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In the current ice-sheet models calving of ice shelves is based on phenomenological approaches. To obtain physics-based calving criteria, a viscoelastic Maxwell model is required accounting for short-term elastic and long-term viscous deformation. On timescales of months to years between calving events, as well as on long timescales with several subsequent iceberg break-offs, deformations are no longer small and linearized strain measures cannot be used. We present a finite deformation framework of viscoelasticity and extend this model by a nonlinear Glen-type viscosity. A finite element implementation is used to compute stress and strain states in the vicinity of the ice-shelf calving front. Stress and strain maxima of small (linearized strain measure) and finite strain formulations differ by ~ 5% after 1 and by ~ 30% after 10 years, respectively. A finite deformation formulation reaches a critical stress or strain faster, thus calving rates will be higher, despite the fact that the exact critical values are not known. Nonlinear viscosity of Glen-type leads to higher stress values. The Maxwell material model formulation for finite deformations presented here can also be applied to other glaciological problems, for example, tidal forcing at grounding lines or closure of englacial and subglacial melt channels.
The focus of this article is understanding the perceptions that Canadian Indigenous teachers and students have on a novel physical activity teaching method that is utilised in the classroom. We explore the role that physical activity plays in the healthy growth and development of children, with special attention to Indigenous children. Through participatory action research and qualitative methods, Indigenous teachers and students share their thoughts on physical activity in the classroom and the barriers and facilitators that exist to implementing classroom physical activity. This work is discussed in the context of the recent Truth and Reconciliation Commission of Canada and provides the reader with tangible suggestions for movement integration in the classroom.
The nucleocapsid protein NC is a crucial component in the human immunodeficiency virus type 1 life cycle. It functions both in its processed mature form and as part of the polyprotein Gag that plays a key role in the formation of new viruses. NC can protect nucleic acids (NAs) from degradation by compacting them to a dense coil. Moreover, through its NA chaperone activity, NC can also promote the most stable conformation of NAs. Here, we explore the balance between these activities for NC and Gag by confining DNA–protein complexes in nanochannels. The chaperone activity is visualized as concatemerization and circularization of long DNA via annealing of short single-stranded DNA overhangs. The first ten amino acids of NC are important for the chaperone activity that is almost completely absent for Gag. Gag condenses DNA more efficiently than mature NC, suggesting that additional residues of Gag are involved. Importantly, this is the first single DNA molecule study of full-length Gag and we reveal important differences to the truncated Δ-p6 Gag that has been used before. In addition, the study also highlights how nanochannels can be used to study reactions on ends of long single DNA molecules, which is not trivial with competing single DNA molecule techniques.
Automated planning has been a continuous field of study since the 1960s, since the notion of accomplishing a task using an ordered set of actions resonates with almost every known activity domain. However, as we move from toy domains closer to the complex real world, these actions become increasingly difficult to codify. The reasons range from intense laborious effort, to intricacies so barely identifiable, that programming them is a challenge that presents itself much later in the process. In such domains, planners now leverage recent advancements in machine learning to learn action models, that is, blueprints of all the actions whose execution effectuates transitions in the system. This learning provides an opportunity for the evolution of the model toward a version more consistent and adapted to its environment, augmenting the probability of success of the plans. It is also a conscious effort to decrease laborious manual coding and increase quality. This paper presents a survey of the machine learning techniques applied for learning planning action models. It first describes the characteristics of learning systems. It then details the learning techniques that have been used in the literature during the past decades, and finally presents some open issues.
This article explores Free French responses to Allied planning for post-war international relief in Europe. A number of French experts in exile, often veterans of the League of Nations, advocated international co-operation with the nascent United Nations Relief and Rehabilitation Administration (UNRRA). For such figures, participating in the UNRRA could bring critical knowledge, political legitimacy, experience, and funds. They also hoped that this participation could bolster French prestige in the wake of the recent experience of defeat and foreign occupation. Their efforts had little impact on the early development of international relief, yet the contacts and exchanges between French and Allied planners resulted in a political imperative that gave a new impetus to the post-war restructuring of French relief abroad. Studying the complex inter-relationship between French foreign policy and humanitarian efforts during the Second World War can offer historians a framework through which to reconsider French attempts to reassert their power globally. Crucially, this article argues that the UNRRA was used by a number of French expert planners as a platform from which to pursue broader political aims, notably the reassertion of republican legitimacy and the re-establishment of national sovereignty.
