Simulated Greenland Surface Mass Balance in the GISS ModelE2 GCM: Role of the Ice Sheet Surface

The rate of growth or retreat of the Greenland and Antarctic ice sheets remains a highly uncertain component of future sea level change. Here we examine the simulation of Greenland ice sheet surface mass balance (GrIS SMB) in a development branch of the ModelE2 version of the NASA Goddard Institute for Space Studies (GISS) general circulation model (GCM). GCMs are often limited in their ability to represent SMB compared with polar region regional climate models. We compare ModelE2-simulated GrIS SMB for present-day (1996-2005) simulations with fixed ocean conditions, at a spatial resolution of 2 degrees latitude by 2.5 degrees longitude (similar to 200km), with SMB simulated by the Modele Atmospherique Regionale (MAR) regional climate model (1996-2005 at a 25-km resolution). ModelE2 SMB agrees well with MAR SMB on the whole, but there are distinct spatial patterns of differences and large differences in some SMB components. The impacts of changes to the ModelE2 surface are tested, including a subgrid-scale representation of SMB with surface elevation classes. This has a minimal effect on ice sheet-wide SMB but corrects local biases. Replacing fixed surface albedo with satellite-derived values and an age-dependent scheme has a larger impact, increasing simulated melt by 60%-100%. We also find that lower surface albedo can enhance the effects of elevation classes. Reducing ModelE2 surface roughness length to values closer to MAR reduces sublimation by similar to 50%. Further work is required to account for meltwater refreezing in ModelE2 and to understand how differences in atmospheric processes and model resolution influence simulated SMB. Plain Language Summary Melting of the Earth's ice sheets represents a substantial contribution to global sea level rise. Global climate model simulations of Earth's climate often model the surface of ice sheets in a fairly simple way because of computational limitations. This study evaluates the representation of the Greenland ice sheet in one such global model simulation (from the NASA Goddard Institute for Space Studies general circulation model) against a regional model that simulates only the local Greenland area in a higher degree of detail. The study finds that the global model simulation of the Greenland ice sheet is sensitive to how the model represents the ice sheet surface, in particular, how it reflects incoming sunlight, stores and freezes liquid water, and simulates surface evaporation. Attempting to improve the simulation by dividing the ice sheet surface into additional grid cells according to surface elevation has a minor impact on the simulation. The study reveals how the representation of the Greenland ice sheet in ModelE2 might be improved to better estimate ice sheet change and the sea level response to global climate changes.