Editing IPCC:AR6/WGI/Chapter-8 (section)

===== 8.5.1.2.2 Regional climate models and convective-permitting models =====

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Regional Climate Models (RCMs) are used to dynamically downscale global model simulations for a particular region (usually at a spatial resolution of the order of 10 to 50 km; see [[IPCC:Wg1:Chapter:Chapter-10#10.3.3|Section 10.3.3]] ). The AR5 reported that RCMs are useful for regions with variable topography and for small-scale phenomena. However, they inherit biases from their driving GCMs and thus may lack physical consistency with them. Since AR5, the application of RCMs has largely increased due to international model intercomparison projects such as CLARIS-LPB ( [[#Sánchez--2015|Sánchez et al., 2015]] ). Many studies have focused on present-day climatological precipitation, showing with ''high confidence'' improvements in its monthly to seasonal accumulation and spatial distribution ( [[#Dosio--2015|Dosio et al., 2015]] ; [[#Giorgi--2016|Giorgi et al., 2016]] ; [[#Bozkurt--2019|Bozkurt et al., 2019]] ; [[#Falco--2019|Falco et al., 2019]] ; [[#Di%20Virgilio--2020|Di Virgilio et al., 2020]] ), although the modelling of precipitation remains the ‘Achilles heel’ of both GCMs and RCMs and should be considered cautiously when informing regional climate change adaptation strategies ( [[#Tapiador--2019b|Tapiador et al., 2019b]] ).

Regional Convective Permitting Models (CPMs), typically run at a resolution less than 10 km, have been implemented over increasingly large domains. Compared to models with parametrized convection ( [[#8.5.1.1.1|Section 8.5.1.1.1]] ), they generally show improved simulation of key features of the water cycle such as orographic precipitation, sea breeze dynamics, the diurnal cycle in precipitation, soil-moisture–precipitation feedbacks, daily precipitation persistence, sub-daily to daily precipitation intensities and related extremes ( [[#8.2.3.2|Section 8.2.3.2]] ; Birch et al. , 2015; Prein et al. , 2015; Kendon et al. , 2017; Leutwyler et al. , 2017; Willetts et al. , 2017; [[#Hohenegger--2018|Hohenegger and Stevens, 2018]] ; Berthou et al. , 2019b; [[#Takahashi--2019|Takahashi and Polcher, 2019]] ; Fumière et al. , 2020; Scaff et al. , 2020; Caillaud et al. , 2021) . A growing number of studies have also assessed the potential added value of using CPMs for regional climate projections (Ban et al. , 2015; Giorgi et al. , 2016; Fosser et al. , 2017; Kendon et al. , 2017, 2019; C. Liu et al. , 2017; Lenderink et al. , 2019; Rasmussen et al. , 2020; see also [[IPCC:Wg1:Chapter:Atlas|Atlas]] 5.6.3) . Although projected changes in rainfall occurrence in CPMs are broadly and qualitatively consistent with the results of GCMs and RCMs ( [[#Kendon--2017|Kendon et al., 2017]] ), there is a tendency towards stronger changes in both wet and dry extremes (Berthou et al. , 2019a; Kendon et al. , 2019; Lenderink et al. , 2019; Finney et al. , 2020a) . While both GCMs and RCMs project an overall decrease in summer precipitation over the Alps, RCMs simulate an increase over the high Alpine elevations that is not present in the global simulations ( [[#Giorgi--2016|Giorgi et al., 2016]] ).

Recent studies based on both GCMs and CPMs indicate that both CAPE and convective inhibition will increase in a warmer climate ( [[#8.2.3.2|Section 8.2.3.2]] ; J. [[#Chen--2020|]] [[#Chen--2020|Chen et al., 2020]] a), consistent with a shift from moderate to less frequent but stronger convective events ( [[#Rasmussen--2020|Rasmussen et al., 2020]] ). If underestimated by models with parametrized convection, such a mechanism could explain the underestimation of both projected increase in precipitation extremes ( [[#Borodina--2017|Borodina et al., 2017]] ; [[#Yin--2018|Yin et al., 2018]] ) and land surface drying ( [[#Douville--2017|Douville and Plazzotta, 2017]] ) in the extratropics. CMIP5 models with a larger increase in extreme precipitation also exhibit larger declines or smaller increases in light to moderate events ( [[#Thackeray--2018|Thackeray et al., 2018]] ).

In summary, there is ''high confidence'' that dynamical downscaling using limited area models adds value in simulating precipitation and related water cycle processes at the regional scale, especially in complex orography areas ( [[IPCC:Wg1:Chapter:Chapter-10#10.3.3.5.1|Section 10.3.3.5.1]] ). There is ''high confidence'' that the explicit simulation of atmospheric convection can improve the representation of weather phenomena, including the life cycle of convective storms and related precipitation extremes. Even with an improved simulation of small-scale processes, there is only ''medium confidence'' that there will be an improvement in RCM-based water cycle projections as they rely on GCM boundary conditions.

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