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In recent decades, trends in photovoltaic (PV) technology deployment have shown an overall increase across the world. Comprehensive knowledge of the solar resource and its future evolution is demanded by the energy sector. Solar resource and PV potential have been estimated in several studies using both the global climate model (GCM) and regional climate model (RCM), revealing a GCM-RCM discrepancy in the projected change over Europe. An increase in surface solar radiation (SSR) (and therefore in PV potential production) is projected by GCMs, whereas most RCM simulations project a decrease in SSR over Europe. In this work, we investigate the role of aerosol forcing in RCMs as a key explaining factor of this inconsistency. The results show that RCM simulations including evolving aerosols agree with GCMs in the sign and amplitude of the SSR change over Europe for mid-21st century projections (2021-2050 compared to 1971-2000 for representative concentration pathway climate change scenario RCP8.5). The opposite signal is projected by the rest of the RCMs. The amplitude of the changes likely depends on the RCM and on its aerosol forcing choice. In terms of PV potential, RCMs including evolving aerosols simulate an increase, especially in summer for Central and Eastern Europe, with maximum values reaching +10% in some cases. This study illustrates the key role of the often-neglected aerosol forcing evolution in RCMs. It also suggests that it is important to be very careful when using the multi-model Coordinated Regional Climate Downscaling Experiment (CORDEX) projections for solar radiation and related variables, and argues for the inclusion of aerosol forcing evolution in the next generation of CORDEX simulations.

Global and regional climate models (respectively GCMs and RCMs) are delivering conflicting messages about summertime climate change in Europe, revealing notably a weaker warming in RCMs. A dedicated multimodel ensemble of nine GCM‐RCM pairs is analyzed to assess the role of anthropogenic aerosols in these inconsistencies. The expected decrease of anthropogenic aerosol concentrations is found both to modify the future evolution of shortwave radiation and to generate an extra warming. For every tenth in aerosol optical depth drop in Central Europe, shortwave radiation is increased at the surface by 6.3 Wm−2$\\mathrm{W}{\\mathrm{m}}^{-2}$and decreased at the top of the atmosphere by 5.6 Wm−2$\\mathrm{W}{\\mathrm{m}}^{-2}$ , while near‐surface temperature is increased by 0.3°^{\\circ}$ C. The consideration of time‐varying anthropogenic aerosols in RCMs thus contributes to improving GCM/RCM consistency in Europe for these three variables, but not for water cycle. The results obtained underline the necessity to better consider aerosols in upcoming regional climate simulations. Plain Language Summary As far as summertime climate change in Europe is concerned, global and regional climate models do not provide exactly the same information, insofar as the warming simulated by regional models is notably lower than in global models. The decrease of anthropogenic aerosol concentrations, not always taken into account in regional models, could be one possible explanation of these inconsistencies. This hypothesis is analyzed here with the help of a dedicated multimodel ensemble of simulations with constant and evolving aerosols. This evolution of aerosols is shown to increase surface solar radiation and near‐surface temperature in Europe. The consideration of time‐varying anthropogenic aerosols in regional climate models thus contributes to reducing the differences between global and regional climate simulations. Key Points The role of anthropogenic aerosols in European summertime climate change is assessed using a set of multi‐model regional climate simulations Reduced concentrations of anthropogenic aerosols in Europe lead to an increase in surface solar radiation and an extra warming near surface Current differences in aerosol representation explain a part of inconsistencies between global and regional climate projections in Europe

CMIP6 climate models have difficulties representing the strong absorption properties of biomass‐burning aerosols (BBA) over the Southeast Atlantic and the associated direct forcing. This study takes advantage of unique in situ and remote sensing observations to evaluate the vertical profile and chemical composition of BBA, which are key properties for modeling radiative interactions. Over land, CMIP6 models show a sharper vertical decrease in carbonaceous aerosol concentration than observed, indicating insufficient vertical mixing. Over the ocean, most models transport BBA lower, between 1 and 1.5 km, than observed. This could be due to both excessive subsidence and lack of self‐lofting of BBA. In addition, the ratio between black carbon and organic aerosol mass concentrations is underestimated by about 0.1 compared to observations. These biases limit the ability of models to capture the absorption above clouds in this region, which could have implications for the regional radiative and climatic impact of African BBA.

