Forty-year Simulations of Firn Processes over the Greenland and Antarctic Ice Sheets

Abstract. Conversion of altimetry-derived ice-sheet volume change to mass requires an understanding of the evolution of the combined ice and air content within the firn column. In the absence of suitable techniques to observe the changes to the firn column across the entirety of an ice sheet, the firn column processes are typically modelled. Here, we present new 40-year simulations of firn processes over the Greenland and Antarctic Ice Sheets using the Community Firn Model and atmospheric reanalysis variables. A dataset of more than 250 measured depth-density profiles from both ice sheets provides the basis of the calibration of the dry-snow densification scheme. The resulting scheme results in a reduction in the rate of densification, relative to a commonly used semi-empirical model, through a decreased dependence on the accumulation rate, a proxy for overburden stress. The modelled firn column runoff, when combined with atmospheric variables from MERRA-2, generates realistic mean integrated surface mass balance values for the Greenland (+361 Gt yr−1) and Antarctic (+2623 Gt yr−1) ice sheets when compared to published model-ensemble means. We find that seasonal volume changes associated with firn air content are approximately 3 times larger than those associated with surface mass balance; however, when averaged over multiple years, ice and air-volume fluctuations within the firn column are of comparable magnitudes. Between 1996 and 2019, the Greenland Ice Sheet lost more than 5 % of its firn air content indicating a reduction in the total meltwater retention capability. Nearly all (>98 %) of the meltwater produced over the Antarctic Ice Sheet is retained within the firn column through infiltration and refreezing.