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Flow of Greenland ice as a function of climatically determined ice characteristics, flow history and conditions at depth, studied by laboratory experiments on grip ice samples

Flow of Greenland ice as a function of climatically determined ice characteristics, flow history and conditions at depth, studied by laboratory experiments on grip ice samples

In the following section the objectives of this study are re stated, prior to critically analyzing the test results. The flow law of ice is strongly temperature and strain-rate dependent. Pressure indirectly affects the flow of ice in that it suppresses cracking activity and, providing it is sufficiently high, converts any air bubbles into air hydrates. It also has the effect of depressing the melting temperature of the ice. The physical characteristics of the ice also have a profound effect on its response to applied stress. Crystallographic anisotropy, a feature common to all deep glacier ice, is associated with strongly anisotropic flow stress. As ice is buried to greater depths or moves along a flow line its degree of crystallographic anisotropy changes and so will its strength. Due to this complexity ice sheet rheology cannot be explained by non-newtonian flow laws derived from tests performed on laboratory manufactured isotropic ice samples. It is only by testing samples of real glacier ice, together with all its complex physical characteristics, under in-situ conditions of pressure and temperature that an accurate picture of ice sheet rheology and its variation with depth can be derived. Ideally a number of samples should be tested from each depth, cut at different orientations to the vertical axis of the ice core. However in this study samples were taken from the vertical direction only due to size limitations imposed by the cutting scheme of the core (see chapter 3, Fig 3.2). Flow law parameters can therefore be derived in the vertical direction only. The results will have significant implications for the dating of the GRIP core and consequently the dating of pre historic global warming and cooling events.
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Tidal bending of ice shelves as a mechanism for large-scale  temporal variations in ice flow

Tidal bending of ice shelves as a mechanism for large-scale temporal variations in ice flow

The Filchner–Ronne, Larsen and to a lesser extent Ross ice shelves are situated in tidally energetic regions and thereby subjected to large vertical motion at tidal frequencies. By far the largest tidal amplitudes are in the Weddell Sea region, particularly at the grounding line of large ice streams such as Rutford and Evans (Padman et al., 2002). In the grounding zone (here defined as a band along the grounding lines that extends several kilometres into the main shelf) the ice bends to accommodate these large vertical tidal motions. This bend- ing generates longitudinal and shear stresses within the ice which contribute to the effective stress and are strongest near the grounding line during high and low tide. Since ice is a non-Newtonian shear thinning fluid, its effective viscosity will be altered by these tidal stresses. A schematic showing how vertical tidal motion can lead to a reduction in effective viscosity of ice shelf shear margins is shown in Fig. 1. This effect, which we will call “flexural ice softening”, leads to an increase in ice velocity during high and low tide. We will show that this is a direct consequence of the non-linearity of Glen’s flow law.
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Variability in ice motion at a land terminating Greenlandic outlet glacier: the role of channelized and distributed drainage systems

Variability in ice motion at a land terminating Greenlandic outlet glacier: the role of channelized and distributed drainage systems

Modelled water pressures follow the same trends as moulin water levels during the short period in which these observations exist (Fig. 7b). Peak daily water pressures rise steadily throughout the last days of May before dropping again, while the predicted spike in water pressure on 5 June is recorded as a period of high water level at L7. The modelled trunk and moulin water pressures are well equili- brated at night, when local supraglacial inputs are often absent. During the day however, modelled moulin water pressures rise much higher than trunk channel water pres- sures. The magnitude of these fluctuations reflects the degree of variability in discharge through the tributary and trunk channels. Because the local meltwater input to the tributary fluctuates greatly over diurnal cycles, the channel cross section is poorly adjusted to discharge at any time and pressure may depart significantly from the steady-state value (as reflected in the results of Bartholomew and others (2012), who used only the local melt input to model water pressure). Discharge in the trunk channel (as evidenced by the proglacial discharge; Fig. 5g) varies comparatively little over diurnal cycles because it reflects the integration of numerous water sources over a large catchment (Bartholomew and others, 2011b). As such, the channel cross section remains relatively well adjusted and the pres- sure fluctuations are smaller (Fig. 8). The effect of varying the flow law parameter A is discussed in the following section (Section 4.1).
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The effect of rock particles and D2O replacement on the flow behaviour of ice

