Aerosols Distribution During a Tramontane/Mistral Episode

Home , East, Tramontane, West

Atmos. Chem. Phys. Discuss., 6, 11913–11956, 2006 Atmospheric www.atmos-chem-phys-discuss.net/6/11913/2006/ Chemistry ACPD © Author(s) 2006. This work is licensed and Physics 6, 11913–11956, 2006 under a Creative Commons License. Discussions

Aerosols distribution during a Tramontane/Mistral Aerosol distribution over the western episode

Mediterranean basin during a T. Salameh et al. Tramontane/Mistral event Title Page T. Salameh1, P. Drobinski2, L. Menut1, B. Bessagnet3, C. Flamant2, A. Hodzic4, and R. Vautard5 Abstract Introduction

1Institut Pierre Simon Laplace/Laboratoire de Met´ eorologie´ Dynamique, Ecole´ Conclusions References Polytechnique/ENS/UPMC/CNRS, Palaiseau, France Tables Figures 2Institut Pierre Simon Laplace/Service d’Aeronomie,´ UPMC/UVSQ/CNRS, Paris, France 3Institut National de l’Environnement Industriel et des Risques, INERIS, Verneuil en Halatte, France J I 4National Center for Atmospheric Research, Boulder, CO, USA 5Institut Pierre Simon Laplace/Laboratoire des Sciences du Climat et de l’Environnement, J I CEA/UVSQ/CNRS, Gif sur Yvette, France Back Close Received: 13 October 2006 – Accepted: 15 November 2006 – Published: 23 November 2006 Full Screen / Esc Correspondence to: T. Salameh ([email protected]) Printer-friendly Version

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11913 EGU Abstract ACPD This paper investigates experimentally and numerically the time evolution of the spatial distribution of aerosols over the Western Mediterranean basin during 24 March 1998 6, 11913–11956, 2006 Mistral event documented during the FETCH experiment. Mistral and Tramontane are 5 very frequent northerly winds (5–15 days per month) accelerated along the Rhoneˆ and Aerosols distribution Aude valley (France) that can transport natural and anthropogenic aerosols oﬀshore as during a far as a few hundreds of kilometers which can in turn have an impact on the radiation Tramontane/Mistral budget over the Mediterranean Sea and on precipitation. episode The spatial distribution of aerosols was documented by means of the airborne lidar 10 LEANDRE-2 and spaceborne radiometer SeaWIFS, and a validated mesoscale chem- T. Salameh et al. ical simulation using the chemistry-transport model CHIMERE with an aerosol module, forced by the non-hydrostatic model MM5. This study shows that: (1) the Mistral contributes to the oﬀshore exportation of a large Title Page

amount of aerosols originally emitted over continental Europe (in particular ammonium Abstract Introduction 15 nitrate in the particulate phase and sulfates) and along the shore from the industrialized and urban areas of Fos-Berre/Marseille. The Genoa surface low contributes to advect Conclusions References the aerosols along a cyclonic trajectory that skirts the North African coast and reaches Tables Figures Italy; (2) the aerosol concentration pattern is very unsteady as a result of the time evolution of the two winds (or Genoa cyclone position): The Tramontane wind prevails J I 20 in the morning hours of 24 March, leaving room for the Mistral wind and an unusually

strong Ligurian outﬂow in the afternoon. The wakes trailing downstream the Massif J I Central and the Alps prevent any horizontal diﬀusion of the aerosols and can, at times, contribute to aerosol stagnation. Back Close

Full Screen / Esc 1 Introduction Printer-friendly Version

25 The Mediterranean basin is featured by an almost-closed ocean basin surrounded by mountain ranges in which numerous rivers rise, a contrasted climate and vegetation Interactive Discussion

11914 EGU from south to north, numerous and rapidly growing built-up areas along the coast with several major cities having industrial activities emitting a large number of gas sub- ACPD stances and aerosols. Highly aerosol loaded air masses are found from the surface up 6, 11913–11956, 2006 to the upper troposphere and contribute to decrease air quality on a large scale and 5 reduce precipitation in the region (Lelieveld et al., 2002). The aerosols found below 4 km height originate from regional sources whereas above 4 km height they are usu- Aerosols distribution ally linked to transport due to global-scale motions and teleconnections, for instance during a with the Indian monsoon and the North Atlantic Oscillation (Moulin et al., 1997). High Tramontane/Mistral aerosol loads in the mid troposphere may also originate from wind-blown dust in the episode 10 Saharan regions. Near the surface, most of the aerosols originate from western and eastern Europe T. Salameh et al. (e.g. Sciare et al., 2003; Traub et al., 2003; Schneider et al., 2004), and from the Saharan desert (e.g. Bergametti et al., 1992; Moulin et al., 1998; Guieu et al., 2002). Title Page Industrial activity, traffic, forest fires, agricultural and domestic burning are the main 15 source of pollution in Europe whereas the close vicinity of the Saharan desert provides Abstract Introduction a source for considerable amounts of dust. Aerosols are harmful for ecosystems and human health, and they affect the Mediter- Conclusions References

ranean climate and water cycle. Indeed, the microscopic particles affect the atmo- Tables Figures spheric energy budget by scattering and absorbing solar radiation, reducing solar radi- 20 ation absorption by the sea by approximately 10% and altering the heating profile of the J I lower troposphere (Lelieveld et al., 2002). As a consequence, evaporation and moisture transport, in particular to North Africa and the Middle East, are suppressed. This J I aerosol effect is substantial today, although the period with highest aerosol concentra- Back Close tions over the Mediterranean Sea was around 1980 and contributed to the drought in 25 the eastern Sahel (Lelieveld et al., 2002). Aerosols also affect the Mediterranean bio- Full Screen / Esc geochemistry by deposition (wet and dry) of dissolved inorganic phosphorus (Berga- metti et al., 1992; Bartoli et al., 2005), silicon (Bartoli et al., 2005) and iron (Guieu et Printer-friendly Version al., 2002) from soil-derived dust from desert areas of north Africa and anthropogenic emissions from European countries. Apart from internal sources of nutrients, the at- Interactive Discussion

11915 EGU mospheric inputs may constitute an important pathway for nutrients to the photic zone of the open sea where there is little riverine input (Migon et al., 1989; Bergametti et ACPD al., 1992; Prospero et al., 1996; Benitez-Nelson, 2000). This particularly applies to the 6, 11913–11956, 2006 Mediterranean Sea because of its reduced dimensions and because surrounding conti- 5 nental emission sources of nutrients are intense and continuously increasing (Bethoux,´ 1989; Guerzoni et al., 1999; Bethoux´ et al., 2002). Aerosols distribution In the context of climate change in which the Mediterranean basin appears quite during a vulnerable due to hydric stress and ever increasing pollution levels, it is crucial that Tramontane/Mistral knowledge be improved concerning the mechanisms linking the dynamics of the main episode 10 ﬂow regimes and the existing pollution sources scattered around the basin in order to provide insight into future trends at the regional scale. T. Salameh et al. The western Mediterranean climate is frequently aﬀected by the Mistral and its com- panion wind, the Tramontane (Georgelin and Richard, 1996; Drobinski et al., 2001a). Title Page The Mistral is a severe northerly wind that develops along the Rhoneˆ valley (while the 15 Tramontane blows in the Aude valley) (Fig. 1), and occurs between 5 and 15 days per Abstract Introduction month. The development of the Mistral is preconditioned by cyclogenesis over the Gulf of Genoa and the passage of a trough through France. The Mistral occurs all year Conclusions References

long but exhibits a seasonal variability either in terms of its strength and direction, or Tables Figures in terms of its spatial distribution (Mayenc¸on, 1982; Orieux and Pouget, 1984). At the 20 regional scale, the Mistral is frequently observed to extend as far as a few hundreds J I of kilometres from the coast (Jansa,´ 1987) and is thus associated with low continental pollution levels near the coastline as it advects the pollutants away from their sources J I of emission over the Mediterranean Sea (Bastin et al., 2006; Drobinski et al., 2006). Back Close Due to favourable dispersion conditions, air quality studies generally do not focus on 25 Mistral events. However the transport and resulting concentration distribution of chemi- Full Screen / Esc cal compounds and aerosols over the Mediterranean Sea has never been documented. The FETCH (Flux, Etat de mer et Teledetection en Condition de fetcH variable) exper- Printer-friendly Version iment (Hauser et al., 2003) oﬀers an ideal framework for such a study. The FETCH experiment took place from 12 March to 15 April 1998 and was dedicated to improve Interactive Discussion

11916 EGU the knowledge of the interactions between the ocean and the atmosphere in a coastal environment under strong wind conditions (e.g. Drennan et al., 2003; Eymard et al., ACPD 2003; Flamant et al., 2003). The 24 March 1998 Mistral case is a strong episode, typi- 6, 11913–11956, 2006 cal of intermediate season Mistral events compared to weaker summer Mistral events 5 (e.g. Drobinski et al., 2005; Bastin et al., 2006). Flamant (2003) analyzed the complex structure of atmospheric boundary layer (ABL) observed using an airborne lidar over Aerosols distribution the Gulf of Lion at the exit of the Rhoneˆ valley during the 24 March 1998 Mistral event. during a Even though the study by Flamant (2003) suggested the existence of a marked east- Tramontane/Mistral west aerosol concentration gradient oﬀshore to be related to larger concentrations of episode 10 pollution aerosol from the city of Marseille and the industrial petrochemical complex of Fos/Berre, the highly spatially resolved aerosol measurements needed to (un)validate T. Salameh et al. this hypothesis simply did not exist. By combining lidar observations and simulations we better address this question in the present article. Therefore this article is designed Title Page to analyze: Abstract Introduction 15 – the dynamic processes driving the small-scale structure of the Mistral ﬂow at the exit of the Rhoneˆ valley, and their relation with the aerosol distribution observed Conclusions References by the airborne lidar and the satellite imagery, Tables Figures – the aerosol sources, composition and distribution over the whole western Mediter- ranean basin. J I

20 The instrument set-up (i.e. FETCH-related sea-borne, airborne and space-borne ob- J I servations) and the numerical models used in this study are described in Sect. 2. In Sect. 3, the meteorological environment leading to the Mistral episode is analyzed as Back Close

well as the ﬁne-scale structure of the Mistral ﬂow. In Sect. 4, the aerosol distribution Full Screen / Esc over the western Mediterranean basin is discussed, before conclusions are drawn in 25 Sect. 5 and suggestions for future work are presented. Printer-friendly Version

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11917 EGU 2 Measurements and models ACPD 2.1 Numerical models 6, 11913–11956, 2006 2.1.1 Dynamical model Aerosols distribution The dynamical numerical simulations presented in this study have been conducted us- during a 5 ing the ﬁfth generation Penn State-National Center for Atmospheric Research MM5 Tramontane/Mistral model, version 3.6 (Dudhia, 1993; Grell et al., 1995). The model solves the non- hydrostatic equations of motion in a terrain-following sigma coordinates. Three in- episode teractively nested model domains are used, the horizontal mesh size being 27 km, T. Salameh et al. 9 km and 3 km, respectively. Domains 1 (coarse domain), 2 (medium domain) and 3 ◦ ◦ 10 (ﬁne domain) are centered at 43.7 N, 4.6 E and cover an area of 1350 km×1350 km, 738 km×738 km and 120 km×174 km, respectively (see Fig. 1). Domain 1 covers half Title Page of France and the western Mediterranean Sea. Domain 2 covers the Rhoneˆ valley, the western Alps, the Massif Central and Corsica. Domain 3 covers the Rhoneˆ val- Abstract Introduction

ley delta. The model orography of the three domains is interpolated from terrain data Conclusions References 00 15 with 30 resolution. It is ﬁltered by a two-pass smoother-desmoother (Guo and Chen, 1994) in order to remove two-grid interval waves that would induce numerical noise. Tables Figures Information on land use was obtained from USGS (United States Geological Survey) data with the same horizontal resolution as for orography. In the vertical, 43 unevenly J I spaced full sigma-levels are used, corresponding to 42 half-sigma levels where most of J I 20 the variables are computed. The lowermost half-sigma level (σ=0.999) is about 12 m above ground. The vertical distance between the model levels is about 50 m close to Back Close the ground and increases up to 1200 m near the upper boundary which is located at 100 hPa. Full Screen / Esc A complete set of physics parameterizations is used. Cloud microphysics is treated 25 with a sophisticated scheme having prognostic equations for cloud water, cloud ice, Printer-friendly Version cloud ice particle number concentration, rain, snow and graupel (Reisner et al., Interactive Discussion 1998), using modiﬁcations proposed by Chiriacco et al. (2005). The Grell cumulus