A narrow bridge of floating ice that connected the Wilkins Ice Shelf, Antarctica, to two confining islands eventually collapsed in early April 2009. In the month preceding the collapse, we observed deformation of the ice bridge by means of satellite imagery and from an in situ GPS station. TerraSAR-X images (acquired in stripmap mode) were used to compile a time series. The ice bridge bent most strongly in its narrowest part (westerly), while the northern end (near Charcot Island) shifted in a northeasterly direction. In the south, the ice bridge experienced compressive strain parallel to its long axis. GPS position data were acquired a little south of the narrowest part of the ice bridge from 19 January 2009. Analysis of these data showed both cyclic and monotonic components of motion. Meteorological data and re-analysis of the output of weather-prediction models indicated that easterly winds were responsible for the cyclic motion component. In particular, wind stress on the rough ice melange that occupied the area to the east exerted significant pressure on the ice bridge. The collapse of the ice bridge began with crack formation in the southern section parallel to the long axis of the ice bridge and led to shattering of the southern part. Ultimately, the narrowest part, only 900 m wide, ruptured. The formation of many small icebergs released energy of >125 ×106 J.
King George Island is located at the northern tip of the Antarctic Peninsula, which is influenced by maritime climate conditions. The observed mean annual air temperature at sea level is –2.4˚C. Thus, the ice cap is regarded as sensitive to changing climatic conditions. Ground-penetrating radar surveys indicate a partly temperate ice cap with an extended water layer at the firn/ice transition of the up to 700 m high ice cap. Measured firn temperatures are close to 0˚C at the higher elevations, and they differ considerably from the measured mean annual air temperature. The aim of this paper is to present ice-flow dynamics by means of observations and simulations of the flow velocities. During several field campaigns from 1997/98 to 2008/09, ice surface velocities were derived with repeated differential GPS measurements. Ice velocities vary from 0.7 m a−1 at the dome to 112.1 m a−1 along steep slopes. For the western part of the ice cap a three-dimensional diagnostic full-Stokes model was applied to calculate ice flow. Parameters of the numerical model were identified with respect to measured ice surface velocities. The simulations indicate cold ice at higher elevations, while temperate ice at lower elevations is consistent with the observations.
Numerical simulations of the temperature regime of the ice shelf Fimbulisen, Antarctica, are presented. A vertical temperature profile (S1) of Fimbulisen has been measured at the extension of Jutulstraumen, in which the temperature decreases with depth. The three-dimensional steady-state temperature field was computed by a finite-element technique. Horizontal flow velocities and surface accumulation rates were derived from observations. The basal melt rate distribution arose from an assumption of balance in the mass continuity equation. The computed basal melt rate distribution (ab) indicates that the highest basal melt rates, up to 15 m a 1 occur at the inflow gate of Jutulstraumen, and low basal melt rates (<0.6ma 1) occur in the slower moving parts. Where the ice shelf overhangs the continental shelf, ab ~1.2ma−1 . The resulting temperature field indicates that Fimbulisen consists of a cold middle part, built up by the extension of Jutulstraumen, and warmer ice masses in slow-moving areas to the west and east. Furthermore, model runs were set up in which the atmospheric temperatures increased in +1 K steps. The results suggest that the warming effectively increases the temperatures throughout the ice column in the slower-moving parts, therefore enhancing shear at the margins of the extension of Jutulstraumen.
