The Effect of Physically Based Ice Radiative Processes on Greenland Ice Sheet Albedo and Surface Mass Balance in E3SM
A new model that improves how the reflectivity, or albedo, of ice, is calculated was implemented into the US Department of Energy's Energy Exascale Earth System Model (E3SM). Simulations with the new ice reflectivity, which is informed by satellite observations, show that the old bare ice reflectivity, which was spatially and temporally constant, was too high, leading to biases in the surface mass balance and surface energy budget over the Greenland Ice Sheet (GrIS).
Improving the representation of bare ice albedo on the GrIS to physically simulate bare ice radiative transfer processes, rather than parameterize bare ice albedo to a constant value, allows for better estimations of surface melt and more realistic calculations of the surface energy budget. More accurate estimations of surface melt and energy fluxes will improve our ability to quantify the GrIS’s current and future contributions to sea-level rise.
The goal of this work is to improve E3SM’s treatment of ice radiative transfer modeling, specifically over the GrIS. A large portion of melt from the GrIS comes from regions with dark bare ice, however, Earth system models, such as E3SM, treat bare ice albedo as a constant. To improve the default bare ice albedo parameterization in E3SM, we incorporated a physically based radiative transfer model (SNICAR‐ADv4) into the E3SM land model (ELM). We utilized satellite observations to determine spatially and temporally varying bare ice physical properties over the GrIS ablation zone to inform SNICAR-ADv4 in ELM. We assessed the impact of the new scheme on the simulated bare ice albedo and surface mass balance. We found that the GrIS‐wide bare ice albedo in the old E3SMv2 scheme is overestimated by ∼4% in the visible and ∼7% in the near-infrared wavelengths compared to satellite observations. We also found that light-absorbing constituents on the ice surface, ice crustal surfaces, and melt ponds reduce visible albedo by 30% in the bare ice region of the GrIS ablation zone. The new physically realistic bare ice scheme reduces surface mass balance by ∼145 Gt, or 0.4 mm of sea‐level equivalent, between 2000 and 2021, compared to the default E3SM.