This study documents the high-resolution model, highlights major improvements in the representation of atmospheric dynamical core and physical processes. BCC-CSM2-HR is evaluated for historical climate simulations from 1950 to 2014, performed under CMIP6-prescribed historical forcing, in comparison with its previous medium-resolution version BCC-CSM2-MR. Observed global warming trends of surface air temperature from 1950 to 2014 are well captured by both BCC-CSM2-MR and BCC-CSM2-HR. Present-day basic atmospheric mean states during the period from 1995 to 2014 are then evaluated at global scale, followed by an assessment on climate variabilities in the tropics including the tropical cyclones (TCs), the El Niño–Southern Oscillation (ENSO), the Madden–Julian Oscillation (MJO), and the quasi-biennial oscillation (QBO) in the stratosphere. It is shown that BCC-CSM2-HR represents the global energy balance well and can realistically reproduce the main patterns of atmospheric temperature and wind, precipitation, land surface air temperature, and sea surface temperature (SST). It also improves the spatial patterns of sea ice and associated seasonal variations in both hemispheres. The bias of the double intertropical convergence zone (ITCZ), obvious in BCC-CSM2-MR, almost disappears in BCC-CSM2-HR. TC activity in the tropics is increased with resolution enhanced. The cycle of ENSO, the eastward propagative feature and convection intensity of MJO, and the downward propagation of QBO in BCC-CSM2-HR are all in a better agreement with observations than their counterparts in BCC-CSM2-MR. Some imperfections are, however, noted in BCC-CSM2-HR, such as the excessive cloudiness in the eastern basin of the tropical Pacific with cold SST biases and the insufficient number of tropical cyclones in the North Atlantic.
Our work shows that enhancing resolution does not noticeably improve climate mean state, and deterioration is even possible. For example, the decrease in JJA precipitation over the warm pool in our high-resolution model is still an important issue which certainly deserves further investigations with multiple models and simulations. Actually, other studies also reported similar issues. Haarsma et al. (2020) shows that increasing resolution in the EC-Earth model deteriorated the wet bias over the western Pacific warm pool. Bacmeister et al. (2014) analysed the high-resolution climate simulations performed with the Community Atmosphere Model (CAM) and showed that dry bias over the same region with enhanced resolution. Over the western Pacific warm pool, the atmospheric circulation and precipitation undergoes not only the impact of tropical variations such as MJO and TC, but also strong regional air–sea coupling.
This paper was devoted to the presentation of the high-resolution version BCC-CSM2-HR and to the description of its climate simulation performance. We focused on its updates and differential characteristics from its predecessor, the medium-resolution version BCC-CSM2-MR. BCC-CSM2-HR is our model version participating in HighResMIP, while BCC-CSM2-MR is our basic model version participating in other CMIP6-endorsed MIPs.
The atmosphere resolution is increased from T106L46 in BCC-CSM2-MR to T266L56 in BCC-CSM2-HR, and the ocean resolution from 1∘×1∘ to 1/4∘×1/4∘. A few novel developments were implemented in BCC-CSM2-HR for both the dynamical core and model physics in the atmospheric component: first, a spatially varying damping for the divergence field was used to improve the atmospheric temperature simulation in the stratosphere at polar areas – it helps to control high-frequency noise in the stratosphere and above; second, the deep cumulus convection scheme originally described in Wu (2012) was further ameliorated to allow detrained cloud water be transported to adjacent grids and downward to lower troposphere; third, we modified the relevant schemes for the boundary layer turbulence and shallow cumulus convection to improve the simulation of ITCZ precipitation; fourth, the UWMT scheme is used to improve the simulation of low-level clouds over eastern basins of subtropical oceans. The land model configuration in BCC-CSM2-HR is the same as in BCC-CSM2-MR. Major land surface biophysical and plant physiological processes of BCC-AVIM2 implemented in BCC-CSM2-MR and BCC-CSM2-HR were kept the same, and the only differences are in the sub-grid surface classification. The ocean component of BCC-CSM2-HR is upgraded from MOM4 in BCC-CSM2-MR to MOM5. The sea-ice component is also updated from SIS4 to SIS5.