Two diagnostic, dynamic/thermodynamic ice-shelf models are applied to the Brunt Ice Shelf/Stancomb-Wills Ice Tongue system, located off Caird Coast, Coats Land, Antarctica. The Brunt Ice Shelf/Stancomb-Wills Ice Tongue system is characterized as a thin, unbounded ice shelf with an atypical and highly heterogeneous structure. In contrast to other ice shelves, a composite mass of icebergs that calved at the grounding line and were then locked within fast (sea) ice exists between the fast-moving Stancomb-Wills Ice Stream and the slow-moving Brunt Ice Shelf. We simulate the present flow regime of the ice shelf that results from the ice-thickness distribution and the inflow at the grounding line with two different models, and compare the model results with feature tracking and InSAR flow velocities. We then incorporate two observed features, a rift and a shear margin, into the models with two different approaches, and demonstrate the effects of variations in numerical values for the shear strength and viscosity in these zones on the simulated velocity field. A major result is that both kinds of implementation of the rifts lead to similar effects on the entire velocity field, while there are discrepancies in the vicinity of the rifts.
A diagnostic, dynamic/thermodynamic ice-shelf model is applied to the George VI Ice Shelf, situated in the Bellinghausen Sea, Antarctica. The George VI Ice Shelf has a peculiar flow geometry which sets it apart from other ice shelves. Inflow occurs along the two longest, and almost parallel, sides, whereas outflow occurs on the two ice fronts that are relatively short and situated at opposite ends of the ice shelf. Two data sources were used to derive the ice thickness distribution: conventional radioecho sounding from the British Antarctic Survey was combined with thickness inferred from surface elevation obtained by the NASA GLAS satellite system assuming hydrostatic equilibrium. We simulate the present ice flow over the ice shelf that results from the ice thickness distribution, the inflow at the grounding line and the flow rate factor. The high spatial resolution of the ice thickness distribution leads to very detailed simulations. The flow field has some extraordinary elements (e.g. the stagnation point characteristics resulting from the unusual ice-shelf geometry).
To understand the dynamics of ice shelves, a knowledge of their internal and basal structure is very important. As the capacity to perform local surveys is limited, remote sensing provides an opportunity to obtain the relevant information. We must prove, however, that the relevant information can be obtained from remote sensing of the surface. That is the aim of this study. The Jelbart Ice Shelf, Antarctica, exhibits a variety of surface structures appearing as stripe-like features in radar imagery. We performed an airborne geophysical survey across these features and compared the results to TerraSAR-X imagery. We find that the stripe-like structures indicate surface troughs coinciding with the location of basal channels and crevasse-like features, revealed by radio-echo sounding. HH and VV polarizations do not show different magnitude. In surface troughs, the local accumulation rate is larger than at the flat surface. Viscoelastic modelling is used to gain an understanding of the surface undulations and their origin. The surface displacement, computed with a Maxwell model, matches the observed surface reasonably well. Our simulations show that the surface troughs develop over decadal to centennial timescales.
We study the presence and effect of subglacial water on the motion of inland ice in western Dronning Maud Land, Antarctica. A full-Stokes model including three routing schemes for a thin film of subglacial water and a modification of a Weertman-type sliding relation, to account for higher sliding velocities under wet basal conditions, were used to perform 200 ka spin-up simulations on a 2.5 km grid. Subsequent 30 ka simulations with wet and dry basal conditions were analysed for the effects of sliding on the thermal regime and velocities. The occurrence of the major ice streams in this area is mainly controlled by the ice and bedrock geometry. Smaller glaciers only appear as pronounced individual glaciers when subglacial water is taken into account. The thermal regime is affected by creep instabilities produced by an ice rheology including a microscopic water content, leading to cyclic behaviour on millennial timescales of the ice flow and occurrence of temperate ice at the base.
Predictions of marine ice-sheet behaviour require models able to simulate grounding-line migration. We present results of an intercomparison experiment for plan-view marine ice-sheet models. Verification is effected by comparison with approximate analytical solutions for flux across the grounding line using simplified geometrical configurations (no lateral variations, no buttressing effects from lateral drag). Perturbation experiments specifying spatial variation in basal sliding parameters permitted the evolution of curved grounding lines, generating buttressing effects. The experiments showed regions of compression and extensional flow across the grounding line, thereby invalidating the boundary layer theory. Steady-state grounding-line positions were found to be dependent on the level of physical model approximation. Resolving grounding lines requires inclusion of membrane stresses, a sufficiently small grid size (<500 m), or subgrid interpolation of the grounding line. The latter still requires nominal grid sizes of <5 km. For larger grid spacings, appropriate parameterizations for ice flux may be imposed at the grounding line, but the short-time transient behaviour is then incorrect and different from models that do not incorporate grounding-line parameterizations. The numerical error associated with predicting grounding-line motion can be reduced significantly below the errors associated with parameter ignorance and uncertainties in future scenarios.
