Improved Representation of Surface Emissivity Effects on Longwave Radiation in a Coupled Earth System Model

Abstract Many Earth System Models (ESMs) approximate surface emissivity as constant over all wavelengths (broadband). This approximation misrepresents the spectral distribution of longwave (LW) flux and introduces error in the LW atmospheric heating rate. Many ESMs employ the “Effective Greybody” (EG) method, which creates bias in the effective temperature used by the atmosphere component. We implement a novel “Full Greybody” (FG) method, modifying the treatment of LW flux in the atmosphere to match that of the ocean surface component in the Energy Exascale Earth System model. We find that, relative to greybody emissivity, the ocean blackbody approximation over the ocean overestimates instantaneous mean upwelling LW surface flux by 1.380.05 W/. The common EG method introduces a mean bias of −0.24 K in the effective temperature. The FG method reduces this bias to 0.0039 K, comparable to the default blackbody bias (0.0070 K) while making ocean emission in ESMs more physically realistic.