Quantifying the processes at the root of the observed acceleration of icestreams from inverse methods

• Main Author: Mosbeux, Cyrille
• Other Authors: Institut des Géosciences de l’Environnement (IGE), Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)-Institut polytechnique de Grenoble - Grenoble Institute of Technology (Grenoble INP )-Institut de Recherche pour le Développement (IRD)-Université Grenoble Alpes 2016-2019 (UGA 2016-2019 ), Université Grenoble Alpes, Olivier Gagliardini, Fabien Gillet-Chaulet
• Format: Doctoral or Postdoctoral Thesis
• Language: French
• Published: HAL CCSD 2016
• Subjects: Inverse methods; Acceleration of icestreams; Processes quantification; Méthodes inverses; Accélération glaciers; Quantification des processus; [SPI.OTHER]Engineering Sciences [physics]/Other; [SDU.STU]Sciences of the Universe [physics]/Earth Sciences; Greenland; Wilkes Land; Antarc*; Antarctica; antartic*; Ice Sheet
• Online Access: https://tel.archives-ouvertes.fr/tel-01688570; https://tel.archives-ouvertes.fr/tel-01688570/document; https://tel.archives-ouvertes.fr/tel-01688570/file/Mosbeux_2016_archivage.pdf

The current global warming has direct consequences on ice-sheet mass loss. Reproducing the responsible mechanisms and forecasting the potential ice-sheets contribution to 21st century sea level rise is one of the major challenges in ice-sheet and ice flow modelling. Ice flow models are now routinely used to forecast the potential ice-sheets contribution to sea level rise. Such short term simulations are very sensitive to model initial state, usually build from field observations. However, some parameters, such as the basal friction between icesheet and bedrock as well as the basal topography, are still badly known because of a lake of direct observations or large uncertainty on measurements. Improving the knowledge of these two parameters for Greenland and Antarctica is therefore a prerequisite for making reliable projections. Data assimilation and inverse methods have been developed in order to overcome this problem. This thesis presents two different assimilation algorithms to better constrain simulaneouslybasal friction and bedrock elevation parameters using surface observations. The first algorithm is entierly based on adjoint method while the second algorithm uses a cycling method coupling inversion of basal friction with adjoint method and inversion of bedrock topography with nudging method. Both algorithms have been implemented in the finite element ice sheet and ice flow model Elmer/Ice and tested in a twin experiment showing a clear improvement of both parameters knowledge. The application of both algorithms to regions such as the Wilkes Land in Antartica reduces the uncertainty on basal conditions, for instance providing more details to the bedrock geometry when compared to usual DEM. Moreover,the reconstruction of both bedrock elevation and basal friction significantly decreases ice flux divergence anomalies when compared to classical methods where only friction is inversed. We finaly sudy the impact of such inversion on pronostic simulation in order to compare the efficiency of the two algorithms to ...