The effect of rock particles and D2O replacement on the flow behaviour of ice

at all conditions. (a) pure ice; (b) ice + 10 vol.% fluorite samples; (c) ice + 25 vol.% fluorite; (d) ice + 50 vol.% fluorite. As well as an increase of measured differential stress due to an increased strain-rate and higher rock fraction, the differential stress also increases with lower temperature and higher confining pressure. Higher strain-rate data shows a good fit to the calculated flow laws for T = 253 K, whereas lower strain-rate data for pure ice, 25 and 50 vol.% rock may be better described by a GSS regime represented by the n = 1.8 flow law. The temperature dependence of the flow law is well described for pure ice, but for 50 vol.% rock, the flow law underestimates the measured strengths. Lighter blue diamonds are lower bounds on steady-state stresses due to extended transient creep effect discussed in text. Errors on stresses are 1 s.d., flow law parameters are detailed in Table 2.
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Persistent tracers of historic ice flow in glacial stratigraphy near Kamb Ice Stream, West Antarctica

Persistent tracers of historic ice flow in glacial stratigraphy near Kamb Ice Stream, West Antarctica

Abstract. Variations in properties controlling ice flow (e.g., topography, accumulation rate, basal friction) are recorded by structures in glacial stratigraphy. When anomalies that disturb the stratigraphy are fixed in space, the structures they produce advect away from the source and can be used to trace flow pathways and reconstruct ice-flow patterns of the past. Here we provide an example of one of these persistent tracers: a prominent unconformity in the glacial layering that originates at Mt. Resnik, part of a subglacial volcanic com- plex near Kamb Ice Stream in central West Antarctica. The unconformity records a change in the regional thinning be- havior seemingly coincident ( ∼ 3440 ± 117 a) with stabiliza- tion of grounding-line retreat in the Ross Sea Embayment. We argue that this feature records both the flow and thin- ning history far upstream of the Ross Sea grounding line, indicating a limited influence of observed ice-stream stag- nation cycles on large-scale ice-sheet routing over the last ∼ 5700 years.
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Antarctic palaeo-ice streams

Antarctic palaeo-ice streams

(2008)). Similar calculations by Shipp et al. (1999) gave a retreat rate of ~50 m yr -1 . Further rates of retreat to the current grounding line position (900 km further inshore) may have been considerably faster 89-140 m yr -1 , whilst grounding line retreat from Drygalski Ice Tongue to Ross Island was also thought to be rapid (317 m yr -1 ). Large GZW on the outer shelf and MSGL preserved along entire length of mapped trough (Shipp et al. 1999).

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Transition of flow regime along a marine-terminating outlet glacier in East Antarctica

Transition of flow regime along a marine-terminating outlet glacier in East Antarctica

Abstract. We present results of a multi-methodological ap- proach to characterize the flow regime of West Ragnhild Glacier, the widest glacier in Dronning Maud Land, Antarc- tica. A new airborne radar survey points to substantially thicker ice (> 2000 m) than previously thought. With a dis- charge estimate of 13–14 Gt yr −1 , West Ragnhild Glacier thus becomes of the three major outlet glaciers in Dronning Maud Land. Its bed topography is distinct between the up- stream and downstream section: in the downstream section (< 65 km upstream of the grounding line), the glacier over- lies a wide and flat basin well below the sea level, while the upstream region is more mountainous. Spectral analy- sis of the bed topography also reveals this clear contrast and suggests that the downstream area is sediment covered. Furthermore, bed-returned power varies by 30 dB within 20 km near the bed flatness transition, suggesting that the water content at bed/ice interface increases over a short dis- tance downstream, hence pointing to water-rich sediment. Ice flow speed observed in the downstream part of the glacier ( ∼ 250 m yr −1 ) can only be explained through very low basal friction, leading to a substantial amount of basal sliding in the downstream 65 km of the glacier. All the above lines of evi- dence (sediment bed, wetness and basal motion) and the rel- atively flat grounding zone give the potential for West Ragn- hild Glacier to be more sensitive to external forcing com- pared to other major outlet glaciers in this region, which are more stable due to their bed geometry (e.g. Shirase Glacier).
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Interpolation methods for Antarctic ice-core timescales: application to Byrd, Siple Dome and Law Dome ice cores

