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.