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A Refined Understanding of the Cloud Longwave Scattering Effects in Climate Model
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  • Chongxing Fan,
  • Yi-Hsuan Chen,
  • Xiuhong Chen,
  • Wuyin Lin,
  • Ping Yang,
  • Xianglei Huang
Chongxing Fan
University of Michigan

Corresponding Author:cxfan@umich.edu

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Yi-Hsuan Chen
Princeton University
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Xiuhong Chen
University of Michigan
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Wuyin Lin
Brookhaven National Laboratory
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Ping Yang
Texas A&M University
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Xianglei Huang
University of Michigan
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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.