a. Model and experiments
We use the High-Resolution Atmospheric Model (HiRAM; Zhao et al. (2009))
developed at the Geophysical Fluid Dynamics Laboratory (GFDL). All
simulations are conducted with prescribed climatological monthly means
of sea surface temperature (SST) and sea ice from Hadley Centre Sea Ice
and Sea Surface Temperature (HadISST) dataset (Rayner et al., 2003)
based on the 20-year period from 1986 to 2005, and are integrated for 50
years with constant atmospheric CO2, greenhouse gases
and aerosol concentrations (at 1990 levels). The first simulation
(referred to as the Control run) follows the default model configuration
and thus has fully interactive radiation. The second simulation
(referred to as the ClimRad run) overwrites the model-generated
atmospheric radiative cooling rates with its monthly-varying
climatological values computed from the Control run. Specifically, the
overwriting process is implemented as follows: (a) monthly atmospheric
radiative cooling rates are retrieved from the last 20 years of the
Control run; (b) a multiyear average is applied to the 20-year data to
get monthly-varying climatological radiative cooling rates; and (c) each
time when the radiation code is called in the ClimRad run, the
atmospheric radiative cooling rates are overwritten by its
monthly-varying climatological values that are temporally interpolated
to the current time step. A summary of these simulations is listed in
Table 1. These simulations are also used in Zhang et al. (2021).