Thesis (M.S.)--University of Wisconsin--Madison, 1974. / Bound typescript (Photocopy). eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 89-91).

Thesis (Ph.D.)--Aberdeen University, 2009. / Title from web page (viewed on Dec. 2, 2009). Includes bibliographical references.

In this study, near-surface stratigraphy was measured in snowpits and shallow ice cores across all snow facies of a High Arctic ice cap in order to examine variability at different spatial (metre to &gt; kilometre) and temporal (seasonal and inter-annual) resolutions.  Additionally, dye tracing was employed to investigate processes controlling meltwater percolation and refreezing.  <i>Pre-melt </i>snowpack densities were low at all spatial scales, whilst different inter-annual <i>post-melt </i>stratigraphies at &gt; 1 km scales showed high variability.  This is attributed to different extrinsic conditions generating dissimilar surface melt and percolation regimes.  Variability at small spatial scales is related to the intrinsic stratigraphic properties of the pre-melt snowpack. A snowpack model was applied to a single point at 1400 m.a.s.l. over the course of two summers, to assess its simulation of the observed dissimilar annular density evolution. The model performed poorly due to an inability to simulate ice layers in the near surface snowpack, which are the main property controlling variability.  Modelled thermal regimes and densification were also unrealistic and found to be highly sensitive to albedo.  The model may be improved by incorporating diurnal Albedo variations; known to be a strong control on surface melting at high latitudes. Overall, combined <i>in-situ</i> measurements and modelling results show that the relationship between near-surface densification and air temperature is not straightforward.  Extrapolation of generalised trends between the two, across large spatial and temporal scales, should be avoided since spatial and temporal variability can be high.  This should be considered when up-scaled modelling of surface mass balance and firm densification is utilised during interpretations of SRA-based estimates of elevation change across large ice masses over several years.

Thesis (M.Sc.)--Memorial University of Newfoundland, 1999. / Bibliography: leaves 152-164. Also available online.

Thesis (Ph. D.)--Oregon State University, 2008. / Printout. Includes bibliographical references (leaves 138-151). Also available on the World Wide Web.

Les calottes polaires, ou inlandsis, sont parmi les principaux contributeurs à la montée des océans. Ces systèmes dynamiques gagnent de la masse par accumulation de neige, et en perdent par fonte au contact de l’océan et à la surface, ainsi que par le vêlage d’icebergs. Depuis plus de trois décennies, les observations ont montré que les calottes polaires de l’Antarctique et du Groenland perdent plus de masse qu’ils n’en gagnent. L’évolution des glaciers suite à ce déséquilibre de masse est devenue aujourd’hui l’une des problématiques les plus importantes des implications du changement climatique. Le Groupe d’experts intergouvernemental sur l’évolution du climat (GIEC) a identifié la contribution des glaciers comme l’un des facteurs clés d’incertitude de prédiction de l’élévation du niveau des mers. La modélisation numérique est le seul outil efficace pour répondre à cette question. Cependant, modéliser l’écoulement de glace à l’échelle du Groenland ou de l’Antarctique représente un défi à la fois scientifique et technique. Deux aspects clés de l’amélioration de la modélisation des glaciers sont abordés dans cette thèse. Le premier consiste à déterminer certaines propriétés non mesurables de la glace par méthode inverse. La friction ou la rigidité des barrières de glace, sont des paramètres qui ne peuvent être mesurés directement et doivent donc être déduits à partir d’observations par télédétection. Nous appliquons ici ces inversions pour trois modèles d’écoulement de glace de complexité croissante: le modèle bidimensionnel de MacAyeal/Morland, le modèle dit d’ordre supérieur de Blatter/Pattyn et le modèle full-Stokes. Les propriétés ainsi calculées sont ensuite utilisées pour initialiser des modèles grande-échelle et pour déterminer le degré de complexité minimum nécessaire pour reproduire correctement la dynamique des glaciers. Le second aspect abordé dans ce travail est l’amélioration de la consistance des données pour la modélisation numérique. Les données disponibles sont souvent issues de campagnes de mesures s’étalant sur plusieurs années et dont résolutions spatiales varient, ce qui rend leur utilisation pour des simulations numériques difficiles. Nous présentons ici un algorithme basé sur la conservation de la masse et les méthodes inverses pour construire des épaisseurs de glace qui sont consistantes avec les mesures de vitesse. Cette approche empêche la redistribution artificielle de masse qu’engendrent généralement les autres méthodes de cartographie de l’épaisseur de glace, ce qui améliore considérablement l’initialisation des modèles d’écoulement de glace. Les avancées présentées ici sont des étapes importantes afin de mieux caractériser de manière précise les glaciers et de modéliser leur évolution de manière réaliste. / Ice sheets are amongst the main contributors to sea level rise. They are dynamic systems; they gain mass by snow accumulation, and lose it by melting at the ice-ocean interface, surface melting and iceberg calving at the margins. Observations over the last three decades have shown that the Greenland and Antarctic ice sheets have been losing more mass than they gain. How the ice sheets respond to this negative mass imbalance has become today one of the most urgent questions in understanding the implications of global climate change. The Intergovernmental Panel on Climate Change (IPCC) has indeed identified the contribution of the ice sheets as a key uncertainty in sea level rise projections. Numerical modeling is the only effective way of addressing this problem. Yet, modeling ice sheet flow at the scale of Greenland and Antarctica remains scientifically and technically very challenging. This thesis focuses on two major aspects of improving ice sheet numerical models. The first consists of determining non-observable ice properties using inverse methods. Some parameters, such as basal friction or ice shelf hardness, are difficult to measure and must be inferred from remote sensing observations. Inversions are developed here for three ice flow models of increasing complexity: MacAyeal/Morland’s shelfy-stream model, Blatter/Pattyn’s higher order model and the full-Stokes model. The inferred parameters are then used to initialize large-scale ice sheet models and to determine the minimum level of complexity required to capture ice dynamics correctly. The second aspect addressed in this work is the improvement of dataset consistency for ice sheet modeling. Available datasets are often collected at different epochs and at varying spatial resolutions, making them not readily usable for numerical simulations. We devise here an algorithm based on the conservation of mass principle and inverse methods to construct ice thicknesses that are consistent with velocity measurements. This approach therefore avoids the artificial mass redistributions that occur in existing algorithms for mapping ice thickness, hence considerably improving ice sheet model initialization. The advances made here are important steps towards the ultimate objective of accurate characterization of ice sheets and the realistic modeling of their evolution.

