This study explores the treatments of two missing radiative processes in the E3SM v2 (as well as in a dominant majority of mainstream models), namely the ice-cloud longwave scattering and the spectral dependence of surface longwave emissivity. Because longwave (LW) absorption by greenhouse gases and clouds is more significant than the LW scattering effect by clouds, most climate models neglect cloud LW scattering to save computational costs. Ignoring cloud LW scattering directly overestimates outgoing longwave radiation (OLR). We first incorporated ice-cloud LW scattering treatment into E3SM v2. Then, we used a myriad of simulations and offline radiative transfer calculations to comprehensively assess the impact of ice-cloud LW scattering on global climate simulation. The instantaneous effect due to ice-cloud LW scattering reduces the OLR by ∼1 W/m² on the global average and 2 W/m² on the tropical average. Tropospheric warming and high cloud amount reduction partially compensate for such instantaneous OLR reduction caused by the inclusion of LW scattering. When the simulation reaches the equilibrium, the surface warms by 0.66 K on average with respect to the simulation without LW scattering, with the Arctic surface temperature differences more than twice as large as that of the global mean. The impact of including LW scattering on the simulated climate change in response to 4 × CO₂ is also assessed. While including the cloud LW scattering does not significantly modify radiative forcing and total radiative feedback under such a scenario, it results in a 10% more positive cloud feedback.

The atmospheric component of E3SM v2 assumes the surface to be always a blackbody, but in reality, the surface emissivity of different surface types indeed has different spectral dependence, and it can be noticeably deviating from a blackbody. When a more realistic representation of surface spectral emissivity is included in the E3SM v2 on top of the inclusion of ice-cloud longwave scattering, together they can affect the simulated global-mean surface temperature by ~1.1K. The impact on long-term Arctic surface-air temperature can be as large as 4.1K. The effects of the two physical processes on the surface air temperature are linearly additive. 

Arctic amplification is a well-known phenomenon in response to the increase of CO₂. Compared to the E3SM v2 default version, including the two missing longwave physics reduces the strength of Arctic amplification in response to 4xCO₂ by 10%, with the largest reductions in the contributions from surface albedo feedback and lapse-rate feedback. When both LW missing physics are included, the initial mean climate state is warmer than its counterpart from E3SM v2 PI control runs. As a result, the surface albedo and lapse-rate feedback in such simulations are not as strong as in the v2 PI control runs, which can explain the reduced strength of Arctic Amplification.