Pattern Of Melt Research Articles

Mixing and Thermal Transport Behavior in a Pin or Non-Pin Extruder Equipped with a Field Synergy Elongation Screw.

The ductile forming process of a polymer in a standard screw extruder and pin-barrel extruder, equipped with or without a field synergy elongation screw, was investigated by the finite element method. In order to assess the mixing and heat transfer capabilities of screws, characteristic parameters such as the mixing efficiency, segregation scale, and temperature distribution of different structures were analyzed and compared. The results indicated that the flow pattern of the polymer melt in the extruder was significantly influenced by the screw structure and was improved by the newly designed field synergy screw configuration, which brought a desirable elongational flow to enhance the radial convection. This was attributed to the unique radial wedge-shaped repeated convergence region of the field synergy elongation screw, increasing the synergistic effect between the velocity field, velocity gradient field, and temperature gradient field and thus improving the heat transfer and mixing efficiency. After adding barrel pins, the flow was forced to split, resulting in a more significant stretching effect on the melt. The field synergy effect in the pin mixed region was strengthened, which further increased the heat and mass transfer efficiency of the screw. However, increasing barrel pins could also lead to undesired temperature fluctuation and flow resistance, which have a negative impact on the melt uniformity. This study offers an important reference for optimizing screw structure to obtain strong mixing and heat transfer performances.

Study on root hump formation mechanism in full penetration laser welding with backing gas for aluminum alloy

The dynamic behavior of molten pool in full penetration laser welding (FPLW) with backing gas or without backing gas for aluminum alloy was observed. It is found that the backing gas can greatly improve the root hump formation. A transient and multi-phase 3D numerical model considering the metallic vapor ejection process was developed to explain this phenomenon, in which an evaporation and condensation model was established to simulate the production of metallic vapor. Based on the proposed model, the metallic vapor ejection, periodic keyhole variation profiles and molten pool evolution were presented, which indicated that the flow pattern of melt in the molten pool was deeply affected by the vapor ejection pattern. The ejection velocity of vapor is 45–70 m s −1 . The high-speed vapor makes the melt flow (MF) velocity within 1 mm of the keyhole wall change from 5.5 m s −1 to 0.2 m s −1 . The high velocity gradient near the keyhole is the main reason to form an invert cone-shaped sagging under the keyhole and some globular spatters near the keyhole outlet. The surface tension plays an important role to shorten the invert cone-shaped sagging and improves root hump formation. Alumina film formed on the back surface of molten pool can worsen the root hump formation due to the lack of surface tension. • An innovative transient and multi-phase 3D numerical model considered the metallic vapor ejection process was developed to simulate the root hump formation process in full penetration laser welding of aluminum alloy. • The flow pattern of melt in the molten pool evolution was presented in the full penetration laser welding with backing gas. • Proposing that the high-speed metallic vapor results in a significant velocity gradient near the keyhole wall, which is the main reason to form an invert cone-shaped sagging under the keyhole. • Proposing that the surface tension plays an important role to shorten the invert cone-shaped sagging and backing gas is helpful for improving the formation of root hump.

The effect of lateral variations in Earth structure on Last Interglacial sea level

SUMMARYIt is generally agreed that the Last Interglacial (LIG; ∼130–115 ka) was a time when global average temperatures and global mean sea level were higher than they are today. However, the exact timing, magnitude and spatial pattern of ice melt is much debated. One difficulty in extracting past global mean sea level from local observations is that their elevations need to be corrected for glacial isostatic adjustment (GIA), which requires knowledge of Earth’s internal viscoelastic structure. While this structure is generally assumed to be radially symmetric, evidence from seismology, geodynamics and mineral physics indicates that large lateral variations in viscosity exist within the mantle. In this study, we construct a new model of Earth’s internal structure by converting shear wave speed into viscosity using parametrizations from mineral physics experiments and geodynamic constraints on Earth’s thermal structure. We use this 3-D Earth structure, which includes both variations in lithospheric thickness and lateral variations in viscosity, to calculate the first 3-D GIA prediction for LIG sea level. We find that the difference between predictions with and without lateral Earth structure can be metres to 10s of metres in the near field of former ice sheets, and up to a few metres in their far field. We demonstrate how forebulge dynamics and continental levering are affected by laterally varying Earth structure, with a particular focus on those sites with prominent LIG sea level records. Results from four 3-D GIA calculations show that accounting for lateral structure can act to increase local sea level by up to ∼1.5 m at the Seychelles and minimally decrease it in Western Australia. We acknowledge that this result is only based on a few simulations, but if robust, this shift brings estimates of global mean sea level from these two sites into closer agreement with each other. We further demonstrate that simulations with a suitable radial viscosity profile can be used to locally approximate the 3-D GIA result, but that these radial profiles cannot be found by simply averaging viscosity below the sea level indicator site.

