scholarly journals Ice deformation very close to the ice-sheet margin in West Greenland

1992 ◽  
Vol 38 (128) ◽  
pp. 3-8 ◽  
Author(s):  
Peter G. Knight
Keyword(s):  
Detailed Analysis ◽  
Flow Direction ◽  
Surface Strain ◽  
Small Scale ◽  
Strain Rates ◽  
Glacier Surface ◽  
West Greenland ◽  
Small Areas ◽  
Strain Pattern ◽  
Fine Resolution

AbstractThis paper describes fine-resolution measurements of glacier surface strain rates very close to the margin of Russell Glacier, West Greenland. Measurements at a small scale make possible detailed analysis of strain patterns close to the glacier margin, and suggest that strain rates vary over small areas. The strain pattern is determined by ice flexure over subglacial obstacles as well as by seasonally variable marginal retardation and by the orientation of the ice margin relative to the flow direction.

scholarly journals Ice deformation very close to the ice-sheet margin in West Greenland

1992 ◽  
Vol 38 (128) ◽  
pp. 3-8 ◽  
Author(s):  
Peter G. Knight
Keyword(s):  
Detailed Analysis ◽  
Flow Direction ◽  
Surface Strain ◽  
Small Scale ◽  
Strain Rates ◽  
Glacier Surface ◽  
West Greenland ◽  
Small Areas ◽  
Strain Pattern ◽  
Fine Resolution

AbstractThis paper describes fine-resolution measurements of glacier surface strain rates very close to the margin of Russell Glacier, West Greenland. Measurements at a small scale make possible detailed analysis of strain patterns close to the glacier margin, and suggest that strain rates vary over small areas. The strain pattern is determined by ice flexure over subglacial obstacles as well as by seasonally variable marginal retardation and by the orientation of the ice margin relative to the flow direction.

scholarly journals Derived bedrock elevations, strain rates and stresses from measured surface elevations and velocities: Jakobshavns Isbræ, Greenland

1995 ◽  
Vol 41 (137) ◽  
pp. 161-173 ◽  
Author(s):  
James L. Fastook ◽  
Henry H. Brecher ◽  
Terence J. Hughes
Keyword(s):  
Flow Direction ◽  
Greenland Ice Sheet ◽  
Surface Strain ◽  
Curvilinear Coordinates ◽  
Surface Velocity ◽  
Strain Rates ◽  
Bed Topography ◽  
Ice Surface ◽  
Basal Sliding ◽  
Grid Points

AbstractJakobshavns Isbræ (69 °10′ N, 49 °59′ W) drains about 6.5% of the Greenland ice sheet and is the fastest ice stream known. The Jakobshavns Isbræ basin of about 10 000 km2was mapped photogrammetrically from four sets of aerial photography, two taken in July 1985 and two in July 1986. Positions and elevations of several hundred natural features on the ice surface were determined for each epoch by photogrammetric block aerial triangulation, and surface velocity vectors were computed from the positions. The two flights in 1985 yielded the best results and provided most common points (716) for velocity determinations and are therefore used in the modeling studies. The data from these irregularly spaced points were used to calculate ice elevations and velocity vectors at uniformly spaced grid points 3 km apart by interpolation. The field of surface strain rates was then calculated from these gridded data and used to compute the field of surface deviatoric stresses, using the flow law of ice, for rectilinear coordinates,X, Ypointing eastward and northward, and curvilinear coordinates.L, Τpointing longitudinally and transversely to the changing ice-flow direction, Ice-surface elevations and slopes were then used to calculate ice thicknesses and the fraction of the ice velocity due to basal sliding. Our calculated ice thicknesses are in fair agreement with an ice-thickness map based on seismic sounding and supplied to us by K. Echelmeyer. Ice thicknesses were subtracted from measured ice-surface elevations to map bed topography. Our calculation shows that basal sliding is significant only in the 10–15 km before Jakobshavns Isbræ becomes afloat in Jakobshavns Isfjord.

scholarly journals Derived bedrock elevations, strain rates and stresses from measured surface elevations and velocities: Jakobshavns Isbræ, Greenland

