowers, GE.Subglacial modulation of the hydrograph from glacierized basins.Hydrol. Process., 2008, 22: 3903-3918 Subglacial modulation of the hydrograph from glacierized basins
subglacial drainage; seasonal transitions; hydrograph; glacierized basins
The extent of basin glacierization has important implications for the hydrograph in part, because snow, firm and ice impart different delays in water transport through the system. Here, the significance of subglacial drainage morphology in modulating the hydrograph is examined with a one-dimensional physically based model. The conceptual model of subglacial drainage comprises both 'fast' and 'slow' elements, respectively associated with summer and winter drainage regimes. The additional possibility of a permeable glacier substrate is taken into account by allowing water transport in a subglacial aquifer. Forced by prescribed rates of melt-water delivery to the glacier bed, the model predicts glacier discharge by drainage system provenance. The effects of (1) 'hard' versus 'soft' glacier beds, (2) subsurface permeability and groundwater flow, and (3) glacier geometry are then investigated. Hydrograph character, in the form of peak timing and amplitude, symmetry with respect to the forcing, and the amplitude of diurnal fluctuations, is affected by the partitioning of water through the various flow elements. Hard beds and impermeable substrates maximize the discharge routed through the fast-drainage system in the simulations, generally resulting in higher seasonal discharge maxima and stronger diurnal variations in discharge. High hydraulic transmissivities, either at the glacier bed or in underlying strata, hinder the development of the fast-drainage system in the simulations, producing hydrographs of lower amplitude. Glacier geometry has a modest effect, with adverse bed slopes, very thick or very thin ice and short glacier lengths favouring prolonged drainage through the slow system. These results suggest that the morphology and evolution of the subglacial drainage system may play a significant role in determining the character of the hydrograph from glacierized basins. Copyright (c) 2008 John Wiley & Sons, Ltd. DOI
4.Flowers, GE; Bjornsson, H; Geirsdottir, A; Miller, GH; Black, JL; Clarke, GKC.Holocene climate conditions and glacier variation in central Iceland from physical modelling and empirical evidence.Quat. Sci. Rev., 2008, 27: 797-813 Holocene climate conditions and glacier variation in central Iceland from physical modelling and empirical evidence
Lacustrine sediment cores from proglacial lake Hvitarvatn, central Iceland, reveal a detailed chronology of Holocene sedimentation, from which environmental conditions and the attendant fluctuations of Langjokull ice cap have been interpreted. We apply a numerical ice-sheet model to determine the climatic conditions under which the empirical reconstruction is glaciologically plausible. Modelling constraints are derived from core records of diatom concentration, benthic diatom fraction, and ice-rafted debris occurrence, as well as lake bottom morphology and the present-day ice-cap geometry. Holocene simulations driven by the NGRIP delta O-18 record that are consistent with the empirical constraints show the most extensive advance of Langjokull ice cap to be its most recent, beginning somewhere between 5 and 3 ka BP. Ice advance in response to the 8.2 ka BP cold event is followed by several thousand years of nearly ice-free conditions in the mid-Holocene. All simulations suggest that the maximum Holocene stand of the ice cap occurred during the Little Ice Age (LIA); those consistent with the constraints show little to no ice advance into Hvitarvatn before similar to 1 ka BP and indicate the lake area occupied by ice was much greater during the LIA than at any previous time. The most plausible simulation results were obtained for a maximum Holocene warming of 3-4 degrees C relative to the 1961-1990 reference, twice the Arctic average, and for Holocene Thermal Maximum (HTM) precipitation amounts comparable to or slightly greater than the modern. Reconciling the simulated subglacial discharge record to the empirically derived sediment volumes and emplacement times requires mean Holocene sediment concentrations of 0.8-1.6 kg m(-3). These estimates increase to 1.4-2.0 kg m(-3) when sedimentation rates are highest. (C) 2008 Published by Elsevier Ltd. DOI
3.Flowers, GE; Bjornsson, H; Geirsdottir, A; Miller, GH; Clarke, GKC.Glacier fluctuation and inferred climatology of Langjokull ice cap through the Little Ice Age.Quat. Sci. Rev., 2007, 26: 2337-2353 Glacier fluctuation and inferred climatology of Langjokull ice cap through the Little Ice Age