Satellite altimetry has been traditionally used in the past few decades to measure elevation of land ice, quantify changes in ice topography and infer the mass balance of large and remote areas such as the Greenland and Antarctic ice sheets. Radar altimetry is particularly well suited to this task due to its all-weather year-round capability of observing the ice surface. However, monitoring of ice caps and ice fields - bodies of ice with areas typically smaller than ~ 10,000 km2 - has proven more challenging. The large footprint of a conventional radar altimeter and coarse ground track coverage are less suited to observing comparatively small regions with complex topography. Since 2010, the European Space Agency’s CryoSat-2 satellite has been collecting ice elevation measurements over ice caps and ice fields with its novel radar altimeter. CryoSat-2’s smaller inter-track spacing provides higher density of observations compared to previous satellite altimeters. Additionally, it generates more accurate measurements because (i) the footprint size is reduced in the along-track direction by means of synthetic aperture radar processing and (ii) interferometry allows to precisely locate the the across-track angle of arrival of a reflection from the surface. Furthermore, the interferometric capabilities of CryoSat-2 allow for the processing of the delayed surface reflections after the first echo. When applied over a sloping surface, this procedure generates a swath of elevations a few km wide compared to the conventional approach returning a single elevation. In this thesis, swath processing of CryoSat-2 interferometric data is exploited to generate topographic data over ice caps and ice fields. The dense elevation field is then used to compute maps of elevation change rates at sub-kilometer resolution with the aim of quantifying ice volume change and mass balance. A number of algorithms have been developed in this work, partly or entirely, to form a complete processing chain from generating the elevation field to calculating volume and mass change. These algorithms are discussed in detail before presenting the results obtained in two selected regions: Iceland and Patagonia. Over Icelandic ice caps, the high-resolution mapping reveals complex surface elevation changes, related to climate, ice dynamics and sub-glacial, geothermal and magmatic processes. The mass balance of each of the six largest ice caps (90% of Iceland’s permanent ice cover) is calculated independently for the first time using spaceborne radar altimetry data. Between October 2010 and September 2015 Icelandic ice caps have lost a total of 5.8± 0.7 Gt a ̄1, contributing 0.016± 0.002 mm a ̄1 to eustatic sea level rise. This estimate indicates that over this period the mass balance was 40% less negative than the preceding 15 years, a fact which partly reflects the anomalous positive balance year across the Vatnaj ̈okull ice cap (~ 70% of the glaciated area) in 2014/15. Furthermore, it is demonstrated how swath processing of CryoSat-2 interferometric data allows the monitoring of glaciological processes at the catchment scale. Comparison of the geodetic estimates of mass balance against those based on in situ data shows good agreement. The thesis then investigates surface elevation change on the Northern and Southern Patagonian Ice Fields to quantify their mass balance. This area is characterized by some of the fastest flowing glaciers in the world, displaying complex interactions with the proglacial environments (including marine fjords and freshwater lakes) they often drain into. Field observations are sparse due to the inaccessibility of these ice fields and even remotely sensed data are limited, often tied to comparisons to the topography in 2000 as measured by the Shuttle Radar Topography Mission. Despite gaps in the spatial coverage, in particular due to the complex topography, CryoSat-2 swath radar altimetry provides insight into the patterns of change on the ice fields in the most recent period (2011 to 2017) and allows to independently calculate the mass balance of glaciers or catchments as small as 300 km2. The northern part of the Southern Patagonian ice field displays the strongest losses due to a combination between ice dynamics and warming temperatures. In contrast Pio XI, the largest glacier on this ice field and in South America, is advancing and gaining mass. Between April 2011 and march 2017, the two ice fields combined have lost an average of 21.29± 1.98 Gt a ̄1 (equivalent to 0.059± 0.005 mm a ̄1 eustatic sea level rise), 24% and 42% more negative when compared to the periods 2000-2012/14 and 1975-2000. In particular the Northern Patagonian ice field, responsible for one third of the mass loss, is losing mass 70% faster compared to the first decade of the 21st century. These results confirm the overall strong mass loss of the Patagonian ice fields, second only to glaciers and ice caps in Alaska and the Canadian Arctic, and higher than High Mountain Asia, which all extend over areas ~ 5-8 times larger (excluding glaciers at the periphery of the Greenland and Antarctic ice sheets).