1995 ◽  
Vol 41 (137) ◽  
pp. 161-173 ◽  
Author(s):  
James L. Fastook ◽  
Henry H. Brecher ◽  
Terence J. Hughes
Keyword(s):  
Flow Direction ◽  
Greenland Ice Sheet ◽  
Surface Strain ◽  
Curvilinear Coordinates ◽  
Surface Velocity ◽  
Strain Rates ◽  
Bed Topography ◽  
Ice Surface ◽  
Basal Sliding ◽  
Grid Points

AbstractJakobshavns Isbræ (69 °10′ N, 49 °59′ W) drains about 6.5% of the Greenland ice sheet and is the fastest ice stream known. The Jakobshavns Isbræ basin of about 10 000 km2 was mapped photogrammetrically from four sets of aerial photography, two taken in July 1985 and two in July 1986. Positions and elevations of several hundred natural features on the ice surface were determined for each epoch by photogrammetric block aerial triangulation, and surface velocity vectors were computed from the positions. The two flights in 1985 yielded the best results and provided most common points (716) for velocity determinations and are therefore used in the modeling studies. The data from these irregularly spaced points were used to calculate ice elevations and velocity vectors at uniformly spaced grid points 3 km apart by interpolation. The field of surface strain rates was then calculated from these gridded data and used to compute the field of surface deviatoric stresses, using the flow law of ice, for rectilinear coordinates, X, Y pointing eastward and northward, and curvilinear coordinates. L, Τ pointing longitudinally and transversely to the changing ice-flow direction, Ice-surface elevations and slopes were then used to calculate ice thicknesses and the fraction of the ice velocity due to basal sliding. Our calculated ice thicknesses are in fair agreement with an ice-thickness map based on seismic sounding and supplied to us by K. Echelmeyer. Ice thicknesses were subtracted from measured ice-surface elevations to map bed topography. Our calculation shows that basal sliding is significant only in the 10–15 km before Jakobshavns Isbræ becomes afloat in Jakobshavns Isfjord.

scholarly journals Geocoding cryptosporidiosis cases in Ireland (2008–2017)—development of a reliable, reproducible, multiphase geocoding methodology

2021 ◽  
Author(s):  
Lisa Domegan ◽  
Patricia Garvey ◽  
Paul McKeown ◽  
Howard Johnson ◽  
Paul Hynds ◽  
...  
Keyword(s):  
Infectious Disease ◽  
Spatial Data ◽  
Application Programming Interface ◽  
Epidemiological Studies ◽  
National Standards ◽  
Small Areas ◽  
Infectious Disease Reporting ◽  
Health Related ◽  
Application Programming ◽  
Fine Resolution

Abstract Background Geocoding (the process of converting a text address into spatial data) quality may affect geospatial epidemiological study findings. No national standards for best geocoding practice exist in Ireland. Irish postcodes (Eircodes) are not routinely recorded for infectious disease notifications and > 35% of dwellings have non-unique addresses. This may result in incomplete geocoding and introduce systematic errors into studies. Aims This study aimed to develop a reliable and reproducible methodology to geocode cryptosporidiosis notifications to fine-resolution spatial units (Census 2016 Small Areas), to enhance data validity and completeness, thus improving geospatial epidemiological studies. Methods A protocol was devised to utilise geocoding tools developed by the Health Service Executive’s Health Intelligence Unit. Geocoding employed finite-string automated and manual matching, undertaken sequentially in three additive phases. The protocol was applied to a cryptosporidiosis notification dataset (2008–2017) from Ireland’s Computerised Infectious Disease Reporting System. Outputs were validated against devised criteria. Results Overall, 92.1% (4266/4633) of cases were successfully geocoded to one Small Area, and 95.5% (n = 4425) to larger spatial units. The proportion of records geocoded increased by 14% using the multiphase approach, with 5% of records re-assigned to a different spatial unit. Conclusions The developed multiphase protocol improved the completeness and validity of geocoding, thus increasing the power of subsequent studies. The authors recommend capturing Eircodes ideally using application programming interface for infectious disease or other health-related datasets, for more efficient and reliable geocoding. Where Eircodes are not recorded/available, for best geocoding practice, we recommend this (or a similar) quality driven protocol.

scholarly journals Structural glaciology of the fast-moving Jakobshavn Isbræ, Greenland, compared to the surging Bering Glacier, Alaska, U.S.A.

