What Knowledge of Satellite Gravimetry and Altimetry can Reveal about the Continental-scale Evolution of Antarctic Firn

Abstract:

Variability in Antarctica’s firn air content constitutes a key uncertainty in altimetry-based estimates of ice-sheet mass balance. Thus, accurate model representation of firn evolution is integral for the quantification of observed ice-sheet mass trends, the improvement of ice-sheet model estimates of mass change, and ice-sheet model projections of Antarctica’s future sea-level contribution. Here, we take advantage of the Jet Propulsion Laboratory's 1-degree joint inversion product of Antarctic Ice Sheet mass balance, which is spatially informed by satellite altimetry patterns. Using this joint-inversion GRACE solution, we derive a remote-sensing-based firn air content (FAC) product, from the difference between the joint-inversion mass balance and that estimated from altimetry - “uncorrected” by FAC. We then model daily estimates of Antarctic Ice Sheet firn evolution from 1979-2019, using the glacier and energy mass balance (GEMB) surface radiation balance model, forced with ERA-5 reanalysis. We evaluate GEMB historical estimates of firn evolution, including trends and spatial/temporal variability, against the observationally-derived FAC. Analysis reveals model skill in the interior of East Antarctica, but indicates that GEMB underestimates an observed decline in FAC over the West Antarctic Ice Sheet. GEMB has recently been integrated into JPL's Ice-Sheet and Sea-level System Model (ISSM) framework. As a result, accurate model representation of FAC evolution will be required to effectively use ISSM for ice-sheet model assimilation of surface altimetry (which must be corrected for density to be converted to mass). This work will also drive improvements to model estimates of FAC, and will therefore help reduce discrepancies between gravimetric- and altimetric- based estimates of ice sheet mass balance.

This work is performed at the California Institute of Technology's Jet Propulsion Laboratory under a contract with the National Aeronautics and Space Administration's Cryosphere, Sea Level Change Team, and MAP Programs and the ICESat-2 and GRACE Science Teams.