Ice dynamics and mass balance in the grounding zone of outlet glaciers in the Transantarctic Mountains

The Antarctic grounding zone has a disproportionately large effect on glacier dynamics and ice sheet stability relative to its size but remains poorly characterised across much of the continent. Accurate ice velocity and thickness information is needed in the grounding zone to determine glacier outflow and establish to what extent changing ocean and atmospheric conditions are affecting the mass balance of individual glacier catchments. This thesis describes new satellite remote sensing techniques for measuring ice velocity and ice thickness, validated using ground measurements collected on the Beardmore, Skelton and Darwin Glaciers and applied to other Transantarctic Mountain outlet glaciers to determine ice discharge. Outlet glaciers in the Transantarctic Mountains provide an important link between the East and West Antarctic Ice Sheets but remain inadequately studied. While long-term velocities in this region are shown here to be stable, instantaneous velocities are sensitive to stresses induced by ocean tides, with fluctuations of up to 50% of the mean observed in GPS measurements. The potential error induced in averaged satellite velocity measurements due to these effects is shown to be resolvable above background noise in the grounding zone but to decrease rapidly upstream. Using a new inverse finite-element modelling approach based on regularization of the elastic-plate bending equations, tidal flexure information from differential InSAR is used to calculate ice stiffness and infer thickness in the grounding zone. This technique is shown to be successful at reproducing the thickness distribution for the Beardmore Glacier, eliminating current issues in the calculation of thickness from freeboard close to the grounding line where ice is not in hydrostatic equilibrium. Modelled thickness agrees to within 10% of ground penetrating radar measurements. Calibrated freeboard measurements and tide-free velocities in the grounding zones of glaciers in the western Ross Sea are used to calculate grounding zone basal melt rates, with values between 1.4 and 11.8 m/a⁻¹ in this region. While strongly dependent on grounding line ice thickness and velocity, melt rates show no latitudinal trend between glaciers, although detailed error analysis highlights the need for much improved estimates of firn density distribution in regions of variable accumulation such as the Transantarctic Mountains

Long-term records of Antarctic outlet glacier dynamics from historical data and novel remote sensing techniques

Quantifying how the cryosphere responds to various climate forcings is essential in accurate forecasting of ice sheet stability as well as sea level rise. In order to better predict how future climates will impact ice sheet and glacier mass balance, it is first necessary to understand whether the observed changes in glaciers are from internal dynamics or responses to climate forcings. Equally necessary is the ability to identify if current glacier transformations are due to discrete events or ongoing phenomena. Unfortunately, most records for the world’s glaciers only extend for the last two decades, thus making long-term projections difficult. The overall aim of each project is to improve our understanding of cryosphere-climate relationships through long-term observational records and numerical modeling. The three projects are: (1) validating the use of historic aerial imagery with modern-day image processing techniques and manually extracted ground control from high-resolution imagery; (2) quantifying ~60-year surface elevation changes of outlet glaciers flowing through the Transantarctic Mountains from trimetrogon aerial photos; and (3) investigating basal crevasse initiation and determining if the formation of anomalously large crevasses can be proxies for historic Antarctic subglacial flooding events. The first project uses historic vertical aerial imagery and high-resolution optical satellite imagery; the second employs oblique aerial imagery from the 1960s and high-resolution optical satellite imagery; and the last project relies on a combination of airborne radar, in situ GPS, and optical satellite imagery. Innovative remote sensing techniques are used to acquire information that is either analyzed or integrated into numerical models (depending on the project) to draw conclusions about the stability of the ice sheet. These results further our understanding of the manner in which ice sheets and glaciers respond to changes in climate and will ultimately allow for more accurate constraints in modeling future sea level rise

Flow dynamics of Byrd Glacier, East Antarctica

This is the published version. Copyright 2014Force-balance calculations on Byrd Glacier, East Antarctica, reveal large spatial variations in the along-flow component of driving stress with corresponding sticky spots that are stationary over time. On the large scale, flow resistance is partitioned between basal (∼80%) and lateral (∼20%) drag. Ice flow is due mostly to basal sliding and concentrated vertical shear in the basal ice layers, indicating the bed is at or close to the pressure-melting temperature. There is a significant component of driving stress in the across-flow direction resulting in nonzero basal drag in that direction. This is an unrealistic result and we propose that there are spatial variations of bed features resulting in small-scale flow disturbances. The grounding line of Byrd Glacier is located in a region where the bed slopes upward. Nevertheless, despite a 10% increase in ice discharge between December 2005 and February 2007, following drainage of two subglacial lakes in the catchment area, the position of the grounding line has not retreated significantly and the glacier has decelerated since then. During the speed-up event, partitioning of flow resistance did not change, suggesting the increase in velocity was caused by a temporary decrease in basal effective pressure

