scholarly journals Modeling the WorldView-derived seasonal velocity evolution of Kennicott Glacier, Alaska

2016 ◽  
Vol 62 (234) ◽  
pp. 763-777 ◽  
Author(s):  
W. H. ARMSTRONG ◽  
R. S. ANDERSON ◽  
JEFFERY ALLEN ◽  
H. RAJARAM
Keyword(s):  
Stress Gradient ◽  
Surface Expression ◽  
Surface Velocity ◽  
Cross Sectional ◽  
Ice Surface ◽  
Hydrologic System ◽  
Glacier Velocity ◽  
Glacier Flow ◽  
Insight Into ◽  
Gaining Insight

ABSTRACTGlacier basal motion generates diurnal to multi-annual fluctuations in glacier velocity and mass flux. Understanding these fluctuations is important for prediction of future sea-level rise and for gaining insight into glacier physics and erosion. Here, we derive glacier velocity through cross-correlation of WorldView satellite imagery to document the evolution of ice surface velocity on Kennicott Glacier, Alaska, over the 2013 melt season. The summer speedup is spatially uniform over a ~12 km2 area, over which the spring velocity varies significantly. Velocity increases by 1.4-fold to tenfold across the study domain, with larger values where spring velocities are low. To investigate the cross-glacier distribution of basal motion required to explain the observed surface speedup, we employ a two-dimensional cross-sectional glacier flow model. We find the model is insensitive to the spatial distribution of basal slip because stress gradient ice coupling diffuses the surface expression of the basal velocity field. While the temporal evolution of the subglacial hydrologic system is critical for predicting a glacier's response to meltwater inputs, our work suggests that glacier and ice-sheet models do not require a detailed representation of subglacial hydrology to accurately capture the spatial pattern of glacier speedup.

scholarly journals Present dynamics and future prognosis of a slowly surging glacier

2011 ◽  
Vol 5 (1) ◽  
pp. 299-313 ◽  
Author(s):  
G. E. Flowers ◽  
N. Roux ◽  
S. Pimentel ◽  
C. G. Schoof
Keyword(s):  
Mass Balance ◽  
Force Balance ◽  
Surface Velocity ◽  
Internal Dynamic ◽  
Ice Dynamics ◽  
Surface Mass ◽  
Ice Surface ◽  
Glacier Ice ◽  
Glacier Flow ◽  
Negative Balance

Abstract. 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 (∼5 km2) 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.

scholarly journals Present dynamics and future prognosis of a slowly surging glacier

2010 ◽  
Vol 4 (4) ◽  
pp. 1839-1876
Author(s):  
G. E. Flowers ◽  
N. Roux ◽  
S. Pimentel
Keyword(s):  
Mass Balance ◽  
Surface Profile ◽  
Yukon Territory ◽  
Force Balance ◽  
Surface Velocity ◽  
Internal Dynamic ◽  
Ice Dynamics ◽  
Surface Mass ◽  
Ice Surface ◽  
Glacier Flow

Abstract. 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 (e.g. period, mechanism) may depend on the glacier's environmental or climate setting. We examine the dynamics of a small (~5 km2) valley glacier in the Yukon Territory of 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 Coulomb-friction sliding law in both diagnostic and prognostic simulations. Diagnostic (force balance) calculations capture the measured ice-surface velocity profile only when high basal water pressures (55–90% of flotation) are prescribed over the central region of the glacier, consistent with where evidence of the surge has been identified. This leads to sliding accounting for 50–100% of the total surface motion. Prognostic simulations, where the glacier geometry evolves in response to a prescribed surface mass balance, reveal a significant role played by a large bedrock bump beneath the current equilibrium line of the glacier. This bump provides resistance to ice flow sufficient to cause the formation of a bulge in the ice-surface profile. We suggest that the bedrock bump contributes to the propensity for surges in this glacier, such that conditions suppressing ice-bulge formation over the bump may also inhibit surges. In our calculations such a situation arises for sufficiently negative values of mass balance. Collectively, these results corroborate our interpretation of the current glacier flow regime as indicative of a "slow surge", and confirm 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.

