Meltwater retention in the firn layer of the Greenland Ice Sheet is has the potential to buffer sea level rise due to ice sheet melt. The capacity of the firn layer to store meltwater is unclear, however, because refrozen ice layers can act as impermeable barriers to meltwater percolation, promoting runoff rather than retention. We present time-domain reflectometry and thermistor data which demonstrates that meltwater successfully penetrates ice layers up to 12 cm thick in the near-surface firn at Dye2, Greenland. Our observations indicate that ice layers within polar firn can become permeable when summer warming and latent heat release from refreezing meltwater raise temperatures to the melting point. This facilitates meltwater retention, and indicates that the depth of penetration of the summer melting front (the 0°C isotherm) represents the primary control on meltwater infiltration in the percolation zone of the Greenland Ice Sheet.

Meltwater infiltration and refreezing in snow and firn are important processes for Greenland Ice Sheet mass balance, acting to reduce meltwater runoff and mass loss. To advance understanding of meltwater retention processes in firn, we deployed vertical arrays of time-domain reflectometry sensors and thermistors to continuously monitor meltwater infiltration, refreezing, and wetting-front propagation in the upper 4 m of snow and firn over the 2016 melt season at DYE-2, Greenland. The dataset provides a detailed record of the co-development of the firn wetting and thawing fronts through the melt season. These data are used to constrain a model of firn thermodynamics and hydrology that is then used in simulations of the long-term firn evolution at DYE-2, forced by ERA5 climate reanalyses over the period 1950-2020. Summer 2016 meltwater infiltration reached a depth of 1.8 m below the surface, which is close to the modelled long-term mean at this site. Modelled meltwater infiltration increased at DYE-2 from 1990-2020, driving increases in firn density, ice content, and temperature; 10-m firn temperatures increased by 1°C per decade over this period. Modelled meltwater infiltration reached 6 to 7 m depth during extreme melt seasons in Greenland such as 2012 and 2019, causing 3-4°C increases in 10-m firn temperatures which persist for several years. A similar event occurred in 1968 in the model reconstructions. These deep infiltration events strongly impact the firn at DYE-2, and may be more influential than the background warming trend in governing meltwater retention capacity in the Greenland percolation zone.

As sources of fresh water and critical components of the global climate system, terrestrial glaciers are important features to monitor, particularly in light of anthropogenic climate change. Remote sensing techniques are being increasingly used to gather information on Earth’s shrinking complex glacial terrains. However, these methods possess critical challenges, including capturing firn dynamics and the presence of ice lenses. Meltwater percolation and retention, as well as thermodynamic effects on snow and firn density can complicate the relationship between surface height and mass balance changes; lowering of the glacier surface may masquerade as a mass change as detected by remote sensing technologies. The St. Elias Mountains, straddling the border between Yukon Territory, Canada and Alaska, USA, are home to extensive icefields. While numerous mass balance studies have been conducted in this region using remote sensing, there is a significant lack of in situ measurements of accumulation zone processes and firn properties. Our research examines refrozen ice layers and firn densification processes in the accumulation zone of Kaskawulsh Glacier in the St. Elias Mountains. In spring 2018, we extracted two firn cores (20 m and 35 m) from the study area and conducted a snow stratigraphy and ice lens survey on both core sections. After subsampling and melting the cores, we analyzed major ion and isotope chronology to identify extreme meltwater percolation and refreezing events, both of which critically affect firn density. The snow stratigraphy analysis from both of the cores showed numerous refrozen ice layers, indicating surface melt and refreezing processes in the accumulation zone. Preliminary results from isotope chronology analysis reveal a wash-out of the glaciochemical pattern in the 35 m and the 20 m ice core at 15 m depth, thus indicating severe surface warming events and subsequent changes in the density of the firn. This may indicate errors in the assumed density of the accumulation zone snow and firn when using remote sensing technologies to infer mass balance of Kaskawulsh Glacier.