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On 18 September 2020, medicane Ianos hit the western coast of Greece resulting in flooding and severe damage at several coastal locations. In this work, we aim at evaluating its impact on sea conditions and the associated uncertainty through the use of an ensemble of numerical simulations. We applied a coupled wave-current model to an unstructured mesh representing the whole Mediterranean Sea, with a grid resolution increasing in the Ionian Sea along the cyclone path and the landfall area. To investigate the uncertainty of modelling sea levels and waves for such an intense event, we performed a multimodel ensemble of ocean simulations using several coarse (10 km) and high-resolution (2 km) meteorological forcings from different mesoscale models. The performance of the ocean and wave models was evaluated against observations retrieved from fixed monitoring stations and satellites. All model runs emphasized the occurrence of severe sea conditions along the cyclone path and at the coast. Due to the rugged and complex coastline, extreme sea levels are localised at specific coastal sites. However, numerical results show a large spread of the simulated sea conditions for both the sea level and waves highlighting the large uncertainty in simulating this kind of extreme event. The multi-model / multi-physics approach allows us to assess how the uncertainty propagates from meteorological to ocean variables and the subsequent coastal impact. The ensemble mean and standard deviation were combined to prove the hazard scenarios of the potential impact of such an extreme event to be used in a flood risk management plan.

The Mediterranean Sea (MS) has been experiencing significant surface warming over the past decades, greater than the global ocean and particularly higher during summers. The present study proposes the concept of Extreme Marine Summers (EMS) and investigates their characteristics in the MS in a climatological framework, based on ECMWF ERA5 daily sea surface temperature (SST) data for the period 1950-2020. Main objectives are to explore the SST substructures within EMSs, the contribution of Marine Heatwaves (MHW) during EMSs and the driving role of air-sea heat fluxes in the EMS formation.EMSs are defined as the summers (July-August-September) exhibiting a mean SST greater than the 95th percentile of the mean summer SST values within the study period. A marine summer may evolve as extreme under different SST substructures within the season, e.g., due to uniformly increased SST values throughout the summer or due to higher than usual SSTs of a specific part of the SST distribution during the season. Results suggest that EMSs identified in the greatest part of the basin are formed due to the warmest part of the ranked daily SST distribution being warmer than normal. SSTs within EMSs are organised under high dependency on the climatological SST variability: locations where the warmest (coldest) part of the ranked daily SST distribution is more variable climatologically, experience EMSs primarily due to the contribution of the warmest (coldest) part of the SST distribution.MHWs in EMSs present greater intensity, duration and occurrence frequency with respect to mean MHW conditions, in the northern flanks of the Mediterranean basin and particularly in the Aegean and Adriatic Seas. Although the north-western part of the basin experiences the most intense EMSs and summer MHWs, the role of MHWs in the formation of EMSs appears more pronounced in the central and eastern MS. In the rest of the basin, and particularly in southern MS regions, MHWs in EMSs are less intense but longer lasting and more frequent than usual.To quantify the driving role of the net surface heat flux (Qnet) in the EMS formation, a metric is proposed based on the surface heat budget equation. The proposed metric represents the mean contribution of Qnet during summer sub-periods within which SST is kept above climatology via a) faster warming or b) slower cooling compared to the corresponding climatological period. Results show that EMSs are largely driven by Qnet in the northern MS regions: a latitudinal gradient is generally observed in the basin with increasing contribution percentages while moving northerly. In areas where the observed SST anomalies are not entirely explained by surface heat fluxes, negative wind speed and mixed layer depth seasonal anomalies relative to climatology are commonly observed, suggesting that wind-induced mixed layer shoaling is a complementary EMS contributing mechanism. Moreover, results reveal a strong link between MHW properties and surface heat fluxes during EMSs, suggesting that Qnet modulates particularly the intensity of MHWs.

A large number of intense cyclones occur every year in the Mediterranean basin, a relatively small and densely populated region, but also a worldwide climate-change hotspot. Given their importance for the variability of the regional climate and its extremes, Mediterranean cyclones have lately attracted much of attention, especially due to the broad range of severe socio-economic and environmental impacts that they produce.This talk aims at summarizing the concentrated knowledge of the last decade on the dynamics, climatology and relevant impacts of Mediterranean cyclones. We will especially focus on the processes that take place in different spatiotemporal scales triggering cyclogenesis and turning Mediterranean cyclones into catastrophic storms. We will also discuss the role of the unique regional geographical features therein, along with the influence of the latitudinal location of the Mediterranean basin. Finally, we will discuss the different subtypes of Mediterranean cyclones that develop in the region, devoting special attention to medicanes, i.e. cyclones with tropical characteristics and subjects of numerous recent studies. Througout the talk, research perspectives that advance the field of Mediterranean cyclones as a whole will be highlighted, along with current trends in community efforts within the framework of MedCyclones COST Action that address relevant topics to the complex dynamics of Mediterranean cyclones and consequent severe socio-economic impacts.

Mid-latitude storms are essential features of atmospheric variability in the cold season. The subsequent damages are caused by high wind speeds and heavy precipitation. Among such events, explosive cyclones can lead to extreme impacts when they make landfall. Climate change is affecting the underlying characteristics of such types of extremes. Being able to understand the way it modifies their dynamics is of great importance. In this work, we assess the influence of anthropogenic climate change on observed explosive cyclones in an Extreme Event Attribution framework using a large ensemble dataset. We evaluate three storms that hit different parts of Europe: Xynthia in February 2010, Alex in October 2020, and Eunice in January 2022. We use three ensembles of 35 members of the Community Earth System Model (CESM). We compare two periods of 6-hourly data: present-day climate [1991-2001] and future climate [RCP8.5 scenario, 2091–2101]. We find analogues of the trajectories of the three storms before their highest intensity in both periods. We do that by tracking all cyclones in the dataset and selecting the cyclone tracks that have the minimum Euclidean distance in km from the trajectories of Xynthia, Alex, and Eunice. We explore the characteristics of the analogues of the trajectories in both periods such as frequency of explosive cyclogenesis and intensity to evaluate whether the dynamics of the storms have been affected by climate change. We further compare the analogues in terms of precipitation and low-level wind in the regions of impact.

