Controls on Greenland moulin geometry and evolution from the Moulin Shape model

Abstract

Abstract. Nearly all meltwater from glaciers and ice sheets is routed englacially through moulins. Therefore, the geometry and evolution of moulins has the potential to influence subglacial water pressure variations, ice motion, and the runoff hydrograph delivered to the ocean. We develop the Moulin Shape (MouSh) model, a time-evolving model of moulin geometry. MouSh models ice deformation around a moulin using both viscous and elastic rheologies and melting within the moulin through heat dissipation from turbulent water flow, both above and below the water line. We force MouSh with idealized and realistic surface melt inputs. Our results show that, under realistic surface melt inputs, variations in surface melt change the geometry of a moulin by approximately 10 % daily and over 100 % seasonally. These size variations cause observable differences in moulin water storage capacity and moulin water levels compared to a static, cylindrical moulin. Our results suggest that moulins are important storage reservoirs for meltwater, with storage capacity and water levels varying over multiple timescales. Implementing realistic moulin geometry within subglacial hydrologic models may therefore improve the representation of subglacial pressures, especially over seasonal periods or in regions where overburden pressures are high.

Observed and modeled moulin heads in the Pâkitsoq region of Greenland suggest subglacial channel network effects

“…In the recent development of the MouSh model (Andrews et al, 2022), results for typical Greenland inputs and ice parameters yielded moulins with radii ∼0.5-1 m at the water line, approximately the size of the moulin we study here. Modeled moulins are larger than this above the water line by a factor of ∼2-5 (Andrews et al, 2022).…”

“…Inventories of moulins in this same area suggest that reuse for 2-3 years is common among lake-draining moulins (Poinar and Andrews, 2021;Andrews, 2015), however, the stream typically flows in from a consistent direction. MouSh model runs over a single melt season (Andrews et al, 2022) are able to predict neither this size nor the overhung shape observed in Phobos Moulin.…”

“…We simulate the size, shape, and hydraulic head of a moulin instrumented in the field using the Moulin Shape (MouSh) model (Andrews et al, 2022) coupled to a subglacial channel model (Covington et al, 2020) based on the Schoof (2010) equations for melt and creep closure.…”

“…To simulate the shape of the moulin and the hydraulic head fluctuations, we use the Moulin Shape (MouSh) model (Andrews et al, 2022). The MouSh model is a two-dimensional physically based model that simulates the depth-dependent size and shape of a moulin for site-specific glacier properties (e.g., ice thickness, ice temperature, external stress, etc.)…”

Modeling the Influence of Moulin Shape on Subglacial Hydrology

“…In general, moulins will evolve through a combination of melt due to the turbulent flow of water and viscous and elastic deformation of the ice (Andrews et al, 2022;Catania & Neumann, 2010;Poinar et al, 2017). Recent model experiments suggest that moulin shape evolves relatively quickly (days to weeks) within a melt season to a near-equilibrium, with diurnally forced oscillations superimposed (Andrews et al, 2022). Hence, the size of a moulin should be correlated to the size of the supraglacial stream feeding it.…”