scholarly journals Glacier Flow in a Curving Channel

1987 ◽  
Vol 33 (115) ◽  
pp. 281-292 ◽  
Author(s):  
Keith Echelmeyer ◽  
Kamb Barclay
Keyword(s):  
Stress Exponent ◽  
Surface Velocity ◽  
Radius Of Curvature ◽  
Center Line ◽  
Cross Sectional ◽  
Parabolic Form ◽  
Flow Law ◽  
Characteristic Features ◽  
Glacier Flow ◽  
Sectional Shape

AbstractThe flow of a glacier along a channel of constant longitudinal curvature is analyzed using analytical and finite-element methods. Channels of various cross–sectional shape are investigated, ranging from a simple rectangular form with zero shear traction along the bed to realistic profiles taken from Blue Glacier, Washington. Terms in the equilibrium and rate-of-deformation equations which are inversely dependent on radius and a body force which varies transversely across the glacier introduce several characteristic features into the stress and velocity fields of the curving glacier. The stress center line is shifted toward the inside of the bend, causing an asymmetric crevasse pattern and non‒zero stress magnitude at the surface on the geometric center line of the channel. The stress field is dependent on the stress exponent in the flow law and is non-linear across the surface. The surface–velocity pattern shows a “tilting” of the usual high‒order parabolic form, being skewed toward the inside of the bend. There is a shift in the velocity maximum from the deepest part of the channel. All of these curvature‒induced features are dependent on the radius of curvature, actual channel geometry, and stress exponent in the flow law. Model results show excellent agreement with the velocity and crevasse patterns on the curving Blue Glacier.

scholarly journals Glacier Flow in a Curving Channel

1987 ◽  
Vol 33 (115) ◽  
pp. 281-292 ◽  
Author(s):  
Keith Echelmeyer ◽  
Kamb Barclay
Keyword(s):  
Stress Exponent ◽  
Surface Velocity ◽  
Radius Of Curvature ◽  
Center Line ◽  
Cross Sectional ◽  
Parabolic Form ◽  
Flow Law ◽  
Characteristic Features ◽  
Glacier Flow ◽  
Sectional Shape

AbstractThe flow of a glacier along a channel of constant longitudinal curvature is analyzed using analytical and finite-element methods. Channels of various cross–sectional shape are investigated, ranging from a simple rectangular form with zero shear traction along the bed to realistic profiles taken from Blue Glacier, Washington. Terms in the equilibrium and rate-of-deformation equations which are inversely dependent on radius and a body force which varies transversely across the glacier introduce several characteristic features into the stress and velocity fields of the curving glacier. The stress center line is shifted toward the inside of the bend, causing an asymmetric crevasse pattern and non‒zero stress magnitude at the surface on the geometric center line of the channel. The stress field is dependent on the stress exponent in the flow law and is non-linear across the surface. The surface–velocity pattern shows a “tilting” of the usual high‒order parabolic form, being skewed toward the inside of the bend. There is a shift in the velocity maximum from the deepest part of the channel. All of these curvature‒induced features are dependent on the radius of curvature, actual channel geometry, and stress exponent in the flow law. Model results show excellent agreement with the velocity and crevasse patterns on the curving Blue Glacier.

scholarly journals Ice flux of Plogbreen, a small ice stream in Dronning Maud Land, Antarctica

2004 ◽  
Vol 39 ◽  
pp. 409-416
Author(s):  
Jim Hedfors ◽  
Veijo Allan Pohjola
Keyword(s):  
Mass Balance ◽  
Ice Core ◽  
Austral Summer ◽  
Snow Accumulation ◽  
Surface Velocity ◽  
Cross Sectional ◽  
Ice Surface ◽  
Dronning Maud Land ◽  
Using Data

AbstractAs part of a long-term mass-balance program run by SWEDARP since 1988, a detailed study on Plogbreen, Dronning Maud Land, Antarctica, was undertaken during the austral summer of 2003 to investigate the long-term mass balance. We compare ice outflux, φout, through a cross-sectional gate with ice influx, φin, from the upstream catchment area. The φin is based on calculations of snow accumulation upstream of the gate using data available from published ice-core records. The φout is based on Glen’s flow law aided by thermodynamic modeling and force-budget calculations. Input data from the field consist of measurements of ice surface velocity and ice geometry. The ice surface velocity was measured using repeated differential global positioning system surveying of 40 stakes over a period of 25 days. The ice geometry was determined by 174 km of ground-penetrating radar profiling using ground-based 8MHz dipole antennas. This study presents the collected velocity and geometry data as well as the calculated ice flux of Plogbreen. The results show a negatively balanced system within the uncertainty limits; φout = 0.55 ± 0.05 km3 a–1 and φin = 0.4 ± 0.1 km3 a–1. We speculate that the negative balance can be explained by recent eustatic increase reducing resistive stresses and inducing accelerated flow.