Some mesoscale cyclones in the Mediterranean attain tropical-like characteristics, such as a deep warm core, absence of fronts, and a cloud-free “eye”. Some others have less symmetry, a shallow warm core structure, and a weak frontal activity, but both categories of cyclones are so-called “Medicanes”. Due to the vast variety of features that resemble tropical cyclones, polar lows and extratropical cyclones, Medicanes still do not have an official definition. The rapid intensity changes and tropical transitions of Medicanes, the evolution of associated deep convection, and the large spread of forecast tracks in numerical weather prediction models, as well as scarce in-situ observations highlight the challenges in understanding the dynamics of these weather systems. In this study, we try to improve our understanding of the atmospheric ingredients that build Medicanes, and most importantly, how much contribution does diabatic heating has on Medicanes’ intensification process. To achieve this, numerical diagnostics are used to explain the evolution and contribution of diabatic processes in case studies of Medicanes observed between 2005 and 2021. The WRF model is utilized in high-resolution mode, and its results are evaluated against satellite and surface observations in order to filter out the problematic simulations. Then, the Pressure Tendency Equation (PTE) is applied, which helps to decompose the contribution of dynamic atmospheric components in each stage of a Medicane’s lifetime. Results show that the contribution of diabatic processes and large-scale forcing varies considerably among the studied Medicanes. Moreover, an online Potential Vorticity (PV) tracer module in WRF provides an additional metric for the role of latent heating during the same Medicanes. Both approaches help to identify important differences among the case studies and shed a new light on a new pathway of Medicane development that was previously ignored.

A large number of intense cyclones occur every year in the Mediterranean basin, one of the climate change hotspots. Producing a broad range of severe socio-economic and environmental impacts in such a densely populated region, Mediterranean cyclones call for coordinated and interdisciplinary research efforts. This article aims at supporting these efforts by reviewing the status of knowledge in the broad field of Mediterranean cyclones. First, we focus on the climatology of Mediterranean cyclone tracks, their relationship to large-scale atmospheric circulation and their future trends. Second, we discuss the dynamics and atmospheric processes that govern the genesis and development of Mediterranean cyclones. Then, we present the different subtypes of Mediterranean cyclones, devoting special attention to medicanes, i.e. cyclones with tropical characteristics and subjects of numerous recent studies. In a subsequent section, we review the state of the art in forecasting cyclones and relevant high-impact weather, and we discuss in detail the challenges and recent efforts to increase their forecast skill. Finally, we discuss the main impacts produced by cyclones, namely heavy precipitation, windstorms, dust transport, storm surges and sea wave extremes. In the last section of this review article, we thoroughly outline the future directions of research that would advance the broader field of Mediterranean cyclones.

Mediterranean cyclones are extratropical cyclones, typically of smaller size and weaker intensity than other cyclones that develop over the main open ocean storm tracks. Nevertheless, Mediterranean cyclones can attain high intensities, even comparable to the ones of tropical cyclones, and thus cause large socioeconomic impacts in the densely populated coasts of the region. After cyclogenesis takes place, a large variety of processes are involved in the cyclone’s development, contributing with positive and negative potential vorticity (PV) changes to the lower-tropospheric PV anomalies in the cyclone center. Although the diabatic processes that produce these PV anomalies in Mediterranean cyclones are known, it is still an open question whether they occur locally within the cyclone itself or remotely in the environment (e.g., near high orography) with a subsequent transport of high-PV air into the cyclone center. This study introduces a Lagrangian method to determine the origin of the lower-tropospheric PV anomaly, which is applied climatologically to ERA5 reanalysis and to 12 monthly simulations, performed with the integrated forecasting system (IFS) model. We define and quantify so-called “cyclonic” and “environmental” PV and find that the main part of the lower-tropospheric PV anomaly (60 %) is produced within the cyclone, shortly prior (−12 h) to the cyclones' mature stage. Nevertheless, in 19.5 % of the cyclones the environmental PV production near the mountains surrounding the Mediterranean Basin plays a significant role in forming the low-tropospheric PV anomaly and therefore in determining the intensity of these cyclones. The analysis of PV tendencies from the IFS simulations reveals that the major PV production inside the cyclone is typically due to convection and microphysics, whereas convection and turbulent momentum tendencies cause most of the positive PV changes in the environment., Weather and Climate Dynamics, 4 (1), ISSN:2698-4016, ISSN:2698-4008

Many cyclone detection and tracking methods (CDTMs) have been developed in the past to study the climatology of extratropical cyclones. However, all CDTMs have different approaches in defining and tracking cyclone centers. This naturally leads to cyclone track climatologies of inconsistent physical characteristics. More than that, it is typical for CDTMs to produce a non-negligible amount of bogus tracks which can be perceived as “false positives”, or more generally as CDTM artifacts, i.e. tracks of weak atmospheric features that do not correspond to large or mesoscale vortices. Lack of consensus in CDTM outputs and the inclusion of significant amounts of bogus tracks therein, has long prohibited the production of a commonly accepted reference dataset of extratropical cyclone tracks. Such a dataset could allow comparable results on the analysis of storm track climatologies and could also contribute to the evaluation and improvement of CDTMs. To cover this gap, we present a new methodological approach that combines overlapping tracks from different CDTMs and produces composite tracks that concentrate the agreement of more than one CDTM. In this study we apply this methodology to the outputs of 10 well-established CDTMs which were originally applied to ERA5 reanalysis in the 42-year period of 1979–2020. We tested the sensitivity of our results to the spatio-temporal criteria that identify overlapping cyclone tracks, and for benchmarking reasons, we produced five reference datasets of subjectively tracked cyclones. Results show that climatological numbers of composite tracks are substantially lower than the ones of individual CDTM, while benchmarking scores remain high (i.e. counting the number of subjectively tracked cyclones captured by the composite tracks). This suggests that our method is able to filter out a large portion of bogus tracks. Indeed, our results show that composite tracks tend to describe more intense and longer-lasting cyclones with more distinguished early, mature and decay stages than the cyclone tracks produced by individual CDTMs. Ranking the composite tracks according to their confidence level (defined by the number of contributing CDTMs), it is shown that the higher the confidence level, the more intense and long-lasting cyclones are produced. Given the advantage of our methodology in producing cyclone tracks with physically meaningful, distinctive life stages and including a minimum number of bogus tracks, we propose composite tracks as reference datasets for climatological research in the Mediterranean. The supplementary material provides the composite Mediterranean tracks for all confidence levels and in the conclusion we discuss their adequate use for scientific research and applications.

