Lionel Favier

Impact of bedrock description on modeling ice sheet dynamics

Recent glaciological surveys have revealed a significant increase of ice discharge from polar ice caps into the ocean. In parallel, ice flow models have been greatly improved to better reproduce current changes and forecast the future behavior of ice sheets. For these models, surface topography and bedrock elevation are crucial input parameters that largely control the dynamics and the ensuing overall mass balance of the ice sheet. For obvious reasons of inaccessibility, only sparse and uneven bedrock elevation data is available. This raw data is processed to produce Digital Elevation Models (DEMs) on a regular 5 km grid. These DEMs are used to constrain the basal boundary conditions of all ice sheet models. Here, by using a full‐Stokes finite element code, we examine the sensitivity of an ice flow model to the accuracy of the bedrock description. In the context of short‐term ice sheet forecast, we show that in coastal regions, the bedrock elevation should be known at a resolution of the order of one kilometer. Conversely, a crude description of the bedrock in the interior of the continent does not affect modeling of the ice outflow into the ocean. These findings clearly indicate that coastal regions should be prioritized during future geophysical surveys. They also indicate that a paradigm shift is required to change the current design of DEMs describing the bedrock below the ice sheets: they must give users the opportunity to incorporate high‐resolution bedrock elevation data in regions of interest.

Antarctic ice rise formation, evolution, and stability

Antarctic ice rises originate from the contact between ice shelves and one of the numerous topographic highs emerging from the edge of the continental shelf. While investigations of the Raymond effect indicate their millennial-scale stability, little is known about their formation and their role in ice shelf stability. Here we present for the first time the simulation of an ice rise using the BISICLES model. The numerical results successfully reproduce several field-observable features, such as the substantial thinning downstream of the ice rise and the successive formation of a promontory and ice rise with stable radial ice flow center, showing that ice rises are formed during the ice sheet deglaciation. We quantify the ice rise buttressing effect, found to be mostly transient, delaying grounding line retreat significantly but resulting in comparable steady state positions. We demonstrate that ice rises are key in controlling simulations of Antarctic deglaciation.

Predicting the drag coefficient of a granular flow using the discrete element method

Passive-protection structures, against snow avalanches, are designed with rough estimates of the drag coefficient, depending straightforwardly on the obstacles geometry. In this paper, assuming an avalanche as a dry granular flow, a numerical model is presented for studying the influence on the drag coefficient of both the shape and size of an obstacle impacted by a granular flow. Small-scale laboratory experiments were conducted to validate the numerical model. During the experiments, velocity profiles were estimated using the particle image velocimetry method applied to the pictures recorded by a fast camera, which focused perpendicular to the lateral wall. Flow thickness variations along the slope were estimated by post-processing the images recorded by another fast camera, which films a laser line projected on the free surface flow. From the lateral motion of that line, the thickness could be determined. The granular impact force was measured through an instrumented obstacle positioned at the lower end of a canal. A 3D numerical model, based on the discrete element method using the YADE code, was set up to reproduce the experimental configuration. The law of contact between discrete elements involved elastic components (normal and tangential stiffnesses) and dissipative components (a normal restitution coefficient and a friction coefficient based on the Coulomb friction law). The model was validated by comparisons with both the experimental flow characteristics (velocity profiles and thicknesses) and the impact load history. Once validated, the numerical model was used to investigate the contribution of the height and shape of the obstacle to the drag coefficient. Finally, results are discussed and compared with ones from other studies.

Progress in Numerical Modeling of Antarctic Ice-Sheet Dynamics

Numerical modeling of the Antarctic ice sheet has gone through a paradigm shift over the last decade. While initially models focussed on long-time diffusive response to surface mass balance changes, processes occurring at the marine boundary of the ice sheet are progressively incorporated in newly developed state-of-the-art ice-sheet models. These models now exhibit fast, short-term volume changes, in line with current observations of mass loss. Coupling with ocean models is currently on its way and applied to key areas of the Antarctic ice sheet. New model intercomparisons have been launched, focusing on ice/ocean interaction (MISMIP+, MISOMIP) or ice-sheet model initialization and multi-ensemble projections (ISMIP6). Nevertheless, the inclusion of new processes pertaining to ice-shelf calving, evolution of basal friction, and other processes, also increase uncertainties in the contribution of the Antarctic ice sheet to future sea-level rise.

Validation of a DEM granular flow model aimed at forecasting snow avalanche pressure

Structural design of passive‐protection against snow avalanches is limitated because of the lack of knowledge about loading pressure on structures. Consequently, that design is based on rough estimas of the drag coefficient, considering a straightforward dependence on the obstacle’s geometry. In this paper, we first assume a flowing snow avalanche as a granular flow and we study numerically and experimentally the impact of a dry granular flow against an obstacle. Small‐scale laboratory experiments were conducted to validate a numerical model. Experimental velocity profiles and thicknesses are estimated and granular impact force is measured. A 3D numerical model, based on the discrete element method (DEM), was set‐up to reproduce the experimental configuration. The contact law involved elastic and dissipative components. The model is validated by comparisons with both the experimental flow characteristics and the impact load history. Once validated, the model is used to investigate the contribution of the ...