Evaluating Greenland surface-mass-balance and firn-densification data usingICESat-2 altimetry

Surface-mass-balance (SMB) and firn-densification (FD)models are widely used in altimetry studies as a tool to separateatmospheric-driven from ice-dynamics-driven ice-sheet mass changes and topartition observed volume changes into ice-mass changes and firn-air-contentchanges. Until now, SMB models have been principally validated based oncomparison with ice core and weather station data or comparison with widelyseparated flight radar-survey flight lines. Firn-densification models havebeen primarily validated based on their ability to match net densificationover decades, as recorded in firn cores, and the short-term time-dependentcomponent of densification has rarely been evaluated at all. The advent ofsystematic ice-sheet-wide repeated ice-surface-height measurements fromICESat-2 (the Ice Cloud, and land Elevation Satellite, 2) allows us tomeasure the net surface-height change of the Greenland ice sheet atquarterly resolution and compare the measured surface-height differencesdirectly with those predicted by three FD-SMB models: MARv3.5.11 (ModeleAtmospherique Regional version 3.5.11) and GSFCv1.1 and GSFCv1.2 (theGoddard Space Flight Center FD-SMB models version 1.1 and 1.2). Bysegregating the data by season and elevation, and based on the timing andmagnitude of modelled processes in areas where we expect minimalice-dynamics-driven height changes, we investigate the models' accuracy inpredicting atmospherically driven height changes. We find that while allthree models do well in predicting the large seasonal changes in thelow-elevation parts of the ice sheet where melt rates are highest, two ofthe models (MARv3.5.11 and GSFCv1.1) systematically overpredict, by around afactor of 2, the magnitude of height changes in the high-elevation partsof the ice sheet, particularly those associated with melt events. Thisoverprediction seems to be associated with the melt sensitivity of themodels in the high-elevation part of the ice sheet. The third model,GSFCv1.2, which has an updated high-elevation melt parameterization, avoidsthis overprediction.