As a contribution to phase 6 of the Coupled Model Intercomparison Project (CMIP6), the global climate simulated by an atmospheric general circulation model (GCM), the Seoul National University Atmosphere Model version 0 with a Unified Convection Scheme (SAM0-UNICON), is compared with observation and climates simulated by the Community Atmosphere Model version 5 (CAM5) and Community Earth System Model version 1 (CESM1), on which SAM0-UNICON is based. Both SAM0-UNICON and CESM1 successfully reproduce observed global warming after 1970. The global mean climate simulated by SAM0-UNICON is roughly similar to that of CAM5/CESM1. However, SAM0-UNICON improves the simulations of the double intertropical convergence zone, shortwave cloud forcing, near-surface air temperature, aerosol optical depth, sea ice fraction, and sea surface temperature (SST), but is slightly poorer for the simulation of tropical relative humidity, Pacific surface wind stress, and ocean rainfall. Two important biases in the simulated mean climate in both models are a set of horseshoe-shaped biases of SST, sea level pressure, precipitation, and cloud radiative forcings in the central equatorial Pacific and a higher sea ice fraction in the Arctic periphery and Southern Hemispheric circumpolar regions. Both SAM0-UNICON and CESM1 simulate the observed El Niño–Southern Oscillation (ENSO) reasonably well. However, compared with CAM5/CESM1, SAM0-UNICON performs better in simulating the Madden–Julian oscillation (MJO), diurnal cycle of precipitation, and tropical cyclones. The aerosol indirect effect (AIE) simulated by SAM0-UNICON is similar to that from CAM5 but the magnitudes of the individual shortwave and longwave AIEs are substantially reduced.

The previously proposed parameterization for the integrated vertical overlap of cumulus and stratus is implemented online into the cloud microphysics and radiation schemes of the Seoul National University Atmosphere Model version 0 with a Unified Convection Scheme (SAM0-UNICON). Instead of a single-merged cloud, the modified radiation scheme handles cumulus, stratus, and stratiform snow, separately, with each type having its own optical properties and vertical overlap structures. The integrated cloud overlap parameterization implemented into the cloud microphysics schemes do not reduce the biases of surface precipitation rate (PRECT) and cloud radiative forcing. Although it changes the overlap structures of clouds and precipitation areas, as well as the associated cloud microphysical processes either directly or indirectly, strong cancelation occurs among these terms, resulting in small changes to the global-mean PRECT and cloud radiative forcing. The integrated cloud overlap parameterization implemented into the radiation scheme has a substantial impact on the simulated climate: the global-mean cloud radiative forcing decreases substantially, mainly due to the separate treatment of radiative properties of individual cumulus, stratus, and stratiform snow, and PRECT exhibits strong regional responses. Sensitivity simulations showed that vertical cloud overlap exerts a weaker influence on the global-mean PRECT than the previous off-line simulations, implying that the indirect effect offsets the direct effect. In contrast to the off-line simulations, the enhanced randomness of cumulus overlap increases PRECT over the western Pacific warm pool region. Our study indicates that vertical cloud overlap has substantial impacts on global climate through complex interactions with other physical processes.

Recent studies have reported a 9% decrease in global carbon emissions during the COVID-19 lockdown period; however, its impact on the variation of atmospheric CO2 level remains under question. Using atmospheric CO2 observed at Anmyeondo station (AMY) in South Korea, downstream of China, this study examines whether the decrease in China’s emissions due to COVID-19 can be detected from the enhancement of CO2 mole fraction (ΔCO2) relative to the background value. The Weather Research and Forecasting–Stochastic Time-Inverted Lagrangian Transport model was applied to determine when the observed mole fractions at AMY were affected by air parcels from China. Atmospheric observations at AMY showed up to a −20% (−1.92 ppm) decrease in ΔCO2 between February and March 2020 compared to the same period in 2018 and 2019, particularly with a −34% (−3.61 ppm) decrease in March. ΔCO, which was analyzed to explore the short-term effect of emission reductions, had a decrease of −43% (−80.66 ppb) during the lockdown in China. Particularly in East China, where emissions are more concentrated than in Northeast China, ΔCO2 and ΔCO decreased by −44% and −65%, respectively. The ΔCO/ΔCO2 ratio (24.8 ppb ppm−1), which is the indicator of emission characteristics, did not show a significant difference before and after the COVID-19 lockdown period (α = 0.05), suggesting that this decrease in ΔCO2 and ΔCO was associated with emission reductions rather than changes in emission sources or combustion efficiency in China. Reduced carbon emissions due to limited human activity resulted in a decrease in the short-term regional contribution to the observed atmospheric CO2.

The previously proposed parameterization for the integrated vertical overlap of cumulus and stratus is generalized to handle both conventional exponential-random stratus overlap and nonconventional (i.e., other than exponential-random) cumulus overlap in a simultaneous way. With the parameterization of the decorrelation length scale of stratus as a function of vertical wind shear, our parameterization simulates various interactive feedback between vertical cloud overlap and other physical processes. This interactive vertical overlap parameterization of cumulus and stratus was implemented into all relevant physics parameterizations (i.e., convection, stratus microphysics, radiation, aerosol wet deposition, and aerosol activation at the base of stratus) of the Seoul National University Atmosphere Model version 0 with a Unified Convection Scheme (SAM0-UNICON) in a fully consistent way. It is shown that the overall performance of the interactive cloud overlap parameterization to simulate the observed mean climate is similar to that of the original overlap parameterization. Given that an intensive tuning has not yet been performed with the new overlap parameterization, this result is quite encouraging.