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

==== Atlas.1.4.4 Regional Model Data (CORDEX) ====

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Global model data,as generated by the CMIP ensembles, although available globally, have spatial resolutions that are limited for reproducing certain processes and phenomena relevant for regional analysis (around 2° and 1° for CMIP5 and CMIP6, respectively). The Coordinated Regional Climate Downscaling Experiment (CORDEX; [[#Gutowski%20Jr.--2016|Gutowski Jr. et al., 2016]] ) facilitates worldwide application of Regional Climate Models (RCMs, see [[IPCC:Wg1:Chapter:Chapter-10#10.3.1.2|Section 10.3.1.2]] ), focusing on a number of regions (Figure Atlas.6) with a typical resolution of 0.44° (but also at 0.22° and 0.11° over some domains, such as Europe). However, only a few simulations are available for some domains (Annex II, Table AII.1), thus limiting the level of analysis and assessment that can be performed using CORDEX data in some regions. Moreover, there are regions where several domains overlap, thus providing additional lines of evidence. The use of multi-domain grand ensembles to work globally with CORDEX data have recently been proposed ( [[#Legasa--2020|Legasa et al., 2020]] ; [[#Spinoni--2020|Spinoni et al., 2020]] ). Ongoing efforts, such as the multi-domain CORDEX-CORE simulations are promoting more homogeneous coverage and thus more systematic treatment of CORDEX domains (Box Atlas.1).

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[[File:ef03a1f34ddd4ff9d8eb0722558a8ac8 IPCC_AR6_WGI_Atlas_Figure_6.png]]

'''Figure Atlas.6''' '''|''' '''CORDEX domains showing the curvilinear domainboundaries resulting from the original rotated domains.''' The topography corresponding to the standard CORDEX 0.44° resolution is shown to illustrate the orographic gradients over the different regions.

A lot of progress has been made by the regional climate modelling community since AR5 (Table AII.1) to produce and make available evaluation (reanalysis-driven) simulations over the different CORDEX domains along with downscaled CMIP5 historical and future climate projection information under a range of emissions scenarios, mainly RCP2.6, RCP4.5 and RCP8.5 (Tables AII.3 and AII.4). However, these ensembles cover only a fraction of the uncertainty range spanned by the full CMIP5 ensemble in the different domains (e.g., Figures Atlas.16, Atlas.17, Atlas.21, Atlas.22, Atlas.24, Atlas.26, Atlas.28 and Atlas.29; [[#Ito--2020b|Ito et al., 2020b]] ). Therefore, comparison of CMIP5 and CORDEX results should be performed carefully, providing results not only for the full CMIP5 ensemble but also for the sub-ensemble formed by the driving models since results can diverge ( [[#Fernández--2019|Fernández et al., 2019]] ; [[#Iles--2020|Iles et al., 2020]] ).

The Atlas chapter and the Interactive Atlas use CORDEX information for the following 11 individual CORDEX domains (out of the 14 domains shown in Figure Atlas.6): North, Central and South America; Europe; Africa; South, East and South East Asia; Australasia; Arctic and Antarctica; in addition, oceanic information has been used from the Mediterranean domain, which provides simulations from coupled atmosphere–ocean regional climate models (RCMs). In order to harmonize the information across domains and to maximize the size of the resulting ensembles, all the available simulations for each individual CORDEX domain (including the standard 0.44° CORDEX and the 0.22° CORDEX-CORE) have been interpolated to a common regular 0.5°-resolution grid to provide a grand ensemble covering the historical and future emissions scenarios RCP2.6, RCP4.5 and RCP8.5, and also the reanalysis-driven simulations for evaluation purposes. In the case of the European domain, the dataset considered is the 0.11° simulations (CORDEX EUR-11, the same dataset as used in Chapter 12), which has been interpolated to a regular 0.25° resolution grid (the same used for the regional observations). In the case of the Mediterranean domain, oceanic information (sea surface temperature, SST) is interpolated to a regular 0.11° grid. In all cases, the indices are computed on the original grids and the interpolation process is applied to the resulting indices. Moreover, for the sake of comparability with CMIP6 results (in particular when using baseline periods beyond 2005), the historical period of the CORDEX datasets has been extended to 2006–2014 using the first years of RCP8.5-driven transient projections in which the emissions are close to those observed (see [[#Atlas.1.3.1|Atlas.1.3.1]] ); note that this procedure is also applied to CMIP5 simulations.

For the different CORDEX domains, the full ensembles of models (GCM-RCM matrix) used in the Atlas for the different scenarios and variables are described in the Supplementary Material (Tables Atlas.SM.3–Atlas.SM.14) and in the Atlas repository ( [[#Iturbide--2021|Iturbide et al., 2021]] ), including full metadata relative to ESGF versions used and the periods with data available for the different simulations. In particular, the historical scenario information is only available from 1970 onwards for some models and therefore the common period 1970–2005 is used for historical CORDEX data in the Atlas. As a result, the WMO baseline period 1961–1990 is not available in the Interactive Atlas for CORDEX data.

Sections [[#Atlas.4|Atlas.4]] to [[#Atlas.11|Atlas.11]] assess research on CORDEX simulations over different regions, analysing past and present climate as well as future climate projections. They also focus on regional model evaluation in order to extend and complement the validation of global models in Chapter 3, considering the specific regional climate and relevant large-scale and regional phenomena, drivers and feedbacks ( [[IPCC:Wg1:Chapter:Chapter-10#10.3.3|Section 10.3.3]] ). Besides the literature assessment, some simple evaluation diagnostics have been computed for the simulations used in the Atlas chapter to provide some basic information on model performance across regions. In particular, biases for mean temperature and precipitation have been calculated for the 11 CORDEX domains analysed.

Figure Atlas.7 shows mean temperature and precipitation biases over the North American domain in RCM simulations driven by reanalysis and historical GCM simulations ( [[IPCC:Wg1:Chapter:Chapter-10#10.3.2.5|Section 10.3.2.5]] ). Annual and seasonal (December–January–February (DJF) and June–July–August (JJA)) biases are computed for both the RCMs and driving GCMs. Biases in the reanalysis-driven RCMs result from intrinsic model errors, with the results displayed being spatially aggregated for each reference region. This same analysis is performed for the GCM-driven RCM simulations over the historical period 1986–2005. This allows comparison of the intrinsic bias of the RCMs with the biases resulting when driven by the different GCMs and patterns of behaviour in the RCMs, for example intrinsic warm and dry biases in ENA and WNA respectively or reduced RCM warm biases compared to the CCCma GCM in NEN and ENA. Similar results for the other CORDEX domains are included as Supplementary Material (Figures Atlas.SM.1–Atlas.SM.10).

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'''Figure Atlas.7''' '''|''' '''Evaluation of annual and seasonal air temperature and precipitation for the six North America sub-regions, NWN, NEN, WNA, CNA, ENA and NCA (land only) for CORDEX-NAM RCM simulations driven by reanalysis or historical GCMs.''' Seasons are June–July–August (JJA) and December–January–February (DJF). Rows represent sub-regions and columns correspond to the models. Magenta text indicates the driving historical CMIP5 GCMs (including ERA-Interim in the first set of slightly separated columns) and the black text to the right of the magenta text represents the driven RCMs. The colour matrices show the mean spatial biases; all biases have been computed for the period 1985–2005 relative to the observational reference (E5W5, see [[#Atlas.1.4.2|Atlas.1.4.2]] ). Further details on data sources and processing are available in the chapter data table (Table Atlas.SM.15).

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