Thermal tracking of meltwater retention in Greenland's accumulation area

Abstract

[1] Poorly understood processes controlling retention of meltwater in snow and firn have important implications for Greenland Ice Sheet's mass balance and flow dynamics. Here we present results from a 3 year (2007)(2008)(2009)) field campaign studying firn thermal profiles and density structure along an 85 km transect of the percolation zone of west Greenland. We installed one or two thermistor strings at 14 study sites, each string having 32 sensors spaced between 0 and 10 m depth. Data from our network of over 500 sensors were collected at 15-60 min intervals for 1-2 years, thereby recording the thermal signature of meltwater infiltration and refreezing during annual melt cycles. We document three types of heating of firn related to different mechanisms of meltwater motion and freezing, including heterogeneous breakthrough events, wetting front advance, and year-round heating from freezing of residual deep pore water. Vertically infiltrating meltwater commonly penetrates through cold firn accumulated over decades, even where ice layers are present at the previous summer surface and where ice layer thickness exceeds several decimeters. The offset between the mean annual air temperature and the 10 m firn temperature reveals the elevation dependency of meltwater retention along our transect. The firn is > 10°C warmer than the mean annual air temperature at the region where meltwater runoff initiates. During 2007-2009, runoff was limited to elevations lower than $1500 m with no sharp "runoff limit"; rather, the ratio of retention to runoff transitioned from all retention to all runoff across a $20 km wide zone.

Greenland meltwater storage in firn limited by near-surface ice formation

“…2), indicating that post-1998 ice lenses are now present in pre-1998 firn. We interpret this as evidence of deep (>10 m) meltwater percolation 14 that has modified firn stratigraphy over the entire depth of the 2013 cores (Fig. 3).…”

“…the firn column contains ice lenses whose average thickness and amount decreases with elevation. Active percolation of meltwater takes place in this zone 14 which we accordingly term the "firn percolation regime." (2) At 1840 m a.s.l.…”

“…3). Limited amounts of subsurface runoff cannot be ruled out where deep percolation prevails 11,14,18 . The low permeability of the thick near-surface ice layers is corroborated by a network of summer 2012 surface meltwater channels ( Fig.…”

“…At higher elevations, firn density and ice lens frequency have increased due to active melt water retention, including processes of deep percolation 3,14 . At lower elevations, where melt is most abundant, thick near-surface ice layers have formed which limit deep percolation, render existing deep pore space difficult to access and force the majority of meltwater to migrate along the surface.…”

Dynamic perennial firn aquifer on an Arctic glacier

“…Storage of summer meltwater warms the ice beneath [e.g., Humphrey et al, 2012], potentially affecting ice rheology. More significantly, PFAs likely drain to the bed through moulins and crevasses, as suggested by Alley et al [2005] and Koenig et al [2014]; this has a variety of implications.…”