Rheological rationalization of in situ nanofibrillar structure development: Tailoring of nanohybrid shish-kebab superstructures of poly (lactic acid) crystalline phase

The key rheological parameters for in situ development of poly (butylene terephthalate) nanofibrils (PBT NFs) in poly (lactic acid) (PLA) matrix using a tuned melt spinning process were studied. Observation of ellipsoidal PBT domains oriented along a short die was attributed to a low interfacial tension between the blend constituents and fulfilled melt rheological criteria for fibrillation in a simple shear flow. The stability of the deformed droplets under the shear and elongational flows was studied by comparing the droplet breakup time and residence time in each flow field. It was demonstrated that the deformed droplets (ellipses) with an increased robust interface underwent coalescence through pinch-off in the subsequent elongational flow field, and transformed to the nanofibrils with an average diameter comparable to the gyration radius (Rg) of PLA chains. In the fabricated isotropic in situ nanofibrillar composites (NFCs), a random orientation of 2D nanohybrid shish-kebab (NHSK) superstructure was observed as a result of the orthogonal pattern of melt crystallized PLA nanodomains on the PBT NFs surface. The NHSK superstructure established in quiescent bulk through mechanism of geometric confinement (soft epitaxy) enabled us to tailor the crystalline morphology via annealing temperature. In a constant nanofibril content, increasing the isothermal temperature (Tiso) up to 114 °C led to an increase in the kebabs length and interlocking of the adjacent superstructures. A transition to spherulitic crystal morphology was observed due to further temperature increase (Tiso≥117 °C). The observed transition indicated that at higher Tiso, the fewer raw nuclei, which were initiated by soft epitaxy mechanism of PLA chain segments on the PBT NFs at the early stage of nucleation, could conduct self-nucleation and 3D lateral growth. The Avrami model was found to be applicable for interpreting variations of crystallite morphology, crystallization rate, and nucleation mechanisms with PBT NF content and isotherm temperature.

Controls on jökulhlaup-transported buried ice melt-out at Skeiðarársandur, Iceland: Implications for the evolution of ice-marginal environments

High-magnitude jökulhlaups, glacier margin position and ice-thickness have been identified as key controls on sandur evolution. Existing models however have focused primarily on observations made during short windows of time and often do not account for the subsequent modification of proglacial landsystems by repeated jökulhlaups or post-depositional modification due to melt out over decadal time-scales. Digital Elevation Models (DEMs) were used to reconstruct the development of large depressions on Skeiðarársandur, an outwash plain in southeast Iceland. These depressions measure up to 1 km in width and up to 13 m in depth and are associated with ice bodies up to 1 km in length and up to 150 m in height emplaced during a high-magnitude jökulhlaup in 1903 and subsequently buried by jökulhlaups in 1913 and 1922. The continued melting of the Harðaskriða ice bodies over a century following their emplacement, together with subsequent repeated burial, by high-magnitude jökulhlaups demonstrates that jökulhlaups may continue to serve as important controls on sandur evolution on a decadal to centennial timescale (101–102 years). The Harðaskriða depressions developed only following the retreat of the glacier margin after 1945, which highlights the controls of margin position on the evolution of the sandur. Margin position and thickness of the glacier profile was seen to affect not only the distribution and thickness of sediment emplaced during jökulhaups but also the rate and pattern of melt in the decades following the decoupling of the margin from the sandur. The jökulhlaup landsystem model signatures identified at this site may provide a useful analogue for interpreting landforms and strata emplaced by glacier margin fluctuations, jökulhlaups and melt out generated by retreating continental Pleistocene ice sheets.