11918 EGU parametrization (Grell, 1993) is used in model domains 1 and 2. In model domain 3, a cumulus parametrization is not needed because convection is resolved explicitly at ACPD such high resolution. The radiation scheme accounts for the interaction with moisture 6, 11913–11956, 2006 and clouds (Grell et al., 1995; Mlawer et al., 1997). The ABL is parameterized using 5 the Hong and Pan scheme (Hong and Pan, 1996). It is an efficient scheme based on Troen and Mahrt representation of countergradient term and eddy viscosity profile in Aerosols distribution the well mixed ABL, and is suitable for high resolution in ABL. during a The initial and boundary conditions are taken from the ECMWF (European Centre Tramontane/Mistral for Medium Range Weather Forecast) reanalyses ERA-40. These reanalysis data are episode ◦ ◦ 10 available every six hours on a 1 ×1 latitude-longitude grid. Since the interpolation routine of the MM5 modeling system needs pressure level data, the standard-level- T. Salameh et al. pressure version of the ECMWF data is used. In order for the synoptic flow to stay close to meteorological analyzes, the meteorological simulation uses nudging for all Title Page variables with a coefficient of 10−4 s−1. The initialization date is 23 March 1998 at 15 12:00 UTC and the simulation ends on 25 March 1998 at 18:00 UTC. Abstract Introduction In the following, MM5 simulations are used to provide the three-dimensional environment necessary for the interpretation of the measurements, and are also the meteoro- Conclusions References

logical forcing for the regional chemistry-transport model CHIMERE (see next section). Tables Figures 2.1.2 Chemistry-transport model J I

20 The chemistry-transport model forced by the MM5 meteorology is CHIMERE. It is a J I 3-D chemistry-transport model, developed cooperatively by the French Institute Pierre- Simon Laplace (IPSL), Laboratoire Interuniversitaire des Systemes` Atmospheriques´ Back Close (LISA) and Institut National de l’Environnement industriel et des RISques (INERIS). Full Screen / Esc CHIMERE is a 3-D chemistry transport model that simulates gas-phase and aerosol- 25 phase chemistry and its transport at the scale of the continent (Schmidt et al., 2001) or at regional scale (Vautard et al., 2001, 2003). It simulates the concentration of 44 Printer-friendly Version gaseous species and 6 aerosol chemical compounds. Sea salt is not accounted for Interactive Discussion in our simulations. In CHIMERE aerosol version (Bessagnet et al., 2004; Hodzic et 11919 EGU al., 2004), the population of aerosol particles is represented using a sectional formula- tion, assuming discrete aerosol size sections and considering the particles of a given ACPD section as homogeneous in composition (internally mixed). Like in the work of Bessag- 6, 11913–11956, 2006 net et al. (2004), we use six diameter bins ranging between 10 nm and 40 µm, with 5 a geometric increase of bin bounds. The aerosol module accounts for both inorganic and organic species, of primary or secondary origin, such as, primary particulate mat- Aerosols distribution ter (PPM), sulfates, nitrates, ammonium, secondary organic species (SOA) and water. during a PPM is composed of primary anthropogenic species such as elemental and organic Tramontane/Mistral carbon and crustal materials. In the model, ammonia, nitrate, and sulfate are consid- episode 10 ered in aqueous, gaseous, and particulate phases. In the present application, the model is run at mesoscale over a domain covering the T. Salameh et al. western Mediterranean basin (Fig. 1). The model domains match the MM5 domains with a vertical resolution of 12 sigma-pressure levels extending up to 500 hPa that cov- Title Page ers the ABL and the lower part of the free troposphere. The meteorological variables 15 used as input are wind, temperature, mixing ratio for water vapor and liquid water in Abstract Introduction clouds, 2 m temperature, surface heat and moisture ﬂuxes and precipitation. Bound- ary conditions of CHIMERE simulations are taken from climatologies of LMDZ-INCA Conclusions References

chemistry transport model for the gazeous species, and from GOCART (The Georgia Tables Figures Tech/Goddard Global Ozone Chemistry Aerosol Radiation Transport) aerosol forecast- 20 ing model for the aerosols. Emissions of primary particulate matter PM10 and PM2.5 J I (particulate matter of diameter less than 10 µm and 2.5 µm) and the anthropogenic emissions for NOx, CO, SO2, NMVOC (Non Methane Volatile Organic Compounds) J I and NH3 gas-phase species for 10 anthropogenic activity sectors (as deﬁned by the Back Close SNAP categories) are provided by EMEP (available at http://www.emep.int) with spa- 25 tial resolution of 50 km (Vestreng 2003). Full Screen / Esc The CHIMERE simulation covers the same period as the MM5 simulation. How- ever, for aerosol-related spin-up needs, a three-day simulation was ran from 20 March Printer-friendly Version 12:00 UTC to 23 March 12:00 UTC to account for some of the aerosol distribution build-up prior to the event under scrutiny. Interactive Discussion

11920 EGU 2.2 Observations ACPD Over the continent, we mainly use the measurements from the operational radiosoundings and synoptic meteorological stations of Met´ eo-France.´ On 24 March 1998, oper- 6, 11913–11956, 2006 ational radiosondes were released every twelve hours from Lyon and Nˆımes. At the 5 surface, the operational meteorological surface station network of Met´ eo´ France gave Aerosols distribution access to the surface thermodynamical ﬁelds (precipitation, wind speed and direction, during a temperature, humidity, pressure). The locations of the operational meteorological sur- Tramontane/Mistral face stations used in this study are shown in Fig. 1b. episode Over the Mediterranean Sea, we use the measurements of meteorological and 10 aerosols variables collected on board the research vessel (R/V) Atalante, the ASIS T. Salameh et al. (Air-Sea Interaction Spar) buoy and the Avion de Recherche Atmospherique´ et Tel´ ed´ etection´ (ARAT), as well as from satellites (the Sea-viewing Wide Field-of-view Sensor – SeaWIFS – and the Active Microwave Instrumentation-Wind Scatterometer Title Page

AMI/Wind on board the European Remote Sensing Satellite ERS). The complemen- Abstract Introduction 15 tarity of the data provided by these various instruments is an essential aspect of this study. The locations of the measurement sites used in this study are shown in Fig. 1. Conclusions References The R/V Atalante had balloon launching capability and carried an instrumented mast Tables Figures for mean and turbulent measurements at a height of 17 m above the sea surface. The ASIS buoy made measurements of mean and turbulent atmospheric variables J I 20 7 m above the air-sea interface as well as wave directional spectra. The ARAT was

equipped with standard in situ sensors as well as sensors dedicated to the analysis J I of aerosol properties (nephelometer, particle and cloud condensation nuclei counters). The ARAT also embarked the diﬀerential absorption lidar LEANDRE-2 (Flamant, 2003; Back Close

Flamant et al., 2003). Full Screen / Esc 25 The ARAT afternoon mission on 24 March 1998 (between 16:20 and 18:50 UTC) was designed in such a manner that long levelled legs be ﬂown along and across the Printer-friendly Version mean ABL wind direction. The structure of the ﬂow and its evolution with time has been analyzed using high resolution measurements (15 m in the vertical and 160 m Interactive Discussion

11921 EGU in the horizontal) of atmospheric reflectivity at 730 nm, made with the airborne lidar LEANDRE-2. Because it is sensitive to relative humidity as well as aerosol proper- ACPD ties and concentration, lidar-derived reflectivity is extremely useful to investigate ABL 6, 11913–11956, 2006 structural properties (e.g. Flamant and Pelon, 1996; Drobinski et al., 2001a; Flamant, 5 2003). Here we shall discuss reflectivity measurements made with a nadir pointing system from an altitude of 4 km above mean sea level (MSL). Aerosols distribution The high spatial and temporal documentation of the 150 km×200 km FETCH target during a area (i.e. Gulf of Lion) allows a very detailed insight on the dynamics of the Mistral Tramontane/Mistral event as well as on the aerosol distribution. However, this dataset was complemented episode 10 by satellite data from AMI-Wind/ERS scatterometer providing daily mean surface wind speed and wind stress over the whole Mediterranean Sea at 1◦ resolution, and from T. Salameh et al. SeaWIFS providing the aerosol optical depth at 865 nm while passing over the western Mediterranean at 11:32 UTC. Title Page

Abstract Introduction 3 The 24 March 1998 Mistral event Conclusions References 15 3.1 Synoptic environment Tables Figures The 24 March 1998 Mistral event was featured by the existence of an upper level

trough associated with a cold front progressing toward the Alps and a shallow vor- J I tex (1014 hPa) over the Tyrrhenian Sea between Sardinia and continental Italy, at 06:00 UTC. As the day progressed, the low over the Tyrrhenian deepened (from 1014 J I

20 to 1008 hPa between 06:00 and 15:00 UTC) while remaining relatively still. From Back Close 15:00 UTC on, the low continued to deepen (from 1008 to 1002 hPa) while moving to the southeast. It was located over Sicily on 25 March 1998 at 06:00 UTC. Full Screen / Esc This two-phase evolution of Alpine lee cyclogenesis has been observed before (Eg- ger, 1972; Buzzi and Tibaldi, 1978). Alpert et al. (1996) have shown that topographical Printer-friendly Version 25 blocking is the dominant factor in the ﬁrst and most rapid phase of the cyclone deepening. During the second phase, the growth rate drops to baroclinic values and the Interactive Discussion

11922 EGU structure of the cyclone approaches that of typical extratropical systems. Convection as well as sensible and latent heat fluxes play an important role in the second phase of ACPD Alpine lee cyclogenesis development (Alpert et al., 1996; Grotjahn and Wang, 1989). 6, 11913–11956, 2006 These authors have also shown that convection has a tendency to move the cyclone 5 to the eastnortheast while topography has a tendency to tie the cyclone to the lee of the Alps. The sea moisture fluxes tend to move the cyclone toward the warm bodies of Aerosols distribution water (i.e., toward Sicily in our case). during a The multistage evolution of the Alpine lee cyclone over the Tyrrhenian Sea induced Tramontane/Mistral a very nonstationary wind regime over the Gulf of Lion (also see Flamant, 2003). The episode 10 diurnal evolution of the Mistral and the Tramontane on 24 March 1998 are evidenced on the wind field simulated in the ABL (at 950 hPa) by the MM5 model at 06:00, 09:00, T. Salameh et al. 12:00, 15:00, 18:00 and 21:00 UTC (Fig. 2). In the early stage (low at 1014 hPa, 06:00 UTC), the Tramontane flow prevailed over the Gulf of Lion. The large westerly Title Page flow component (leading to prevailing Tramontane conditions) resulted from the rather 15 high position (in terms of latitude) of the depression. The Mistral extended all the way Abstract Introduction to Southern Corsica, wrapping around the depression. To the north, a weak easterly outflow was observed over the Gulf of Genoa. As the low deepened (1010 hPa), the Conclusions References

prevailing wind regime shifted to a well-established Mistral peaking around 12:00 UTC. Tables Figures The Mistral was observed to reach Southern Sardinia where it wrapped around the 20 depression. At this time, the outflow from the Ligurian Sea (i.e., Gulf of Genoa) had J I become stronger. In the afternoon, the Mistral was progressively disrupted by the strengthening outflow coming from the Ligurian Sea in response to the deepening low J I over the Tyrrhenian Sea (1008 hPa, 15:00 UTC) and the channelling induced by the Back Close presence of the Apennine range (Italy) and the Alps. In the evening, the Mistral was 25 again well established over the Gulf of Lion as the depression continued to deepen Full Screen / Esc (1002 hPa, 21:00 UTC), but moved to the southeast reducing the influence of outflow from the Ligurian Sea on the flow over the Gulf of Lion. During this period, the Tra- Printer-friendly Version montane flow appeared to be much steadier than the Mistral and less disrupted by the return flow associated with the depression. The cold air outbreak episode over Interactive Discussion