Les glaciers des régions sub-polaires entre 45 et 60°S ont reculé dramatiquement au cours du dernier siècle. L'archipel des Kerguelen (49°S, 69°E) constitue un site unique dans ces régions où peu d'observations sont disponibles pour comprendre le recul glaciaire. Situés à faible altitude et proches de l'océan, ses glaciers ont montré une sensibilité particulière aux variations atmosphériques et océaniques. Ainsi, depuis les années 60, la calotte Cook (~400 km2) a reculé de manière spectaculaire, perdant 20% de sa surface en 40 ans. L'objectif de mon travail de thèse était d'évaluer l'état actuel et futur de la calotte, et de comprendre les causes de ce recul tout en les replaçant dans un contexte global. Pour ce faire, un réseau météorologique et glaciologique a été mis en place en 2010 sur l'archipel et des campagnes de mesures ont depuis été réalisées annuellement. L'analyse de ces mesures nous permet de confirmer le bilan de masse négatif de la calotte. Parallèlement, l'étude de l'albédo de l'ensemble de la calotte Cook à partir d'images satellites MODIS (MODerate resolution Imaging Spectroradiometer) permet d'évaluer l'évolution de la ligne de neige de la calotte depuis 2000, mettant en évidence une réduction importante de sa zone d'accumulation au cours des dix dernières années. La modélisation du bilan de masse de la calotte Cook à l'aide d'un modèle degré-jour couplé à une routine dynamique révèle par ailleurs que son retrait est principalement dû à une forte diminution des précipitations sur l'archipel depuis les années 60. Afin de replacer le recul des glaciers aux îles Kerguelen dans un contexte global, les tendances climatiques sur l'ensemble des zones subpolaires sont étudiées, faisant apparaître que la zone sub-Antarctique est actuellement celle où les retraits sont les plus forts à l'échelle du globe. Pour comprendre ces variations, nous analysons un ensemble complet de jeux d'observations de terrain, de satellites et de résultats de modélisation : réanalyses, modèles de l'exercice CMIP5 (Coupled Model Intercomparison Project phase 5), observations de température atmosphérique et océanique, précipitations, etc. Ceux-ci révèlent un réchauffement et un assèchement quasi généralisé de l'ensemble de la zone 40-60° S, lié à un déplacement vers le sud des zones dépressionnaires en réponse aux phases de plus en plus fréquemment positives du mode annulaire austral (Southern Annular Mode, SAM). Le recul récent des glaciers des îles Kerguelen, mais également d'autres zones glaciaires des régions subpolaires de l'hémisphère sud, est donc principalement lié à un déficit d'accumulation causé par le SAM, et amplifié par le réchauffement atmosphérique. L'évolution future du bilan de masse de la calotte Cook aux îles Kerguelen est évaluée grâce au Modèle Atmosphérique Régional (MAR), forcé à ses frontières par les modèles de l'exercice CMIP5. Des simulations du bilan de masse récent sont d'abord effectuées sur base des réanalyses ERA-Interim et NCEP1, et comparées aux observations in situ. Parallèlement, des simulations d'un an sont réalisées avec le désagrégateur de précipitations SMHiL (Surface Mass balance High resolution downscaLing) en sortie du MAR, à différentes échelles, afin d'évaluer l'impact du changement d'échelle sur la représentation des précipitations. Une évaluation des modèles CMIP5 par rapport à ERA-Interim sur la période récente est ensuite réalisée sur base de certaines variables climatiques-clé. Le modèle le plus proche d'ERA-Interim sur la période récente, et les deux modèles les plus extrêmes sont ensuite utilisés pour forcer le MAR sur le prochain siècle, et les sorties de bilan de masse de surface sont analysées de manière critique. L'analyse