On the evening of November 12, 2019, an exceptional high tide hit the city of Venice and the central-southern area of its lagoon, damaging a large part of its historical center. The main cause of the event was a small warm-core mesoscale cyclone, which formed in the central Adriatic Sea and intensified during its northwestward movement. Simulations with different initialization times were carried out with the Weather Research and Forecasting (WRF) model, showing a strong sensitivity to the initial conditions, since the track (and strength) of the cyclone was determined by the exact position of an upper-level potential vorticity (PV) streamer. The factors responsible for the cyclone development are also investigated. The pre-existence of positive low-level cyclonic vorticity, associated with the convergence of the Sirocco and Bora winds in the Adriatic, made the environment favorable for cyclone development. Also, the interaction between the upper-level PV anomaly and the low-level baroclinicity, created by the advection of warm, humid air associated with the Sirocco, was responsible for the cyclone’s intensification, in a manner similar to a transitory (stable) baroclinic interaction at small horizontal scales.Conversely, convection and sea surface fluxes did not play a significant role, thus the warm-core feature appears mainly as a characteristic of the environment in which the cyclone developed rather than a consequence of diabatic processes. The cyclone does not fall into any of the existing categories for Adriatic cyclones.

The classification of Mediterranean cyclones has always been a challenging task, especially when grouping warm-core and small-scale cyclones such as the Mediterranean tropical-like cyclones (Medicanes). The sudden intensity changes of Medicanes, the evolution of deep convection and the large spread of forecast tracks in numerical weather forecast models highlight the challenges in understanding the dynamics of these weather systems. In this study, numerical diagnostics are used to explain the evolution and contribution of diabatic processes in case studies of Medicanes. High-resolution simulations with WRF model are utilized and are evaluated against observations, before implementing the Pressure Tendency Equation (PTE). The decomposition of PTE shows interesting results about the contribution of diabatic processes and large-scale forcing during each Medicane. Moreover, an online Potential Vorticity (PV) tracer module is implemented in the WRF model, that provides another metric for the role of latent heating during the same Medicanes. Both approaches help to identify important differences among the case studies and shed a new light on a new pathway of Medicane development.

The Mediterranean Basin is a region characterized by intense cyclonic activity associated with events of extreme and adverse weather, often having high social and economic impact. The genesis and further development of these cyclones is strongly dependent to preceding Rossby wave breaking over the Atlantic Ocean. However, the accurate reproduction of such Rossby wave breaking is a challenging issue for numerical weather prediction (NWP) limiting the predictability of Mediterranean cyclones to lead times of typically no more than 5 days. To better understand the sensitivity of forecast systems in predicting Mediterranean cyclogenesis and for further advancing with adequate ensemble approaches for NWP, we have developed a new scheme of stochastic parameter perturbation of physical tendencies (SPPT). The new SPPT scheme has been implemented into WRF and consists of random perturbations which are applied solely to coherent 3D objects of potential vorticity (PV) components that are produced by physical parametrisations. Given the invertibility trait of PV, these 3D objects are expected to significantly affect the forecasted atmospheric state with inherent uncertainties to physical parametrisations and consequent model errors.A scenario tested for the needs of the study is the Mediterranean tropical-like cyclone (medicane) Zorbas that took place in September 2018; an event characterized by difficulties in adequately forecasting the cyclogenesis stage. Overall, our results quantify the sensitivity of NWP of Mediterranean cyclogenesis and highlight our scheme as an attractive alternative to classic approaches producing significant ensemble spread.

Research on extreme weather typically investigated the physical and dynamical processes involved in the formation of specific meteorological events that occur on time scales of hours to a several days (e.g., heavy precipitation events, windstorms, heat waves). Such events can be extremely hazardous, but for certain socioeconomic sectors the seasonal aggregation of weather is particularly harmful. These sectors include, for instance, agriculture, forestry, energy, and reinsurance. This presentation introduces the concept of “extreme seasons” as an important and not yet thoroughly investigated research field at the interface of weather and climate science. Extreme seasons are defined as seasons during which a particular meteorological or impact-related parameter (or a combination thereof) strongly deviates from climatology. An important conclusion of the presentation will be that large ensemble climate simulations (here using an extended CESM1-LENS data set with 6-hourly output of 3D fields), with about 1000 simulated years per climate period, are an essential resource enabling novel quantitative insight into the processes leading to and characteristics of extreme seasons. The presentation provides examples for the identification of extreme seasons and emphasizes the importance of studying their substructure, including the occurrence of specific weather systems. A first approach to systematically study extreme seasons is to consider the top 10 seasons (for a given metric) in the large ensemble at every grid point, e.g., the 10 wettest winters or hottest summers, or the 10 summers with the largest vapour pressure deficit (as an example for a more impact-related metric). Alternatively, one can look at anomalies in a multi-dimensional parameter phase space, identifying extreme seasons that result from a highly unusual combination of, e.g., surface temperature, precipitation, and surface energy balance. Or, using a pragmatic method based on fitting a statistical model to seasonal mean values at each grid point, spatially coherent extreme season objects can be identified that exceed a local return period threshold of, e.g., 40 years. The same statistical approach can be applied to ERA5 reanalyses to compare characteristics of extreme season objects (e.g., their size and intensity) in climate models with observation-based data. With this approach we can meaningfully estimate how often, e.g., an observed extreme winter like the cold North American 2013/14 winter is expected anywhere in midlatitude regions. The last part of the presentation addresses the substructure and weather system characteristics of extreme seasons. Illustrative results are shown that address the questions: (i) Where are extremely hot summers the result of the warmest days being anomalously hot vs. the coldest days being anomalously mild? (ii) Where are wettest seasons the result of more frequent wet days vs. more intense precipitation on wet days? and (iii) How does the frequency of weather systems and their precipitation efficiency change during the wettest seasons? The answers to these questions reveal interesting and large regional differences.