Interpolation methods for Antarctic ice-core timescales: application to Byrd, Siple Dome and Law Dome ice cores

The age uncertainty at the methane tie points is well de- scribed (e.g., Blunier et al., 1998, 2007; Pedro et al., 2011; Stenni et al., 2011), but uncertainty introduced by the inter- polation scheme is often given less attention. Two common approaches have been used to interpolate between methane tie points. In the first approach, variations of linear interpo- lation were used to construct timescales for Taylor Dome (Steig et al., 1998), Byrd (Blunier and Brook, 2001), and Siple Dome (Brook et al., 2005). Law Dome (Pedro et al., 2011) does not use linear interpolation, instead assuming constant accumulation rates between tie points and a thinning function computed with a one-dimensional ice-flow model. The resulting timescale, however, has many of the same fea- tures as a timescale constructed with linear interpolation, as is shown below. In the second approach, Bayesian sta- tistical methods are applied to multiple ice cores simulta- neously (Lemieux-Dudon et al., 2010). This method starts with initial timescales derived from ice-flow modeling driven by accumulation-rate histories based on the stable-isotope records. The initial timescales are then adjusted to optimize the agreement among initial timescales and age constraints by taking into account their respective confidence intervals. This method has been used to construct consistent timescales for EDML, EDC, Vostok, TALDICE, and NGRIP (Lemieux- Dudon et al., 2010; Buiron et al., 2011; Schüpbach et al., 2011; Veres et al., 2013; Bazin et al., 2013). Uncertainties are
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Incorporation of rheological properties into ice sheet flow models

Incorporation of rheological properties into ice sheet flow models

The functional relation for the enhancement factor with position was based on the laboratory studies of the development of anisotropic tertiary flow rates for different stress configurat[r]

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Ice-stream flow switching by up-ice propagation of instabilities along glacial marginal troughs

Ice-stream flow switching by up-ice propagation of instabilities along glacial marginal troughs

al., 2017), and therefore the competition for the ice drainage basin should have a major influence on the growth and decay of an ice sheet. Accordingly, abrupt changes in ice-stream networks are expected to influence ice sheet stability. Where switching occurs and leads to the merging of two ice streams, the ice flow acceleration can propagate upstream in both glacial drainage basins (i.e., in both Scott and Sam Ford in the study area). Merged ice streams can then affect a greater area of the ice sheet and lead to ice-divide migration (Green- wood and Clark, 2009). Such flow switching of ice streams can provide more efficient and rapid pathways for continental ice to reach the ocean, possibly leading to a more rapid draw- down of the ice sheet. The merging of ice streams through ice piracy should result in an increase in ice discharge and ero- sion rates in the “winning” trough. The winning ice stream gains mass balance and should consequently equilibrate by advancing its margins if it has not already reached the shelf break. The seaward extent to the shelf break of Scott Trough and its overdeepened bedrock basin cannot be unequivocally attributed to ice-stream switching. However, the longitudi- nal basins originating from Sam Ford Fiord represent al- most half of the width of Scott Trough. These basins sug- gest that the switch could account for up to half of the ero- sion of Scott Trough. Ice-stream switching through marginal troughs can therefore lead to more extensive glaciations and to the erosion of deeper glacial troughs on high-latitude con- tinental shelves. Increasing depths behind grounding zones are known to create a positive feedback during glacial retreat that results in faster retreat of the ice margin in progressively deeper waters (Mercer, 1978; Schoof, 2007; Joughin and Al- ley, 2011). Therefore, the erosion of a marginal trough lead- ing to a flow switch is expected to make ice sheets occupying similar trough systems on continental shelves more sensitive to climate forcing, in both the short (within a glacial cycle) and long term (over multiple glacial cycles).
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Greenland Ice Mapping Project: ice flow velocity variation at sub-monthly to decadal timescales

Greenland Ice Mapping Project: ice flow velocity variation at sub-monthly to decadal timescales