11923 EGU the Gulf of Lion ended on 25 March at 06:00 UTC and anticyclonic conditions then prevailed over the Gulf of Lion. ACPD An important feature of the cold air outbreak over the Gulf of Lion is also observed 6, 11913–11956, 2006 in the form of banners of weaker winds (sheltered region) separating (i) the Mistral 5 and the Tramontane (in the lee of the Massif Central) and (ii) the Mistral and the Lig- urian Sea outflow (in the lee of the western Alps, see Fig. 2). Such sheltered regions Aerosols distribution are a common feature over the Mediterranean because of the presence of numerous during a mountain ranges and are related to potential vorticity banners (i.e., regions of important Tramontane/Mistral wind shear) generated downstream of the mountain ranges (e.g. Aebischer and Schar,¨ episode 10 1998; Drobinski et al., 2005; Guenard´ et al., 2006), which contribute to the deepening of the Alpine lee cyclones. The unsteady nature of the western Alps wake (as opposed T. Salameh et al. to the steadier Massif Central wake) is caused by the complex topography of the Alps which amplifies any variation of the wind speed and direction, or ABL depth upstream Title Page of the Alps (Guenard´ et al., 2006). Abstract Introduction 15 3.2 Structure of the Mistral over the continent Conclusions References The radiosoundings launched from Lyon on 24 March 1998 at 00:00 and 12:00 UTC show the synoptic northwesterly flow blowing above 1 km a.s.l. (Fig. 3, top row). Below Tables Figures 1 km height, a low-level jet blows at about 5–8 m s−1 from the north and is aligned with the Rhoneˆ valley axis displaying channelling in the valley. Between 1 and 3 km a.s.l., J I 20 the wind direction veers to the northeast and the wind speed increases from about J I 5 m s−1 to about 15 m s−1 at 3 km a.s.l. The Mistral jet blows within the ABL. The radiosoundings launched from Nˆımes (Fig. 3, middle row) shows that the Mistral experi- Back Close ences channelling in the Rhoneˆ valley and accelerates with the wind speed increasing Full Screen / Esc from 5–8 m s−1 in Lyon to 15–18 m s−1 in Nˆımes. The daytime cooling between the 25 entrance and exit of the Rhoneˆ valley is reversed during nighttime because of the large continental diurnal cycle in Lyon and the smoothed diurnal cycle near the sea shore. Printer-friendly Version The MM5 model (solid line) accurately reproduces the vertical profiles of wind speed Interactive Discussion and direction, and potential temperature with at most 2 m s−1 difference, less than 20◦ 11924 EGU and 1 K on average, respectively. Except for near surface temperature, the radiosoundings launched from Lyon and Nˆımes on 24 March 1998 at 00:00 and 12:00 UTC show ACPD that the Mistral characteristics are fairly persistent with time. 6, 11913–11956, 2006 The simulated surface wind and temperature fields are evaluated by comparing the 5 measured 10-m horizontal wind components and 2-m temperature to their simulated counterparts interpolated at the location of the meteorological surface stations in south- Aerosols distribution ern France (see dots in Fig. 1b). Quantitatively, Fig. 4 shows the histograms of the during a difference between the three-hourly simulated and measured 10-m zonal and merid- Tramontane/Mistral ional wind components and 2-m temperature during the 24 h on 24 March 1998. The episode −1 10 measured 10-m wind speed and 2-m temperature accuracies are 1 m s and about 0.1◦ K, respectively. The bias between MM5 and the measured wind speed is about T. Salameh et al. −1 m s−1 for the two horizontal components which is thus non significant. The standard deviation is also small considering the accuracy of the wind observations. As for Title Page the temperature, the largest discrepancies are found in the steep orography regions 15 where the height of the topography is not accurately represented in the model and at Abstract Introduction night when numerical diffusion slightly deteriorates the simulation of the near surface temperature and the katabatic flows. Conclusions References

Tables Figures 3.3 Structure of the Mistral over the Mediterranean Sea

Over the Mediterranean Sea, the data are generally sparse, and available from J I 20 satellite-borne sensors thus restricted to the surface and reliable few hundreds of kilo- J I meters away from the shore. Figure 5 shows the daily mean values of the sea surface wind speed provided by the satellite-borne AMI-Wind/ERS scatterometer over the Back Close Mediterranean Sea at 1◦ resolution (no wind vectors are available for year 1998). It Full Screen / Esc shows three regions of sea surface wind speed exceeding 20 m s−1: to the west, the ◦ 25 Tramontane merge with the Spanish Cierzo; at around 7 E longitude, the Mistral blows around the Genoa cyclone and immediately to the east of Sicily and Sardinia, the Lig- Printer-friendly Version urian outflow. The MM5 models reproduces the same structure but with lower wind Interactive Discussion speed which can reach 5 m s−1 in the regions of strong flows. From an observational 11925 EGU point of view, this can be attributed to the fact that the largest AMI-Wind/ERS errors are found near the shore, whereas from a modelling point of view, the absence of coupling ACPD between the sea and the atmosphere for winter Mistral events which generate at this 6, 11913–11956, 2006 period of the year intense air-sea heat exchanges (Flamant, 2003) and sea surface 5 cooling (Millot, 1979) can also be a source of error. The ASIS buoy data allows a more detailed comparison at a single point. Figure 6 shows the good agreement between Aerosols distribution the surface wind speed and direction, temperature, relative humidity and surface stress during a from the ASIS buoy measurements and from the MM5 simulations (better than 2 m s−1, Tramontane/Mistral 10◦, 1◦C, 20% and 0.1 N m−2 for wind speed and direction, temperature, relative hu- episode 10 midity and wind stress, respectively, with some intermittent exceptions). This figure illustrates the unsteady aspect of the Mistral/Tramontane episode with the Tramontane T. Salameh et al. blowing over the buoy between 04:00 and 08:00 UTC, followed by a calm period until the Mistral breaks through after 12:00 UTC. Title Page The radiosoundings launched at 09:00 UTC and 11:00 UTC from the research ves- ◦ ◦ ◦ ◦ 15 sel Atalante at (42.75 N; 4.37 E longitude) and (42.95 N; 4.27 E), respectively show a Abstract Introduction northerly wind flow from the surface up to 5 km and more (Figs. 3g, h and i). The vertical profiles of potential temperature at 09:00 and 11:00 UTC show a remarkable persis- Conclusions References

tence of the thermodynamical characteristics of the Mistral with a potential temperature Tables Figures inversion at about 2 km height which corresponds to the ABL depth. At 09:00 UTC, the −1 −1 20 wind blows at approximately 15 m s within the ABL and increases up to 28 m s J I above (Fig. 3g). The wind speed is stronger at 11:00 UTC than at 09:00 UTC (20– −1 22 m s below 3 km) since the research vessel Atalante moves closer to the Mistral J I core. The MM5 model reproduces the vertical proﬁles of wind speed and direction, Back Close and potential temperature with at most 2 m s−1 (09:00 UTC) to 5 m s−1 (11:00 UTC) ◦ 25 diﬀerence and less than 30 and 2 K on average for the wind speed and direction and Full Screen / Esc potential temperature, respectively. The deepening of the ABL between Nˆımes and the Atalante can be attributed to the occurrence of a hydraulic jump associated with en- Printer-friendly Version hanced turbulence and mixing (e.g. Drobinski et al., 2001b) as previously observed at the exit of the Rhoneˆ valley for many other Mistral events by Pettre´ (1982), Corsmeier Interactive Discussion

11926 EGU et al. (2005) and Drobinski et al. (2005). ACPD 4 Aerosol distribution at the scale of the western Mediterranean basin 6, 11913–11956, 2006

4.1 Sources of atmospheric pollutants Aerosols distribution The western Mediterranean basin is characterized by a meridional gradient of surface during a 5 land types, mainly associated with vegetation and aerosol sources. South of the basin, Tramontane/Mistral the desert is a primary source of mineral dust. West and north of the basin, some episode aerosol sources are large industrial zones (reﬁneries, power plants,. . . ) surrounded by T. Salameh et al. residential areas with important traﬃc, such as Barcelona, Marseille, Lyon, Milano and Rome. Rural areas prone to agriculture or covered by Mediterranean natural landscape

10 are the main sources of biogenic emissions. Finally emissions related to ship traffic also Title Page impact Mediterranean pollution levels and climate (Marmer and Langmann, 2005). Some anthropogenic species such as nitrogen oxides (Fig. 7a) are mainly emitted Abstract Introduction via traffic, industries and domestic combustion. For instance, over the continent, max- Conclusions References ima of NO emissions are seen in the main cities Toulouse, Lyon, Marseille as well as 15 Milano. Over the Sea, evidence of emissions associated with maritime traffic is also Tables Figures seen along the northern African coastline (i.e. ship routes between Gibraltar and the

Suez canal), as well as along the Spanish coastline between Marseille and Gibraltar J I or along the Thyrrennean coast. SO2 (Fig. 7c) is mainly emitted via industrial activities and domestic combustion, as shown by the more scattered emission pattern. The J I

20 emissions associated with maritime traffic are significant. CO and coarse primary par- Back Close ticulate matter (PPM, of size less than 10 µm and more than 2.5 µm) emissions are shared by traffic and various sources of combustion. These sources are significant in Full Screen / Esc the main western Mediterranean cities as shown in Figs. 7b and d. Printer-friendly Version

Interactive Discussion

11927 EGU 4.2 Analysis at the scale of the Gulf of Lion ACPD On the shore of the Gulf of Lion, the area around the city of Marseille and the Berre pond, in southeastern France is a source region of large urban and industrial emission 6, 11913–11956, 2006 with high occurrence of photochemical pollution events (Drobinski et al., 2006). The 5 aerosol optical depth (AOD) at 865 nm derived from the SeaWIFS pass over the Gulf of Aerosols distribution Lion at 11:32 UTC on 24 March 1998 (obtained from the SeaDAS software) is shown during a in Fig. 8c. Figure 8 evidences that most of the western Mediterranean was covered Tramontane/Mistral by clouds, with the notable exception of Gulf of Lion, which was partly cloud-free, on episode account of the strong Mistral jet blowing in the region. AODs at 865 nm could only be 10 retrieved in the cloud-free part of the Gulf of Lion, but displays a minimum south of the T. Salameh et al. city of Montpellier (0.11 around 4◦ E) and a maximum south of the Fos/Berre industrial area (0.18 around 4.7◦ E) along leg AF of the ARAT aircraft (Fig. 1). In the Mistral region (leg FC), the AOD at 865 nm varied from over 0.2 near the coast (Fos/Berre region Title Page

and further east) to 0.17 around way-point F (Fig. 1). The aerosol distribution simulated Abstract Introduction 15 by CHIMERE using the recently developed methodology by Hodzic et al. (2004), can be compared to the aerosol optical depth retrieved from SeaWIFS in the cloud-free re- Conclusions References gions. Figure 8b shows the AOD at 865 nm simulated with CHIMERE over the domain Tables Figures documented by SeaWIFS. The location of the Marseille/Fos-Berre aerosol plume is reproduced by CHIMERE (shifted by about 20 km to the east) with an AOD of about 0.15, J I 20 as well as the aerosol plume from the industrial city of Toulon. The plume emerging

from the Aude valley channelling the Tramontane as well as the plume skirting the east- J I ern Spanish coast (Fig. 8a) are not visible in the SeaWIFS data because these regions are covered by clouds preventing from reliable retrieval of aerosol products (Fig. 8b, c). Back Close

Despite the global good agreement, the AOD simulated in some plumes can be half Full Screen / Esc 25 smaller than the observed AOD. In the afternoon 24 March 1998, the vertical structure of aerosols as well as the ABL Printer-friendly Version were documented by the lidar LEANDRE-2 along legs AF, FC and CE (Fig. 9a) as detailled by Flamant (2003). Lidar measurements on leg AF (Fig. 9b) show an internal Interactive Discussion

11928 EGU boundary layer developing from the coast (within the advected continental ABL) and reaching a depth of 1200 m. Close to way-point F, at approximately 42.6◦ N, the ma- ACPD rine ABL structure characteristics over the sea changed: it is observed to be shallower 6, 11913–11956, 2006 (700 m in Fig. 9b) and is characterized by larger values of atmospheric reflectivity. This 5 is confirmed by measurements made on leg FC (Fig. 9c), along which the marine ABL is characterized by values of atmospheric reflectivity similar to those observed near Aerosols distribution way-point F. Also, the marine ABL is observed to remain shallow, its depth gradually during a decreasing from 700 to 500 m with the distance to the coast. As discussed in Flamant Tramontane/Mistral (2003), the larger atmospheric reflectivity values in the eastern part of the sampled episode 10 region is thought to be related to larger concentrations of pollution aerosol from the city of Marseille and the industrial petrochemical complex of Fos/Berre. On leg CE T. Salameh et al. (Fig. 9d), close to the coast, the ABL structure is similar to that observed along FC (i.e., shallow). The depth of the ABL increases westward from 500 to 1200 m (between Title Page 43.25 and 43◦ N) with reflectivity values on the order of those observed on leg CE. 15 Further to the west, a continental ABL is observed in the sheltered region in the lee Abstract Introduction of the Massif Central, reflectivity values being significantly smaller than to the east of 43◦ N. In this region, lidar measurements evidence the presence of gravity waves in- Conclusions References

side the continental ABL and above, which are common features during Mistral events Tables Figures (Caccia et al., 2004; Drobinski et al., 2005). The vertical structure of the aerosol dis- 20 tribution in the atmosphere derived from LEANDRE-2 reflectivity is used to validate J I the CHIMERE simulations along the ARAT legs. To do that, we use the methodology by Hodzic et al. (2004) that involves simulating the lidar backscattering profiles from J I the model relative humidity and concentration outputs (optical properties varying with Back Close chemical composition and mass vertical distribution). Figures 9 and 10 show the sim- 25 ulated lidar reflectivity and relative humidity along legs AF, CE and CF. To obtain a Full Screen / Esc quantitative comparison between the measured and simulated reflectivities, the simulated lidar profiles are normalized by the measured values in the aerosol-free layer Printer-friendly Version (at about 3.8 km in the troposphere). Figure 10 shows a fairly horizontally homogeneous relative humidity field meaning that the lidar reflectivity can be directly related Interactive Discussion