2000 ◽  
Vol 30 ◽  
pp. 243-249 ◽  
Author(s):  
Helmut Mayer ◽  
Ute Christina Herzfeld
Keyword(s):  
Fast Moving ◽  
Flow Direction ◽  
Kinematics And Dynamics ◽  
Similar Data ◽  
Brittle Deformation ◽  
West Greenland ◽  
Video Images ◽  
Ice Stream ◽  
Bering Glacier

AbstractCrevasse patterns revealing mostly brittle deformation on glacier surfaces are analyzed based on video images collected during systematic overflights of Jakobshavn Isbræ, West Greenland, the Earths continuously fastest moving ice stream, in 1996 and 1997. Crevasse patterns on the surface of the central ice stream are distinct. All crevasses are closed, the surface appears rather smooth. Towards the margins, typical shear patterns with conjugate shears and still-closed crevasses prevail, curved patterns indicate the bending of crevasse lines into the flow direction. Outward from this zone different patterns of open crevasses occur. This suite of patterns is compared to similar data collected over Bering Glacier, Alaska, U.S.A., during its recent surge from 1993–95. There a number of patterns of mostly open crevasses is characteristic: parallel crevasses, two-directional orthogonal open crevasses, arrays of wavy crevasses, en-echelon crevasses. These patterns of the surging glacier are completely different from those of the fast-moving ice stream indicating different underlying kinematics and dynamics.

scholarly journals Latitudinal patterns in intertidal ecosystem structure in West Greenland suggest resilience to climate change

2021 ◽  
Author(s):  
Jakob Thyrring ◽  
Susse Wegeberg ◽  
Martin E Blicher ◽  
Dorte Krause-Jensen ◽  
Signe H&oslashgslund ◽  
...  
Keyword(s):  
Climate Change ◽  
Ascophyllum Nodosum ◽  
Tidal Range ◽  
Environmental Drivers ◽  
Small Scale ◽  
Ecosystem Structure ◽  
Assemblage Composition ◽  
West Greenland ◽  
Functional Changes ◽  
Time Substitution

Climate change has ecosystem-wide cascading effects. Little is known, however, about the resilience of Arctic marine ecosystems to environmental change. Here we quantify and compare large-scale patterns in rocky intertidal biomass, coverage and zonation in six regions along a north-south gradient of temperature and ice conditions in West Greenland (60-72°N). We related the level and variation in assemblage composition, biomass and coverage to latitudinal-scale environmental drivers. Across all latitudes, the intertidal assemblage was dominated by a core of stress-tolerant foundation species that constituted >95% of the biomass. Hence, canopy-forming macroalgae, represented by Fucus distichus subsp. evanescens and F. vesiculosus and, up to 69 °N, also Ascophyllum nodosum, together with Semibalanus balanoides, occupied >70% of the vertical tidal range in all regions. Thus, a similar functional assemblage composition occurred across regions, and no latitudinal depression was observed. The most conspicuous difference in species composition from south to north was that three common species (the macroalgae Ascophyllum nodosum, the amphipod Gammarus setosus and the gastropod Littorina obtusata) disappeared from the mid-intertidal, although at different latitudes. There were no significant relationships between assemblage metrics and air temperature or sea ice coverage as obtained from weather stations and satellites, respectively. Although the mean biomass decreased >50% from south to north, local biomass in excess of 10 000 g ww m-2 was found even at the northernmost site, demonstrating the patchiness of this habitat and the effect of small-scale variation in environmental characteristics. Hence, using the latitudinal gradient in a space-for-time substitution, our results suggest that while climate modification may lead to an overall increase in the intertidal biomass in north Greenland, it is unlikely to drive dramatic functional changes in ecosystem structure in the near future. Our dataset provides an important baseline for future studies to verify these predictions for Greenlands intertidal zone.

