Abstract
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 (2007–2009) 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.
Highlights
[2] Significant increases in intensity and areal extent of surface melt have been documented recently on the Greenland Ice Sheet [Hanna et al, 2005; Mote, 2007; Steffen et al, 2004]
The principal weakness in the 1-D Colbecktype model of wetting front migration is the absence of heterogeneous infiltration, which is known to be dominant in subfreezing firn [Marsh and Woo, 1984; Parry et al, 2007; Pfeffer and Humphrey, 1996], but is difficult to model in any way that provides robust, useful large-scale constraints on meltwater retention
[6] Our understanding of the eventual fate of meltwater in most of the percolation zone remains poor owing to our lack of knowledge of heterogeneous infiltration and processes related to ice layers
Summary
[2] Significant increases in intensity and areal extent of surface melt have been documented recently on the Greenland Ice Sheet [Hanna et al, 2005; Mote, 2007; Steffen et al, 2004]. Understanding the motion and ultimate fate of meltwater produced in the percolation zone of the Greenland Ice Sheet is critical to understanding the implications of the observed increases in surface melt. 3. Data Collection [11] We measured high time resolution thermal profiles in the upper 10 m of the firn column by installing temperature sensors in core holes drilled at each of our 14 intensive study sites (Figure 1). The two thermistor strings installed 10 m apart at site T1 exhibited temperature profiles which did not differ by more than their 0.5°C accuracy (Figure 2), implying repeatability of measurements at the “site” scale
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