A Refined Understanding of the Ice Cloud Longwave Scattering Effects in Climate Model
The impacts of ice cloud longwave scattering on the simulated mean-state climate and climate change projection were thoroughly evaluated in this study.
As more than 90% of the current models used in the IPCC assessment do not include the physics of cloud longwave scattering, our study presented a comprehensive evaluation of its impacts on the simulated mean-state climate and climate change projection. This helps reveal compensating biases in the simulations when cloud longwave scattering is not included.
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 an abrupt 4 × CO2 increase. We concluded that it can increase the cloud feedback strength by ∼10%, but overall, the impact is not statistically significant.