X-ray diffraction study of structure of CaO-Al2O3-SiO2 ternary compounds in molten and crystalline states

Anorthite and gehlenite crystalline structure and short-range order of anorthite melt have been studied by X-ray diffraction in the temperature range from room temperature up to ~ 1923 K. The corresponding anorthite and gehlenite phases were identified as well as amorphous component for anorthite samples having identical shape to XRD pattern of the anorthite melt. The structure factor and the radial distribution function of atoms of the anorthite melt were calculated from the X-ray high-temperature experimental data. The partial structural parameters of the short-range order of the melt were reconstructed using Reverse Monte Carlo simulations.

La Réunion Island dunites as analogs of the Martian chassignites: Tracking trapped melts with incompatible trace elements

Fractional crystallization of basalts produces a variety of ultramafic cumulates poor in incompatible elements. In order to discuss the effect of trapped liquids on the concentrations of incompatible elements in these cumulates, we studied a series of dunites and pyroxenites from the Piton de la Fournaise (La Réunion Island, Indian Ocean). These rocks have a high variability in the abundance of these elements, which is only partially explained by their proportions of olivines and pyroxenes. Mass balance calculations indicate that ≈0.25 to 1 wt% of trapped melts are required to account for the budget of the highly incompatible element abundances such as U, Rb, Nb or light Rare Earth Elements (REE). Using our results, we revisited the REE geochemistry of chassignites, a grouplet of Martian meteorites. Chassignites are mineralogically, texturally and chemically very similar to the dunites from La Réunion Island, which can be seen as very close terrestrial analogs. The variations of REE abundances and ratios in Chassigny, one of the three known chassignites, can be explained by inhomogeneous distribution of trapped melts in this meteorite at the scale of the size of the fragments (typically <1 g) used for the chemical analyses. Using these variations, we deduced the shape of the REE pattern and abundances of the Chassigny parent melt.

A Southern Ocean Mechanism for the Interhemispheric Coupling and Phasing of the Bipolar Seesaw

AbstractThe last glacial period is punctuated by abrupt changes in Northern Hemisphere temperatures that are known as Dansgaard–Oeschger (DO) events. A striking and largely unexplained feature of DO events is an interhemispheric asymmetry characterized by cooling in Antarctica during periods of warming in Greenland and vice versa—the bipolar seesaw. Methane-synchronized ice core records indicate that the Southern Hemisphere lags the Northern Hemisphere by approximately 200 years. Here, we propose a mechanism that produces observed features of both the bipolar seesaw and the phasing of DO events. The spatial pattern of sea ice formation and melt in the Southern Ocean imposes a rigid constraint on where water masses are modified: waters are made denser near the coast where ice forms and waters are made lighter farther north where ice melts. This pattern, coupled to the tilt of density surfaces across the Southern Ocean and the stratification of the ocean basins, produces two modes of overturning corresponding to different bipolar seesaw states. We present evolution equations for a simplified ocean model that describes the transient adjustment of the basin stratification, the Southern Ocean surface density distribution, and the overturning strength as the ocean moves between these states in response to perturbations in North Atlantic Deep Water formation, which we take as a proxy for Greenland temperatures. Transitions between different overturning states occur over a multicentennial time scale, which is qualitatively consistent with the observed Southern Hemisphere lag. The volume of deep density layers varies inversely with the overturning strength, leading to significant changes in residence times. Evidence of these dynamics in more realistic circulation models is discussed.