Emerging paleoclimate records from proglacial lake Hvitarvatn, central Iceland, suggest that Langjokull ice cap attained its maximum Holocene extent within the last 400 years. With the aim of constructing glaciological models and appropriate model inputs for Holocene simulations of Langjokull, we begin by simulating the evolution of Langjokull through the Little Ice Age to present, a period for which we have some constraint on ice-cap geometry. Using modern measured mass balance distributions (1997-2003) and meteorological data from nearby Hveravellir, we derive a reference precipitation field for the period 1961-1990 over the ice cap. Our simulations suggest Langjokull attained its maximum Little Ice Age volume around 1840 with a second local maximum around 1890. The two outlet glaciers terminating in Hvitarvatn, Norourjokull and Suourjokull, advance slowly into the lake, occupying their maximum lake area in the late 19th century, and retreat comparatively rapidly in the mid- to late 20th century. Simulations of Norourjokull are much more faithful to the geomorphic evidence than are simulations of Suourjokull, potentially suggesting a difference in dynamics between these two glaciers. While only 35% of the Hvitarvatn catchment area is ice-covered, meltwater from Langjokull comprises similar to 70% of the water input to the lake. Two-thirds of this input from the ice cap is transported as groundwater. Simulated glacier-derived discharge to the lake through the Little Ice Age suggests that a sediment concentration of 1.5 kg m(-3) would have resulted in the transport of 1.5 x 10(11) kg of material to the lake over the last 300 years, comparable to the estimated mass of sediment in the most recently deposited sedimentary unit in the lake. (C) 2007 Elsevier Ltd. All rights reserved. DOI
2. Huntington, HP; Boyle, M; Flowers, GE; Weatherly, JW; Hamilton, LC; Hinzman, L; Gerlach, C; Zulueta, R; Nicolson, C; Overpeck, J.The influence of human activity in the Arctic on climate and climate impacts.Clim. Change, 2007, 82: 77-92 The influence of human activity in the Arctic on climate and climate impacts
Human activities in the Arctic are often mentioned as recipients of climate-change impacts. In this paper we consider the more complicated but more likely possibility that human activities themselves can interact with climate or environmental change in ways that either mitigate or exacerbate the human impacts. Although human activities in the Arctic are generally assumed to be modest, our analysis suggests that those activities may have larger influences on the arctic system than previously thought. Moreover, human influences could increase substantially in the near future. First, we illustrate how past human activities in the Arctic have combined with climatic variations to alter biophysical systems upon which fisheries and livestock depend. Second, we describe how current and future human activities could precipitate or affect the timing of major transitions in the arctic system. Past and future analyses both point to ways in which human activities in the Arctic can substantially influence the trajectory of arctic system change. DOI
1.Flowers, GE; Marshall, SJ; Bjornsson, H; Clarke, GKC.Sensitivity of Vatnajokull ice cap hydrology and dynamics to climate warming over the next 2 centuries.J. Geophys. Res.-Earth Surf., 2005, 110 Sensitivity of Vatnajokull ice cap hydrology and dynamics to climate warming over the next 2 centuries
The sensitivity of Vatnajokull ice cap to future climate change is examined using spatially distributed coupled models of ice dynamics and hydrology. We simulate the evolving ice cap geometry, mass balance, velocity structure, subglacial water pressures and fluxes, and basin runoff in response to perturbations to a 1961-1990 reference climatology. For a prescribed warming rate of 2 degrees C per century, simulated ice cap area and volume are reduced by 12-15% and 18-25% within 100 years, respectively. Individual outlet glaciers experience 3-6 km of retreat in the first 100 years and a total retreat of 10-30 km over 200 years. For the same applied warming our results suggest a maximum increase in glacier-derived runoff of similar to 25% after 130 years. Ice cap thinning and retreat alters Vatnajokull's subglacial hydraulic catchment structure in the simulations, with up to several kilometers of local hydraulic divide migration. This serves to redistribute water among the major outlet rivers and, in extreme cases, to isolate river basins from glacially derived runoff. Glacier discharge from northern and northwestern Vatnajokull (distal from the coast) appears to be the most robust to climate warming, while discharge from Vatnajokull's southern margin (proximal to the coast) is particularly vulnerable. The latter reflects pronounced changes in the geometry of the southern outlet glaciers and has implications for glacier flood routing and frequency. DOI