11929 EGU to the aerosol concentration without any modulation of the aerosol size by hygroscopic effect. Despite some differences in the lidar reflectivity field, the simulation displays ACPD many similar features. The main difference is the thin free-aerosol layer (reflectivity 6, 11913–11956, 2006 values of about 50 a.u.) just above the ABL (reflectivity values larger than 150 a.u.) 5 which is not reproduced by CHIMERE. Otherwise, the second layer above the ABL (the advected continental ABL with reflectivity values of about 100–120 a.u.) is accu- Aerosols distribution rately simulated for all legs. The transition between the marine ABL and the advected during a continental ABL is thus too smooth in the simulation probably because of too large Tramontane/Mistral diffusivity which prevents the model to predict this aerosol-free layer between the ma- episode 10 rine ABL and the advected continental ABL. Along leg CF, the aerosol backscatter is homogeneous in the ABL both in the observations and the simulation since the aircraft T. Salameh et al. flies along the aerosol plume (Fig. 8), with a deepening of the ABL from east to west (the CHIMERE simulation, however, overpredicts by about 500 m the ABL depth west Title Page of leg CF). Along legs CE and AF, the aerosol backscatter in the ABL varies along the 15 legs, with larger reflectivity on the eastern part of the leg. On leg CE, the CHIMERE Abstract Introduction model produces large lidar reflectivity east of 3.9◦ E. The location of the aerosol plume is accurately reproduced with however a more diffuse plume edge between 3.8 and Conclusions References ◦ ◦ 4.3 E in the LEANDRE-2 observations and smaller simulated reflectivity east of 4.3 E. Tables Figures West of 3.8◦ E, the aerosol content decreases. Along leg AF, the model accurately ◦ ◦ 20 reproduces the heavily loaded aerosol plumes east of 4.2 E and west of 3.8 E. Be- J I tween 3.8 and 4.2◦ E, CHIMERE model simulates the incursion of a low aerosol streak within the ABL, but is not able to reproduce its fine-scale structure and the simulated J I reflectivity is larger than the measured reflectivity. Back Close To assess the origin air masses responsible for the aerosol variability in ABL in the 25 Gulf of Lion area, we computed back trajectories ending at various locations in the Full Screen / Esc aerosol plume documented by SeaWIFS (Fig. 11) and LEANDRE-2 (Figs. 12 and 13), using a lagrangian particle dispersion model (Menut et al., 2000). The trajectories use Printer-friendly Version the MM5 wind fields. During the convective period, air parcels are mixed within the ABL. Hence neither constant-altitude trajectories nor trajectories following the mean Interactive Discussion

11930 EGU three-dimensional wind would be realistic. In order to overcome this problem, air trajectories are assumed to undergo a random altitude change every hour during the ACPD convective period. They were nevertheless bounded to stay between 0 and 1000 m 6, 11913–11956, 2006 (tests with the upper boundary of 2000 and 3000 m have been conducted without any 5 impact). Fifty back trajectories are calculated in this way (due to the convective processes, an ending point does not correspond to the same origin, In order to take these Aerosols distribution various origins into account, 50 trajectories are studied). Figures 11a and b show that during a the air masses following similar trajectories and ending on the western edge of the Tramontane/Mistral aerosol plume at 11:00 UTC can be clustered in two groups: one cluster follows the episode 10 Aude valley channelling the Tramontane flow, the other follows the western boundary of the Mistral flow. There is no major urban and industrial aerosol sources along the T. Salameh et al. two trajectories, except maybe the cities of Toulouse and Montpellier (Figs. 1b and 7). Figures 11c and d show that the center and eastern regions of the aerosol plume originate from the Marseille/Fos-Berre area but also from further sources: To the north, Title Page 15 emissions from Lyon are transported long range by the Mistral (see also Corsmeier Abstract Introduction et al., 2005, for a summer Mistral) and to the east, a cross-Alpine flow carries the emissions from the heavily industrialized regions of the Po Valley in Italy. The re- Conclusions References

gions of Marseille/Fos-Berre, Lyon and Torino are major emission sources compared Tables Figures to Toulouse or Montpellier, thus explaining why the western region of the plume is 20 optically thinner than the middle and eastern regions of the plume. J I The back plumes ending along the ARAT legs at two different altitudes (50 m within the ABL, 2000 m within the advected aerosol-rich continental ABL above the marine J I ABL) also explain the differences in aerosol concentration observed by LEANDRE-2. At Back Close 50 m (and higher within the ABL, not shown), large-scale transport from Lyon is visible 25 on the four clusters of back trajectories. However, at ending points A and E, the air Full Screen / Esc masses do not flow over the Marseille/Fos-Berre area or other major cities, explaining the lower aerosol content observed by LEANDRE-2 within the ABL. On the contrary, Printer-friendly Version the air masses ending at points C and F are enriched in aerosols as they pass over the Marseille/Fos-Berre area explaining the higher reflectivity values in the ABL. Along Interactive Discussion

11931 EGU leg CF, the ARAT follows the Marseille/Fos-Berre plume explaining the homogeneous concentration of aerosols along the leg. ACPD At 2000 m, the aerosols of the layer just above the marine ABL (reflectivity values of 6, 11913–11956, 2006 about 100 a.u.) originate from remote sources (e.g. Lyon) which are then transported 5 over a long range (Corsemeier et al., 2005) (Fig. 13). One exception is at ending point C which is just above this aerosol-rich layer, in the aerosol-free layer (Fig. 9). Figure 13c Aerosols distribution shows that the air mass flows over the advected continental ABL above the western during a Alps and originate from the northern Alps. The absence of significant aerosol sources Tramontane/Mistral in this area explains the very low reflectivity values at 2000 m at ending point C. One episode 10 can finally notice the absence of trans-alpine transport between 16:00 and 17:00 UTC contrary to the situation at 11:00 UTC. This is the result of an evolution of the regime of T. Salameh et al. the flow impinging the Alpine ridge: at 11:00 UTC, part of the upstream air mass flows over the mountain whereas at 16:00 and 17:00 UTC, the air mass is mostly blocked Title Page by the Alps and flows around. In Fig. 2, it is illustrated by the larger extension of the 15 western Alpine wake in the afternoon. Abstract Introduction

4.3 Aerosol distribution over the western Mediterranean from the MM5-CHIMERE Conclusions References model Tables Figures Insight into the aerosol distribution, origin and composition over the western Mediter- ranean can be obtained from the CHIMERE aerosol products, in close relationship J I 20 with the mesoscale dynamical processes accessible from the validated MM5 meteoro- J I logical fields. In the following, the aerosol distribution, origin and composition will be discussed as a function of the position and intensity of the “eye” of the Genoa cyclone Back Close on 24 March. Full Screen / Esc During the morning period, the Genoa cyclone is located immediately to the east 25 of Corsica, the surface pressure low gradually decreasing from 1014 to 1010 hPa. Over the southwestern region of the Mediterranean basin, large aerosol loading can Printer-friendly Version be attributed to ship emissions (see Figs. 7 and 14a and b). South of Sardinia, the Interactive Discussion air circulating around the Genoa surface low brings ship emissions to southern Italy. 11932 EGU North of this band, most of the aerosol loading over the Mediterranean Sea is of local origin (inter-continental ship tracks and coastal urban and industrialized area such as ACPD Barcelona, Marseille/Fos-Berre, Toulon, see Figs. 7 and 14a and b), with one exception 6, 11913–11956, 2006 along the Aude valley where the Tramontane transports offshore continental aerosols. 5 The plume over Toulon stagnates in the western Alps wake trailing downstream across the Alps, an area of very weak wind. Finally, the close location of the Genoa cyclone Aerosols distribution from Corsica and the absence of strong winds during the morning period lead to the during a fact that the region north and northwest of Corsica is only influenced by marine clean Tramontane/Mistral air. episode 10 During the afternoon period, the Genoa cyclone intensifies (from 1010 to 1004 hPa) and moves southeastward, about 300 km east of Sardinia over the Tyrrhenian Sea. T. Salameh et al. The Tramontane has nearly vanished so that the polluted air of the morning stagnates in the Aude valley. Conversely, the Mistral has intensified and most of the aerosols Title Page exported offshore originates from central France and the Marseille/Fos-Berre plumes. 15 Moreover, the Ligurian outflow brings in the aerosols emitted from the major industrial- Abstract Introduction ized Italian cities (Milano and Torino) over the western part of the Mediterranean basin. This is consistent with aerosol optical depth values of 0.1 and 0.3 at 870 and 440 nm, Conclusions References

respectively, measured at Ispra (Italy) with the photometer of the AERONET network Tables Figures and with spectral distribution revealing high accumulation mode aerosol concentration. 20 The CHIMERE AOD at 865 nm at Ispra is about 0.09. A narrow band of clean air sepa- J I rates the two major plumes extending more than 300 km over the sea. This cleaner air corresponds to the western Alps wake and, associated with large horizontal shear of J I wind, acts as a dynamical barrier, preventing any lateral exchanges between the Mis- Back Close tral and the Ligurian outflow. The southeastward displacement and intensification of 25 the Genoa cyclone increases the meridian extension of the Mistral that sweeps away Full Screen / Esc the band of aerosols emitted from the ships that skirt the North African coast in the morning (Figs. 14c and d). Printer-friendly Version Despite the difficulty in validating the simulated composition of the aerosol loading over the basin, measurements of nitrates available in Netherlands and Italy clearly Interactive Discussion

11933 EGU show the development of an ammonium nitrate episode over the western Europe from 22 to 31 March (see measured and simulated values in Table 1, NL09, IT04). These ACPD episodes are often observed in the late winter during such meteorological conditions 6, 11913–11956, 2006 with high pressure over Great Britain and central Europe (Bessagnet et al., 2005). The 5 large nitrogen oxides emissions in the Netherlands, Belgium, Germany and northern France produce by oxidation a large amount of nitric acid which is then neutralized by Aerosols distribution ammonia emitted in the same regions and form ammonium nitrate in the particulate during a phase. Moreover, the coarse simulation displays the formation of a sulfate bubble in Tramontane/Mistral Great Britain on 22 March going to southern France on 23 and 24 March (not shown). episode 10 In the same time, the sulfate concentration increases when crossing western France (Normandy) due to high sulfur oxide emissions. This behavior is supported by sulfate T. Salameh et al. observations in three EMEP background sites in Great Britain and France (see Table 1, GB02, GB14 and FR03). During the development of this episode, in particular on 24 Title Page March, according to the model results, the Genoa cyclone drives the pollution plume 15 towards southern and western France. Afterwards, the ammonium nitrate is advected Abstract Introduction through the Aude and Rhoneˆ valleys to the Mediterranean basin. A similar ammonium nitrate plume is also transported from the Po valley (Fig. 14). The formation of ammo- Conclusions References

nium nitrate is favored by low temperatures in southern France which reach a maximum Tables Figures of about 10◦C near the surface on 24 March at 12:00 UTC and avoid the evaporation 20 of ammonium nitrate. J I

J I 5 Conclusions Back Close Finally, this article presents a ﬁrst attempt to characterize the aerosol distribution over the western Mediterranean basin during a Mistral event from the combination of inno- Full Screen / Esc vative instrumentation (passive and active remote sensors) and the aerosol products 25 of the ﬁne-scale chemistry transport model CHIMERE forced with the non-hydrostatic Printer-friendly Version mesoscale model MM5. This issue is motivated by (1) the potential impact of aerosols Interactive Discussion on the Mediterranean radiate budget, the water cycle (through the modulation of precip-

11934 EGU itations) and the marine ecosystems; and (2) the very frequent occurrence of Mistral- type weather regimes (5 to 15 days per month all year long). After validating the dy- ACPD namical and chemical simulations, this paper shows that: 6, 11913–11956, 2006 1. the Mistral contributes to export far oﬀshore a large amount of aerosols emitted 5 over continental Europe (in particular ammonium nitrate in the particulate phase and sulfates) and along the shore from the industrialized and urban areas of Fos- Aerosols distribution Berre/Marseille, as well as from the Po valley. The Genoa surface low contributes during a to advect the aerosols along a cyclonic trajectory which skirts the North African Tramontane/Mistral coast and reach Italy. episode

10 2. due to the time evolution of the Genoa cyclone position, the aerosol concentration T. Salameh et al. pattern is very unsteady, as the Tramontane prevails in the morning hours of 24 March, leaving place to the Mistral and an unusually strong Ligurian outﬂow in the afternoon. The wakes trailing downstream the Massif Central and the Alps prevent Title Page any horizontal diﬀusion of the aerosols and can, in some occasions, contribute to Abstract Introduction 15 aerosol stagnation. Future work will be dedicated to assess the representativity of the aerosol distribution Conclusions References during the very numerous Mistral events. Tables Figures Acknowledgements. This work was conducted at Service d’Aeronomie´ and Laboratoire de Met´ eorologie´ Dynamique of the Institut Pierre Simon Laplace. The authors would like to thank J I 20 T. Elias, G. Bergametti, S. Bastin and C. Moulin for fruitful discussion; J. L. Monge for technical support; M. C. Lanceau for help in collecting the referenced papers; and D. Hauser for J I the coordination of the experiment. This research has received funding from the Agence de l’Environnement et de la Maˆıtrise de l’Energie´ (ADEME) and support from Met´ eo-France´ that Back Close provided the data from the operational radiosoundings and meteorological surface stations. Full Screen / Esc

