Assimilation of surface velocities acquired between 1996 and 2010 to constrain the form of the basal friction law under Pine Island Glacier

Abstract

In ice‐sheet models, slip conditions at the base between the ice and the bed are parameterized by a friction law. The most common relation has two poorly constrained parameters, C and m. The basal slipperiness coefficient, C, depends on local unobserved quantities and is routinely inferred using inverse methods. While model results have shown that transient responses to external forcing are highly sensitive to the stress exponent m, no consensus value has emerged, with values commonly used ranging from 1 to ∞ depending on the slip processes. By assimilation of Pine Island Glacier surface velocities from 1996 to 2010, we show that observed accelerations are best reproduced with m≥5. We conclude that basal motion, in much of the fast flowing region, is governed by plastic deformation of the underlying sediments. This implies that the glacier bed in this area cannot deliver resistive stresses higher than today, making the drainage basin potentially more sensitive to dynamical perturbations than predicted with models using standard values m = 1 or 3.

Regularized Coulomb Friction Laws for Ice Sheet Sliding: Application to Pine Island Glacier, Antarctica

“…The largest model‐data disagreement occurs for the linear‐viscous case ( m = 1), which is a result of this model's basal friction having the greatest sensitivity to speed so that most of the additional resistance occurs near the grounding line, limiting inland propagation of the speedup. For larger m values (Figures b and c), the reduced sensitivity to speed allows the speedup to migrate farther inland, producing the best agreement on the trunk for a power law with m = 8 (approaching Coulomb friction), which is consistent with other findings (Gillet‐Chaulet et al, ). Although the larger exponents provide better agreement on the fast‐flowing trunk (speeds >300 m/year), they produce poorer results for the slower‐moving regions.…”

Projections of Future Sea Level Contributions from the Greenland and Antarctic Ice Sheets: Challenges Beyond Dynamical Ice Sheet Modeling

“…Higher order (HO) models include additional stresses to allow stress transmission across floating and grounded ice, while Full-Stokes (FS) models resolve all stresses. Adapted from Pattyn et al (2006) of the nature of the underlying bed (Christianson et al, 2013) and the evolution in properties of basal conditions (Gillet-Chaulet et al, 2016). Such processes and understanding are starting to be implemented in regional ISMs and should be more systematically included in future simulations.…”

Progress in Numerical Modeling of Antarctic Ice-Sheet Dynamics

“…New developments in data assimilation methods led to improved initializations in which the initial ice-sheet geometry and velocity field are kept as close as possible to observations by optimizing other unknown fields, such as basal friction coefficient (Advances in Process Understanding and Modeling) and ice stiffness (accounting for crevasse weakening and ice anisotropy; [4,5,28,78,83,84]). Motivated by the increasing ice-sheet imbalance of the ASE glaciers over the last 20 years [118], and supported by the recent boom in satellite data availability, data-assimilation methods are progressively used to evaluate unknown fields using time-evolving states accounting for the transient nature of observations and the model dynamics [51,56,58,59].…”