Studying the impact of proglacial lakes on ice sheet dynamics

Proglacial lakes have an important impact on the dynamics of ice sheets by imposing a marine-like boundary condition to the ice margin. At the ice-ocean interface the ice sheet is know to have a complex dynamical behavior. These processes can include the formation of ice shelves that can buttress glaciers, increase subglacial melting, reduce basal friction and calving of icebergs. Such processes also act at the boundaries at ice-marginal lakes, which arose along the ice margins of past continental ice sheets. Some of these lakes spanned several hundreds of kilometers. We have implemented a module into the Parallel Ice Sheet Model (PISM) to study the impacts of proglacial lakes on the ice dynamics. This model dynamically updates the lake basins by filling all depressions of the domain until they overflow. This simple approach does not calculate a water budget and therefore does not rely on an advanced and computationally expensive hydrology model. The lake boundaries are treated in a similar way as a marine boundary. We test this model by reconstructing the evolution of the North American ice sheets by using a transient climate forcing for the last glacial cycle. These reconstructions are then compared to geological records, such as ice margins and paleo-lake shorelines

The impact of spatially varying ice sheet basal conditions on sliding at glacial time scales

Spatially variable basal conditions are thought to govern how ice sheets behave at glacial time scales (>1000 years) and responsible for changes in dynamics between the core and peripheral regions of the Laurentide and Fennoscandian ice sheets. Basal motion is accomplished via the deformation of unconsolidated sediments, or via sliding of the ice over an undeformable bed. We present an ice sheet sliding module for the Parallel Ice Sheet Model (PISM) that takes into account changes in sediment cover and incorporates surface meltwater. This model routes meltwater, produced at the surface and base of the ice sheet, toward the margin of the ice sheet. Basal sliding is accomplished through the deformation of water saturated sediments, or sliding at the ice-bed interface. In areas with continuous, water saturated sediments, sliding is almost always accomplished through sediment deformation. In areas with incomplete cover, sliding has a stronger dependence on the supply of water. We find that the addition of surface meltwater to the base is a more important factor for ice sheet evolution than the style of sliding. In a glacial cycle simulation, our model causes a more rapid buildup of the Laurentide Ice Sheet

The sensitivity of Northern Hemisphere ice sheets to atmospheric forcing during the last glacial cycle using PMIP3 models

The evolution of Northern Hemisphere ice sheets through the last glacial cycle is simulated with the glacial index method by using the climate forcing from one General Circulation Model, COSMOS. By comparing the simulated results to geological reconstructions, we first show that the modelled climate is capable of capturing the main features of the ice-sheet evolution. However, large deviations exist, likely due to the absence of nonlinear interactions between ice sheet and other climate components. The model uncertainties of the climate forcing are examined using the output from nine climate models from the Paleoclimate Modelling Intercomparison Project Phase III. The results show a large variability in simulated ice sheets between the different models. We find that the ice-sheet extent pattern resembles summer surface air temperature pattern at the Last Glacial Maximum, confirming the dominant role of surface ablation process for high-latitude Northern Hemisphere ice sheets. This study shows the importance of the upper boundary condition for ice-sheet modelling, and implies that careful constraints on climate output is essential for simulating realistic glacial Northern Hemisphere ice sheets

Investigating the role of subglacial geology on ice sheet dynamics

Basal boundary conditions, such as basal geology, the presence of unconsolidated sediments, and hydrology, play a dominant role in the dynamics of ice sheets. One problem when studying existing ice sheets in Greenland and Antarctica is the lack of direct observations of these basal conditions. Studying paleo-ice sheet behaviour is advantageous in this respect, because these conditions are preserved in glacial landforms and sediments. By studying past ice sheet behaviour, we can provide analogues for modern behaviour. We investigate the role of basal geological conditions and hydrology on ice sheet dynamics using the ice sheet model PISM. We specifically focus on the North American ice sheet complex. We present datasets for this region that can be used in ice sheet models to investigate basal conditions including information on bedrock geology, the distribution of unconsolidated sediments, and the dominant grain size of the sediments. We use these datasets to investigate how they impact ice sheet dynamics with simulations over the last glacial cycle. We include a simple hydrology model that includes meltwater derived from the surface, and property changes depending on the basal geology from our datasets. Our results show that the behaviour of the ice sheet changes when there is a transition from regions with sparse sediment cover to complete sediment cover. One impact is that because ice can flow faster in regions like Hudson Bay, it can cause the Laurentide Ice Sheet to stabilize into a single continent-wide ice sheet faster than in a situation where sediments are not present. We also investigate the role of changes in basal geological conditions have in causing unstable behaviour in ice sheets

Das vaskuläre Trauma: Analyse der Versorgungsrealität in einer deutschlandweiten Umfrage

