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. Increasing melt over the Greenland Ice Sheet (GrIS) recordedover the past several years has resulted in significant changes of the percolationregime of the ice sheet. It remains unclear whether Greenland's percolationzone will act as a meltwater buffer in the near future through graduallyfilling all pore space or if near-surface refreezing causes the formation ofimpermeable layers, which provoke lateral runoff. Homogeneous ice layerswithin perennial firn, as well as near-surface ice layers of several meterthickness have been observed in firn cores. Because firn coring is adestructive method, deriving stratigraphic changes in firn and allocation ofsummer melt events is challenging. To overcome this deficit and providecontinuous data for model evaluations on snow and firn density, temporalchanges in liquid water content and depths of water infiltration, weinstalled an upward-looking radar system (upGPR) 3.4 m below the snowsurface in May 2016 close to Camp Raven(66.4779 ∘  N, 46.2856 ∘  W) at 2120 m a.s.l. The radar iscapable of quasi-continuously monitoring changes in snow and firnstratigraphy, which occur above the antennas. For summer 2016, we observedfour major melt events, which routed liquid water into various depths beneaththe surface. The last event in mid-August resulted in the deepest percolationdown to about 2.3 m beneath the surface. Comparisons with simulations fromthe regional climate model MAR are in very good agreement in terms ofseasonal changes in accumulation and timing of onset of melt. However,neither bulk density of near-surface layers nor the amounts of liquid waterand percolation depths predicted by MAR correspond with upGPR data. Radardata and records of a nearby thermistor string, in contrast, matched verywell for both timing and depth of temperature changes and observed waterpercolations. All four melt events transferred a cumulative mass of56 kg m −2 into firn beneath the summer surface of 2015. We find thatcontinuous observations of liquid water content, percolation depths and ratesfor the seasonal mass fluxes are sufficiently accurate to provide valuableinformation for validation of model approaches and help to develop a betterunderstanding of liquid water retention and percolation in perennial firn.

the cryosphere

Seasonal monitoring of melt and accumulation within the deep percolation zone of the Greenland Ice Sheet and comparison with simulations of regional climate modeling