The Mediterranean basin occasionally hosts small intense vortices that evolve in tropical-like cyclones, also called “medicanes”. Although they are more intense over the sea, their landfall may be associated with destructive extreme events, such as heavy precipitation, windstorms, flooding, and marine storminess. On 18 September 2020, medicane Ianos hit the western coast of Greece resulting in flooding and severe damages at specific coastal locations. In this work, we aim at evaluating the impact of medicane Ianos on the sea state and water level through the use of numerical simulations. We applied a coupled wave-current model to an unstructured mesh representing the whole Mediterranean Sea, with a grid resolution varying from 15 km in the open sea to 2 km along the predetermined cyclone path, and up to 500 m along the landfall area (the western Greek coast). In order to investigate the uncertainty of the ocean model derived by the atmospheric modelling of such an intense event, we performed an ensemble of simulations using several coarse (10 km) and high-resolution (2 km) meteorological forcings from different mesoscale models. Also, results obtained using ERA5 reanalysis or IFS analysis are considered as a benchmark. The multi-model approach allows us to assess how the uncertainty propagates from meteorological fields to the ocean quantities and the subsequent coastal impact. The model performance was evaluated against observations retrieved from fixed monitoring stations and satellites. The numerical results show a large spread of the simulated sea conditions. Due to the rugged and complex coastline, extreme sea levels are localized at specific coastal sites. The ensemble results were combined for proving a set of indicators of the potential impact of such an intense event, in order to assess the effectiveness of this multi-model ensemble approach. This work is part of the COST action CA19109 MEDCYCLONES (European Network for Mediterranean Cyclones in weather and climate) and of the Interreg Italy-Croatia STREAM project (Strategic development of flood management, project ID 10249186).

Here, we analyze future projections of cyclone activity in the Mediterranean region at the end of the twenty-first century based on an ensemble of state-of-the-art fully-coupled Regional Climate System Models (RCSMs) from the Med-CORDEX initiative under the Representative Concentration Pathway (RCP) 8.5. Despite some noticeable biases, all the RCSMs capture spatial patterns and cyclone activity key characteristics in the region and thus all of them can be considered as plausible representations of the future evolution of Mediterranean cyclones. In general, the RCSMs show at the end of the twenty-first century a decrease in the number and an overall weakening of cyclones moving across the Mediterranean. Five out of seven RCSMs simulate also a decrease of the mean size of the systems. Moreover, in agreement with what already observed in CMIP5 projections for the area, the models suggest an increase in the Central part of the Mediterranean region and a decrease in the South-eastern part of the region in the cyclone-related wind speed and precipitation rate. These rather two opposite tendencies observed in the precipitation should compensate and amplify, respectively, the effect of the overall reduction of the frequency of cyclones on the water budget over the Central and South-eastern part of the region. A pronounced inter-model spread among the RCSMs emerges for the projected changes in the cyclone adjusted deepening rate, seasonal cycle occurrence and associated precipitation and wind patterns over some areas of the basin such as Ionian Sea and Iberian Peninsula. The differences observed appear to be determined by the driving Global Circulation Model (GCM) and influenced by the RCSM physics and internal variability. These results point to the importance of (1) better characterizing the range of plausible futures by relying on ensembles of models that explore well the existing diversity of GCMs and RCSMs as well as the climate natural variability and (2) better understanding the driving mechanisms of the future evolution of Mediterranean cyclones properties.

This paper presents and analyzes the three-dimensional dynamical structure of intense Mediterranean cyclones. The analysis is based on a composite approach of the 200 most intense cyclones during the period 1989–2008 that have been identified and tracked using the output of a coupled ocean–atmosphere regional simulation with 20 km horizontal grid spacing and 3-hourly output. It is shown that the most intense Mediterranean cyclones have a common baroclinic life cycle with a potential vorticity (PV) streamer associated with an upper-level cyclonic Rossby wave breaking, which precedes cyclogenesis in the region and triggers baroclinic instability. It is argued that this common baroclinic life cycle is due to the strongly horizontally sheared environment in the Mediterranean basin, on the poleward flank of the quasi-persistent subtropical jet. The composite life cycle of the cyclones is further analyzed considering the evolution of key atmospheric elements as potential temperature and PV, as well as the cyclones’ thermodynamic profiles and rainfall. It is shown that most intense Mediterranean cyclones are associated with warm conveyor belts and dry air intrusions, similar to those of other strong extratropical cyclones, but of rather small scale. Before cyclones reach their mature stage, the streamer’s role is crucial to advect moist and warm air towards the cyclones center. These dynamical characteristics, typical for very intense extratropical cyclones in the main storm track regions, are also valid for these Mediterranean cases that have features that are visually similar to tropical cyclones., Climate Dynamics, 44 (9-10), ISSN:0930-7575, ISSN:1432-0894

In this study, we use convection-permitting high resolution (3 km) simulations to quantify and analyse the water budget, precipitation efficiency and water sources of 100 intense Mediterranean cyclones. To this end, we calculate the water content, advection and microphysical processes of water vapour and rain water by implementing new diagnostics to the Weather Research and Forecasting (WRF) model. The 100 intense cyclones have been randomly selected from a 500 intense cyclones dataset, identified and tracked in an 11-year time period in part I of this study. Results are presented in a composite approach showing that most rainfall takes place to the north-east side of the cyclones, close to their centre. Rainfall location is concomitant to the area of horizontal moisture flux convergence and is quasi-equal to the amount of water vapour loss due to microphysical processes. Similar results were found regardless if cyclones produce high or low rainfall amounts. Vertical profiles of the water budget terms revealed deeper clouds for the cyclones producing high rainfall, consistent with higher values of vertical advection of both water vapour and rain water. Finally, cyclones were analysed with respect to their precipitation efficiency, i.e. the ratio between the rainwater produced in an atmospheric column and the consequent rainfall, and showed that cyclones tend to be more efficient when their rainfall production takes place over land. Therefore, there is a complex relation between water vapour advection, precipitation efficiency and rainfall which is discussed through the comparison of two tropical-like cyclones with two cyclones that produced low rainfall amounts. Finally, our analysis is complemented by applying a Lagrangian approach to all 100 cyclones in order to quantify the water vapour source regions that contribute to the cyclones’ rainfall due to local surface evaporation. Results showed that these regions are located over both the Atlantic and the Mediterranean, however we show that cyclones producing high rainfall are related with higher water transport from both the subtropical Atlantic and the Mediterranean Sea. © 2019, Springer-Verlag GmbH Germany, part of Springer Nature.

