Humbert, Angelika
Lade...
Preferred name
Humbert, Angelika
Official Name
Humbert, Angelika
Alternative Name
Himmler, Angelika
Humbert, A.
Himmler, A.
Email
angelika.humbert@awi.de
ORCID
GND
Scopus Author ID
26027014300
3 Ergebnisse
Gerade angezeigt 1 - 3 von 3
- Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Analysis of Recent Dynamic Changes of Jakobshavn Isbrae, West Greenland, using a Thermomechanical Model(2017-06-16); ; ; Jakobshavn Isbrae is a major marine terminating outlet glacier of the western Greenland Ice Sheet, which has been undergoing widespread acceleration and strong mass loss since the disintegration of its floating ice tongue in the late 1990s. The underlying mechanisms are poorly understood despite a wealth in observational and modelling studies. This doctoral thesis analyses the dynamic changes of Jakobshavn Isbrae using the Ice Sheet System Model (ISSM), a state-of-the-art finite-element ice flow model. Two missing model features for 1) the modelling the polythermal regime of glaciers and ice sheets, and 2) the dynamic evolution of its horizontal calving front position are designed and implemented into ISSM. A three-dimensional, thermodynamically coupled model of Jakobshavn Isbrae is set up and calibrated using modern observational data products. Low basal drag in the trough under the ice stream requires that its high driving stress is balanced by lateral drag in the shear margins, which allows for high flow velocities, as the ice viscosity is strain-rate-dependent. The developed modules are applied to the glacier model, which captures 90% of the observed changes from 1985 to 2015. Analysis of the model results reveals that calving front retreat is able to trigger widespread inland acceleration due to a rheological ice viscosity drop in the shear margins. Thermal feedbacks contribute 5 to 10% to the total acceleration. The study shows that Jakobshavn Isbrae will continue to contribute to eustatic sea level rise for at least the next century due to ongoing geometry adjustment to the new calving front position. Future fields of research include deriving a suitable calving rate parametrisation for large-scale ice flow models, a material law for temperate ice with a microscopic water content larger than 1%, and technical refinements of the modules developed for this thesis.Dissertation449 274 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Glaciological observations using phase-sensitive radar(2022-02-15); ; ; The large ice sheets in Greenland and Antarctica are losing mass due to global warming. In particular, the acceleration of ice streams and thus the increased discharge into the ocean contributes significantly to global sea-level rise. The floating extensions of the ice streams counteract this, but intense basal melting can destabilise the ice shelves. In this thesis, a contribution is made to determine the melt rates of two ice shelves, which are crucial for the future mass losses of the respective ice sheets. In the north, the focus is on the Northeast Greenland Ice Stream (NEGIS) that feeds the Nioghalvfjerdsbrae (79°N Glacier). My analysis of phase-sensitive radar measurements indicates high melt rates near the onset of the ice stream and thus the presence of subglacial melt water, which is associated with the formation of the ice flow. An extensive study in my thesis reveals that the 79°N Glacier has been thinned out considerably in recent years due to extreme melt rates and that large channels have been formed. Melt rates of the Filchner Ice Shelf, Antarctica, which I also determined using phase-sensitive radar measurements, are comparatively low. I was able to attribute significant deviations from remote sensing-derived melt rates to inaccuracies in the used ice flow velocity field. Furthermore, I show that the use of newer velocity fields improves the determination of the melt rates from remote sensing. My analysis of melt rate time series in the vicinity of a channel indicates higher melt rates in the summer as well as several melt events spread over the entire measurement period. Another study combines measurements and numerical modelling and shows that higher melt rates must have occurred in the past than those that were measured. These would lead to the closure of the channel within 250 years. Thus, neither the channel itself nor the present day melt rates endanger the stability of one of the largest Antarctic ice shelves at present.Dissertation410 206 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Viskoelastische Modellierung der Dynamik eines Gletschers als Antwort auf basales Schmelzen und die Oberflächenmassenbilanz(2017-12-19); An der Basis der Eisschilde, die Grönland und die Antarktis bedecken, finden sich Kanalstrukturen. Sie stellen einen wichtigen Teil des subglazialen hydrologischen Systems der Eisschilde und der Schelfeise dar. Mithilfe eines numerischen Modells der viskoelastischen Rheologie von Eis wird die zeitliche Entwicklung dieser Strukturen in dieser Arbeit untersucht. Ein Abfall des Wasserdrucks innerhalb eines subglazialen Kanals an der Basis eines gegründeten Gletschers führt zu einer Verringerung seiner Querschnittsfläche. Die Geschwindigkeit, mit der diese Verringerung erfolgt, hängt wesentlich vom Wasserdruck und der initialen Form des Kanals ab. Ein Teil der Querschnittsflächenverringerung erfolgt instantan durch die elastische Deformation des viskoelastischen Eises. Am Support-Force Gletscher in der Westantarktis findet sich ein Schmelzkanal an der Unterseite des Filchner Schelfeises. Aufgrund des Schmelzkanals ist das hydrostatische Gleichgewicht des Schelfeises gestört. Der Kanal beginnt sich durch ein Fließen des Eises zu schließen. Die Geschwindigkeit dieses Eisflusses wird wesentlich durch die nichtlineare Viskosität des Eises beeinflusst. Basales Schmelzen und oberflächliche Akkumulation stellen zusätzliche Einflussfaktoren auf die Entwicklung des Eiskörpers dar. Der elastische Anteil der viskoelastischen Deformation von Eis trägt in signifikantem Maß zur Verringerung der Querschnittsfläche subglazialer Kanäle großer Breite und geringer Höhe bei. In der Reaktion des Schelfeises auf eine zyklische Belastung spiegelt sich die viskoelastische Rheologie des Eises ebenfalls wider. Zukünftige Betrachtungen der untersuchten Strukturen mithilfe numerischer Modelle sollten daher die viskoelastischen Materialeigenschaften des Eises mit berücksichtigen.Masterarbeit285 321