Editing IPCC:Wg1:Chapter:Chapter-1-comments (section)

===== ''1.5.3.1.1 Model grids'' ''and resolution'' =====

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The horizontal resolution and the number of vertical levels in ESMs is generally higher in CMIP6 than in CMIP5 (Figure 1.19). Global models with finer horizontal grids better represent many aspects of the circulation of the atmosphere ( [[#Gao--2020|Gao et al., 2020]] ; [[#Schiemann--2020|Schiemann et al., 2020]] ) and ocean ( [[#Bishop--2016|Bishop et al., 2016]] ; [[#Storkey--2018|Storkey et al., 2018]] ), bringing improvements in the simulation of the global hydrological cycle ( [[#Roberts--2018|Roberts et al., 2018]] ). CMIP6 includes a dedicated effort (HighResMIP, [[#Haarsma--2016|Haarsma et al., 2016]] ) to explore the effect of higher horizontal resolution, such as ~50 km, ~25 km and even ~10 km ( [[#1.5.4.2|Section 1.5.4.2]] and Annex II, Table AII.6). Improvements are documented in the highest-resolution coupled models used for HighResMip ( [[#Hewitt--2017|]] [[#Hewitt--2017|Hewitt et al., 2017]] ; [[#Roberts--2019|Roberts et al., 2019]] ). Flexible grids allowing spatially variable resolution in the atmosphere ( [[#McGregor--2015|McGregor, 2015]] ; [[#Giorgetta--2018|Giorgetta et al., 2018]] ) and in the ocean ( [[#Wang--2014|Wang et al., 2014]] ; [[#Petersen--2019|Petersen et al., 2019]] ) are more widely used than at the time of the AR5.

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'''Figure 1.19 |''' '''Resolution of the atmospheric and oceanic components of global climate models participating in CMIP5, CMIP6 and HighResMIP:''' '''(a, b)''' horizontal resolution (km), and '''(c, d)''' number of vertical levels. Darker-colour circles indicate high-top models (in which the top of the atmosphere is above 50 km). The crosses are the median values. These models are documented in Annex II. Note that duplicated models in a modelling group are counted as one entry when their horizontal and vertical resolutions are the same. For HighResMIP, one atmosphere–ocean coupled model with the highest resolution from each modelling group is used. The horizontal resolution (rounded to 10 km) is the square root of the surface area of the Earth divided by the number of grid points, or the area of the ocean surface divided by the number of surface ocean grid points, for the atmosphere and ocean, respectively.
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The number of vertical levels in the atmosphere of global models has increased (Figure 1.19), partly to enable simulations to include higher levels in the atmosphere and better represent stratospheric processes ( [[#Charlton-Perez--2013|Charlton-Perez et al., 2013]] ; [[#Kawatani--2019|Kawatani et al., 2019]] ). Half the modelling groups now use ‘high-top’ models with a top level above the stratopause (a pressure of about 1 hPa). The number of vertical levels in the ocean models has also increased in order to achieve finer resolution over the water column and especially in the upper mixed layer and to better resolve the diurnal cycle ( [[IPCC:Wg1:Chapter:Chapter-3#3.5|Section 3.5]] and Annex II; [[#Bernie--2008|Bernie et al., 2008]] ).

Despite the documented progress of higher resolution, the model evaluation carried out in subsequent chapters shows that improvements between CMIP5 and CMIP6 remain modest at the global scale ( [[IPCC:Wg1:Chapter:Chapter-3#3.8.2|Section 3.8.2]] ; [[#Bock--2020|Bock et al., 2020]] ). Lower resolution alone does not explain all model biases, for example, a low blocking frequency ( [[#Davini--2020|Davini and D’Andrea, 2020]] ) or a wrong shape of the Intertropical Convergence Zone ( [[#Tian--2020|Tian and Dong, 2020]] ). Model performance depends on model formulation and parameterizations as much as on resolution (Chapters 3, 8 and 10).

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