An extreme aggregation of precipitation on the seasonal timescale, leading to a so-called extreme wet season, can have substantial environmental and socio-economic impacts. In contrast to extreme precipitation events on hourly to daily timescales, which are typically caused by single weather systems, an extreme wet season may be attributed to a combination of different and/or recurring weather systems. In fact, extreme wet seasons may be formed by almost continuously occurring moderate events, or by more frequent and/or more intense short-duration extreme events, or by a combination of these scenarios. This study aims at identifying and statistically characterizing extreme wet seasons around the globe, and elucidating their relationship with specific weather systems. To define extreme wet seasons, we used 40 years (1979–2018) of ERA-Interim reanalyses. Primary extreme seasons were defined independently at every grid point as the consecutive 90-day period with the highest accumulated precipitation. Secondary extreme seasons were also considered, if accumulated precipitation amounts to at least 90 % of the precipitation in the primary season at the same grid point. A high number of secondary extreme seasons was found for instance in the extratropical storm tracks, suggesting that these regions are less likely to experience an exceptional amount of precipitation in a particular 90-day period. In most continental regions, the extreme seasons occur during the warm months of the year, especially in the mid-latitudes. Nevertheless, colder periods might be also relevant to extreme seasons within the same continent, especially in coastal areas. All identified extreme seasons were statistically characterised in terms of anomalies compared to the climatology of the number of wet days and daily extreme events. Results show that daily extremes are decisive for the occurrence of extreme wet seasons in regions of frequent precipitation, e.g. in the tropics. In contrast, e.g., in arid regions where wet days are scarce, extreme seasons may occur only due to anomalously high numbers of wet days. In the subtropics and more precisely within the transitional zones between arid areas and regions of frequent precipitation, both an anomalously high occurrence of daily extremes and wet days are related to the formation of extreme wet seasons. The spatial extent of regions affected by the same extreme wet season is variable and can reach continental scales, although the vast majority of extreme seasons is limited to scales of the order of 20 × 105 km2. Finally, the relationship of extreme seasons to synoptic-scale weather systems was investigated on the basis of four objectively identified weather systems that are known to be associated with intense precipitation: cyclones, warm conveyor belts, tropical moisture exports and breaking Rossby waves. A grid-to-grid association of these weather systems to daily precipitation allows quantifying their role for extreme wet seasons. In particular, cyclones and warm conveyor belts contribute strongly to extreme wet seasons in most regions of the globe. But interlatitudinal influences are also shown to be important: tropical moisture exports, i.e., the poleward transport of tropical moisture, can contribute to extreme wet seasons in the mid-latitudes, while breaking Rossby waves, i.e., the equatorward intrusion of stratospheric air, may decisively contribute to the formation of extreme wet seasons in the tropics. Four illustrative examples provide insight into the synergetic effects of the four identified weather systems on the formation of extreme wet seasons in the Arctic, the mid-latitudes, Australia, and the tropics.

In the last decades, extremely hot summers (hereafter extreme summers) have challenged societies worldwide through their adverse ecological, economic and public health effects. In this study, extreme summers are identified at all grid points in the Northern Hemisphere in the upper tail of the July–August (JJA) seasonal mean 2-meter temperature (T2m) distribution, separately in ERA-Interim reanalyses and in 700 simulated years with the Community Earth System Model (CESM) large ensemble for present-day climate conditions. A novel approach is introduced to characterize the substructure of extreme summers, i.e., to elucidate whether an extreme summer is mainly the result of the warmest days being anomalously hot, or of the coldest days being anomalously mild, or of a general shift towards warmer temperatures on all days of the season. Such a statistical characterization can be obtained from considering so-called rank day anomalies for each extreme summer, that is, by sorting the 92 daily mean T2m values of an extreme summer and by calculating, for every rank, the deviation from the climatological mean rank value of T2m. Applying this method in the entire Northern Hemisphere reveals spatially strongly varying extreme summer substructures, which agree remarkably well in the reanalysis and climate model data sets. For example, in eastern India the hottest 30 days of an extreme summer contribute more than 70% to the total extreme summer T2m anomaly, while the colder days are close to climatology. In the high Arctic, however, extreme summers occur when the coldest 30 days are substantially warmer than climatology. Furthermore, in roughly half of the Northern Hemisphere land area, the coldest third of summer days contribute more to extreme summers than the hottest third, which highlights that milder than normal coldest summer days are a key ingredient of many extreme summers. In certain regions, e.g., over western Europe and western Russia, the substructure of different extreme summers shows large variability and no common characteristic substructure emerges. Furthermore, we show that the typical extreme summer substructure in a certain region is directly related to the region’s overall T2m rank day variability pattern. This indicates that in regions where the warmest summer days vary particularly strongly from one year to the other, these warmest days are also particularly anomalous in extreme summers (and analogously for regions where variability is largest for the coldest days). Finally, for three selected regions, thermodynamic and dynamical causes of extreme summer substructures are briefly discussed, indicating that, for instance, the onset of monsoons, physical boundaries like the sea ice edge, or the frequency of occurrence of Rossby wave breaking, strongly determine the substructure of extreme summers in certain regions.

Mediterranean thunderstorms climatology is analyzed for a 10-year period from 2005 to 2014. For this analysis, we applied a clustering method to the cloud-to-ground lightning stroke dataset, derived from the ZEUS detection system, operated by the National Observatory of Athens. Results showed that the annual days of thunderstorm occurrence are dependent on the diurnal cycle of insolation and on the underlying topographic features. Therefore, the majority of thunderstorms occur over land in spring and summer, while winter and autumn thunderstorms tend to occur over the sea. On average, winter thunderstorms are less frequent and of lower intensity than the ones of summer, but last longer, tend to attain larger sizes and are associated with significantly higher velocity. Regardless the season, all thunderstorms tend to propagate towards the east, while their speed is higher in winter and autumn. In particular, for thunderstorms occurring over land, we found that the higher the duration of the lifetime of a thunderstorm, the earlier in the day it is expected to occur. Mediterranean thunderstorms size and duration have been found to be correlated, while a linear relation was determined between thunderstorms intensity and rainfall over the Mediterranean region, where the rain yield was estimated to be about 1.8 108 kg of rainfall per CG lightning.