Since a decade, convection-permitting regional climate models (CPRCM) have emerged showing promising results, especially in improving the simulation of precipitation extremes. In this article, the CPRCM CNRM-AROME developed at the Centre National de Recherches Météorologiques (CNRM) since a few years is described and evaluated using a 2.5-km 19-year long hindcast simulation over a large northwestern European domain using different observations through an added-value analysis in which a comparison with its driving 12-km RCM CNRM-ALADIN is performed. The evaluation is challenging due to the lack of high-quality observations at both high temporal and spatial resolutions. Thus, a high spatio-temporal observed gridded precipitation dataset was built from the collection of seven national datasets that helped the identification of added value in CNRM-AROME. The evaluation is based on a series of standard climatic features that include long-term means and mean annual cycles of precipitation and near-surface temperature where CNRM-AROME shows little improvements compared to CNRM-ALADIN. Additional indicators such as the summer diurnal cycle and indices of extreme precipitation show, on the contrary, a more realistic behaviour of the CNRM-AROME model. Moreover, the analysis of snow cover shows a clear added-value in the CNRM-AROME simulation, principally due to the improved description of the orography with the CPRCM high resolution. Additional analyses include the evaluation of incoming shortwave radiation, and cloud cover using satellite estimates. Overall, despite some systematic biases, the evaluation indicates that CNRM-AROME is a suitable CPRCM that is superior in many aspects to the RCM CNRM-ALADIN.

The present study investigates the radiative effects of dust aerosols in the Mediterranean region during summer 2012 using a coupled regional aerosol–atmosphere–ocean model (CNRM-RCSM5). A prognostic aerosol scheme, including desert dust, sea salt, organic, black-carbon and sulphate particles, has been integrated to CNRM-RCSM5 in addition to the atmosphere, land surface and ocean components. An evaluation of this aerosol scheme of CNRM-RCSM5, and especially of the dust aerosols, has been performed against in situ and satellite measurements, showing its ability to reproduce the spatial and temporal variability of aerosol optical depth (AOD) over the Mediterranean region in summer 2012. The dust vertical and size distributions have also been evaluated against observations from the TRAQA/ChArMEx campaign. Three simulations have been carried out for summer 2012 with CNRM-RCSM5, including the full prognostic aerosol scheme, only monthly-averaged AOD means from the aerosol scheme or no aerosols at all, in order to focus on the radiative effects of dust particles and the role of the prognostic scheme. Surface short-wave aerosol radiative forcing variability is found to be more than twice as high over regions affected by dust aerosols, when using a prognostic aerosol scheme instead of monthly AOD means. In this case downward surface solar radiation is also found to be better reproduced according to a comparison with several stations across the Mediterranean. A composite study over 14 stations across the Mediterranean, designed to identify days with high dust AOD, also reveals the improvement of the representation of surface temperature brought by the use of the prognostic aerosol scheme. Indeed the surface receives less radiation during dusty days, but only the simulation using the prognostic aerosol scheme is found to reproduce the observed intensity of the dimming and warming on dusty days. Moreover, the radiation and temperature averages over summer 2012 are also modified by the use of prognostic aerosols, mainly because of the differences brought in short-wave aerosol radiative forcing variability. Therefore this first comparison over summer 2012 highlights the importance of the choice of the representation of aerosols in climate models.

