Application of GRACE to the evaluation of an ice flow model of the Greenland Ice Sheet

Abstract. Quantifying the Greenland Ice Sheet’s future contribution to sea level rise is a challenging task that requires accurate estimates of ice flow sensitivity to climate change. Forward models of ice flow dynamics are promising tools for estimating future ice sheet behavior, yet confidence is low because evaluation of historical simulations is so challenging due to the scarcity of highly-resolved (spatially and temporally) continental-wide validation data. Recent advancements in processing of Gravity Recovery and Climate Experiment (GRACE) data using Bayesian-constrained mass concentration ("mascon") functions have led to improvements in spatial resolution and noise reduction of estimated monthly global gravity fields. Specifically, the Jet Propulsion Laboratory’s JPL RL05M GRACE mascon solution (GRACE-JPL) now offers an opportunity for ice sheet model evaluation within independently resolved 300 km mascons. Here, we investigate how Greenland Ice Sheet mass balance captured through observations - GRACE-JPL - differs from that simulated by the ice flow model - the Ice Sheet System Model (ISSM). For the years 2003-2012, ISSM is forced with regional climate model (RCM) surface mass balance (SMB), and resulting mass balance is directly compared against GRACE-JPL within individual mascons. Overall, we find good agreement in the Northeast, Southwest, and the interior of the ice sheet, where mass balance is primarily controlled by SMB. In the Northwest, seasonal amplitudes match well, but trends in ISSM are muted relative to GRACE-JPL. In the Southeast, GRACE-JPL exhibits larger seasonal amplitude than that predicted by SMB while simultaneously having more pronounced trends. These results indicate that discrepancies in the Northwest are controlled by changes in ice dynamics that are not currently modeled by ISSM, i.e. transient processes driven by ice sheet hydrology and ice-ocean interaction, while discrepancies in the Southeast are controlled by a combination of these missing dynamics and errors in modeled SMB. Along the margins, we find that transient dynamics are responsible for consistent intra-annual variations in regional mass balance that ultimately contribute to the steeper negative mass trends observed by GRACE-JPL. Consequently, ice-ocean interactions and hydrologically-driven processes at relatively high (monthly-to-seasonal) temporal resolutions must be considered for improving upon ice flow models.