Hintergrund und Ziel der Arbeit Die vaskuläre Beteiligung im Rahmen von Traumen ist selten. Für die Versorgung der Verletzungen gibt es nur wenige konkrete Handlungsempfehlungen, sodass von einer großen Varianz auszugehen ist. Ziel der vorliegenden Umfrage war die Statuserhebung der aktuellen Versorgungsrealität des Gefäßtraumas in Deutschland sowie die Eruierung des Bedarfs und der Form von entsprechenden Fortbildungsangeboten. Material und Methoden Es wurde eine Online-Umfrage über SurveyMonkey® mit den Mitgliedern der Gesellschaft für Gefäßchirurgie und Gefäßmedizin (DGG) durchgeführt. Ergebnisse An der Umfrage haben sich 10,6 % der angeschriebenen Mitglieder der DGG beteiligt. Hieraus ergab sich, dass die meisten Kliniken 5–10 traumatische Gefäßverletzungen pro Jahr versorgen, wobei die höchsten Behandlungszahlen erwartungsgemäß in den überregionalen Traumazentren erreicht werden. Die Versorgung des Gefäßtraumas ist nicht einheitlich, sondern findet abhängig von der anatomischen Lokalisation durch unterschiedliche Fachabteilungen statt. Kliniken für Gefäßchirurgie sind selten an der Versorgung beteiligt. Bei den meisten Befragten bestand der Wunsch nach Fortbildungen zum Erlernen von gefäßtraumatologischen Techniken. Diskussion Gefäßchirurgische Kliniken sind nach den vorliegenden Umfrageergebnissen selten an der Versorgung des vaskulären Traumas beteiligt. Mögliche Erklärungen hierfür liegen in lokalen und infrastrukturellen Gegebenheiten. Eine aktive Einbindung von gefäßchirurgisch ausgebildeten Ärztinnen und Ärzten in die Diagnostik und Therapie der vaskulären Traumata ist wünschenswert und sollte aktiv angeboten werden. Das Erlernen der hierfür notwendigen Techniken kann beispielsweise im Rahmen praktischer Kurse stattfinden

Key stages in mammary gland development: The cues that regulate ductal branching morphogenesis

Part of how the mammary gland fulfills its function of producing and delivering adequate amounts of milk is by forming an extensive tree-like network of branched ducts from a rudimentary epithelial bud. This process, termed branching morphogenesis, begins in fetal development, pauses after birth, resumes in response to estrogens at puberty, and is refined in response to cyclic ovarian stimulation once the margins of the mammary fat pad are met. Thus it is driven by systemic hormonal stimuli that elicit local paracrine interactions between the developing epithelial ducts and their adjacent embryonic mesenchyme or postnatal stroma. This local cellular cross-talk, in turn, orchestrates the tissue remodeling that ultimately produces a mature ductal tree. Although the precise mechanisms are still unclear, our understanding of branching in the mammary gland and elsewhere is rapidly improving. Moreover, many of these mechanisms are hijacked, bypassed, or corrupted during the development and progression of cancer. Thus a clearer understanding of the underlying endocrine and paracrine pathways that regulate mammary branching may shed light on how they contribute to cancer and how their ill effects might be overcome or entirely avoided

Hydrodynamical experiments with Lattice Boltzmann models

In this thesis different numerical models based on the lattice Boltzmann equation presented and tested. Therefore the models are applied to two classical two-dimensional hydrodynamical problems. Parameter studies are performed with particular regard to changes in flow dynamics at hydrodynamical instabilities. The iLBGK model [Z. Guo et al., 2000] with a D2Q9 lattice is used to study the 2D flow past a cylinder placed between two walls. The transition from a steady flow to a vortex shedding regime is analyzed by varying the Reynolds number and the distance of the cylinder to one wall. Due to interaction of the cylinder's wake with the wall vorticity, the transition is delayed as the cylinder approaches the wall. The results are compared with the findings of Zovatto & Pedrizzetti [2001]. For the simulation of thermal flows, the multi-distribution-function (MDF) approach [Z. Guo et al., 2002a; He et al., 1998] is used. This approach uses the Boussinesq approximation to separate the liquid and the thermal components of the flow, which are solved on separate lattices. Two implementations of this approach are carried out using the LBGK and MRT models [Ginzburg, 2005; Wang et al., 2013]. These thermal models are used to study the 2D Rayleigh-Bénard problem for a fixed Prandtl number Pr = 0.71. The transition from the solely conductive to the convective regime is found to be dependent on the wavenumber k of a perturbation. The usage of lateral periodic boundary conditions restricts the possible values for k, which depend on the aspect ratio of the numerical domain. Checked against theoretical results, the critical Rayleigh numbers obtained with the MRT model are found to be more accurate than those obtained with the LBGK model

Impact of glacio-lacustrine interactions on ice-sheet dynamics

Geological records show that large proglacial lakes existed along the land-terminating margins of the Quaternary ice sheets. The major lakes formed in basins that were left deeply depressed due to delayed glacial isostatic adjustment (GIA) when the ice sheets retreated. As the environment around the retreating ice margin is quite dynamic, the lake basins constantly change. Where an ice sheet and proglacial lake are in contact, the dynamics are similar to a marine terminating ice-sheet margin. Such lacustrine boundary conditions cause changes in the ice-sheet’s geometry, stress balance and frontal ablation and therefore affect its entire mass balance. Despite this, dynamically evolving proglacial lakes have rarely been considered in detail in ice-sheet modelling endeavors. In this Ph.D. project, the impact of proglacial lakes on the ice-sheet dynamics are investigated using numerical methods. For this reason, I implemented a new proglacial lake boundary model into the Parallel Ice Sheet Model (PISM). This model computes the lake basins each timestep according to the dynamic geometry of the ice-sheet and topography. The lake–ice interface is implemented as a generalization of the marine boundary of PISM. In the first study, the underlying algorithm, LakeCC, is tested to determine if it is capable of reconstructing known lakes. Therefore, it is applied to paleo-topographic reconstructions of North America. In a second study, the impact of the PISM-LakeCC model on the ice-sheet dynamics is tested by running several simplified experiments of the glacial retreat of the North American ice sheets after the Last Glacial Maximum (LGM). Compared to control runs, the lake experiments exhibit an accelerated glacial retreat. Furthermore, for the Laurentide ice sheet (LIS), the presence of lakes triggers a process similar to marine ice sheet instability (MISI), which causes the final collapse of the ice saddle over Hudson Bay