This paper describes the main characteristics of CNRM‐CM6‐1, the fully coupled atmosphere‐ocean general circulation model of sixth generation jointly developed by Centre National de Recherches Météorologiques (CNRM) and Cerfacs for the sixth phase of the Coupled Model Intercomparison Project 6 (CMIP6). The paper provides a description of each component of CNRM‐CM6‐1, including the coupling method and the new online output software. We emphasize where model's components have been updated with respect to the former model version, CNRM‐CM5.1. In particular, we highlight major improvements in the representation of atmospheric and land processes. A particular attention has also been devoted to mass and energy conservation in the simulated climate system to limit long‐term drifts. The climate simulated by CNRM‐CM6‐1 is then evaluated using CMIP6 historical and Diagnostic, Evaluation and Characterization of Klima (DECK) experiments in comparison with CMIP5 CNRM‐CM5.1 equivalent experiments. Overall, the mean surface biases are of similar magnitude but with different spatial patterns. Deep ocean biases are generally reduced, whereas sea ice is too thin in the Arctic. Although the simulated climate variability remains roughly consistent with CNRM‐CM5.1, its sensitivity to rising CO2 has increased: the equilibrium climate sensitivity is 4.9 K, which is now close to the upper bound of the range estimated from CMIP5 models. Key Points Description of CNRM‐CM6‐1 model components, their coupling, and tuning procedures are described Historical simulations and DECK experiments are assessed Preindustrial simulation is stable and mean climate and variability in historical runs is realistic

Aerosol absorption properties are of high importance to assess aerosol impact on regional climate. This study presents an analysis of aerosol absorption products obtained over the Mediterranean basin or land stations in the region from multi-year ground-based AERONET observations with a focus on the Absorbing Aerosol Optical Depth (AAOD), Single Scattering Albedo (SSA) and their spectral dependence. The AAOD and Absorption Angström Exponent (AAE) dataset is composed of daily averaged AERONET level 2 data from a total of 22 Mediterranean stations having long time series, mainly under the influence of urban-industrial aerosols and/or soil dust. This dataset covers the 17-yr period 1996–2012 with most data being from 2003–2011 (~89% of level-2 AAOD data). Since AERONET level-2 absorption products require a high aerosol load (AOD at 440 nm > 0.4), which is most often related to the presence of desert dust, we also consider level-1.5 SSA data, despite their higher uncertainty, and filter out data with an Angström exponent < 1.0 in order to study absorption by carbonaceous aerosols. The SSA dataset includes AERONET level-2 products. Sun-photometer observations show that values of AAOD at 440 nm vary between 0.024 ± 0.01 (resp. 0.040 ± 0.01) and 0.050 ± 0.01 (0.055 ± 0.01) for urban (dusty) sites. Analysis shows that the Mediterranean urban-industrial aerosols appear \"moderately\" absorbing with values of SSA close to ~0.94–0.95 ± 0.04 (at 440 nm) in most cases except over the large cities of Rome and Athens, where aerosol appears more absorbing (SSA ~0.89–0.90 ± 0.04). The aerosol Absorption Angström Exponent (AAE, estimated using 440 and 870 nm) is found to be larger than 1 for most sites over the Mediterranean, a manifestation of mineral dust (iron) and/or brown carbon producing the observed absorption. AERONET level-2 sun-photometer data indicate a possible East-West gradient, with higher values over the eastern basin (AAEEast = 1.39/AAEWest = 1.33). The North-South AAE gradient is more pronounced, especially over the western basin. Our additional analysis of AERONET level-1.5 data also shows that organic absorbing aerosols significantly affect some Mediterranean sites. These results indicate that current climate models treating organics as nonabsorbing over the Mediterranean certainly underestimate the warming effect due to carbonaceous aerosols.

The Mediterranean is expected to be one of the most prominent and vulnerable climate change “hotspots” of the twenty-first century, and the physical mechanisms underlying this finding are still not clear. Furthermore, complex interactions and feedbacks involving ocean–atmosphere–land–biogeochemical processes play a prominent role in modulating the climate and environment of the Mediterranean region on a range of spatial and temporal scales. Therefore, it is critical to provide robust climate change information for use in vulnerability–impact–adaptation assessment studies considering the Mediterranean as a fully coupled environmental system. The Mediterranean Coordinated Regional Downscaling Experiment (Med-CORDEX) initiative aims at coordinating the Mediterranean climate modeling community toward the development of fully coupled regional climate simulations, improving all relevant components of the system from atmosphere and ocean dynamics to land surface, hydrology, and biogeochemical processes. The primary goals of Med-CORDEX are to improve understanding of past climate variability and trends and to provide more accurate and reliable future projections, assessing in a quantitative and robust way the added value of using high-resolution and coupled regional climate models. The coordination activities and the scientific outcomes of Med-CORDEX can produce an important framework to foster the development of regional Earth system models in several key regions worldwide.

