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