25 References Printer-friendly Version

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11936 EGU pollution transport and vertical mixing, Atmos. Res., 74, 275–302, 2005. Drennan, W., Graber, H. C., Hauser, D., and Quentin, C.: On the wave age dependence of ACPD wind stress over pure wind seas, J. Geophys. Res., 108, 8062, doi:10.1029/2000JC000715, 2003. 6, 11913–11956, 2006 5 Drobinski, P., Sa¨ıd, F., Ancellet, G., Arteta, J., Augustin, P., Bastin, S., Brut, A., Caccia, J. L., Campistron, B., Cautenet, S., Colette, A., Coll, I., Cros, B., Corsmeier, U., Dabas, A., Delbarre, H., Dufour, A., Durand, P., Guenard,´ V., Hasel, M., Kalthoff, N., Kottmeier, C., Aerosols distribution Lemonsu, A., Lasri, F., Lohou, F., Masson, V., Menut, L., Moppert, C., Peuch, V. H., Puygre- during a nier, V., Reitebuch, O., and Vautard, R.: Regional transport and dilution during high pollution Tramontane/Mistral 10 episodes in southern France: Summary of findings from the ESCOMPTE experiment, J. episode Geophys. Res., in press, 2006. Drobinski, P., Bastin, S., Guenard,´ V., Caccia, J. L., Dabas, A. M., Delville, P., Protat, A., T. Salameh et al. Reitebuch, O., and Werner, C.: Summer Mistral at the exit of the Rhoneˆ valley, Quart. J. Roy. Meteorol. Soc., 131, 353–375, 2005. 15 Drobinski, P., Flamant, C., Dusek, J., Flamant, P. H., and Pelon, J.: Observational evidence Title Page and modeling of an internal hydraulic jump at the atmospheric boundary-layer top during a Tramontane event, Bound.-Layer Meteorol., 98, 497–515, 2001a. Abstract Introduction Drobinski, P.,Dusek, J., and Flamant, C.: Diagnostics of hydraulic jump and gap flow in stratified flows over topography, Bound.-Layer Meteorol., 98, 475–495, 2001b. Conclusions References 20 Dudhia, J. A.: Nonhydrostatic version of the Penn State-NCAR mesoscale model: validation tests and simulation of an Atlantic cyclone and cold front, Mon. Wea. Rev., 121, 1493–1513, Tables Figures 1993. Egger, J.: Numerical experiments on the cyclogenesis in the Gulf of Genoa, Contrib. Atmos. J I Phys., 45, 320–346, 1972. 25 Eymard, L., Weill, A., Bourras, D., Guerin, C., Le Borgne, P., and Lefevre, J. M.: Use of ship J I mean data for validating model and satellite flux fields during the FETCH experiment, J. Geophys. Res., 108, 8060, doi:10.1029/2001JC001207, 2003. Back Close Flamant, C.: Alpine lee cyclogenesis influence on air-sea heat exchanges and marine atmo- Full Screen / Esc spheric boundary layer thermodynamics over the western Mediterranean during a Tramon- 30 tane/Mistral event, J. Geophys. Res., 108, 8057, doi:10.1029/2001JC001040, 2003. Flamant, C., Pelon, J., Hauser, D., Quentin, C., Drennan, W. M., Gohin, F., Chapron, B., Printer-friendly Version and Gourrion, J.: Analysis of surface wind and roughness length evolution with fetch using a combination of airborne lidar and radar measurements, J. Geophys. Res., 108, 8058, Interactive Discussion

11937 EGU doi:10.1029/2002JC001405, 2003. Flamant, C. and Pelon, J.: Atmospheric boundary-layer structure over the Mediterranean during ACPD a Tramontane event, Quart. J. Roy. Meteorol. Soc., 122, 1741–1778, 1996. Georgelin, M. and Richard, E.: Numerical simulation of flow diversion around the Pyrenees:´ a 6, 11913–11956, 2006 5 Tramontana case study, Mon. Wea. Rev., 124, 687–700, 1996. Grell, G. A.: Prognostic evaluation of assumptions used by cumulus parameterizations, Mon. Wea. Rev., 121, 764–787, 1993. Aerosols distribution Grell, G. A., Dudhia, J., and Stauffer, D. R.: A description of the fifth-generation Penn during a State/NCAR mesoscale model (MM5), NCAR Technical Note, NCAR/TN-398+STR, 122 pp, Tramontane/Mistral 10 1995. episode Grotjahn, R. and Wang, C. H.: On the source of air modified by surface fluxes to enhance frontal cyclone development, Ocean-Air Interact., 1, 257–288, 1989. T. Salameh et al. Guenard,´ V., Drobinski, P., Caccia, J. L., Tedeschi,´ G., and Currier, P.: Dynamics of the MAP- IOP 15 severe Mistral event: observations and high-resolution numerical simulations, Quart. 15 J. Roy. Meteorol. Soc., 132, 757–778, 2006. Title Page Guenard,´ V., Drobinski, P., Caccia, J. L., Campistron, B., and Benech,´ B.: An observational study of the mesoscale Mistral dynamics, Bound.-Layer Meteorol., 115, 263–288, 2005. Abstract Introduction Guerzoni, S., Chester, R., Dulac, F., Herut, B., Loye-Pilot,¨ M. D., Measures, C., Mignon, C., Molinaroli, E., Moulin, C., Rossini, P., Saydam, C., Soudine, A., and Ziveri, P.: The role Conclusions References 20 of atmospheric deposition in the biogeochemistry of the Mediterranean Sea, Progress in Oceanography, 44, 147–190, 1999. Tables Figures Guieu, C., Bozec, Y., Blain, S., Ridame, C., Sarthou, G., and Leblond, N.: Impact of high Saharan dust inputs on dissolved iron concentrations in the Mediterranean Sea, Geophys. J I Res. Let., 29, 1911, doi:10.1029/2001GL014454, 2002. 25 Guo, Y. R. and Chen, S.: Terrain and land use for the fifth-generation Penn State/NCAR J I mesoscale modeling system (MM5), NCAR Technical Note, NCAR/TN-397+IA, 114 pp, 1994. Back Close Hauser, D., Branger, H., Bou es-Cloche,´ S., Despiau, S., Drennan, W. M., Dupuis, H., Durand, ffi Full Screen / Esc P., Durrieu de Madron, X., Estournel, C., Eymard, L., Flamant, C., Graber, H. C., Guerin,´ C., 30 Kahma, K., Lachaud, G., Lefevre,` M. P., Pelon, J., Pettersson, H., Piguet, B., Queffeulou, P., Tailliez, D., Tournadre, J., and Weill, A.: The FETCH experiment: an overview, J. Geophys. Printer-friendly Version Res., 108, 8053, doi:10.1029/2001JC001202, 2003. Interactive Discussion Hong, S. Y. and Pan, H. L.: Nonlocal boundary layer vertical diffusion in a medium-range

11938 EGU forecast model, Mon. Wea. Rev., 124, 2322–2339, 1996. Hodzic, A., Chepfer, H., Vautard, R., Chazette, P., Beekmann, M., Bessagnet, B., Chatenet, B., ACPD Cuesta, J., Drobinski, P., Goloub, P., Haeﬀelin, M., and Morille, Y.: Comparison of aerosol chemistry-transport model simulations with lidar and sun-photometer observations at a site 6, 11913–11956, 2006 5 near Paris, J. Geophys. Res., 109, D23201, doi:10.1029/2004JD004735, 2004. Jansa,´ A.: Distribution of the mistral: A satellite observation, Meteorol. Atmos. Phys., 36, 201– 214, 1987. Aerosols distribution Lelieveld, J., Berresheim, H., Borrmann, S., Crutzen, P. J., Dentener, F. J., Fischer, H., Fe- during a ichter, J., Flatau, P. J., Heland, J., Holzinger, R., Korrmann, R., Lawrence, M. G., Levin, Z., Tramontane/Mistral 10 Markowicz, K. M., Mihalopoulos, N., Minikin, A., Ramanathan, V., de Reus, M., Roelofs, G. episode J., Scheeren, H. A., Sciare, J., Schlager, H., Schultz, M., Siegmund, P., Steil, B., Stephanou, E. G., Stier, P., Traub, M., Warneke, C., Williams, J., and Ziereis, H.: Global air pollution T. Salameh et al. crossroads over the Mediterranean, Science, 298, 794–799, 2002. Marmer, E. and Langmann, B.: Impact of ship emissions on the Mediterranean summertime 15 pollution and climate: A regional model study, Atmos. Environ., 39, 4659–4669, 2005. Title Page Mayenc¸on, R.: Met´ eorologie´ pratique, Editions Maritimes D’outre-mer, 1982. Menut, L., Vautard, R., Flamant, C., Abonnel, C., Beekmann, M., Chazette, P., Flamant, P. H., Abstract Introduction Gombert, D., Guedalia,´ D., Kley, D., Lefebvre, M. P.,Lossec, B., Martin, D., Megie,´ G., Perros, P.,Sicard, M., and Toupance, G.: Measurements and modelling of atmospheric pollution over Conclusions References 20 the Paris area: an overview of the ESQUIF project, Ann. Geophys., 18, 1467–1481, 2000. Migon, C., Copin-Montegut,´ G., Elegant,´ L., and Morelli, J.: Etude de l’apport atmospherique´ en Tables Figures sels nutritifs au milieu cotierˆ mediterran´ een´ et implications biogeochimiques,´ Oceanologica Acta, 12, 187–191, 1989. J I Millot, C.: Wind induced upwellings in the Gulf of Lions, Oceanol. Acta, 2, 261–274, 1979. 25 Mlawer, E. J., Taubman, S. J., Brown, P. D., Iacono, M. J., and Clough, S. A.: Radiative transfer J I for inhomogeneous atmosphere: RRTM, a validated correlated-k model for the longwave, J. Geophys. Res., 102(D14), 16 663–16 682, 1997. Back Close Moulin, C., Lambert, C. E., Dayan, U., Masson, V., Ramonet, M., Bousquet, P., Legrand, M., Full Screen / Esc Balkanski, Y. J., Guelle, W., Marticorena, B., Bergametti, G., and Dulac, F.: Satellite climatol- 30 ogy of African dust transport in Mediterranean atmosphere, J. Geophys. Res., 103, 137–144, 1998. Printer-friendly Version Moulin, C., Lambert, C. E., Dulac, F., and Dayan, U.: Control of atmospheric export of dust from North Africa by the North Atlantic Oscillation, Nature, 387, 691–694, 1997. Interactive Discussion

11939 EGU Orieux, A. and Pouget, E.: Le Mistral: Contribution a` l’etude´ de ses aspects synoptiques et regionaux,´ Direction de la Met´ eorologie,´ Monographie, 5, 1984. ACPD Pettre,´ P.: On the problem of violent valley wind, J. Atmos. Sci., 39, 542–554, 1982. Prospero, J. M., Barrett, K., Church, T., Dentener, F., Duce, R. A., Galloway, J. N., Levy II, H., 6, 11913–11956, 2006 5 Moody, J., and Quinn, P.: Atmospheric deposition of nutrients to the North Atlantic Basin, Biogeochemistry, 35, 27–73, 1996. Reisner, J., Rasmussen, R. J., and Bruintjes, R. T.: Explicit forecasting of supercooled liquid Aerosols distribution water in winter storms using the MM5 mesoscale model, Quart. J. Roy. Meteorol. Soc., 124, during a 1071–1107, 1998. Tramontane/Mistral 10 Schar,¨ C. and Smith, R. B.: Shallow-water ﬂow past isolated topography. Part I: Vorticity pro- episode duction and wake formation, J. Atmos. Sci., 50, 1373–1400, 1993. Schmidt, H., Derognat, C., Vautard, R., and Beekmann, M.: A comparison of simulated and T. Salameh et al. observed ozone mixing ratios for the summer of 1998 in Western Europe, Atmos. Environ., 35, 6277–6297, 2001. 15 Schneider, J., Borrmann, S., Wollny, A. G., Blasner,¨ M., Mihalopoulos, N., Oikonomou, K., Title Page Sciare, J., Teller, A., Levin, Z., and Worsnop, D. R.: Online mass spectrometric aerosol measurements during the MINOS campaign (Crete, August 2001), Atmos. Chem. Phys., 4, Abstract Introduction 65–80, 2004. Sciare, J., Cachier, H., Oikonomou, K., Ausset, P., Sarda-Esteve,` R., and Mihalopoulos, N.: Conclusions References 20 Characterization of carbonaceous aerosols during the MINOS campaign in Crete, July- August 2001: a multi-analytical approach, Atmos. Chem. Phys., 3, 1743–1757, 2003. Tables Figures Traub, M., Fischer, H., de Reus, M., Kormann, R., Heland, J., Ziereis, H., Schlager, H., Holzinger, R., Williams, J., Warneke, C., de Gouw, J., and Lelieveld, J.: Chemical char- J I acteristics assigned to trajectory clusters during the MINOS campaign, Atmos. Chem. Phys., 25 3, 459–468, 2003. J I Vautard, R., Martin, D., Beekmann, M., Drobinski, P., Friedrich, R., Jaubertie, A., Kley, D., Lattuati, M., Moral, P., Neininger, B., and Theloke, J.: Paris emission inventory diagnostics Back Close from the ESQUIF airborne measurements and a chemistry-transport model, J. Geophys. Full Screen / Esc Res., 108, 8564, doi:10.1029/2002JD002797, 2003. 30 Vautard, R., Beekmann, M., Roux, J., and Gombert, D.: Validation of a hybrid forecasting system for the ozone concentrations over the Paris area, Atmos. Environ., 35, 2449–2461, Printer-friendly Version 2001. Interactive Discussion Vestreng, V.: EMEP/MSC-W Technical report. Review and Revision. Emission data reported to