The Impact of Föhn Winds on Surface Energy Balance During the 2010–2011 Melt Season Over Larsen C Ice Shelf, Antarctica

AbstractWe use model data from the Antarctic Mesoscale Prediction System (AMPS), measurements from automatic weather stations and satellite observations to investigate the association between surface energy balance (SEB), surface melt, and the occurrence of föhn winds over Larsen C Ice Shelf (Antarctic Peninsula) over the period November 2010 to March 2011. Föhn conditions occurred for over 20% of the time during this period and are associated with increased air temperatures and decreased relative humidity (relative to nonföhn conditions) over the western part of the ice shelf. During föhn conditions, the downward turbulent flux of sensible heat and the downwelling shortwave radiation both increase. However, in AMPS, these warming tendencies are largely balanced by an increase in upward latent heat flux and a decrease in downwelling longwave radiation so the impact of föhn on the modeled net SEB is small. This balance is highly sensitive to the representation of surface energy fluxes in the model, and limited validation data suggest that AMPS may underestimate the sensitivity of SEB and melt to föhn. There is broad agreement on the spatial pattern of melt between the model and satellite observations but disagreement in the frequency with which melt occurs. Satellite observations indicate localized regions of persistent melt along the foot of the Antarctic Peninsula mountains which are not simulated by the model. Furthermore, melt is observed to persist in these regions during extended periods when föhn does not occur, suggesting that other factors may be important in controlling melt in these regions.

Evolution of the lithospheric mantle beneath Mt. Baekdu (Changbaishan): Constraints from geochemical and Sr–Nd–Hf isotopic studies on peridotite xenoliths in trachybasalt

Major and trace element compositions of minerals as well as Sr–Nd–Hf isotopic compositions of clinopyroxenes from spinel peridotite xenoliths entrained in Late Cenozoic trachybasalt from Mt. Baekdu (Changbaishan) were used to elucidate lithospheric mantle formation and evolution in the eastern North China Craton (NCC). The analyzed peridotites were mainly spinel lherzolites with rare harzburgites. They consisted of olivine (Fo89.3–91.0), enstatite (Wo1–2En88–90Fs8–11), diopside (Wo45–50En45–51Fs4–6), and spinel (Cr#=8.8–54.7). The peridotite residues underwent up to ~25% partial melting in fertile mid-ocean-ridge basalt (MORB) mantle. Plots of the Cr# in spinel against the Mg# in coexisting olivine or spinel suggested an affinity with abyssal peridotites. Comparisons of Cr# and TiO2 in spinel were also compatible with an abyssal peridotite-like composition; however, harzburgites were slightly enriched in TiO2 because of the reaction with MORB-like melt. Temperatures estimated using two-pyroxene thermometry ranged from 750 to 1010°C, reflecting their lithospheric mantle origin. The rare earth element (REE) patterns in clinopyroxenes of the peridotites varied from light REE (LREE) depleted to spoon shaped to LREE enriched, reflecting secondary overprinting effects of metasomatic melts or fluids on the residues from primordial melting. The calculated trace element pattern of metasomatic melt equilibrated with clinopyroxene in Mt. Baekdu peridotite showed strong enrichment in large-ion lithophile elements, Th and U together with slight fractionation in heavy REEs (HREEs) and considerable depletion in Nb and Ti. The Sr–Nd–Hf isotopic compositions of clinopyroxenes separated from the peridotites varied from more depleted than present-day MORB to bulk Earth values. However, some clinopyroxene showed a decoupling between Nd and Sr isotopes, deviating from the mantle array with a high 87Sr/86Sr ratio. This sample also showed a significant Nd–Hf isotope decoupling lying well above the mantle array. The Lu–Hf and Sm–Nd model ages of residual clinopyroxenes yielded Early Proterozoic to Phanerozoic ages. No signature of Archean cratonic mantle was present. Therefore, Mt. Baekdu peridotite is residual lithospheric mantle that has undergone variable degrees of diachronous melt extraction and infiltration metasomatism involving subduction-related, fluid-bearing silicate melts. The predominance of Phanerozoic Hf model ages indicates that the lherzolites represent lithospheric mantle fragments newly accreted underneath the eastern NCC.