Mediterranean cyclones with tropical-like characteristics such as spiral cloud coverage and a central cloud-free “eye” are referred to as medicanes. These systems have been analyzed due to their relation with high-impact weather. In previous studies, the identification of medicanes has been typically performed subjectively, using satellite pictures, but also objectively through three-dimensional diagnosis of a warm core and an axisymmetric structure. Despite the presence of these characteristics, it is still unclear if medicanes show dynamical similarities with tropical cyclones. We analyse the (thermo-)dynamics of a recognized medicane that occurred in December 2005 by applying different diagnostics to a high-resolution simulation. These diagnostics are focused on the intensification, dynamical structure and water budget of this representative case, aiming to highlight extratropical and tropical cyclone characteristics. Three stages in the medicane life cycle are identified. In stage I, a potential vorticity (PV) streamer reaches the Mediterranean, triggering deep convection and deepening the medicane's central surface pressure due to diabatic heating. When the lowest central pressure is reached (stage II), the medicane presents a warm core and an axisymmetric structure. However, convection is rather weak and the PV streamer evolves into a cut-off system which contributes to the deepening of the medicane's surface pressure. Finally, stage III corresponds to the decay phase where the medicane tends to weaken and lose its axisymmetric structure. Our results highlight the detrimental role of deep convection prior to the mature stage of the medicane, as well as the possibility of positive or negative feedback of upper-tropospheric dynamics on the central surface pressure. In addition, we show that the medicane warm core might be achieved due to front seclusion, while the “eye” formation is associated with dry air intrusions. Our analysis suggests that medicanes are hybrid systems combining characteristics of both tropical and extratropical cyclones and thus they plausibly correspond to subtropical cyclones. Fil: Fita Borrell, Lluís. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Centro de Investigaciones del Mar y la Atmósfera. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Centro de Investigaciones del Mar y la Atmósfera; Argentina Fil: Flaounas, E.. National Observatory of Athens; Grecia

This study aims to assess the skill of regional climate models (RCMs) at reproducing the climatology of Mediterranean cyclones. Seven RCMs are considered, five of which were also coupled with an oceanic model. All simulations were forced at the lateral boundaries by the ERA-Interim reanalysis for a common 20-year period (1989–2008). Six different cyclone tracking methods have been applied to all twelve RCM simulations and to the ERA-Interim reanalysis in order to assess the RCMs from the perspective of different cyclone definitions. All RCMs reproduce the main areas of high cyclone occurrence in the region south of the Alps, in the Adriatic, Ionian and Aegean Seas, as well as in the areas close to Cyprus and to Atlas mountains. The RCMs tend to underestimate intense cyclone occurrences over the Mediterranean Sea and reproduce 24–40 % of these systems, as identified in the reanalysis. The use of grid nudging in one of the RCMs is shown to be beneficial, reproducing about 60 % of the intense cyclones and keeping a better track of the seasonal cycle of intense cyclogenesis. Finally, the most intense cyclones tend to be similarly reproduced in coupled and uncoupled model simulations, suggesting that modeling atmosphere–ocean coupled processes has only a weak impact on the climatology and intensity of Mediterranean cyclones. This paper is a contribution to the special issue on Med-CORDEX, an international coordinated initiative dedicated to the multi-component regional climate modelling (atmosphere, ocean, land surface, river) of the Mediterranean under the umbrella of HyMeX, CORDEX, and Med-CLIVAR and coordinated by Samuel Somot, Paolo Ruti, Erika Coppola, Gianmaria Sannino, Bodo Ahrens, and Gabriel Jordà.

In this study, we provide an insight to the role of deep convection (DC) and the warm conveyor belt (WCB) as leading processes to Mediterranean cyclones’ heavy rainfall. To this end, we use reanalysis da-ta, lighting and satellite observations to quantify the relative contribution of DC and the WCB to cyclone rainfall, as well as to analyse the spatial and temporal variability of these processes with respect to the cy-clone centre and life cycle.\ud \ud Results for the period 2005-2015 show that the relationship between cyclone rainfall and intensity has high variability and demonstrate that even intense cyclones may produce low rainfall amounts. However, when considering rainfall averages for cyclone intensity bins, a linear relationship was found. We focus on the 500 most intense tracked cyclones (responsible for about 40-50% of the total 11-year Mediterrane-an rainfall) and distinguish between the ones producing high and low rainfall amounts. DC and the WCB are found to be the main cause of rainfall for the former (producing up to 70% of cyclone rainfall), while, for the latter, DC and the WCB play a secondary role (producing up to 50% of rainfall). Further analysis showed that rainfall due to DC tends to occur close to the cyclones’ centre and to their eastern sides, while the WCBs tend to produce rainfall towards the northeast. In fact, about 30% of rainfall produced by DC overlaps with rainfall produced by WCBs but this represents only about 8% of rainfall produced by WCBs. This suggests that a considerable percentage of DC is associated with embedded convection in WCBs. Finally, DC was found to be able to produce higher rain rates than WCBs, exceeding 50 mm in 3-hourly accumulated rainfall compared to a maximum of the order of 40 mm for WCBs. \ud \ud Our results demonstrate in a climatological framework the relationship between cyclone intensity and pro-cesses that lead to heavy rainfall, one of the most prominent environmental risks in the Mediterranean. Therefore, we set perspectives for a deeper analysis of the favourable atmospheric conditions that yield high impact weather.

Two deep cyclones occurred in the Mediterranean between 25–31 October 2012, during the first Special Observation Period (SOP1) of the Hydrological cycle in Mediterranean Experiment (HyMeX). Both cyclones were associated with extreme rainfall covering a large part of the western Mediterranean Sea, where 24-h accumulated precipitation measurements exceeded 150 mm. We combine complementary observations from airborne radar and lidar systems, ZEUS lightning detection network and meteorological surface stations along with satellite diagnostics on deep convection, for a detailed microphysics and (thermo-)dynamical analysis of the two extreme rainfall cases. In addition, we use operational analysis data from the European Centre for Medium-Range Weather Forecasts (ECMWF) for analyzing the synoptic conditions and diagnosing strongly ascending air masses in the vicinity of the cyclones, so-called warm conveyor belts (WCBs). The analysis revealed the different physical characteristics of the two cyclones responsible for the extreme rainfalls. Both cyclones were associated with a WCB and a comma cloud, but deep convection, intense lightning and very cold cloud tops occurred only for the first case while the second cyclone was mostly associated with stratiform rainfall, a strong WCB, and only few embedded cells of deep convection.