11940 EGU CLRTAP, MSC-W Status Report 2003, EMEP/MSC-W Note 1/2003, ISSN 0804-2446, 2003. Wrathall, J. E.: The Mistral and forest ﬁres in Provence-Coteˆ d’Azur-South France, Weather, ACPD 40, 119–124, 1985. 6, 11913–11956, 2006

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11941 EGU Table 1. Nitrate concentrations measured/simulated in the Netherlands (NL09) and Italy (IT03) ACPD and sulfate concentrations measured/simulated in Great Britain (GB02 and GB14) and France (FR03) between 22 and 31 March. The stations are from the EMEP measurement network and 6, 11913–11956, 2006 the concentrations are daily averaged. The simulated values are given between brackets. Aerosols distribution Nitrates (µg m−3) Date NL09 IT04 (6.28◦ E; 53.33◦ N) (8.62◦ E; 45.82◦ N) during a Tramontane/Mistral 22 March 1998 1.28 (0.79) 4.43 (7.11) 23 March 1998 3.23 (1.96) 5.00 (9.35) episode 24 March 1998 5.49 (4.41) 4.21 (3.12) T. Salameh et al. 25 March 1998 7.71 (13.66) 9.83 (6.24) 26 March 1998 7.26 (8.22) 17.80 (22.66) 27 March 1998 8.99 24.98 28 March 1998 8.99 27.90 Title Page 29 March 1998 11.69 21.17 30 March 1998 9.48 23.25 Abstract Introduction 31 March 1998 10.23 18.11 Conclusions References Sulfates (µg m−3) Date GB02 GB14 FR03 ◦ ◦ ◦ ◦ ◦ ◦ (−3.20 E; 55.32 N) (−0.80 E; 54.33 N) (1.38 E; 46.13 N) Tables Figures 22 March 1998 8.34 (3.62) 4.95 (3.16) 0.30 (1.59) 23 March 1998 8.97 (8.58) 5.82 (4.61) 5.61 (2.36) J I 24 March 1998 1.05 (1.33) 6.30 (3.57) 13.20 (3.18)? 25 March 1998 1.14 (0.80) 2.76 (2.02) 1.83 (2.55) J I 26 March 1998 0.81 (0.63) 1.62 (1.04) 3.00 (2.01) 27 March 1998 2.61 3.30 1.83 Back Close 28 March 1998 5.85 6.60 4.71 29 March 1998 5.52 6.84 2.40 Full Screen / Esc 30 March 1998 1.20 3.00 5.07 31 March 1998 0.48 1.14 3.69 Printer-friendly Version ?: the simulated sulfate plume with concentrations of about 8–9 µg m−3 is about 50 km to the west Interactive Discussion of the measurement station, thus explaining the large diﬀerence.

11942 EGU T. Salameh et al.: Aerosols distribution during a Tramontane/Mistral episode 3

LYO (b) MIL 45 VAL TOR ACPD GEN Rhône valley 44 NIM 6, 11913–11956, 2006 Aude valley Gulf TLN 52 43 of Lion (a) 42 Corsica BAR Aerosols distribution

Latitude (°N) 41 48 FRANCE Sardinia during a 40 Balearic Atlantic Islands Tramontane/Mistral Ocean Massif 39 Central ITALY Alps Appenine range 44 1 2 3 4 5 6 7 8 9 10 episode Ligurian Longitude (°E) Pyrénées Sea 44.2 (c) Latitude (°N) NIM T. Salameh et al. Mediterranean Sea Tyrrhenian 43.8 MON 40 Sea A AIX 43.4 FOS C MRS 43 Title Page 36 42.6 E −5 0 5 10 15 (°N) Latitude Longitude (°E) 42.2 F Abstract Introduction 41.8 3.4 3.9 4.4 4.9 5.4 Longitude (°E) Conclusions References

Fig. 1. Panel a: Map of France with the topography shaded in grey when higher than 500 m above Tables Figures Fig. 1. Panel (a):sea Maplevel. ofThe Francetwo rectangles within dashed theline topographydisplay the large and shadedmedium domains in grey(hereafter whencalled higher than 500 m domain 1 and domain 2, respectively) of the MM5 simulations. Panel b: Domain 2 of the MM5 above sea level.simulation The twowith its rectanglesnested smaller domain in dashed(hereafter called linedomain display3) in the therectangle largein dashed andline. medium domains (hereafter called domainThe acronyms 1BAR, andGEN, domainMIL, NIM, 2,LYO, respectively)TLN, TLS, TOR and ofVAL thestand MM5for the city simulations.names Panel (b): J I Barcelona, Genoa, Milano, Nîmes, Lyon, Toulon, Toulouse, Torino and Valence, respectively. The Domain 2 of the MM5dots indicate simulationthe locations withof the itsoperational nestedmeteorological smallersurf domainace stations. (hereafterPanel c: Zoom on called domain 3) in the rectangle in dashedthe Gulf du line.Lyon area. TheThe acronymsrectangle in dashed BAR,line displays GEN,the small MIL,domain NIM,(domain LYO,3) of TLN,the TLS, TOR and J I MM5 simulations. The dotted line corresponds to the flight track of the ARAT carrying the water ˆ VAL stand for the cityvapor lidar namesLEANDRE-2 Barcelona,on March 24, Genoa,1998 in the afternoon Milano,and Ntheımes,thick solid Lyon,line to the Toulon,route of Toulouse, Torino Back Close and Valence, respectively.R/V ATALANTE. TheThe diamond dotsmark indicateer shows the thelocation locationsof the ASIS ofbuoy the. The acron operationalyms AIX, meteorological FOS, MON, MRS and NIM correspond to the city names Aix en Provence, Fos/Berre, Montpellier, surface stations.Marseille Paneland(c)Nˆ:ımes, Zoomrespectiv onely. the Gulf du Lyon area. The rectangle in dashed line Full Screen / Esc displays the small domain (domain 3) of the MM5 simulations. The dotted line corresponds to the flight track of thetral ARATcase is a carryingstrong episode, thetypical waterof vaporfrom lidarthe city LEANDRE-2of Marseille and the onindus- 24 March 1998 in the Printer-friendly Version afternoon and theintermediate thick solidseason lineMistral to theevents routecom- oftrial R/Vpetrochemical ATALANTE.comple Thex of Fos/Berre, diamond marker shows pared to weaker summer Mistral events the highly spatially resolved aerosol mea- the location of the(e.g. ASISDrobinski buoy.et Theal. acronyms2005, Bastin et AIX,surements FOS, MON,needed to MRS(un)validate andthis NIMhy- correspond to the Interactive Discussion city names Aix enal. Provence,2006). Flamant Fos/Berre,(2003) analyzed Montpellier,the pothesis Marseillesimply did not andexist. NˆıBymes,combin- respectively. complex structure of atmospheric bound- ing lidar observations and simulations we ary layer (ABL) observed using an air11943- better address this question in the present borne lidar over the Gulf of Lion at the article. Therefore this article is designed to EGU exit of the Rhoneˆ valley during the March, analyze: 24 1998 Mistral event. Even though the study by Flamant (2003) suggested the ex- – the dynamic processes driving the istence of a marked east-west aerosol con- small-scale structure of the Mistral centration gradient offshore to be related to flow at the exit of the Rhoneˆ valley, larger concentrations of pollution aerosol and their relation with the aerosol dis-

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Fig. 2. Horizontal wind ﬁelds simulated with MM5 (at 950 hPa) and extracted from domain 2 on March 24, 1998 at 0600 (a), 0900 (b), 1200 (c), 1500 (d), 1800 (e) and 2100 UTC (f), respectively. Fig. 2. Horizontal wind ﬁelds simulated with MM5 (at− 9501 hPa) and−1 extracted from domain 2 The isocontours represent the isotachs. Contour interval is 2 m s from 10 to 20 m s . The black Printer-friendly Version on 24 March 1998 atsolid 06:00line represent(a)the, 09:00coastline. (b), 12:00 (c), 15:00 (d), 18:00 (e) and 21:00 UTC (f), −1 respectively. The isocontours represent the isotachs. Contour interval is 2 m s from 10 to Interactive Discussion −1 tion of the meteorological surface stations simulated and measured 10-m zonal and 20 m s . The black solidin southern lineFrance represent(see dots in theFig. 1b). coastline.meridional wind components and 2-m tem- Quantitatively, Fig. 4 shows the histograms11944perature during the 24 hours on March 24, of the difference between the three-hourly 1998. The measured 10-m wind speed and EGU

Atmos. Chem. Phys., 0000, 0001–25, 2006 www.atmos-chem-phys.org/acp/0000/0001/ ACPD T. Salameh et al.: Aerosols distribution during a Tramontane/Mistral episode 9 6, 11913–11956, 2006 5 5 5 (a) (b) (c) 4 4 4 3 3 3 Aerosols distribution 2 MM5 0000 UTC 2 2 Obs. 0000 UTC 1 MM5 1200 UTC 1 1 during a Obs. 1200 UTC Height ASL (km) 0 0 0 Tramontane/Mistral 3 13 23 33 43 180 270 0 90 180 274 284 294 304 314 5 5 5 episode (d) (e) (f) 4 4 4 3 3 3 T. Salameh et al. 2 2 2 1 1 1

Height ASL (km) 0 0 0 3 13 23 33 43 180 270 0 90 180 274 284 294 304 314 Title Page 5 5 5 (g) (h) (i) 4 4 4 Abstract Introduction 3 3 3 2 2 2 Conclusions References 1 1 1

Height ASL (km) 0 0 0 3 13 23 33 43 180 270 0 90 180 274 284 294 304 314 Tables Figures Wind speed (m s−1) Wind direction (o) Potential temperature (oK)

Fig. 3. Vertical profiles of wind speed, wind direction and potential temperature (left, middle and right J I Fig. 3. Verticalcolumns, profilesrespecti of windvely) o speed,ver Lyon, N windˆımes and directionon the location andof potentialthe R/V ATALANTE temperature(top, middle (left, middle and right columns,and respectively)bottom rows, respecti overvely). Lyon,The Nmeasuredˆımes andsimulated on theprofiles locationare displayed of thewith R/Vdashed ATALANTE (top, J I middle and bottomand solid rows,lines, respecti respectively).vely. On the first Theand the measuredsecond rows, thin andlines simulatedrepresent measurements profilesat are displayed 0000 UTC and bold lines represent measurements at 1200 UTC. On the third row, thin lines are for with dashed andmeasurements solid lines,at 0900 respectively.UTC and bold lines Onare thefor measurements first and theat 1100 secondUTC. The rows,location thinof the lines represent Back Close measurementsR/V atA 00:00TALANTE UTCwas 42.75 and◦N/4.37 bold◦E linesat 0900 UTC representand 42.95◦ measurementsN/4.27◦E at 1100 UTC. at 12:00 UTC. On the third row, thin lines are for measurements at 09:00 UTC and bold lines are for measurements Full Screen / Esc −1 ◦ ◦ at 11:00 UTC.2-m Thetemperature locationaccuracies of theare R/V1 m ATALANTEs surface wasand reliable 42.75feN/4.37w hundredsEof atkilo- 09:00 UTC and ◦ ◦ ◦ 42.95 N/4.27 andE atabout 11:000.1 UTC.K, respectively. The bias meters away from the shore. Figure 5 Printer-friendly Version between MM5 and the measured wind shows the daily mean values of the sea sur- speed is about -1 m s−1 for the two hori- face wind speed provided by the satellite- zontal components which is thus non sig- borne AMI-Wind/ERS scatterometer over Interactive Discussion nificant. The standard deviation is also the Mediterranean Sea at 1◦ resolution (no small considering the accuracy of the wind11945wind vectors are available for year 1998). observations. As for the temperature, the It shows three regions of sea surface wind EGU largest discrepancies are found in the steep speed exceeding 20 m s−1: to the west, orography regions where the height of the the Tramontane merge with the Spanish topography is not accurately represented Cierzo; at around 7◦E longitude, the Mis- in the model and at night when numerical tral blows around the Genoa cyclone and diffusion slightly deteriorates the simula- immediately to the east of Sicily and Sar- tion of the near surface temperature and the dinia, the Ligurian outflow. The MM5 katabatic flows. models reproduces the same structure but with lower wind speed which can reach −1 3.3 Structure of the Mistral over the 5 m s in the regions of strong flows. Mediterranean Sea From an observational point of view, this can be attributed to the fact that the largest AMI-Wind/ERS errors are found near the Over the Mediterranean Sea, the data shore, whereas from a modelling point of are generally sparse, and available from view, the absence of coupling between the satellite-borne sensors thus restricted to the

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300 300 ACPD Bias = −1.0387 Bias = 0.0997 6, 11913–11956, 2006 Std = 3.1059 Std = 3.5512 200 200 Aerosols distribution during a # data # data 100 100 Tramontane/Mistral episode

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200 Tables Figures # data 100 J I