Cloud–Atmospheric Boundary Layer–Surface Interactions on the Greenland Ice Sheet during the July 2012 Extreme Melt Event

Regional model simulations of the 10–13 July 2012 extreme melt event over the Greenland Ice Sheet (GIS) are used to investigate how low-level liquid-bearing clouds impact surface energy fluxes, and therefore the energy available for melt. A sensitivity study in which the radiation code is modified so that cloud liquid and ice do not emit, absorb, or reflect radiation is used to identify cloud impacts beyond the cloud radiative effect. It is found that Arctic mixed-phase stratocumuli are not produced in the sensitivity experiment, highlighting that cloud radiative fluxes are required to maintain the clouds. A number of feedbacks are found that damp the warming effect of the clouds. Thin mixed-phase clouds increase the downward longwave fluxes by 100 W m−2, but upward daytime surface longwave fluxes increase by 20 W m−2 (60 W m−2 at night) and net shortwave fluxes decrease by 40 W m−2 (partially due to a 0.05 increase in surface albedo), leaving only 40 W m−2 available for melt. This 40 W m−2 is distributed between the turbulent and conductive ground fluxes, so it is only at times of weak turbulent fluxes (i.e., at night or during melt) that this energy goes into the conductive ground flux, providing energy for melt. From these results it is concluded that it is the integrated impact of the clouds over the diurnal cycle (the preconditioning of the snowpack by the clouds at night) that made melt possible during this 3-day period. These findings are extended to understand the pattern of melt observed over the GIS.

Modelling the response of ice shelf basal melting to different ocean cavity environmental regimes

ABSTRACTWe present simulation results from a version of the Regional Ocean Modeling System modified for ice shelf/ocean interaction, including the parameterisation of basal melting by molecular diffusion alone. Simulations investigate the differences in melting for an idealised ice shelf experiencing a range of cold to hot ocean cavity conditions. Both the pattern of melt and the location of maximum melt shift due to changes in the buoyancy-driven circulation, in a different way to previous studies. Tidal forcing increases both the circulation strength and melting, with the strongest impact on the cold cavity case. Our results highlight the importance of including a complete melt parameterisation and tidal forcing. In response to the 2.4°C ocean warming initially applied to a cold cavity ice shelf, we find that melting will increase by about an order of magnitude (24 × with tides and 41 × without tides).

Sensitivity of glacier runoff projections to baseline climate data in the Indus River basin

Quantifying the contribution of glacier runoff to water resources is particularly important in regions such High Mountain Asia, where glaciers provide a large percentage of seasonal river discharge and support large populations downstream. In remote areas, direct field measurements of glacier melt rates are difficult to acquire and rarely observed, so hydro-glaciological modeling and remote sensing approaches are needed. Here we present estimates of glacier melt contribution to the Upper Indus watershed over the last 40 years using a suite of seven reanalysis climate datasets that have previously been used in hydrological models for this region, a temperature-index melt model and > 29,000 km2 of ice cover. In particular, we address the uncertainty in estimates of meltwater flux that is introduced by the baseline climate dataset chosen, by comparing the results derived from each. Mean annual glacier melt contribution varies from 8 km3 yr-1 and 169 km3 yr-1, or between 4-78% of the total annual runoff in the Indus, depending on temperature dataset applied. Under projected scenarios of an additional 2-4°C of regional warming by 2100 AD, we find annual meltwater fluxes vary by >200% depending on the baseline climate dataset used and, importantly, span a range of positive and negative trends. Despite significant differences between climate datasets and the resulting spread in meltwater fluxes, the spatial pattern of melt is highly correlated and statistically robust across all datasets. This allows us to conclude with confidence that fewer than 10% of the >20,000 glaciers in the watershed contribute more than 80% of the total glacier runoff to the Indus. These are primarily large, low elevation glaciers in the Karakoram and Hindu Kush. Additional field observations to ground-truth modeled climate data will go far to reduce the uncertainty highlighted here and we suggest that efforts be focused on those glaciers identified to be most significant to water resources.