Characteristics and radiative forcing of the aerosol and ozone fields of two configurations of the Centre National de Recherches Météoroglogiques (CNRM) and Cerfacs climate model are analyzed over the historical period (1850–2014), using several Coupled Model Intercomparison Project 6 (CMIP6) simulations. CNRM‐CM6‐1 is the atmosphere‐ocean general circulation model including prescribed aerosols and a linear stratospheric ozone scheme, while the Earth System Model CNRM‐ESM2‐1 has interactive tropospheric aerosols and chemistry of the midtroposphere aloft. The representations of aerosols and ozone in CNRM‐CM6‐1 are issued from simulations of CNRM‐ESM2‐1, and this ensures some comparability of both representations. In particular, present‐day anthropogenic aerosol optical depths are similar (0.018), and their spatial patterns correspond to those of reference data sets such as MACv2 and MACv2‐SP despite a negative bias. Effective radiative forcings (ERFs) have been estimated using 30‐year fixed sea surface temperature simulations (piClim) and several calls to the radiative scheme. Present‐day anthropogenic aerosol ERF, aerosol‐radiation ERF, and aerosol cloud ERF are fully within CMIP5 estimates and, respectively, equal to −1.10, −0.36, and −0.81 W m −2 for CNRM‐CM6‐1 and −0.21, −0.61, and −0.74 W m −2 for CNRM‐ESM2‐1. Additional CMIP6‐type piClim simulations show that these differences are mainly due to the interactivity of the aerosol scheme whose impact is confirmed when assessing the response of both climate model configurations to rising CO 2. Present‐day stratospheric ozone ERF, equal to −0.04 W m −2, is in agreement with that of the CMIP6 ozone. No trend appears in the ozone ERF over the historical period although the evolution of the total column ozone is correctly simulated. Plain Language Summary The manuscript documents the Météo‐France Centre National de Recherches Météorologiques aerosol‐chemistry modeling contributions to the sixth Coupled Model Intercomparison Project that supports the sixth IPCC Assessment Report of climate change. It establishes that their results are suitable for use by the scientific community in the analysis of the sixth Coupled Model Intercomparison Project experiments. The authors provide an evaluation of the model performance in both present‐day and historical (1850–2014) contexts, as well as a detailed analysis of the model calculated effective radiative forcing due to ozone and aerosols. Key Points The representations of aerosol and ozone in the CMIP6 CNRM‐CM6‐1 and CNRM‐ESM2‐1 models is described Present‐day and historical aerosol and ozone distributions are assessed, as well as their effective radiative forcing (ERF) The present‐day anthropogenic aerosol ERF (‐1.10 W m −2 for CNRM‐CM6‐1) is sensitive to the interactivity of aerosols

The present study investigates the dust emission and load over the Mediterranean basin using the coupled chemistry–aerosol–regional climate model RegCM-4. The first step of this work focuses on dust particle emission size distribution modeling. We compare a parameterization in which the emission is based on the individual kinetic energy of the aggregates striking the surface to a recent parameterization based on an analogy with the fragmentation of brittle materials. The main difference between the two dust schemes concerns the mass proportion of fine aerosol that is reduced in the case of the new dust parameterization, with consequences for optical properties. At the episodic scale, comparisons between RegCM-4 simulations, satellite and ground-based data show a clear improvement using the new dust distribution in terms of aerosol optical depth (AOD) values and geographic gradients. These results are confirmed at the seasonal scale for the investigated year 2008. This change of dust distribution has sensitive impacts on the simulated regional dust budget, notably dry dust deposition and the regional direct aerosol radiative forcing over the Mediterranean basin. In particular, we find that the new size distribution produces a higher dust deposition flux, and smaller top of atmosphere (TOA) dust radiative cooling. A multi-annual simulation is finally carried out using the new dust distribution over the period 2000–2009. The average SW radiative forcing over the Mediterranean Sea reaches −13.6 W m−2 at the surface, and −5.5 W m−2 at TOA. The LW radiative forcing is positive over the basin: 1.7 W m−2 on average over the Mediterranean Sea at the surface, and 0.6 W m−2 at TOA.