0 J I −20 −15 −10 −5 0 5 10 15 20 Tmodel−Tsurface (°K) Back Close 2 m 2 m Full Screen / Esc FigFig.. 4. 4.HistogramsHistogramsof ofthe thedifference diﬀerencebetween betweenthe three-hourly the three-hourlysimulated simulatedand measured and measured10-m zonal 10-m(a) andzonalmeridional(a) and meridional(b) wind components(b) wind componentsand 2-m temperature and 2-m(c) temperatureduring the 24(c)hoursduringperiod theof 24March h period24, Printer-friendly Version 1998of 24in Marchsouthern 1998France in southern(see dots Francein Fig. (see1b). The dotsmeasured in Fig. 1b).10-m Thewind measuredspeed and 10-m2-m windtemperature speed −1 ◦ accuraciesand 2-m temperatureare 1 m s accuraciesand about 0.1 areK, 1 mrespecti s−1 andvely about. 0.1◦ K, respectively. Interactive Discussion

11946 EGU sea and the atmosphere for winter Mistral longitude) and (42.95◦N;4.27◦E), respec- events which generate at this period of the tively show a northerly wind flow from the year intense air-sea heat exchanges (Fla- surface up to 5 km and more (Fig. 3g, h mant 2003) and sea surface cooling (Mil- and i). The vertical profiles of potential lot 1979) can also be a source of error. temperature at 0900 and 1100 UTC show The ASIS buoy data allows a more detailed a remarkable persistence of the thermo- comparison at a single point. Figure 6 dynamical characteristics of the Mistral shows the good agreement between the with a potential temperature inversion at surface wind speed and direction, temper- about 2 km height which corresponds to ature, relative humidity and surface stress the ABL depth. At 0900 UTC, the wind from the ASIS buoy measurements and blows at approximately 15 m s−1 within from the MM5 simulations (better than the ABL and increases up to 28 m s−1 2 m s−1, 10◦, 1◦C, 20 % and 0.1 N m−2 above (Fig. 3g). The wind speed is for wind speed and direction, temperature, stronger at 1100 UTC than at 0900 UTC relative humidity and wind stress, respec- (20-22 m s−1 below 3 km) since the re- tively, with some intermittent exceptions). search vessel Atalante moves closer to the This figure illustrates the unsteady aspect Mistral core. The MM5 model reproduces of the Mistral/Tramontane episode with the vertical profiles of wind speed and the Tramontane blowing over the buoy direction, and potential temperature with between 0400 and 0800 UTC, followed at most 2 m s−1 (0900 UTC) to 5 m s−1 by a calm period until the Mistral breaks (1100 UTC) difference and less than 30◦ through after 1200 UTC. and 2 K on average for the wind speed The radiosoundings launched at and direction and potential temperature, 0900 UTC and 1100 UTC from the re- respectively. The deepening of the ABL search vessel Atalante at (42.75◦N;4.37◦E between Nˆımes and the Atalante can be

Atmos. Chem. Phys., 0000, 0001–25, 2006 www.atmos-chem-phys.org/acp/0000/0001/ ACPD T. Salameh et al.: Aerosols distribution during a Tramontane/Mistral episode 11 6, 11913–11956, 2006 NSCAT MM5 20 47 (a) 47 (b) )

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Fig. 5. (a), (b) and (c) are the daily mean values of the wind speeds measured on March 24, 1998 Back Close Fig. 5. (a), (b)byandthe AMI-W(c) areind/ERS thesatellite dailyov meaner sea surf valuesace, simulated of theby MM5 wind(domain speeds3) and measuredthe difference on 24 March 1998 by the AMI-Wind/ERSbetween the measured satelliteand simulated overwind seaspeeds, surface,respectively simulated. (a) and (b) are bywith MM5the same (domaincolor 3) and the difference betweenscale. the measured and simulated wind speeds, respectively. (a) and (b) are with Full Screen / Esc the same color scale. attributed to the occurrence of a hydraulic portant traffic, such as Barcelona, Mar- Printer-friendly Version jump associated with enhanced turbulence seille, Lyon, Milano and Rome. Rural and mixing (e.g. Drobinski et al. 2001b) areas prone to agriculture or covered by Interactive Discussion as previously observed at the exit of the Mediterranean natural landscape are the Rhoneˆ valley for many other Mistral main sources of biogenic emissions. Fi- events by Pettre´ (1982), Corsmeier et al.11947nally emissions related to ship traffic also (2005) and Drobinski et al. (2005). impact Mediterranean pollution levels and EGU climate (Marmer and Langmann 2005).

4 Aerosol distribution at the scale of Some anthropogenic species such as ni- the western Mediterranean basin trogen oxides (Fig. 7 a) are mainly emitted via traffic, industries and domestic com- 4.1 Sources of atmospheric pollutants bustion. For instance, over the continent, maxima of NO emissions are seen in the The western Mediterranean basin is main cities Toulouse, Lyon, Marseille as characterized by a meridional gradient of well as Milano. Over the Sea, evidence surface land types, mainly associated with of emissions associated with maritime traf- vegetation and aerosol sources. South of fic is also seen along the northern African the basin, the desert is a primary source coastline (i.e. ship routes between Gibral- of mineral dust. West and north of the tar and the Suez canal), as well as along the basin, some aerosol sources are large in- Spanish coastline between Marseille and dustrial zones (refineries, power plants,...) Gibraltar or along the Thyrrennean coast. surrounded by residential areas with im- SO2 (Fig. 7c) is mainly emitted via indus-

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24 90 (a) (b) ) 20 60 ACPD −1 16 30 6, 11913–11956, 2006 12 0 8 −30 Wind direction ()

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Fig. 6. Surface wind speed (a) and direction (b), temperature (c), relative humidity (d) and surface Full Screen / Esc Fig. 6. Surfacestress wind(e) as speeda function(a)of timeandfrom directionthe ASIS buo(b)y measurements, temperature(dashed(c)line),(see relativediamond humidityin (d) and surface stress (e)Fig. 1c)asand afrom functionthe MM5 ofsimulations time from(solid theline). ASIS buoy measurements (dashed line) (see diamond in Fig. 1c) and from the MM5 simulations (solid line). Printer-friendly Version trial activities and domestic combustion, as 4.2 Analysis at the scale of the Gulf of shown by the more scattered emission pat- Lion Interactive Discussion tern. The emissions associated with maritime traffic are significant. CO and coarse primary particulate matter (PPM, of size11948 On the shore of the Gulf of Lion, the EGU less than 10 µm and more than 2.5 µm) area around the city of Marseille and the emissions are shared by traffic and various Berre pond, in southeastern France is a sources of combustion. These sources are source region of large urban and industrial significant in the main western Mediter- emission with high occurrence of photo- ranean cities as shown in Fig. 7 (b and d). chemical pollution events (Drobinski et al. 2006). The aerosol optical depth (AOD) at 865 nm derived from the SeaWIFS pass over the Gulf of Lion at 1132 UTC on 24 March 1998 (obtained from the SeaDAS software) is shown in Fig. 8 (c). Fig- ure 8 evidences that most of the west-

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Fig. 7. EMEP inventory emissions in Mg/(50 km × 50 km)/year, adjusted on the grid of the domain 3 Back Close Fig. 7. EMEPat inventory1200 UTC. P emissionsanels a, b, c and ind represent Mg/(50the kmconcentrations×50 km)/year,of NO (a), adjustedPPM coarse particles on the(≥ grid of the domain 3 at 12:002.5 UTC.µm and Panels≤ 10 µm) (b),(a)SO, (b)2 (c), and(c)COand(d) during(d) representa labor day of March the concentrations1998. of NO (a), PPM Full Screen / Esc coarse particles (≥2.5 µm and ≤10 µm) (b), SO2 (c) and CO (d) during a labor day of March 1998. ern Mediterranean was covered by clouds, The location of the Marseille/Fos-Berre with the notable exception of Gulf of Lion, aerosol plume is reproduced by CHIMERE Printer-friendly Version which was partly cloud-free, on account of (shifted by about 20 km to the east) with an the strong Mistral jet blowing in the region. AOD of about 0.15, as well as the aerosol Interactive Discussion AODs at 865 nm could only be retrieved plume from the industrial city of Toulon. in the cloud-free part of the Gulf of Lion, The plume emerging from the Aude valley but displays a minimum south of the city11949of channelling the Tramontane as well as the ◦ EGU Montpellier (0.11 around 4 E) and a maxi- plume skirting the eastern Spanish coast mum south of the Fos/Berre industrial area (Fig. 8a) are not visible in the SeaWIFS (0.18 around 4.7◦E) along leg AF of the data because these regions are covered by ARAT aircraft (Fig. 1). In the Mistral re- clouds preventing from reliable retrieval of gion (leg FC), the AOD at 865 nm varied aerosol products (Fig. 8b, c). Despite the from over 0.2 near the coast (Fos/Berre re- global good agreement, the AOD simu- gion and further east) to 0.17 around way- lated in some plumes can be half smaller point F (Fig. 1). The aerosol distribution than the observed AOD. simulated by CHIMERE using the recently developed methodology by Hodzic et al. In the afternoon March 24, 1998, the (2004), can be compared to the aerosol op- vertical structure of aerosols as well as tical depth retrieved from SeaWIFS in the the ABL were documented by the lidar cloud-free regions. Figure 8b shows the LEANDRE-2 along legs AF, FC and CE AOD at 865 nm simulated with CHIMERE (Fig. 9a) as detailled by Flamant (2003). over the domain documented by SeaWIFS. Lidar measurements on leg AF (Fig. 9b) show an internal boundary layer develop-

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Full Screen / Esc Fig.Fig. 8. PPanelanel a:(a)simulated: simulatedaerosol aerosoloptical opticaldepth (A depthOD) at (AOD)865 nm atderi 865ved nmfrom derivedCHIMERE from CHIMEREsimulation simulation(domain 2). (domainThe rectangle 2). Theindicates rectanglethe zoomed indicatesarea thesho zoomedwn in panels area shown(b) and in(c); panelsPanel (b)b: simulatedand (c); Printer-friendly Version PanelAOD (b):at 865 simulatednm over AODthe Gulf at 865of nmLions overmultiplied the Gulfby of Lionsa factor multipliedof 2 with bythe a factorcloud ofmask 2 withderi theved cloud mask derived from the Sea-viewing Wide Field-of-view Sensor (SeaWIFS) observations; from the Sea-viewing Wide Field-of-view Sensor (SeaWIFS) observations; Panel c: AOD at 865 nm Interactive Discussion Panelderived (c):from AODSeaWIFS at 865 nmmeasurements. derived from SeaWIFS measurements. 11950 EGU ing from the coast (within the advected served along FC (i.e., shallow). The depth continental ABL) and reaching a depth of of the ABL increases westward from 500 1200 m. Close to way-point F, at approx- to 1200 m (between 43.25 and 43 ◦N) with imately 42.6◦N, the marine ABL struc- reflectivity values on the order of those ob- ture characteristics over the sea changed served on leg CE. Further to the west, a : it is observed to be shallower (700 m continental ABL is observed in the shel- in Fig. 9b) and is characterized by larger tered region in the lee of the Massif Cen- values of atmospheric reflectivity. This is tral, reflectivity values being significantly confirmed by measurements made on leg smaller than to the east of 43 ◦N. In this FC (Fig. 9c), along which the marine ABL region, lidar measurements evidence the is characterized by values of atmospheric presence of gravity waves inside the con- reflectivity similar to those observed near tinental ABL and above, which are com- way-point F. Also, the marine ABL is ob- mon features during Mistral events (Cac- served to remain shallow, its depth grad- cia et al. 2004; Drobinski et al. 2005). ually decreasing from 700 to 500 m with The vertical structure of the aerosol dis- the distance to the coast. As discussed in tribution in the atmosphere derived from Flamant (2003), the larger atmospheric re- LEANDRE-2 reflectivity is used to vali- flectivity values in the eastern part of the date the CHIMERE simulations along the sampled region is thought to be related to ARAT legs. To do that, we use the method- larger concentrations of pollution aerosol ology by Hodzic et al. (2004) that involves from the city of Marseille and the indus- simulating the lidar backscattering profiles trial petrochemical complex of Fos/Berre. from the model relative humidity and con- On leg CE (Fig. 9d), close to the coast, centration outputs (optical properties vary- the ABL structure is similar to that ob- ing with chemical composition and mass

Atmos. Chem. Phys., 0000, 0001–25, 2006 www.atmos-chem-phys.org/acp/0000/0001/ 16 T. Salameh et al.: Aerosols distribution during a Tramontane/Mistral episode CHIMERE simulation LEANDRE−2 measurements ACPD 3 (a) Leg CE 3 (b) Leg CE 6, 11913–11956, 2006 2 2

Height (km) 1 Height (km) 1 Aerosols distribution during a 3.8 4 4.2 4.4 4.6 3.8 4 4.2 4.4 4.6 Longitude (°E) Longitude (°E) Tramontane/Mistral episode 3 (c) Leg CF 3 (d) Leg CF T. Salameh et al. 2 2