scholarly journals Factors Associated with Cessation Activities amongst a Multiethnic Sample of Transit Workers

2016 ◽  
Vol 13 (1) ◽  
pp. 1-10 ◽  
Author(s):  
Carol B. Cunradi ◽  
Roland S. Moore ◽  
Robynn S. Battle
Keyword(s):  
Low Cost ◽  
Cross Sectional ◽  
Factors Associated ◽  
Quit Attempt ◽  
Action Preparation ◽  
Public Transit Agency ◽  
Insomnia Symptoms ◽  
Insight Into ◽  
Multiethnic Sample ◽  
Gaining Insight

Introduction: Transit workers are a blue-collar occupational group with elevated rates of smoking despite access to free or low-cost cessation services available through their health insurance as a union-negotiated employee benefit. Little is known about the influences on cessation participation in this workforce.Aims: The purpose of this study is to analyse the factors associated with past-year cessation activities amongst a multiethnic sample of transit workers.Methods: Cross-sectional tobacco surveys were completed by 935 workers at an Oakland, California, USA-based public transit agency. Data from 190 current smokers (68% African American; 46% female) were analysed. Adjusted odds ratios were calculated to identify factors associated with past-year cessation activity.Results: Approximately 55% of smokers stopped smoking for one day or longer during the past year in order to quit. Nearly half reported that the most common barrier to quitting was, ‘Not mentally ready to quit because I like smoking’. Workers in the contemplation/precontemplation stage for intention to quit were less likely to have engaged in cessation activities than those in the action/preparation stage (AOR = 0.34). Frequency of coworker encouragement for quitting was positively associated with past-year cessation activities (AOR = 3.25). Frequency of insomnia symptoms was negatively associated with cessation activity participation (AOR = 0.34).Conclusions: Most transit workers who smoke made a past-year quit attempt. Gaining insight into factors associated with participation in cessation activities can aid worksite efforts to promote cessation and reduce tobacco-related disparities.

scholarly journals Spatio-temporal variations in glacier surface velocity in the Himalayas

2021 ◽  
Author(s):  
Yu Zhou ◽  
Jianlong Chen ◽  
Xiao Cheng
Keyword(s):  
Mass Loss ◽  
Temporal Variations ◽  
Surface Velocity ◽  
Glacier Surface ◽  
Environmental Forcing ◽  
Landsat 7 ◽  
Spatio Temporal ◽  
Velocity Changes ◽  
Glacier Velocity ◽  
Glacier Flow

Abstract. Glacier evolution with time provides important information about climate variability. Here we investigate glacier surface velocity in the Himalayas and analyse the patterns of glacier flow. We collect 220 scenes of Landsat-7 panchromatic images between 1999 and 2000, and Sentinel-2 panchromatic images between 2017 and 2018, to calculate surface velocities of 36,722 glaciers during these two periods. We then derive velocity changes between 1999 and 2018, based on which we perform a detailed analysis of motion of each individual glacier, and noted that the changes are spatially heterogeneous. Of all the glaciers, 32 % have speeded up, 24.5 % have slowed down, and the rest 43.5 % remained stable. The amplitude of glacier slowdown, as a result of glacier mass loss, is remarkably larger than that of speedup. At regional scales, we found that glacier surface velocity in winter has uniformly decreased in the western part of the Himalayas between 1999 and 2018, whilst increased in the eastern part; this contrasting difference may be associated with decadal changes in accumulation and/or melting under different climatic regimes. We also found that the overall trend of surface velocity exhibits seasonal variability: summer velocity changes are positively correlated with mass loss, whereas winter velocity changes show a negative correlation. Our study suggests that glacier velocity changes in the Himalayas are more spatially and temporally heterogeneous than previously thought, emphasising complex interactions between glacier dynamics and environmental forcing.

