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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.

Russell Glacier is a land terminating glacier in South West Greenland. Survey site SHR lies at several kilometers from the terminus and is closely monitored. In recent Summer months research site SHR has seen unusual high ice velocity of 400m/a which has been linked to increased Summer melt. To capture the probably changing basal conditions of SHR we carried out two seismic surveys at site SHR, one in September 2013 at the end of the melt seaasn and one in May 2014 at the start of the melt season. Data were recorded using a 300m snow streamer and explosives. The seismic data reveal an ice thickness of about 550m and 30 to 40m thick accreted subglacial sediments with varying degrees of water saturation. We speculate the increased ice velocity is caused by sediments that become temporarily liquefied in the Summer months.

Numerical simulations moved in the recent year(s) from describing the cold-temperate transition surface (CTS) towards an enthalpy description, which allows avoiding incorporating a singular surface inside the model (As- chwanden et al., 2012). In Enthalpy methods the CTS is represented as a level set of the enthalpy state variable. This method has several numerical and practical advantages (e.g. representation of the full energy by one scalar field, no restriction to topology and shape of the CTS). The proposed method is rather new in glaciology and to our knowledge not verified and validated against analytical solutions. Unfortunately we are still lacking analytical solutions for sufficiently complex thermo-mechanically coupled polythermal ice flow. However, we present two experiments to test the implementation of the enthalpy equation and corresponding boundary conditions. The first experiment tests particularly the functionality of the boundary condition scheme and the corresponding basal melt rate calculation. Dependent on the different thermal situations that occur at the base, the numerical code may have to switch to another boundary type (from Neuman to Dirichlet or vice versa). The main idea of this set-up is to test the reversibility during transients. A former cold ice body that run through a warmer period with an associated built up of a liquid water layer at the base must be able to return to its initial steady state. Since we impose several assumptions on the experiment design analytical solutions can be formulated for different quantities during distinct stages of the simulation. The second experiment tests the positioning of the internal CTS in a parallel-sided poly- thermal slab. We compare our simulation results to the analytical solution proposed by Greve and Blatter (2009). Results from three different ice flow-models (COMIce, ISSM, TIMFD3) are presented.