Because strong absorption of infrared radiation by greenhouse gases is more significant than the cloud longwave (LW) scattering effect, most climate models neglect cloud LW scattering to save computational costs. However, ignoring cloud LW scattering directly overestimates the outgoing longwave radiation (OLR). A recent study performed slab-ocean model simulations in the Community Earth System Model and showed that such radiative flux changes due to ice cloud LW scattering can affect the polar surface climate more than other climate zones. In this study, we included the same ice cloud LW scattering treatment in the Exascale Energy Earth System Model (E3SM) version 2 and ran fully-coupled simulations to assess the impact of ice cloud LW scattering on global climate simulation. Including ice cloud LW scattering leads to ~2 W m^-2 instantaneous OLR reduction in the tropics, more than the OLR reduction in other climate zones. Strong surface warming occurs in the Arctic, which is dominantly caused by the polar amplification resulting from the radiative forcing caused by ice cloud LW scattering. In the tropics, when the ice cloud LW scattering effect is included, more liquid clouds form in the middle troposphere, high clouds in the convection zone are lifted, anvil clouds retreat, and stratiform low cloud fraction increases. Most of these effects are similar to the cloud response to the increase of well-mixed greenhouse gases. The present study suggests that the ice cloud LW scattering effect must be incorporated into climate simulations.

Scattering of longwave radiation by cloud particles has been regarded unimportant and hence commonly neglected in global climate models. However, it has been demonstrated by recent studies that cloud longwave scattering plays an unignorable role in modulating the energy budget of the Earth System. Offline radiative transfer calculation showed that excluding cloud longwave scattering could overestimate outgoing longwave radiation and underestimate downward irradiance to the surface, and thus impose excessive cooling onto the atmosphere column. How this physical process interacts with other processes in the Arctic climate system, however, has not been thoroughly evaluated yet. Given the fact that the melting of ice and snow that cover the vast surface of the Arctic region is sensitive to energy budget, and such melting may trigger further feedback mechanisms, the neglection of cloud longwave scattering could bias the regional climate simulations to a considerable extent. We have incorporated cloud longwave scattering into the NCAR CESM and the DoE E3SM and this study analyzed the impact on the simulated polar climates in both earth system models. Cloud longwave scattering leads to a warmer surface air temperature in both models, especially over the wintertime. A detailed surface energy budget analysis is performed, for both the mean state and the temporal variability. Preliminary results suggest that the leading change is downward longwave flux and upward longwave flux, followed by the changes of turbulent heat flux. How the longwave scattering treatments can couple with cloud microphysics and precipitation physics to affect Arctic precipitation is further explored.