Abstract. We describe several new ice velocity maps pro- duced by the Greenland Ice Mapping Project (GIMP) using Landsat 8 and Copernicus Sentinel 1A/B data. We then fo- cus on several sites where we analyse these data in conjunc- tion with earlier data from this project, which extend back to the year 2000. At Jakobshavn Isbræ and Køge Bugt, we find good agreement when comparing results from different sensors. In a change from recent behaviour, Jakobshavn Is- bræ began slowing substantially in 2017, with a midsummer peak that was even slower than some previous winter min- ima. Over the last decade, we identify two major slowdown events at Køge Bugt that coincide with short-term advances of the terminus. We also examined populations of glaciers in north-west and south-west Greenland to produce a record of speed-up since 2000. Collectively these glaciers continue to speed up, but there are regional differences in the timing of periods of peak speed-up. In addition, we computed trends in winter flow speed for much of the south-west margin of the ice sheet and find little in the way of statistically signif- icant changes over the period covered by our data. Finally, although the consistency of the data is generally good over time and across sensors, our analysis indicates that substan- tial differences can arise in regions with high strain rates (e.g. shear margins) where sensor resolution can become a factor. For applications such as constraining model inversions, users should factor in the impact that the data’s resolution has on their results.
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Are long subglacial bedforms indicative of fast ice flow?

Are long subglacial bedforms indicative of fast ice flow?

If subglacial till deformation is an important control on the fast ow of some ice streams (cf. Alley et al. 1986), then the deforming material might be attenuated in the downstream direction. However, if pervasive deformation is inhibited, some workers have suggested that the development of elongated bedforms may be hindered. This scenario was invoked by Boyce & Eyles (1991), who suggested that decreased duration of subglacial sediment deformation was responsible for a downstream decrease in drumlin elongation ratio in the Simcoe Lobe of the Laurentide Ice Sheet. A similar downstream decrease in drumlin elongation ratio characterizes the M’Clintock Channel palaeo-ice stream bed, Victoria Island, Arctic Canada (Clark & Stokes 2001). They argued that it is related to a reduction in the availability of sediment for subglacial deformation immediately prior to ice stream shut-down. Several arguments were given by Clark (1993) to suggest why mega-scale glacial lineations (up to 70 km in length) are most likely to record the location of former ice streams. Assuming point initiation of features and their downstream attenuation, long bed- forms may arise from high velocities over short time- scales or slower velocities over longer timescales. As more evidence is being uncovered which suggests that former ice sheets changed their volume and area rapidly, the latter option becomes less likely (cf. Andrews et al. 1983; Boulton & Clark 1990a, b; MacAyeal 1993). This led Stokes & Clark (1999) to postulate highly attenuated bedforms (elongation ratios > 10:1) as a speciŽc geomorphological criterion for identifying palaeo-ice streams. Hart (1999) performed a more quantitative analysis to substantiate this proposi- tion by matching variations in elongation ratios across the Lake Cayuga area of the New York State drumlin Želd with the lateral velocity patterns across two contemporary ice streams. This lateral variation in elongation ratios, whereby longer bedforms occur towards the central axis of ow within a lobe/ice stream, is well documented (e.g. Dyke & Morris 1988; Mitchell 1994) and provides strong evidence that bedform attenuation is related to ice velocity.
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Halley Research Station, Antarctica: calving risks and monitoring strategies

Halley Research Station, Antarctica: calving risks and monitoring strategies

Before the 1970s, there are three critical ice-front mea- surements that define our knowledge of calving history: one from 1915, and two from 1956 and 1958. Frank Worsely, on the Shackleton-led Endurance expedition in 1915, was the first to survey the SWGT and the BIS (Worsely, 1921). At this time the SWGT was at its most advanced state known (see Fig. 7) whereas the BIS appears far more retreated, as can be seen in Fig. 4. Ice-front positions were not measured again until the visit of an Argentinean ship in 1956 (Expe- dition, 1955). By this time the SWGT had undergone a sig- nificant calving event, with the 1956 front showing its least- advanced known position. The BIS also displays a similar retreated state, implying calving occurred between 1915 and 1956. In 1958, the Royal Society conducted a comprehen- sive survey of the shelf, providing a survey-quality reliable ice-front position (IGY, 1958). This 1958 map is inconsis- tent with the 1956 map, but consistent with maps from sub- sequent surveys. For this reason the 1958 survey is used as the most retreated position of the ice front that can be trusted. In terms of calving cycles, it is evident that significant calving occurred on the BIS between 1915 and 1956, but a specific date is difficult to ascertain. The data does not al- low us to conclude if only one or several calving events oc- curred in this period. A summary of these data can be found in Fig. 5. This figure shows the ice-front position measured
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Representation of basal melting at the grounding line in ice flow models

