Collaborative Research: Refreezing in the firn of the Greenland ice sheet: Spatiotemporal variability and implications for ice sheet mass balance

A substantial fraction of meltwater of the Greenland ice sheet is retained in firn (liquid or refrozen) rather than running off into the ocean. Unusually thick, near-surface, ice lenses have recently been discovered in the firn and are thought to be the result of exceptionally large recent melt events. This suggests that the larger volumes of meltwater produced in recent years may have been prevented from percolating into deeper firn layers, as has typically been observed in the past, and have instead run off immediately. As a result, a qualitatively different and new ’state’ of the firn has to be taken into account when attempting to quantify the mass balance of the ice sheet, and estimates based on our current state of knowledge are probably subject to larger uncertainty than previously thought. Refreezing, as the mechanism which creates impenetrable ice lenses, emerges as a crucial process in the redistribution of surface runoff and therefore in the determination of surface mass balance (SMB) of the Greenland ice sheet. To quantify this impact on the current and future SMB, this project will combine detailed field observations in the Kangerlussaq section of the Greenland ice sheet with numerical modeling of the relevant components of the climate system. Project goals are: 1) to investigate the changes and quantify the spatio-temporal variability of the firn by analyzing shallow firn cores and subsurface data at selected sites covering a wide range of elevations and climatic conditions, and compare these new observations to similar data from past decades and remotely sensed observations of firn structures; 2) to quantify the role of refreezing on the SMB with the help of a regional climate model and a high-resolution distributed energy balance model, both of which include a sub-surface snow/firn model; 3) to perform simulations of the spatio-temporal evolution of the SMB with an improved representation of refreezing process under different emission scenarios through the year 2100.