A Refined Understanding of the Ice Cloud Longwave Scattering Effects in Climate Model

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). This study included ice-cloud LW scattering treatment in the Exascale Energy Earth System Model (E3SM) version 2 and ran fully-coupled simulations, prescribed sea surface temperature 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 similar to 1 W/m2 on the global average and 2 W/m2 on the tropical average. Tropospheric warming and high cloud amount reduction act to 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 x 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. Clouds are an essential mediator in the climate system because they can reflect solar radiation back to space and block longwave radiation emitted below reaching the top of the atmosphere by either absorbing it or scattering it elsewhere. Such longwave scattering physics is deemed less important and thus neglected in most climate models to save computational time. We incorporated this mechanism into a climate model and ran pairs of simulations, with or without cloud scattering, to see how it would affect the simulated global climate. We found that cloud longwave scattering reduces the longwave radiation that goes to space. Such reduction of outgoing longwave radiation is strongest in the tropics. Compared to the simulation without longwave scattering, the mean-state surface temperature change is larger in the Arctic than in the tropics, which is primarily caused by the slow response to the inclusion of scattering. We also assessed to what extent the inclusion of cloud longwave scattering can affect the simulated response to abrupt 4 x CO2 increase. We concluded that it can increase the cloud feedback strength by similar to 10%, but overall, the impact is not statistically significant. Ice-cloud longwave (LW) scattering leads to warming with a pattern similar to the response to quadrupled CO2 in Exascale Energy Earth System Model version 2Strong Arctic warming is part of the global response to radiative perturbation rather than a local impact of LW scatteringIncluding ice-cloud LW scattering does not significantly affect the simulated responses to abrupt 4 x CO2 increases