Representation of basal melting at the grounding line in ice flow models

Experiment 1 simulates the evolution of the glacier when ocean-induced melt is applied under floating ice. The equa- tion that governs the melt rate in this experiment pro- vides limited melt close to the grounding line, as the wa- ter column thickness becomes smaller (see Eq. 3). Fig- ure 5 shows the evolution of the ice volume above floata- tion for this experiment for the different sub-element melt parameterizations, the different mesh resolutions, and the two friction laws. The volume above floatation lost (see also Table 2) varies between 4140 and 6690 Gt for the EXP1_Weertman_2km_NMP and EXP1_Tsai_2km_FMP scenarios, respectively. Experiments performed with the Tsai friction law show a larger mass loss (between 5480 and 6690 Gt over the 100-year period) than the ones per- formed with a Weertman friction law (between 4140 and 5410 Gt). The impact of the sub-element melt parameteri- zation adopted, however, is more pronounced in the case of Weertman sliding law. The Tsai sliding law shows similar results for all sub-element parameterizations if the mesh res- olution is 1 km and under, suggesting that any sub-element melt parameterization can be adopted in this case. Results performed at 2 km resolution all overestimate the mass loss, except when the NMP is adopted, in which case they under- estimate the mass loss. If the Weertman sliding law is ap- plied, the results are strongly dependent on both the sub-
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Comparison of Flow Characteristics of Centrifugal Compressors by Numerical Modelling of Flow

Comparison of Flow Characteristics of Centrifugal Compressors by Numerical Modelling of Flow

The distributions of relative velocities on the radius R= 0.075 m (Figs. 15 - 16) and in outlet section (Figs. 17-18) for both types of impellers are especially illustrative. At first sight it may be observed that in both sections the distributions of relative velocities with “the second type of impeller” are more uniform (Figs. 15 and 17) than with “the first type of impeller” (Figs. 16 and 18). However, it should be noted that in neither type of impeller in the outlet sections the “jet-wake” is not present, only with “the first type of impeller” it is possible to speak about its almost negligible presence. The distributions of velocities both in axial and in circumferential direction are satisfactorily uniform, especially with “the second type of impeller”. This contributes to effective diffuser operation, which is confirmed by Figs. 5-6, where significant pressure increase is observed with uniform distribution. All this tells about the positive influence of the usage of reactive impellers with the aim of preventing the flow non-uniformity at the outlet from impeller, i.e. of “jet- wake” flow, especially of “the second type of impeller”.
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Active subglacial lakes and channelized water flow beneath the Kamb Ice Stream

Active subglacial lakes and channelized water flow beneath the Kamb Ice Stream

The Kamb Ice Stream (KIS), located on the eastern bound- ary of the Ross Ice Shelf, ceased streaming ice flow approx- imately 160 years ago (Retzlaff and Bentley, 1993). This event significantly affected the mass balance of the West Antarctic Ice Sheet (WAIS), locally incurring a net mass gain equal to ∼ 20 % of the net mass loss of the WAIS (Rignot et al., 2008). Hulbe and Fahnestock (2007) and Catania et al. (2012) have suggested that the Siple Coast Ice Streams have experienced several stagnation and reactivation cycles. Abrupt change in the basal hydrological system has been cited as a possible cause for the stagnation in several stud- ies (Anandakrishnan and Alley, 1997; Catania et al., 2006; Retzlaff and Bentley, 1993). Van der Wel et al. (2013) also showed numerically that the period of the long-term velocity cycles in the KIS is strongly associated with its subglacial hydrology, ice thermodynamics, and till regime. All these factors are related to the basal melt rate and upstream sub- glacial water supplies. One hypothesis for the KIS stagnation is that it resulted from a change in the configuration of the subglacial drainage system from sheet flow to channelized water flow (Retzlaff and Bentley, 1993), although no previ- ous direct evidence of channelized flow beneath the KIS has been observed. Another hypothesis suggests that reduced lu- brication of the KIS basal interface, caused by a change in the subglacial water pathway in an upstream region, provoked the stagnation of the downstream region. This is known as the water-piracy hypothesis (Anandakrishnan and Alley, 1997).
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Human Trafficking Victims