This study addresses the question of cyclone contribution to dust transport events over the Mediterranean. For this reason, we tracked and associated all intense cyclones in the period 2005–2012 with satellite estimations of the aerosol optical depth, the aerosol index and the aerosol Angstrom exponent. Results show that cyclones are related with up to 20% of the total dust events over the Mediterranean, especially affecting its eastern side. When considering only the extreme dust events, the cyclone contribution may reach 70%, highlighting cyclones as one of the main factors for extreme dust transport events in the region.

In this study we aim to assess the WRF-Chem model capacity to reproduce dust transport over the eastern Mediterranean. For this reason, we compare the model aerosol optical depth (AOD) outputs to observations, focusing on three key regions: North Africa, the Arabian Peninsula and the eastern Mediterranean. Three sets of four simulations have been performed for the six-month period of spring and summer 2011. Each simulation set uses a different dust emission parametrisation and for each parametrisation, the dust emissions are multiplied with various coefficients in order to tune the model performance. Our assessment approach is performed across different spatial and temporal scales using AOD observations from satellites and ground-based stations, as well as from airborne measurements of aerosol extinction coefficients over the Sahara. Assessment over the entire domain and simulation period shows that the model presents temporal and spatial variability similar to observed AODs, regardless of the applied dust emission parametrisation. On the other hand, when focusing on specific regions, the model skill varies significantly. Tuning the model performance by applying a coefficient to dust emissions may reduce the model AOD bias over a region, but may increase it in other regions. In particular, the model was shown to realistically reproduce the major dust transport events over the eastern Mediterranean, but failed to capture the regional background AOD. Further comparison of the model simulations to airborne measurements of vertical profiles of extinction coefficients over North Africa suggests that the model realistically reproduces the total atmospheric column AOD. Finally, we discuss the model results in two sensitivity tests, where we included finer dust mode (less than 1 μm) and changed accordingly the dust bins mass fraction.

In this study we present a new cyclone identification and tracking algorithm, cycloTRACK. The algorithm describes an iterative process. At each time step it identifies all potential cyclone centers, defined as relative vorticity maxima embedded in smoothed enclosed contours of at least 3 × 10−5 s−1 at the atmospheric level of 850 hPa. Next, the algorithm finds all the potential cyclone paths by linking the cyclone centers at consecutive time steps and selects the most probable track based on the minimization of a cost function. The cost function is based on the average differences of relative vorticity between consecutive track points, weighted by their distance. Last, for each cyclone, the algorithm identifies "an effective area" for which different physical diagnostics are measured, such as the minimum sea level pressure and the maximum wind speed. The algorithm was applied to the ERA-Interim reanalyses for tracking the Northern Hemisphere extratropical cyclones of winters from 1989 until 2009, and we assessed its sensitivity for the several free parameters used to perform the tracking.

In this study we aim at optimizing the WRF-Chem model performance for the purpose of operational forecasting of dust transport over the eastern Mediterranean. For this reason, we compare the model output to observations in order to assess its capacity to realistically reproduce the aerosol optical depth (AOD), focusing on three key regions: North Africa, the Arabian Peninsula and the eastern Mediterranean. Three sets of four simulations each have been performed for the six-month period of spring and summer 2011. Each simulation set uses a different dust emission parametrisation and for each parametrisation, the dust emissions are multiplied with various coefficients in order to tune the model performance. Our approach is based on the model assessment across spatial and temporal scales by comparing its outputs to AOD observations from satellites and ground-based stations, as well as airborne measurements of aerosol extinction coefficients over the Sahara. Tuning the model performance by applying a coefficient to dust emissions may reduce the model AOD bias over a region, but may increase it in other regions. Concerning dust transport over the eastern Mediterranean, the model was shown to realistically reproduce the major transport events, however failing to capture the regional background AOD. Model assessment over the entire domain and simulation period shows that the model presents temporal and spatial variability similar to observed AODs, regardless of the applied dust emission parametrisation. However, when focusing on specific regions, the model’s skill may vary significantly. Further comparison of the model simulations to airborne measurements of the vertical profiles of extinction coefficients over North Africa suggests that the model may realistically reproduce the total atmospheric column AOD. Finally, we show that the inclusion of a finer dust mode (less than 1 μm) in the model presents the advantage of relaxing unrealistically large atmospheric dust loads and yet reproducing realistic AOD values.

International audience; This paper analyzes the impact of cyclones to the atmospheric components on the Mediterranean Sea Water Budget, namely the cyclones contribution to precipitation and evaporation over the Mediterranean Sea. Three regional simulations were performed with the WRF model for the period 1989–2008. The model was run (1) as a standalone model, (2) coupled with the oceanic model NEMO-MED12 and (3) forced by the smoothed Sea Surface Temperature (SST) fields from the second simulation. Cyclones were tracked in all simulations, and their contribution to the total rainfall and evaporation was quantified. Results show that cyclones are mainly associated with extreme precipitation, representing more than 50 % of the annual rainfall over the Mediterranean Sea. On the other hand, we found that cyclone-induced evaporation represents only a small fraction of the annual total, except in winter, when the most intense Mediterranean cyclones take place. Despite the significant contribution of cyclones to rainfall, our results show that there is a balance between cyclone-induced rainfall and evaporation, suggesting a weak net impact of cyclones on the Mediterranean Sea water budget. The sensitivity of our results with respect to rapid SST changes during the development of cyclones was also investigated. Both rainfall and evaporation are affected in correlation with the SST response to the atmosphere. In fact, air feedbacks to the Mediterranean Sea during the cyclones occurrence were shown to cool down the SST and consequently to reduce rainfall and evaporation at the proximity of cyclone centers.

International audience; Heavy precipitation events (HPEs) can be a major cause of damage and casualties in the Mediterranean basin. With the use of atmosphere–ocean coupled regional climate models (AORCMs) and the advantage of 24 years of simulation (1989–2012), we identified regions with potential impact of submonthly air-sea coupling on HPEs among the regions hit by heavy rain during HyMeX 1st special observation period (SOP1): Valencia in Spain, the Cévennes in Southern France, Liguria in Northwestern Italy, Calabria in Southern Italy and Northeastern Italy. A first evaluation of the two AORCMs (MORCE and CNRM-RCSM4) against gridded precipitation datasets showed that 70 to 90% of the 30 most intense HPEs simulated were observed HPEs for most regions. The Cévennes, Valencia and Calabria were the only three regions to show a statistical relationship between rain differences and SST differences where the low-level jets that feed the events most frequently blow. This sensitivity of precipitation to SST changes is due to low-level wind changes related with changes in surface heat fluxes. Based on the calculation of submonthly variations in these regions during HyMeX, HPEs on the 28/09/2012 and 12/10/2012 in Valencia and on 25-26/10/2012 in the Cévennes have most probably been affected by submonthly air-sea coupling.