Horizontal continuous casting process under electromagnetic field for preparing AA3003/AA4045 clad composite hollow billets

A modified horizontal continuous casting process under the electromagnetic field was proposed for preparing AA3003/AA4045 clad composite hollow billets. To investigate the effect of electromagnetic field on this process, a comprehensive three-dimensional model was developed. Two cases with and without electromagnetic field were compared using the simulations. When rotating electromagnetic stirring is applied, the flow pattern of fluid melt is greatly modified; the mushy zone becomes much wider, the temperature profile becomes more uniform, and the solid fraction decreases for both the external and internal alloy melt layers. These modifications are beneficial for the formation of a bimetal interface and fine and uniform grain structure of the clad composite hollow billet. Experiments conducted using the same electrical and casting parameters as the simulations verify that under the electromagnetic field the microstructure of the clad composite hollow billet becomes fine and the diffusion of the elements at the interface is promoted.

Surface melt and ponding on Larsen C Ice Shelf and the impact of föhn winds

AbstractA common precursor to ice shelf disintegration, most notably that of Larsen B Ice Shelf, is unusually intense or prolonged surface melt and the presence of surface standing water. However, there has been little research into detailed patterns of melt on ice shelves or the nature of summer melt ponds. We investigated surface melt on Larsen C Ice Shelf at high resolution using Envisat advanced synthetic aperture radar (ASAR) data and explored melt ponds in a range of satellite images. The improved spatial resolution of SAR over alternative approaches revealed anomalously long melt duration in western inlets. Meteorological modelling explained this pattern by föhn winds which were common in this region. Melt ponds are difficult to detect using optical imagery because cloud-free conditions are rare in this region and ponds quickly freeze over, but can be monitored using SAR in all weather conditions. Melt ponds up to tens of kilometres in length were common in Cabinet Inlet, where melt duration was most prolonged. The pattern of melt explains the previously observed distribution of ice shelf densification, which in parts had reached levels that preceded the collapse of Larsen B Ice Shelf, suggesting a potential role for föhn winds in promoting unstable conditions on ice shelves.

Trends in Antarctic Peninsula surface melting conditions from observations and regional climate modeling

Multidecadal meteorological station records and microwave backscatter time‐series from the SeaWinds scatterometer onboard QuikSCAT (QSCAT) were used to calculate temporal and spatial trends in surface melting conditions on the Antarctic Peninsula (AP). Four of six long‐term station records showed strongly positive and statistically significant trends in duration of melting conditions, including a 95% increase in the average annual positive degree day sum (PDD) at Faraday/Vernadsky, since 1948. A validated, threshold‐based melt detection method was employed to derive detailed melt season onset, extent, and duration climatologies on the AP from enhanced resolution QSCAT data during 1999–2009. Austral summer melt on the AP was linked to regional‐ and synoptic‐scale atmospheric variability by respectively correlating melt season onset and extent with November near‐surface air temperatures and the October–January averaged index of the Southern Hemisphere Annular Mode (SAM). The spatial pattern, magnitude, and interannual variability of AP melt from observations was closely reproduced by simulations of the regional model RACMO2. Local discrepancies between observations and model simulations were likely a result of the QSCAT response to, and RACMO2 treatment of, ponded surface water, and the relatively crude representation of coastal climate in the 27 km RACMO2 grid.

Simulated Internal Storage Buildup, Release, and Runoff from Greenland Ice Sheet at Kangerlussuaq, West Greenland

This study focused on simulated glacier surface conditions (simulated Surface Melt and liquid Precipitation available for supra-, en-, sub-, and proglacial flow processes [after vertical percolation and potential storage within the snowpack] [henceforth SMP]), internal water storage and release, and runoff from the Kangerlussuaq drainage area of the Greenland Ice Sheet (GrIS), West Greenland, for the period 2006/2007 to 2007/2008. GrIS winter accumulation and summer ablation processes, including SMP, was simulated on both daily and hourly time steps. Using hourly meteorological driving data produced more realistic meteorological conditions instead of daily-averaged data, in relation to snow and melt threshold surface processes, and produced 9–17% higher annual cumulative SMP. The difference between simulated SMP and observed catchment runoff showed a decreasing lag time through the summer, and a drainage system storage buildup through approximately June and early July of up to 0.29 × 109 m3, and a storage release through approximately late July and August of up to 0.25 × 109 m3. The simulated total Kangerlussuaq SMP for 2006/2007 and 2007/2008, indicated a reduction of 30%. This reduction in SMP occurred simultaneously with the reduction in the overall pattern of satellite-derived GrIS surface melt from 2007 to 2008.

