Abstract. The development of the climate model MRI-ESM2 (Meteorological Research Institute Earth System Model version 2), which is
planned for use in the sixth phase of the Coupled Model Intercomparison
Project (CMIP6) simulations, involved significant improvements to the
representation of clouds from the previous version MRI-CGCM3 (Meteorological Research Institute Coupled Global Climate Model version 3), which was used
in the CMIP5 simulations. In particular, the serious lack of reflection of
solar radiation over the Southern Ocean in MRI-CGCM3 was drastically
improved in MRI-ESM2. The score of the spatial pattern of radiative fluxes
at the top of the atmosphere for MRI-ESM2 is better than for any CMIP5
model. In this paper, we set out comprehensively the various modifications
related to clouds that contribute to the improved cloud representation and
the main impacts on the climate of each modification. The modifications
cover various schemes and processes including the cloud scheme, turbulence
scheme, cloud microphysics processes, interaction between cloud and
convection schemes, resolution issues, cloud radiation processes,
interaction with the aerosol model, and numerics. In addition, the new
stratocumulus parameterization, which contributes considerably to increased
low-cloud cover and reduced radiation bias over the Southern Ocean, and the
improved cloud ice fall scheme, which alleviates the time-step dependency of
cloud ice content, are described in detail.