scholarly journals Modelling intra-annual dynamics of a major marine-terminating Arctic glacier

2017 ◽  
Vol 58 (74) ◽  
pp. 118-130 ◽  
Author(s):  
Sam Pimentel ◽  
Gwenn E. Flowers ◽  
Martin J. Sharp ◽  
Bradley Danielson ◽  
Luke Copland ◽  
...  
Keyword(s):  
Sea Ice ◽  
Flow Speed ◽  
Surface Velocity ◽  
Devon Island ◽  
Ice Cap ◽  
Ice Surface ◽  
Run Off ◽  
Glacier Flow ◽  
A Minor ◽  
Arctic Glacier

ABSTRACT 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 higher-order ice flow model is used to estimate changes in glacier flow speed as a result of changes in sea-ice 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.

scholarly journals Spatial and temporal evolution of rapid basal sliding on Bench Glacier, Alaska, USA

2005 ◽  
Vol 51 (172) ◽  
pp. 49-63 ◽  
Author(s):  
Kelly R. Macgregor ◽  
Catherine A. Riihimaki ◽  
Robert S. Anderson
Keyword(s):  
Temporal Evolution ◽  
Surface Velocity ◽  
Mean Velocity ◽  
Surface Uplift ◽  
Ice Surface ◽  
Velocity Wave ◽  
Basal Sliding ◽  
Hydrologic System ◽  
Speed Up ◽  
Cavity System

AbstractWe measured the surface velocity field during the summers of 1999 and 2000 on the 7 km long, 185 m thick Bench Glacier, Alaska, USA. In the spring of both years, a short-lived pulse of surface velocity, 2-4 times the annual mean velocity, propagated up-glacier from the terminus at a rate of ~200-250md-1. Displacement attributable to rapid sliding is ~5-10% of the annual surface motion, while the high-velocity event comprised 60-95% of annual basal motion. Sliding during the propagating speed-up event peaked at 6-14 cm d-1, with the highest rates in mid-glacier. Continuous horizontal and vertical GPS measurements at one stake showed divergence and then convergence of the ice surface with the bed as the velocity wave passed, with maximum surface uplift of 8-16 cm. High divergence rates coincided with high horizontal velocities, suggesting rapid sliding on the up-glacier side of bedrock steps. Initiation of the annual speed-up event occurred during the peak in englacial water storage, while the glacier was entirely snow-covered. Basal motion during the propagating speed-up event enlarges cavities and connections among them, driving a transition from a poorly connected hydrologic system to a well-connected linked-cavity system. Sliding is probably halted by the development of a conduit system.

scholarly journals Inversion of a glacier hydrology model

2016 ◽  
Vol 57 (72) ◽  
pp. 84-95 ◽  
Author(s):  
Douglas J. Brinkerhoff ◽  
Colin R. Meyer ◽  
Ed Bueler ◽  
Martin Truffer ◽  
Timothy C. Bartholomaus
Keyword(s):  
Covariance Structure ◽  
Monte Carlo Algorithm ◽  
Surface Velocity ◽  
Stream Discharge ◽  
Ice Surface ◽  
Input Flux ◽  
Outlet Stream ◽  
Hydrologic System ◽  
Parameter Values ◽  
Averaged Model

ABSTRACTThe subglacial hydrologic system exerts strong controls on the dynamics of the overlying ice, yet the parameters that govern the evolution of this system are not widely known or observable. To gain a better understanding of these parameters, we invert a spatially averaged model of subglacial hydrology from observations of ice surface velocity and outlet stream discharge at Kennicott Glacier, Wrangell Mountains, AK, USA. To identify independent parameters, we formally non-dimensionalize the forward model. After specifying suitable prior distributions, we use a Markov-chain Monte Carlo algorithm to sample from the distribution of parameter values conditioned on the available data. This procedure gives us not only the most probable parameter values, but also a rigorous estimate of their covariance structure. We find that the opening of cavities due to sliding over basal topography and turbulent melting are of a similar magnitude during periods of large input flux, though turbulent melting also exhibits the greatest uncertainty. We also find that both the storage of water in the englacial system and the exchange of water between englacial and subglacial systems are necessary in order to explain both surface velocity observations and the relative attenuation in the amplitude of diurnal signals between input and output flux observations.

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