Surface elevation changes at the front of the Ross Ice Shelf: Implications for basal melting

[1] Rapid melting beneath the Ross Ice Shelf (RIS) occurs near the ice front, likely in response to a secondary buoyant plume with tidal mixing, and is sensitive to seasonal water temperatures in front of the ice shelf. The front of the RIS is examined using GLAS ICESat laser altimetry data. Spatial and temporal changes in surface elevations are attributed to enhanced basal melting of the ice shelf near the ice front. Melt rates (b) increase exponentially as the front is approached, from approximately zero at 40 km from the front to an average of 2.8 ± 1.0 m a−1 within the front kilometer. Melt estimates within the front 60 km are best fit by the relationship b = 2.0 × exp(−x/11900) m a−1, where x denotes distance from the front. Frontal melt totals approximately 16 km3 a−1 in the front 40 km, which accounts for between 10% and 40% of current published estimates of total melt beneath the RIS. Spatial averaging along the front reveals a different pattern of melt in regions that have recently calved as compared to regions that have not calved. Frontal melt is modeled as a one-dimensional buoyant plume with tidal currents included. These model results imply that modest increases in sea surface temperatures will lead to considerable increases in melting beneath the ice shelf front.

Spatial distribution of surface ablation in the terminus of Rhonegletscher, Switzerland

AbstarctThe spatial pattern of glacier surface melt was measured with a resolution of 20–100m within a region extending 1 km up-glacier from the terminus of Rhonegletscher, Switzerland. The melt rate was monitored from 6 July to 6 September 2009 using 44 ablation stakes. We also measured the surface albedo near the stakes to investigate the importance of this parameter for the melt-rate distribution. The melt rate varied from 32.8 to 71.9 mm w.e. d–1 in the study area. Our measurements suggest that the spatial variation of the melt rate can be explained by (1) shading of the ice surface by neighbouring mountains, (2) surface albedo and (3) effects of microclimate (e.g. radiation from sidewalls) on the surface energy balance. The observed melt-rate distribution was compared to the results of a temperature-index melt model, which takes into account shading of direct solar illumination but not the other two effects. The model reproduces some important features of the field data, but its spatial variations are generally less than the measured values. Our study shows the importance of albedo and other local conditions in the accurate estimation of the small-scale melt-rate distribution.

Variability of basal melt beneath the Pine Island Glacier ice shelf, West Antarctica

AbstractObservations from satellite and airborne platforms are combined with model calculations to infer the nature and efficiency of basal melting of the Pine Island Glacier ice shelf, West Antarctica, by ocean waters. Satellite imagery shows surface features that suggest ice-shelf-wide changes to the ocean’s influence on the ice shelf as the grounding line retreated. Longitudinal profiles of ice surface and bottom elevations are analyzed to reveal a spatially dependent pattern of basal melt with an annual melt flux of 40.5 Gt a−1. One profile captures a persistent set of surface waves that correlates with quasi-annual variations of atmospheric forcing of Amundsen Sea circulation patterns, establishing a direct connection between atmospheric variability and sub-ice-shelf melting. Ice surface troughs are hydrostatically compensated by ice-bottom voids up to 150 m deep. Voids form dynamically at the grounding line, triggered by enhanced melting when warmer-than-average water arrives. Subsequent enlargement of the voids is thermally inefficient (4% or less) compared with an overall melting efficiency beneath the ice shelf of 22%. Residual warm water is believed to cause three persistent polynyas at the ice-shelf front seen in Landsat imagery. Landsat thermal imagery confirms the occurrence of warm water at the same locations.