Ice-stream dynamics are strongly controlled by processes taking place at the ice/bed interface where subglacial water both lubricates the base and saturates any existing, underlying sediment. Large parts of the former Eurasian ice sheet were underlain by thick sequences of soft, marine sediments and many areas are imprinted with geomorphological features indicative of fast flow and wet basal conditions. Here, we study the effect of subglacial water on past Eurasian ice-sheet dynamics by incorporating a thin-film model of basal water flow into the ice-sheet model SICOPOLIS and use it to better represent flow in temperate areas. The adjunction of subglacial hydrology results in a smaller ice-sheet building up over time and generally faster ice velocities, which consequently reduces the total area fraction of temperate basal ice and ice streaming areas. Minima in the hydraulic pressure potential, governing water flow, are used as indicators for potential locations of past subglacial lakes and a probability distribution of lake existence is presented based on estimated lake depth and longevity.
The goal of pediatric palliative care (PPC) is to maintain the quality of life (QoL) of children whose lives are threatened. However, there are sparse scientific data on the domains of QoL in this particular context, and no measurement strategies are available. The present study aims to describe the domains of QoL in the context of PPC in oncology, according to the perceptions of professional caregivers.
Semistructured interviews were conducted with a random sample of 20 professional caregivers from the Division of Hematology/Oncology at Le Centre Hospitalier Universitaire Sainte-Justine (Montréal, Canada). The caregivers were asked about their perceptions about the QoL of the children they have cared for in this context. The data were analyzed using inductive thematic content analysis.
The analysis allowed us to identify seven domains of QoL: “physical comfort,” “alleviation of psychological suffering,” “fun and the present moment,” “sense of control,” “feeling valued and appreciated,” “feeling that life goes on,” and “meaningful social relationships.”
Significance of Results:
Caregivers recount the regard that should be accorded to maintaining well-being and a sense of fun, as well as fostering the child's abilities, taking account of the progression of the disease, and to fulfilling his or her needs, especially social ones. Our results also demonstrate that all domains were positively referred to by professional caregivers. The data from our study will lead to better assessment of QoL according to the trajectory of a child with advanced cancer while undergoing PPC.
We experimentally study resonant interactions of oblique surface gravity waves in a large basin. Our results strongly extend previous experimental results performed mainly for perpendicular or collinear wave trains. We generate two oblique waves crossing at an acute angle, while we control their frequency ratio, steepnesses and directions. These mother waves mutually interact and give birth to a resonant wave whose properties (growth rate, resonant response curve and phase locking) are fully characterized. All our experimental results are found in good quantitative agreement with four-wave interaction theory with no fitting parameter. Off-resonance experiments are also reported and the relevant theoretical analysis is conducted and validated.
Calving mechanisms are still poorly understood and stress states in the vicinity of ice-shelf fronts are insufficiently known for the development of physically motivated calving laws that match observations. A calving model requires the knowledge of maximum tensile stresses. These stresses depend on different simulation approaches and material models. Therefore, this study compares results of a two-dimensional (2-D) continuum approach using finite elements with results of a one-dimensional (1-D) beam model elaborated in Reeh (1968). A purely viscous model, as well as a viscoelastic Maxwell model, is applied for the 2-D case. The maximum tensile stress usually appears at the top surface of an ice shelf. Its location and magnitude are predominantly influenced by the thickness of the ice shelf and the height of the freeboard, the traction-free part at the ice front. More precisely, doubling the thickness leads to twice the stress maximum, while doubling the freeboard, based on changes of the ice density, results in an increase of the stress maximum by 61%. Poisson's ratio controls the evolution of the maximum stress with time. The viscosity and Young's modulus define the characteristic time of the Maxwell model and thus the time to reach the maximum principal stress.