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.
