Relevance of Topographic Roughness Amplitude, Orientation, and Spatial Frequency for Basal Resistance Inversions: A Case Study on Thwaites Glacier, Antarctica.

Abstract:

Recent radar technology development enables three-dimensional mapping of modern glacier beds over large areas of the polar ice sheets. In this work, we use two densely surveyed swath-mapped topographies (<50m² resolution) of Thwaites Glacier, West Antarctica to investigate the sensitivity of inferred friction proxies to basal topographic roughness magnitude and orientation. Applying traditional snap-shot inverse methods, we use the full-Stokes solver implemented in Elmer/Ice to calculate friction proxy fields for a suite of model geometries that sample versions of the swath-mapped bed progressively filtered at higher spatial resolutions. In agreement with previous work, we show that misfit between observations and model output is reduced using the highest resolution glacier beds. We also show that the orientation of subglacial bedforms affects the magnitude of the inferred friction proxy. Our work agrees with analytic theories of form drag and suggests that roughness influences friction proxy inversions below the 300m² average element area often used to resolve stresses near glacier grounding zones. Furthermore, our diagnostic experiments and spectral analysis suggest that unresolved basal topographic roughness still contributes substantially to form drag below the resolutions of even the swath-mapped topography, especially on subglacial ridges that may stabilize future grounding-line positions. These diagnostic experiments motivate a suite of synthetic prognostic experiments that aim to understand the sensitivity of marine outlet glacier retreat to unresolved form drag in the inferred basal boundary condition.