Meltwater percolation, impermeable layer formation and runoff buffering on Devon Ice Cap, Canada

Abstract

The retention of meltwater in the accumulation area of the Greenland ice sheet and other Arctic ice masses buffers their contribution to sea level change. However, sustained warming also results in impermeable ice layers or ‘ice slabs’ that seal the underlying pore space. Here, we use a 1-D, physically based, high-resolution model to simulate the surface mass balance (SMB), percolation, refreezing, ice layer formation and runoff from across the high-elevation area of Devon Ice Cap, Canada, from 2001 to 2016. We vary the thickness of the ‘impermeable’ ice layer at which underlying firn becomes inaccessible to meltwater. Thick near-surface ice layers are established by an initial deep percolation, the formation of decimetre ice layers and the infilling of interleaving pore space. The cumulative SMB increases by 48% by varying impermeable layer thickness between 0.01 and 5 m. Within this range we identify narrower range (0.25–1 m) that can simulate both the temporal variability in SMB and the observed near-surface density structure. Across this range, cumulative SMB variation is limited to 6% and 45–49% of mass retention takes place within the annually replenished snowpack. Our results indicate cooler summers after intense mid-2000s warming have led to a partial replenishment of pore space.

Spatial characterization of near-surface structure and meltwater runoff conditions across Devon Ice Cap from dual-frequency radar reflectivity

“…Our ice slab thicknesses are thicker than the impermeable thickness threshold of at least 1 m noted by Ashmore et al (2020) and overall similar to the observed thickness of Greenland's ice slabs, which can limit or substantially delay meltwater percolation (Charalampidis et al, 2016;Machguth et al, 2016;MacFerrin et al, 2019). Spatially, the radar response associated with refrozen ice slabs in Zone II is widespread (Fig.…”

“…Field-based techniques have expanded our knowledge of melting and refreezing processes in firn (Bell et al, 2008;Sylvestre et al, 2013;Forster et al, 2014;Gascon et al, 2014;Machguth et al, 2016), and their observations are necessary to help validate firn models that are often relied upon for mass balance estimates (Ashmore et al, 2020). Field studies typically utilize some combination of firn cores, snow pits, and ground-based radar (GPR) measurements, which provide in-situ measurements or high resolution remotely sensed observations of the local firn column.…”

Extreme melt season ice layers reduce firn permeability across Greenland

“…This is analogous to the ice lens aggregation mechanism proposed for the growth of ice slabs 9 , but in these interior regions, an extreme melt season can be the key catalyst that initiates this aggregation, rather than a multi-year excess melt. This implies that the frequency of extreme melt seasons relative to the rate at which the long-term melt to accumulation ratio allows new pore space and cold content to regenerate above the most recent melt layer 36 can alter the water storage capacity of near-surface firn and its response to ongoing surface melting in the ice sheet interior. This timescale is critical because Greenland has already experienced five recordingbreaking melt seasons since 2000 17,[37][38][39][40] , with the 2019 season second only to 2012 in total melt extent 40 .…”

Meltwater Penetration Through Temperate Ice Layers in the Percolation Zone at DYE‐2, Greenland Ice Sheet

“…The effective permeability of ice layers and the relation to ice layer thickness is an open question. There is evidence that thick ice layers in firn can be impermeable (Ashmore et al, 2020; Gascon et al, 2013), but meltwater infiltration through ice layers has also been observed or inferred (Ashmore et al, 2020; Humphrey et al, 2012; Machguth et al, 2016). Direct observations are needed to understand these processes and their efficacy with thicker ice layers, as found in the lower percolation zone of Greenland (MacFerrin et al, 2019; Machguth et al, 2016).…”

“…The extent to which meltwater retention in firn can buffer mass loss and sea level rise is also unclear. Greenland's firn zone covers about 80% of the ice sheet, but refrozen near‐surface ice layers have the potential to act as impermeable barriers, redirecting meltwater percolation into runoff (Ashmore et al, 2020; Gascon et al, 2013; MacFerrin et al, 2019; Noël et al, 2017). In the percolation zone, firn densification associated with warming and refreezing is also leading to a loss of available pore space, imposing further limits on meltwater storage capacity as melting progresses inland (Vandecrux et al, 2019).…”