What is glacier sliding
Robert Law, David Chandler, Phillip Voigt, Ivan Utkin, Andreas Born
TL;DR
This work reframes glacier sliding as the aggregate effect of numerous near-bed sub-processes, introducing a sliding-bulk layer framework that separates tangential (slip) and normal (form drag) resistances and links them to a transmissible near-bed velocity field. It formalizes a practical production-model representation as the sum of a regularised-Coulomb slip term and a power-law drag term, plus an error component, enabling incorporation of soft-bed, hard-bed, cavitation, form drag, temperate basal ice, and hydrology within a single cohesive scheme. The authors argue that a truly universal single-term sliding law is unlikely due to scale dependence, bed heterogeneity, cavitation dynamics, and compensating model errors; instead, they advocate a multi-term or setting-aware approach with explicit attention to cavitation scale ($l_c$) and bed roughness, which can improve model realism and predictive performance for sea-level projections. The framework provides a structured path to unify diverse sub-processes across settings and scales, clarifying where current production-model parameterizations succeed or fail and outlining key open questions in subglacial hydrology, basal rheology, and scale-dependent form drag. This work thus offers a principled route toward more physically grounded, region-specific sliding parameterizations for ice-sheet and glacier models, with implications for better constraining future sea-level rise and water-resource forecasts.
Abstract
Glacier and ice-sheet motion is fundamental to glaciology. However, we still lack a consensus for the optimal way to relate basal velocity to basal traction for large-scale glacier and ice-sheet models (the 'sliding relationship'). Typically, a single tunable coefficient loosely connected to one or a limited number of physical processes is varied spatially to reconcile model output with observations. Yet, process-agnostic studies indicate that the suitability of a given sliding relationship depends on the setting. Here, we suggest that this arises from myriad overlapping setting- and scale-dependent sliding sub-processes, including complicated near-basal stress states not captured by large-scale models, reviewed here as comprising a basal 'sliding layer'. A corresponding 'bulk layer' then accounts for ice deformation only minimally influenced by bed properties. We provide a framework for incorporating arbitrarily many sub-processes within a given region -- separated into normal ('form drag') and tangential ('slip') resistance at the ice-bed interface, stressing that the maximum scale of cavitation is an important contributor to the division between the two. Under reasonable assumptions, our framework implies that sliding relationships should fall within a sum of regularised-Coulomb and power-law components, with a rough-smooth distinction proving more consequential in dictating sliding behaviour than a traditional hard-soft transition.