du retrait de la calotte des îles Kerguelen à l'aide de différents outils a permis de mieux comprendre le lien entre glaciers et climat, mettant en évidence le rôle majeur du SAM, mais a également soulevé de nouvelles questions. / Glaciers of the southern hemisphere sub-polar regions between 45 and 60°S have declined dramatically over the last century. The islands of Kerguelen archipelago (49°S, 69°E) represent a unique location in regions where few data are available to understand glacier retreat. Situated at low altitudes and close to the ocean, their glaciers have shown particular sensitivity to atmospheric and oceanic variations. Thus, since the 1960s, the Cook Ice Cap (~400km2) has retreated spectacularly, losing 20% of its area in 40 years. The aim of my thesis was to assess the present and future state of the ice cap, and to understand the causes of this decline while putting them in a global context. To do so, a meteorological and glaciological network was set up in 2010 on Kerguelen archipelago and field campaigns have been carried out annually since then. Analysis of these measurements confirms the negative mass balance of Cook Ice Cap. In parallel, the study of the albedo over the whole ice cap from MODIS satellite images (MODerate resolution Imaging Spectroradiometer) gives us access to the evolution of the snow line since 2000, highlighting an important reduction of Cook Ice Cap accumulation area over the last decade. Mass balance modelling of the Cook Ice Cap using a degree-day model coupled to a simple ice motion routine further reveals that its retreat is mainly due to a strong decrease in precipitation over the Kerguelen Islands since the 1960s. In order to put the decline of the cryosphere on Kerguelen in a global context, climatic trends over the whole sub-polar regions are studied, revealing that the sub-Antarctic area is currently the one where glacier retreat is the strongest. To understand these variations, we analyse a complete set of field and satellite observations and modelling results : reanalyses, models from the CMIP5 (Coupled Model Intercomparison Project phase 5) experiment, atmospheric and oceanic temperature and precipitation observations, etc. The latter show warming and quasigeneralised drying of the whole 40-60°S area, linked to the southward shift of storm tracks in response to the more frequent positive phases of the Southern Annual Mode (SAM). Recent glacier retreat on Kerguelen archipelago, and for other glaciers and ice caps located at similar latitudes, is thus mainly due to a deficit of accumulation caused by the SAM, and amplified by atmospheric warming. The future evolution of Cook Ice Cap mass balance is evaluated using the MAR (Modèle Atmosphérique Régional) model, forced at its boundaries by CMIP5 models. Recent mass balance simulations are first carried out using ERA-Interim and NCEP1 reanalyses, and compared to in situ observations. In parallel, one-year simulations are produced with the precipitation desagregation scheme SMHiL (Surface Mass balance High resolution downscaLing) on MAR outputs, at various scales, in order to evaluate the impact of downscaling on precipitation. An evaluation of CMIP5 models over the recent period against ERA-Interim is then carried out, considering certain key climatic variables. The model closest to ERA-Interim as well as the two most extreme models are then used to force the MAR model over the next century, and surface mass balance outputs are critically analysed. The analysis of the decline of the Kerguelen ice cap using different tools and techniques brought new insights on the link between glaciers and climate, highlighting the major role of the SAM, but also raised new questions.