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J I 3.8 4 4.2 4.4 4.6 3.8 4 4.2 4.4 4.6 Longitude (°E) Longitude (°E) J I 50 100 150 Reflectivity (a.u.) Back Close Fig. 9. Atmospheric reflectivity at 0.73 nm as obtained from LEANDRE-2 (right column) and Fig. 9. AtmosphericCHIMERE reflectivity(left column) atalong 0.73leg nmCE (a and asb), obtainedleg CF (c and d), fromand leg LEANDRE-2AF (e and f) on the (right column) and afternoon of March 24 1998. Way points A, C, and E are shown in Fig. 1c. The aerosol-ladden Full Screen / Esc CHIMERE (left column)marine alongABL is characterized leg CEby high (a reflectiandvitybvalues,), legwhile CFthe adv (ectedc andcontinentald),ABL andexhibits leg AF (e and f) on the reflectivity values comprised between 80 and 1200 a.u. The free troposphere above the ABL, is afternoon of 24 Marchcharacterized 1998.by Waya reflectivity pointsvalues of A,80 a.u. C,or andless. E are shown in Fig. 1c. The aerosol-ladden marine ABL is characterized by high reflectivity values, while the advected continental ABL Printer-friendly Version exhibits reflectivity valueswhy the western comprisedregion of the betweenplume is op- 80within andthe ABL, 12002000 a.u.m within Thethe freead- troposphere above tically thinner than the middle and eastern vected aerosol-rich continental ABL above Interactive Discussion the ABL, is characterizedregions of bythe aplume. reflectivity valuesthe ofmarine 80 a.u.ABL) oralso less.explain the differences in aerosol concentration observed The back plumes ending along the by LEANDRE-2. At 50 m (and higher ARAT legs at two different altitudes (5011951m EGU

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4 (b) 4 (a) Aerosols distribution 3 3 during a Tramontane/Mistral 2 2 episode Height (km) Height (km) 1 1 T. Salameh et al. 0 0 4 4.2 4.4 4.6 42.6 42.8 43 43.2 Longitude (oE) Latitude (oN) Title Page

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4 (c) 60 Conclusions References 50 3 Tables Figures 40 2 30 Height (km) 20 J I 1

10 Relative Humidity (%) J I 0 3.8 4 4.2 4.4 4.6 Longitude (oE) Back Close

Fig. 10. Vertical profiles of relative humidity simulated with MM5 and extracted from domain 2 Full Screen / Esc Fig. 10. Verticalalong profileslegs AF (a), ofCE relative(b) and CF humidity(c). simulated with MM5 and extracted from domain 2 along legs AF (a), CE (b) and CF (c). Printer-friendly Version within the ABL, not shown), large-scale ported over a long range (Corsemeier et al. transport from Lyon is visible on the 2005) (Fig 13). One exception is at ending Interactive Discussion four clusters of back trajectories. How- point C which is just above this aerosol- ever, at ending points A and E, the air rich layer, in the aerosol-free layer (Fig. 9). masses do not flow over the Marseille/Fos-11952Figure 13c shows that the air mass flows Berre area or other major cities, explain- over the advected continental ABL above EGU ing the lower aerosol content observed by the western Alps and originate from the LEANDRE-2 within the ABL. On the con- northern Alps. The absence of significant trary, the air masses ending at points C aerosol sources in this area explains the and F are enriched in aerosols as they very low reflectivity values at 2000 m at pass over the Marseille/Fos-Berre area ex- ending point C. One can finally notice the plaining the higher reflectivity values in absence of trans-alpine transport between the ABL. Along leg CF, the ARAT fol- 1600 and 1700 UTC contrary to the situa- lows the Marseille/Fos-Berre plume ex- tion at 1100 UTC. This is the result of an plaining the homogeneous concentration evolution of the regime of the flow imping- of aerosols along the leg. ing the Alpine ridge: at 1100 UTC, part of the upstream air mass flows over the moun- At 2000 m, the aerosols of the layer just tain whereas at 1600 and 1700 UTC, the above the marine ABL (reflectivity values air mass is mostly blocked by the Alps and of about 100 a.u.) originate from remote flows around. In Fig. 2, it is illustrated by sources (e.g. Lyon) which are then trans-

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◦ ◦ ◦ ◦ ◦ ◦ Fig. 11. Back trajectories ending at 3.3◦ E/42.5 N ◦(a), 3.39 E/43 N ◦(b), 3.95◦E/43.16 N (c) and◦ ◦ J I Fig. 11. Back4.75 trajectories◦E/42.6◦N (d) ending(shown in Fig. at8). 3.3TheE/42.5altitude ofNthe(a)ending, 3.39pointsE/43is 50 m.NThe(b)ending, 3.95time E/43.16 N (c) ◦ ◦ and 4.75 E/42.6of theNback(d)trajectories(shownis in1100 Fig.UTC8which). Theis the altitudetime of the ofpassage theof endingSeaWIFS pointsover the iswestern 50 m. The ending Back Close time of the backMediterranean. trajectoriesThe iswind 11:00fields UTCused to whichcompute isthese theback timetrajectories of theare passagefrom the 9-km of SeaWIFSmesh over the size domain. Filled black dots indicate the cities of Barcelona, Toulouse, Montpellier, Marseille/Fos- Full Screen / Esc western Mediterranean.Berre, Lyon, Torino Theand windMilano. fields used to compute these back trajectories are from the 9- km mesh size domain. Filled black dots indicate the cities of Barcelona, Toulouse, Montpellier, Marseille/Fos-Berre, Lyon, Torino and Milano. the larger extension of the western Alpine east of Corsica, the surface pressure Printer-friendly Version wake in the afternoon. low gradually decreasing from 1014 to 1010 hPa. Over the southwestern region Interactive Discussion 4.3 Aerosol distribution over the west- of the Mediterranean basin, large aerosol ern Mediterranean from the MM5- loading can be attributed to ship emissions CHIMERE model 11953(see Fig. 7 and 14a and b). South of Sar- EGU dinia, the air circulating around the Genoa Insight into the aerosol distribution, ori- surface low brings ship emissions to south- gin and composition over the western ern Italy. North of this band, most of Mediterranean can be obtained from the the aerosol loading over the Mediterranean CHIMERE aerosol products, in close re- Sea is of local origin (inter-continental ship lationship with the mesoscale dynamical tracks and coastal urban and industrialized processes accessible from the validated area such as Barcelona, Marseille/Fos- MM5 meteorological fields. In the fol- Berre, Toulon, see Fig. 7 and Fig. 14a and lowing, the aerosol distribution, origin and b), with one exception along the Aude val- composition will be discussed as a function ley where the Tramontane transports off- of the position and intensity of the ”eye” of shore continental aerosols. The plume over the Genoa cyclone on 24 March. Toulon stagnates in the western Alps wake trailing downstream across the Alps, an During the morning period, the Genoa area of very weak wind. Finally, the close cyclone is located immediately to the

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◦ ◦ ◦ ◦ Fig. 12. Back trajectories ending at 3.9 E/43.4◦ N (way-point◦ A) (a), 4.65 E/42.5 N (way-point◦F) ◦ ◦ ◦ ◦ ◦ J I Fig. 12. Back(b), trajectories4.7 E/43.2 N (w endingay-point C) at(c) and 3.93.88E/43.4E/42.75 N(w (way-pointay-point E) (d). A)The(a)back,trajectories 4.65 E/42.5 N (way- ◦ ◦ ◦ ◦ point F) (b), 4.7endingE/43.2points correspondN (way-pointto the starting C)and(c)endingandpoints 3.88of theE/42.75ARAT legsNand (way-pointthe aerosol plumes E) (d). The back trajectories endingdetected pointsby the lidar correspondLEANDRE-2 on toboard thethe startingARAT. The altitude and endingof the ending pointspoints is of50 m. theThe ARAT legs and Back Close ending times of the back trajectories are 1600 and 1700 UTC for the first and second row, respectively. the aerosol plumesFilled black detecteddots indicate bythe thecities lidarof Barcelona, LEANDRE-2Toulouse, Montpellier on board, Marseille/F the ARAT.os-Berre, TheLyon, altitude of the Full Screen / Esc ending pointsT isorino 50and m.Milano. The ending times of the back trajectories are 16:00 and 17:00 UTC for the first and second row, respectively. Filled black dots indicate the cities of Barcelona, Toulouse, Montpellier,location of Marseille/Fos-Berre,the Genoa cyclone from Cor Lyon,- the Torinowestern andpart Milano.of the Mediterranean Printer-friendly Version sica and the absence of strong winds dur- basin. This is consistent with aerosol ing the morning period lead to the fact that optical depth values of 0.1 and 0.3 at 870 Interactive Discussion the region north and northwest of Corsica and 440 nm, respectively, measured at is only influenced by marine clean air. Ispra (Italy) with the photometer of the 11954AERONET network and with spectral EGU During the afternoon period, the distribution revealing high accumula- Genoa cyclone intensifies (from 1010 tion mode aerosol concentration. The to 1004 hPa) and moves southeastward, CHIMERE AOD at 865 nm at Ispra is about 300 km east of Sardinia over the about 0.09. A narrow band of clean air Tyrrhenian Sea. The Tramontane has separates the two major plumes extending nearly vanished so that the polluted air of more than 300 km over the sea. This the morning stagnates in the Aude valley. cleaner air corresponds to the western Conversely, the Mistral has intensified and Alps wake and, associated with large hori- most of the aerosols exported offshore zontal shear of wind, acts as a dynamical originates from central France and the barrier, preventing any lateral exchanges Marseille/Fos-Berre plumes. Moreover, between the Mistral and the Ligurian the Ligurian outflow brings in the aerosols outflow. The southeastward displacement emitted from the major industrialized and intensification of the Genoa cyclone Italian cities (Milano and Torino) over

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Latitude ( 42 42 J I 40 40 0 2 4 6 8 10 12 0 2 4 6 8 10 12 J I Longitude (oE) Longitude (oE) Back Close Fig. 13. Same as Fig. 12 for back trajectories ending at 2000 m MSL. Fig. 13. Same as Fig. 12 for back trajectories ending at 2000 m m.s.l. Full Screen / Esc

increases the meridian extension of the displays the formation of a sulfate bub- Mistral that sweeps away the band of ble in Great Britain on March 22 going Printer-friendly Version aerosols emitted from the ships that skirt to southern France on March 23 and 24 the North African coast in the morning (not shown). In the same time, the sul- Interactive Discussion (Fig. 14c and d). fate concentration increases when crossing Despite the difﬁculty in validating the western France (Normandy) due to high simulated composition of the aerosol load-11955sulfur oxide emissions. This behavior is EGU ing over the basin, measurements of ni- supported by sulfate observations in three trates available in Netherlands and Italy EMEP background sites in Great Britain clearly show the development of an ammo- and France (see Table 1, GB02, GB14 and nium nitrate episode over the western Eu- FR03). During the development of this rope from March 22 to 31 (see measured episode, in particular on March 24, accord- and simulated values in Table 1, NL09, ing to the model results, the Genoa cyclone IT04). These episodes are often observed drives the pollution plume towards south- in the late winter during such meteoro- ern and western France. Afterwards, the logical conditions with high pressure over ammonium nitrate is advected through the Great Britain and central Europe (Bessag- Aude and Rhoneˆ valleys to the Mediter- net et al., 2005). The large nitrogen ox- ranean basin. A similar ammonium nitrate ides emissions in the Netherlands, Bel- plume is also transported from the Po val- gium, Germany and northern France pro- ley (Fig. 14). The formation of ammonium duce by oxidation a large amount of ni- nitrate is favored by low temperatures in tric acid which is then neutralized by am- southern France which reach a maximum ◦ monia emitted in the same regions and of about 10 C near the surface on March form ammonium nitrate in the particulate 24 at 1200 UTC and avoid the evaporation phase. Moreover, the coarse simulation of ammonium nitrate.

Atmos. Chem. Phys., 0000, 0001–25, 2006 www.atmos-chem-phys.org/acp/0000/0001/ T. Salameh et al.: Aerosols distribution during a Tramontane/Mistral episode 21 ACPD 6, 11913–11956, 2006

Aerosols distribution during a Tramontane/Mistral episode

T. Salameh et al.

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Abstract Introduction

Conclusions References

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Fig. 14. Aerosol loading with respect to the layer hight, over the western Mediterranean basin. Pan- els a, b, c and d display the particulate concentrations fields modelled with CHIMERE on domain 2 Printer-friendly Version Fig. 14. Aerosol loadingat 0600, with0900, 1200, respect1500, 1800 toand the2100 UTC, layerrespecti hight,vely. Solid overlines represent thethe westerncoastlines and Mediterranean basin. Panels (a), (b), (c) andelev(d)ationsdisplaybeyond 500 m. the particulate concentrations fields modelled with CHIMERE on domain 2 at 06:00, 09:00, 12:00, 15:00, 18:00 and 21:00 UTC, respectively. Solid lines Interactive Discussion 5 Conclusions tion over the western Mediterranean basin represent the coastlines and elevations beyond 500during m.a Mistral event from the combi- Finally, this article presents a first11956at- nation of innovative instrumentation (pas- tempt to characterize the aerosol distribu- EGU

www.atmos-chem-phys.org/acp/0000/0001/ Atmos. Chem. Phys., 0000, 0001–25, 2006