scholarly journals Ice-divide flow at Hans Tausen Iskappe, North Greenland, from surface movement data

2001 ◽  
Vol 47 (156) ◽  
pp. 78-84 ◽  
Author(s):  
C. S. Hvidberg ◽  
K. Keller ◽  
N. Gundestrup ◽  
P. Jonsson
Keyword(s):  
Global Positioning System ◽  
Average Rate ◽  
Horizontal Velocity ◽  
Surface Strain ◽  
Surface Velocity ◽  
Strain Rates ◽  
Ice Cap ◽  
Movement Data ◽  
Global Positioning ◽  
North Greenland

AbstractSurface strain rates around the southeastern dome of Hans Tausen Iskappe in Peary Land, North Greenland (82.5° N, 27.5° W), are determined from global positioning system surveys of a strain net. Average longitudinal surface strain rate increases towards the dome, from (1.4 ± 0.2) × 10−4 a−1 at 5–10 ice thicknesses from the divide to (2.4 ± 1.0) × 10−4 a−1 within 1 ice thickness from the divide. Analysis of the data shows that the ice cap is presently building up within the strain net with an average rate of 〈∂H/∂t〉 = + 0.04 ± 0.02 m a−1. Assuming a uniform thickening, the shape factor of the horizontal velocity (the ratio between the vertically averaged horizontal velocity and the horizontal surface velocity) decreases towards the dome, from 0.9 at a distance of 10 ice thicknesses from the dome to 0.5 at the dome based on application of the continuity equation. Our results indicate that a region with anomalous flow is formed around the dome, supporting recent indications reported by Vaughan and others (1999). It is not possible from our data to constrain parameters of the flow law, because there is no independent estimate of the significant present thickening of the central part of the ice cap and its pattern around the dome.

scholarly journals The Development of Fast Response Aerodynamic Probes for Flow Measurements in Turbomachinery

1994 ◽  
Author(s):  
Roger W. Ainsworth ◽  
John L. Allen ◽  
J. Julian M. Batt
Keyword(s):  
Mach Number ◽  
Large Scale ◽  
Flow Direction ◽  
Three Dimensional ◽  
Pressure Sensors ◽  
Fast Response ◽  
Rotor Blades ◽  
Small Scale ◽  
Hot Wire ◽  
Flow Measurements

The advent of a new generation of transient rotating turbine simulation facilities, where engine values of Reynolds and Mach number are matched simultaneously together with the relevant rotational parameters for dimensional similitude (Dunn et al [1988], Epstein et al [1984]. Ainsworth et al [1988]), has provided the stimulus for developing improved instrumentation for investigating the aerodynamic flows in these stages. Much useful work has been conducted in the past using hot-wire and laser anemometers. However, hot-wire anemometers are prone to breakage in the high pressure flows required for correct Reynolds numbers, Furthermore some laser techniques require a longer runtime than these transient facilites permit, and generally yield velocity information only, giving no data on loss production. Advances in semiconductor aerodynamic probes are beginning to fulfil this perceived need. This paper describes advances made in the design, construction, and testing of two and three dimensional fast response aerodynamic probes, where semiconductor pressure sensors are mounted directly on the surface of the probes, using techniques which have previously been successfully used on the surface of rotor blades (Ainsworth, Dietz and Nunn [1991]). These are to be used to measure Mach number and flow direction in compressible unsteady flow regimes. In the first section, a brief review is made of the sensor and associated technology which has been developed to permit a flexible design of fast response aerodynamic probe. Following this, an extensive programme of testing large scale aerodynamic models of candidate geometries for suitable semiconductor scale probes is described, and the results of these discussed. The conclusions of these experiments, conducted for turbine representative mean and unsteady flows, yielded new information for optimising the design of the small scale semiconductor probes, in terms of probe geometry, sensor placement, and aerodynamic performance. Details are given of a range of wedge and pyramid semiconductor probes constructed, and the procedures used in calibrating and making measurements with them. Differences in performance are discussed, allowing the experimenter to choose an appropriate probe for the particular measurement required. Finally, the application of prototype semiconductor probes in a transient rotor experiment at HP turbine representative conditions is described, and the data so obtained is compared with (PD solutions of the unsteady viscous flow-field.