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., This work is a contribution to the HyMeX program supported by grants MISTRALS and ANR-12-SENV-001 REMEMBER and to the CLIMRUN project (www.climrun.eu) funded under the European Commission’s Seventh Framework Programme (FP7).

Due to its adverse effects on human health, atmospheric particulate matter (PM) constitutes a growing challenge for air quality management. It is also a complex subject of study. The understanding of its atmospheric evolution is indeed made difficult by the wide number of sources and the numerous processes that govern its evolution in the troposphere. As a consequence, the representation of particulate matter in chemistry-transport models needs to be permanently evaluated and enhanced in order to refine our comprehension of PM pollution events and to propose consistent environmental policies. The study presented here focuses on a summer particulate pollution episode that occurred on the French Mediterranean coast. It aims at identifying the constitutive elements of this episode and to discuss its representation within a eulerian model. We first highlight the major role of dust transport from western Africa in the formation of a multi-day PM event. This result shows that dust import has to be regarded as a potentially major participant to PM events in Europe, even when considering moderate peak values. In parallel we focus on a lack of diurnal variability in the model, which is attributed to missing urban sources in standard emission inventories, and notably the resuspension of particles by urban road traffic. Through a sensitivity study based on PM and NOx measurements, we could assess the amplitude of this lack as well as the need to reconsider road traffic PM sources. In parallel, by coupling the CHIMERE-DUST model outputs to our simulation, we could show that the representation of transcontinental dust transport is a necessity for a better simulation of atmospheric particles in Southern Europe, and – in the frame of air quality management – for the quantification of the anthropogenic part of particulate matter pollution.

In this study we present a new cyclone identification and tracking algorithm. Identification is based on a recognition pattern of enclosed contours of 850 hPa filtered relative vorticity values, while tracking is based on the minimization of a cost function. In particular, for each tracked cyclone our algorithm builds all possible tracks and finally chooses the one which presents the least differences of relative vorticity between consecutive track points. In parallel, for each track point the algorithm provides a cyclone area within which different physical diagnostics are calculated (such as pressure and wind speed). The area size is a function of the cyclone relative vorticity. To validate our approach we apply the algorithm on the Northern Hemisphere for the winters of 1989–2009. Three integrations of the algorithm were performed, each by using different filtering strengths. Using the three integrations, we assess the algorithm sensitivity to prior filtering the relative vorticity field. We show that filtering the input relative vorticity fields has an impact only on the weak cyclones, while in their majority the strong cyclones are independently detected and tracked.

International audience; The use of high resolution atmosphere-ocean coupled regional climate models to study possible future climate changes in the Mediterranean Sea requires an accurate simulation of the atmospheric component of the water budget (i.e., evaporation, precipitation and runoff). A specific configuration of the version 3.1 of the weather research and forecasting (WRF) regional climate model was shown to systematically overestimate the Mediterranean Sea water budget mainly due to an excess of evaporation (~1,450 mm yr-1) compared with observed estimations (~1,150 mm yr-1). In this article, a 70-member multi-physics ensemble is used to try to understand the relative importance of various sub-grid scale processes in the Mediterranean Sea water budget and to evaluate its representation by comparing simulated results with observed-based estimates. The physics ensemble was constructed by performing 70 1-year long simulations using version 3.3 of the WRF model by combining six cumulus, four surface/planetary boundary layer and three radiation schemes. Results show that evaporation variability across the multi-physics ensemble (~10 % of the mean evaporation) is dominated by the choice of the surface layer scheme that explains more than ~70 % of the total variance and that the overestimation of evaporation in WRF simulations is generally related with an overestimation of surface exchange coefficients due to too large values of the surface roughness parameter and/or the simulation of too unstable surface conditions. Although the influence of radiation schemes on evaporation variability is small (~13 % of the total variance), radiation schemes strongly influence exchange coefficients and vertical humidity gradients near the surface due to modifications of temperature lapse rates. The precipitation variability across the physics ensemble (~35 % of the mean precipitation) is dominated by the choice of both cumulus (~55 % of the total variance) and planetary boundary layer (~32 % of the total variance) schemes with a strong regional dependence. Most members of the ensemble underestimate total precipitation amounts with biases as large as 250 mm yr-1 over the whole Mediterranean Sea compared with ERA Interim reanalysis mainly due to an underestimation of the number of wet days. The larger number of dry days in simulations is associated with a deficit in the activation of cumulus schemes. Both radiation and planetary boundary layer schemes influence precipitation through modifications on the available water vapor in the boundary layer generally tied with changes in evaporation. © 2014 Springer-Verlag Berlin Heidelberg.

International audience; In order to evaluate the future potential benefits of emission regulation on regional air quality, while taking into account the effects of climate change, off-line air quality projection simulations are driven using weather forcing taken from regional climate models. These regional models are themselves driven by simulations carried out using global climate models (GCM) and economical scenarios. Uncertainties and biases in climate models introduce an additional 'climate modeling' source of uncertainty that is to be added to all other types of uncertainties in air quality modeling for policy evaluation. In this article we evaluate the changes in air quality-related weather variables induced by replacing reanalyses-forced by GCM-forced regional climate simulations. As an example we use GCM simulations carried out in the framework of the ERA-interim programme and of the CMIP5 project using the Institut Pierre-Simon Laplace climate model (IPSLcm), driving regional simulations performed in the framework of the EURO-CORDEX programme. In summer, we found compensating deficiencies acting on photochemistry: an overestimation by GCM-driven weather due to a positive bias in short-wave radiation, a negative bias in wind speed, too many stagnant episodes, and a negative temperature bias. In winter, air quality is mostly driven by dispersion, and we could not identify significant differences in either wind or planetary boundary layer height statistics between GCM-driven and reanalyses-driven regional simulations. However, precipitation appears largely overestimated in GCM-driven simulations, which could significantly affect the simulation of aerosol concentrations. The identification of these biases will help interpreting results of future air quality simulations using these data. Despite these, we conclude that the identified differences should not lead to major difficulties in using GCM-driven regional climate simulations for air quality projections.