Estimating Strain and Rotation tensors of glacier flow from wrapped SAR interferograms

This letter aims to discuss a general framework that allows the direct interpretation of the wrapped DInSAR phase in terms of surface strain S and rotation R components. The methodology is demonstrated showing the estimation of strain and rotation components of a glacier flow using three TerraSAR-X interferometric geometries (ascending right-looking, descending right-looking and descending left-looking. Finally since the left looking geometry can be difficult to obtain on a regular basis, the surface parallel flow assumption is extended to the phase gradients inversion in order to reduce the amount of necessary geometries from three to two

Flow dynamics of Byrd Glacier, East Antarctica

Force-balance calculations on Byrd Glacier, East Antarctica, reveal large spatial variations in the along-flow component of driving stress with corresponding sticky spots that are stationary over time. On the large scale, flow resistance is partitioned between basal (�80%) and lateral (�20%) drag. Ice flow is due mostly to basal sliding and concentrated vertical shear in the basal ice layers, indicating the bed is at or close to the pressure-melting temperature. There is a significant component of driving stress in the across-flow direction resulting in nonzero basal drag in that direction. This is an unrealistic result and we propose that there are spatial variations of bed features resulting in small-scale flow disturbances. The grounding line of Byrd Glacier is located in a region where the bed slopes upward. Nevertheless, despite a 10% increase in ice discharge between December 2005 and February 2007, following drainage of two subglacial lakes in the catchment area, the position of the grounding line has not retreated significantly and the glacier has decelerated since then. During the speed-up event, partitioning of flow resistance did not change, suggesting the increase in velocity was caused by a temporary decrease in basal effective pressure

Inverse Modelling at Recovery Glacier, Antarctica

The future ice loss of Recovery Glacier will probably be the largest of the East Antarctic Ice Sheet over the next millennia. Its evolution can be predicted by models solving the equations of the momentum and mass balance. Ice dynamics are fundamentally driven by bedrock conditions underneath the ice, but these can not simply be measured yet. This thesis utilizes an inverse method implemented in the Ice Sheet System Model (ISSM) to acquire basal parameters. The technique minimizes the difference between horizontal surface velocities derived from remote sensing and computed by the model. False values in the observations can lead to uncertainties in basal parameters. In order to remove such false values, this thesis presents a new filtering method. Data gaps are filled comparing four different interpolation methods. A sensitivity analysis shows that the influence of filtering outliers and interpolation on basal parameters derived from inverse modelling is large in specific regions. The resulting basal parameters do not indicate the existence of the previously proposed subglacial lakes at the onset of Recovery Glacier

Processing of optic and radar images.Application in satellite remote sensing of snow, ice and glaciers

Ce document présente une synthèse de mes activités de recherche depuis la soutenance de ma thèse en 1999. L'activité rapportée ici est celle d'un ingénieur de recherche, et donc s'est déroulée en parallèle d'une activité ``technique'' comprenant des taches d'instrumentation en laboratoire, d'instrumentation de plateformes en montagne, de raids scientifiques sur les calottes polaires, d'élaboration de projets scientifiques, d'organisation d'équipes ou d'ordre administratif. Je suis Ingénieur de recherche CNRS depuis 2004 affecté au laboratoire Gipsa-lab, une unité mixte de recherche du CNRS, de Grenoble-INP, de l'université Joseph Fourier et de l'université Stendhal. Ce laboratoire (d'environ 400 personnes), conventionné avec l'INRIA, l'Observatoire de Grenoble et l'université Pierre Mendès France, est pluridisciplinaire et développe des recherches fondamentales et finalisées sur les signaux et les systèmes complexes.}Lors de la préparation de ma thèse (mi-temps 1995-99) au LGGE, je me suis intéressé au traitement des images de microstructures de la neige, du névé et de la glace. C'est assez naturellement que j'ai rejoint le laboratoire LIS devenu Gipsa-lab pour y développer des activités de traitement des images Radar à Synthèse d'Ouverture (RSO) appliqué aux milieux naturels neige, glace et glaciers. Etant le premier à générer un interférogramme différentiel des glaciers des Alpes, j'ai continué à travailler sur la phase interférométrique pour extraire des informations de déplacement et valider ces méthodes sur le glacier d'Argentière (massif du Mont-Blanc) qui présente l'énorme avantage de se déplacer de quelques centimètres par jour. Ces activités m'ont amené à développer, en collaboration avec les laboratoires LISTIC, LTCI et IETR, des méthodes plus générales pour extraire des informations dans les images RSO.Ma formation initiale en électronique, puis de doctorat en physique m'ont amené à mettre à profit mes connaissances en traitement d'images et des signaux, en électromagnétisme, en calcul numérique, en informatique et en physique de la neige et de la glace pour étudier les problèmes de traitement des images RSO appliqués à la glace, aux glaciers et à la neige