scholarly journals Strain Rates On Rutford Ice Stream, Antarctica (Abstract)

1986 ◽  
Vol 8 ◽  
pp. 207
Author(s):  
N. Stephenson ◽  
C.S.M. Doake
Keyword(s):  
Strain Rate ◽  
Velocity Gradient ◽  
Surface Strain ◽  
Lateral Strain ◽  
Strain Rate Tensor ◽  
Strain Rates ◽  
Average Value ◽  
Divergent Flow ◽  
Ice Stream ◽  
Thickness Variations

In a study of the Rutford Ice Stream, strain rates were measured on a transverse section. Magnitudes ranged up to 40 × 10−3 a−1 but were typically in the order of 3 × 10−3 a−1 with an error of 0.1 χ 10−3 a−1. Variations in the strain rate between adjacent stakes of 0.2 χ 10−3 a−1 to 2 × 10−3 a−1 were matched to the thickness variations on the glacier. For each set of three adjacent stakes, the velocity gradient components of the surface strain rate tensor were calculated by assuming that the gradients were linear over the distance between adjacent stakes. When plotted against distance across the ice stream, each strain rate component revealed different aspects of the flow field. The longitudinal strain rate was compressive, with an almost constant magnitude of 10−3 a−1. The lateral strain rate is extensive, with an average value of 1.1 × 10−3 a−1 which agreed with the angle between the divergent flow lines observed on a Landsat image. Peaks in the lateral strain rate, corresponding to longitudinal bands of thicker ice, showed that these thicker bands were spreading more rapidly at the expense of thinner areas. The two velocity gradient components of the shear rate tensor also reflected differences in ice thickness.

scholarly journals Rhombus and Rhomboid Parallelogram Patterns on Glaciers: Natural Indicators of Strain

1979 ◽  
Vol 22 (87) ◽  
pp. 247-261 ◽  
Author(s):  
Charles J. Waag ◽  
Keith Echelmeyer
Keyword(s):  
Strain Rate ◽  
Flow Direction ◽  
Acute Angle ◽  
Principal Strain ◽  
Obtuse Angle ◽  
Strain Rates ◽  
Ice Flow ◽  
Glacier System ◽  
Glacier Ice

AbstractSubtle rhombus and rhomboid parallelogram patterns occur on Vaughan Lewis Glacier and the Gilkey Glacier System, Juneau Icefield, Alaska. The patterns are within the firn at the firn-ice interface, are formed by differential recrystallization within narrow preferred zones, and are apparently manifestations of stresses transferred upward from the glacier ice. On the glaciers of the Gilkey System the patterns occur where intense lateral shortening is indicated by abrupt convergence of medial moraines and an abundance of extension crevasses. The short axes of the rhombi and the obtuse angle bisectors of the rhomboids are subparallel to the strike of extension crevasses, therefore to the axis of shortening. The long axes of the rhombi and the acute angle bisectors of the rhomboids are parallel to the foliation, and ice-flow direction. The angles of the parallelograms are variable locally, but average 105° and 75°; the variation seems to reflect intensity and duration of stress. Similar parallelograms occur within the troughs of wave bulges below the Vaughan Lewis Icefall. In the wave bulges, the foliation arcs parallel the wave. The long axes of the rhombi and acute angle bisectors of the rhomboids parallel the foliation around the foliation arc. The short axes of the rhombi and the obtuse angle bisectors of the rhomboids parallel the strikes of radial crevasses, are perpendicular to the direction of extension, and form a fan divergent down-stream. The precise mechanisms and conditions of formation of the parallelograms are not yet understood. Preliminary strain-rate measurements suggest, however, that correlations exist between the orientations of the principal strain-rates and the axes of the patterns, and between the magnitude of the strain-rates and the axial lengths of the patterns.

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