Human Trafficking Victims

and will be publically available for anyone who ICE HSI and FLETC, pursuant to funding and resources, will continue to provide training and technical assistance to federal, state, territorial, tribal, and local law enforcement organizations and other targeted groups using DHS’s law enforcement with state and local law enforcement to load DHS’s Web-based state and local law enforcement human learning systems or use it in police academies. This interactive module introduces law enforcement how to recognize indicators that someone may be a victim. This training could be supplemented by the state or locality to create a comprehensive importance of victim-centered practices.
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A Holocene trace chemistry record from law dome ice cores

A Holocene trace chemistry record from law dome ice cores

reference horizons will cause errors in the timescale. Often it is difficult to obtain reliable dates for reference horizons beyond recent history, especially in the Southern Hemisphere (SH), as historical documentation of eruptions is poor. The dating accuracy is compromised by variations in snowfall from the average. It is crucial to avoid circular arguments whereby volcanic signatures in ice core records are assigned a date based on some external information (e.g. historical documents, tree ring records, and other ice cores) and then used to verify the dating of that particular event. The assignment of dates does not advance the dating accuracy of the volcanic record – the record will only be as accurate as the record it has been synchronized to, and has the potential to inadvertently reinforce the acceptance of an initially questionable date. Cores dated by non-independent methods remain essential to understanding the spatial distribution patterns of volcanic sulphate aerosols, and dating of deep ice where layer counting cannot be reliably performed. Ice cores dated via accurate layer counting, independent of previously reported volcanic event dates – such as Law Dome – are key to producing high accuracy volcanic chronologies, and improving the dating of events recorded during the first millennium CE, which offers the opportunity to improve correlations between ice cores, as well as better constraining the timing of past volcanic eruptions.
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Sheet, stream, and shelf flow as progressive ice-bed uncoupling: Byrd Glacier, Antarctica and Jakobshavn Isbrae, Greenland

Sheet, stream, and shelf flow as progressive ice-bed uncoupling: Byrd Glacier, Antarctica and Jakobshavn Isbrae, Greenland

Abstract. The first-order control of ice thickness and height above sea level is linked to the decreasing strength of ice- bed coupling along flowlines from an interior ice divide to the calving front of an ice shelf. Uncoupling progresses as a frozen bed progressively thaws for sheet flow, as a thawed bed is progressively drowned for stream flow, and as lat- eral and/or local grounding vanish for shelf flow. This can reduce ice thicknesses by 90 % and ice elevations by 99 % along flowlines. Original work presented here includes (1) replacing flow and sliding laws for sheet flow with upper and lower yield stresses for creep in cold overlying ice and basal ice sliding over deforming till, respectively, (2) replacing in- tegrating the Navier–Stokes equations for stream flow with geometrical solutions to the force balance, and (3) includ- ing resistance to shelf flow caused by lateral confinement in a fjord and local grounding at ice rumples and ice rises. A comparison is made between our approach and two ap- proaches based on continuum mechanics. Applications are made to Byrd Glacier in Antarctica and Jakobshavn Isbrae in Greenland.
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Effects of Slip Condition on the Characteristic of Flow in Ice Melting Process

Effects of Slip Condition on the Characteristic of Flow in Ice Melting Process

In the experimental work of Carey and Gebhart [18], slow patterns were visualized by seeding the water and illuminating the flow field with a sheet of laser light. Wilson and Vyas [19] applied the thymol – blue technique in order to visualize the velocity profile occurring in the natural convection boundary layer. Oosthuizen and Xu [20] presented evidence that the flow around a horizontal melting ice cylinder is three – dimensional in nature. Gebhart and Wang [21] melted short vertical ice cylinders into cold fresh water in order to visualize the melting convective motion. Fukasako and Yamada [22] have presented an extensive summery of the work carried out on water freezing and ice melting problems.
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