Impact of Inclusion of Realistic Surface Spectral Emissivity in the Global Climate Model on the Simulated Arctic Climate Mean States and Climate Changes

Surface longwave emissivity can be less than unity and vary significantly with frequency. However, most climate models still assume blackbody surface in the longwave (LW) radiation scheme of their atmospheric modules. This study incorporates realistic surface spectral emissivity into the atmospheric component of the NCAR Community Earth System Model (CESM) version 1.1.1 and evaluates its impact on simulated climatology. By ensuring the consistency of the broadband longwave flux across different modules of the CESM, the TOA energy balance in the simulation can be attained without additional tuning of the model. As for the global mean surface energy budget, inclusion of surface spectral emissivity leads to a decrease of net upward longwave flux at surface and a comparable increase of latent heat flux. Global-mean surface temperature difference between the modified and standard CESM simulation is 0.54 K for the fully coupled run and 0.78 K for the slab-ocean run. The surface temperature differences in Polar Regions are statistically significant. Accordingly the sea ice fraction in the modified CESM simulation is less than that in the standard CESM simulation by as much as 0.1, which significantly reduces the biases in the sea ice climatology as simulated by the CESM. When spectral emissivities of sea ice and open ocean surfaces are considered, the broadband LW sea-ice emissivity feedback estimated from 2xCO₂ and control runs is only -0.003 Wm^-2 per K, two orders-of-magnitude smaller than the surface albedo feedback. Therefore, we conclude that inclusion of the surface spectral emssivity into the global climate model cna help reducing biases in the mean state, especially over the polar sea ice region. The impact on climate change simulation, however, is little.