Modelling the transfer of supraglacial meltwater to the bed of Leverett Glacier, Southwest Greenland

Abstract

Abstract. Meltwater delivered to the bed of the GreenlandIce Sheet is a driver of variable ice-motion through changes in effective pressure and enhanced basal lubrication. Ice surface velocities have been shown to respond rapidly both to meltwater production at the surface and to drainage of supraglacial lakes, suggesting efficient transfer of meltwater from the supraglacial to subglacial hydrological systems. Although considerable effort is currently being directed towards improved modelling of the controlling surface and basal processes, modelling the temporal and spatial evolution of the transfer of melt to the bed has received less attention. Here we present the results of spatially distributed modelling for prediction of moulins and lake drainages on the Leverett Glacier in Southwest Greenland. The model is run for the 2009 and 2010 ablation seasons, and for future increased melt scenarios. The temporal pattern of modelled lake drainages are qualitatively comparable with those documented from analyses of repeat satellite imagery. The modelled timings and locations of delivery of meltwater to the bed also match well with observed temporal and spatial patterns of ice surface speed-ups. This is particularly true for the lower catchment ( < 1000 m a.s.l.) where both the model and observations indicate that the development of moulins is the main mechanism for the transfer of surface meltwater to the bed. At higher elevations (e.g. 1250-1500 m a.s.l.) the development and drainage of supraglacial lakes becomes increasingly important. At these higher elevations, the delay between modelled melt generation and subsequent delivery of melt to the bed matches the observed delay between the peak air temperatures and subsequent velocity speed-ups, while the instantaneous transfer of melt to the bed in a control simulation does not. Although both moulins and lake drainages are predicted to increase in number for future warmer climate scenarios, the lake drainages play an increasingly important role in both expanding the area over which melt accesses the bed and in enabling a greater proportion of surface melt to reach the bed.

Recent Advances in Our Understanding of the Role of Meltwater in the Greenland Ice Sheet System

“…The presence of crevasses is dependent on the stress regime within the ice, and recent modelling studies in Greenland have used the von Mises yield criterion [44,46] to predict areas where crevassing will occur. Where crevasses intersect surface drainage, meltwater ponding in the crevasse will occur establishing the potential for hydrofracture to propagate the crevasse to the glacier bed [48].…”

“…Rapid lake drainage results in large volumes of water entering the subglacial drainage system in a few hours with rates of 8700 and 3300 m 3 s −1 recorded [9,41]. In order for surface lakes to drain, an appropriate stress regime in the ice is required with existing crevasses, which can then be exploited and expanded by ponded surface waters draining englacially via hydrofracture [46][47][48]. There is as yet no evidence however to support the idea of a unifying critical lake volume or depthdependent drainage threshold [38].…”

Force Balance along Isunnguata Sermia, West Greenland

“…Surface crevasses provide one mechanism facilitating meltwater transport. It has been suggested that conditions promoting crevasse formation are limited to below ∼1400 m (Clason et al, 2015) to 1600 m elevation (Poinar et al, 2015) in the study area. Meltwater-induced seasonal variations in ice motion have been observed to elevations reaching ∼1500-1600 m (Bartholomew et al, 2011;Palmer et al, 2011).…”

Modelling seasonal meltwater forcing of the velocity of land-terminating margins of the Greenland Ice Sheet

“…Mean winter velocities for [2008][2009] are provided at 500 m resolution by the MEaSUREs Greenland Ice Sheet Velocity Map dataset (Joughin et al, 2010a, b). For the inversion procedure, the winter velocities, along with their associated errors, are reinterpolated to 1000 m. Velocities at 500 m resolution are used to determine surface stresses, assuming an ice temperature of −5 • C. Crevassed areas are then calculated using a von Mises stress criterion following Clason et al (2015). A crevassing threshold is selected by comparing the von Mises stress to observed patterns of crevassing in a Landsat 8 image, acquired on 19 August 2013.…”