Please find below the description of a phd thesis position open at the Laboratoire de Glaciologie et Géophysique de l'Environnement (LGGE, Grenoble, France).
"Quantifying the processes at the root of the observed acceleration of ice-streams from inverse methods"
More informations about the subject: http://www-lgge.ujf-grenoble.fr/pdr/ADAGe/jobs.html
Deadline to apply: 1st June 2012.
Quantifying the processes at the root of the observed acceleration of ice-streams from inverse methods
By gaining and loosing mass, glaciers and ice-sheets play a key role in the sea level evolution. This is obvious when considering the past 20000 years as the collapse of the large northern hemisphere ice-sheets after the Last Glacial Maximum contributed to a 120 m rise in sea level. This is particularly worrying when the future is considered. Indeed, recent observations clearly indicate that important changes in the velocity structure of both Antarctic and Greenland ice-sheets are occurring, suggesting that large and irreversible changes may have been initiated. This has been clearly emphasized in the last report published by the Intergovernmental Panel on Climate Change (IPCC, 2007). IPCC has further insisted on the poor current knowledge of the key processes at the root of the observed accelerations and finally concluded that reliable projections of sea-level rise (SLR) are currently unavailable.
The general aim of this thesis is to increase our understanding of englacial processes at the root of outlet glacier dynamics by the use of data assimilation methods. The final objective is to provide accurate and reliable estimates of the future contribution of ice-sheets to SLR. The economical implications of this project are therefore natural hazards mitigation, insurances reliability and more generally sea-defence planning. Economical and societal impacts are based on IPCC SLR scenarios which suffer from significant uncertainties. In its last report, IPCC has proposed SLR projections for 2100 ranging within 0.18 to 0.59 cm depending on the emission scenario. This thesis, by contributing to a better knowledge of the key processes that lead to loss of continental ice and by developing data assimilation methods, will decrease the uncertainty affecting SLR projections for the near future and consequently bring some responses to the ongoing international debates surrounding coastal adaptation and sea-defence planning.
Antarctic and Greenland outlet glaciers control the ice discharge toward the sea and the resulting contribution to sea level rise. Many different physical processes at the root of the observed acceleration and retreat of many of these glaciers can be invoked. Most of these changes are driven by a modification of the boundaries of the ice-sheet domain, excepted changes in the ice viscosity induced by changes of the ice temperature. This first process is likely to only play a significant role for longer-term changes, and cannot explain the currently observed acceleration. The other processes are induced by an increase of ocean and atmospheric temperatures. Increase of ocean temperature will induce an increase of calving rate at the front of marine terminated glaciers, as well as an increase of the melting below ice-shelves, which in turns reduce the buttressing force exerted by the floating part of the glacier. Increase of atmospheric temperature induces an increase of the surface runoff, both in covered space and on the total amount at a given place. Even if more water doesn’t necessary imply an increase of basal sliding, increase of runoff might have enhanced the basal sliding of outlet glaciers. Surface water, by filling crevasses also induces an increase of calving and a decrease of friction in the highly crevassed lateral margin of ice-streams.
All these processes certainly play a role but their relative contributions have not yet been quantified. The objective of the thesis will be to quantify the sensitivity of each of these processes for different outlet glaciers, both in Antarctica and in Greenland. For that, we will use full-Stokes finite element ice flow model DassFlow/Ice developed at the Institut de Mathématiques de Toulouse in the framework of the ANR ADAGe project (http://www-lgge.ujf-grenoble.fr/pdr/ADAGe/). DassFlow/Ice includes a direct and an adjoint code and full 4d-var data assimilation process in which the control variables are the basal and lateral friction parameters as well as the calving front pressure. For each available date, the sensitivity of each process will be first studied and an optimal distribution will then be inferred from the surface measurements. Using this optimal distribution of these parameters, transient simulations will be performed over the whole dataset period. The relative contributions of the basal friction, lateral friction and front back force will then be discussed under the light of these new results.
The method will be applied to various outlet glaciers in Antarctica and Greenland, for whom a sufficient dataset is available. The ideal dataset is for different given dates, a surface DEM and associated surface velocities and changes in the surface elevation. A relatively good knowledge of the bedrock DEM is also required. For Antarctica, the potential candidates are Pine Island glacier, the largest current contributor to sea level rise in Antarctica, but also the Astrolabe glacier, which is studied by the LGGE in the framework of the DACOTA project. In Greenland, we will focus on the major outlets, namely the Jakobshavn, Helheim, Peterman, Kangerdlugssuaq, Petermann and 79fjords glaciers.
By quantifying the relative contribution of these different processes, for different glaciers in Antarctica and Greenland, we will point out which ones are essentially to be incorporated in ice-sheet models. By analysing how they are related to climate changes, physical parameterizations of these processes will be proposed to allow their implementation in the new generation, currently under development, ice-sheet model Elmer/Ice.
Framework of the thesis:
This thesis is part of the ANR project ADAGe (http://www-lgge.ujf-grenoble.fr/pdr/ADAGe/), which is a collaborative project between Glaciologists (LGGE, LEGOS) and Mathematicians (IMT, LJK). You will conduct the first data assimilation experiments using the newly developed adjoint model DassFlow/Ice (IMT, http://www-gmm.insa-toulouse.fr/~monnier/DassFlow/index.html). You must have strong background in modelling and programming and a real interest in conducting geophysical simulations.
The ideal candidate will have a master degree in mechanics, geophysics or applied mathematics with strong interest in scientific computing. The candidate should also be interested in climate sciences, geophysics and glaciology. Experience in Fortran would be greatly appreciated.
Documents to be send: a CV, a letter of motivation, academic results.Deadline to apply: 1st June 2012.
page perso : http://www-lgge.ujf-grenoble.fr/~gagliardini/
Tel: +33 4 76 82 42 76
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