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International audience; This paper is concerned with a dust-raising cold pool over the Sahara desert that occurred on August 3-5, 2006. Both the quantity of the uplifted dust and its spatio-temporal evolution are examined using satellite observations and a numerical simulation. The dust emission during this event was initiated by a mesoscale cold pool emanating from mesoscale convective systems (MCSs) that developed over northern Niger and Mali on August 3. This event is one of several exceptional northward surges of the West African Monsoon (WAM) during the 2006 wet season. We examine the propagation of the cold pool and associated dust lofting using high temporal resolution false-color dust product images from the Meteosat Second Generation Spinning Enhanced Visible and Infrared Imager (MSG-SEVIRI). Observations from Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) are used to characterize the vertical structure of the dust cloud as it spreads over the Sahara and across the Atlantic coast. The European Centre for Medium-Range Weather Forecasting African Monsoon Multidisciplinary Analysis (ECMWF-AMMA) special reanalysis was used to describe the synoptic conditions that accompanied this event. Furthermore, a numerical simulation using the mesoscale model MesoNH was performed to estimate the emissions and the westward transport of dust during this event. MODIS AOD satellite imagery has then been used to track the dust plume across the Atlantic Ocean to Barbados, where comparisons are made with the local dust record there. The dusty cold pool covered southern Algeria and a large part of northern Mali and Western Niger attaining a total area close to 2 x 106 km2. It extended over 2-3 km in altitude and had an aerosol optical depth on the order of 1.5 and an estimated total dust load of about 1.5 Tg on average. Following daytime heating, the dusty cold pool and associated northward surge of moisture favored the development of new convection and additional precipitation over the Sahara. The northward extension of the dusty cold pool was accompanied by a collapse of the Saharan heat low, a characteristic feature of monsoon surges. A model-estimated quantity of 0.4 Tg of the dust produced during this event was subjected to westward transport toward the Atlantic Ocean after being mixed upward in the thickening boundary layer by the daytime heating over the Sahara to altitudes as high as 5-6 km. The arrival of the dust plume in Barbados in the Caribbean Sea 9 days after its departure from the west coast of Africa was characterized by a peak in dust concentration of 48.5 µg. m-3.

Air pollution can be transported between continents by large-scale atmospheric dynamics, where it can impact climate and regional air quality far from emission regions. During transport, concentrations in polluted plumes are affected by mixing with the background. Impacts of longrange pollution on climate and air quality are usually quantified using global Eulerian models, that tend to overestimate mixing due to their low resolution. This induces errors in the simulated export of pollutants, and in the reaction rates due to the non-linearity of atmospheric chemistry. This thesis aims to better understand and quantify mixing of pollutant plumes in the free troposphere during long-range transport, and its representation in numerical models. It focuses on the Arctic, which is a region vulnerable to climate change, and where greenhouse gases and aerosol distributions are strongly influenced by transport of pollution from mid-latitudes. The work focuses on three case studies of transport into the Arctic troposphere during the POLARCAT campaign, in summer 2008. Results include estimates of the intensity of mixing in the Arctic troposphere, which was found to be lower than in mid-latitudes. The results also show that a Lagrangian modelling approach is able to reproduce the variability in observed concentrations in aged plumes, to simulate the transport of small-scale structures, and to evaluate the variability of ozone evolution in pollutant plumes due to differences in mixing scenarios. The model results are also used to assess pollutant transport simulated by global models in the Arctic, and to define the resolution neeeded for more accurate simulations. Overall, the results highlight the necessity to combine Lagrangian and Eulerian approaches to improve the representation of long-range pollution transport in numerical models., La pollution de l'air peut être transportée sur des distances de plusieurs milliers de kilomètres par la dynamique de grande échelle. Elle affecte ainsi le climat et la qualité de l'air loin des régions sources. Durant leur transport, les panaches de pollution voient leur composition altérée par le mélange avec l'air environnant. Les modèles globaux eulériens utilisés pour calculer les bilans d'export de polluants ne représentent pas bien le transport des panaches du fait de leur faible résolution, ce qui induit des erreurs dans le calcul de ces bilans et dans les vitesses de réaction du fait de la non-linéarité de la photochimie atmosphérique. Cette thèse a pour but de contribuer à améliorer la compréhension et la quantification du mélange subi par les panaches de pollution dans la troposphère libre lors du transport à longue distance, et sa représentation dans les modèles numériques. Elle se focalise sur l'Arctique, une région vulnérable au changement climatique, où la distribution des gaz à effet de serre et des aérosols est fortement influencée par le transport de polluants depuis les moyennes latitudes. Les travaux effectués s'organisent autour de trois études de cas de transport dans la troposphère arctique, dans le cadre de la campagne aéroportée POLARCAT durant l'été 2008. Les résultats obtenus permettent d'estimer que l'intensité du mélange dans cette région est plus faible qu'aux moyennes latitudes. Ils montrent également qu'une approche lagrangienne permet de reproduire la variabilité des concentrations dans les panaches âgés, de simuler le transport des structures de petite échelle, et d'évaluer la variabilité de l'évolution des concentrations en ozone dans les panaches induite par les différents scénarios de mélange. Les résultats de ces modèles lagrangiens permettent aussi de jauger les performances des modèles globaux eulériens en terme de transport des polluants en Arctique, et de définir la résolution spatiale nécessaire pour représenter ce transport de façon satisfaisante. Les trois études présentées concordent sur l'importance de combiner les approches lagrangienne et eulérienne pour simuler le transport à longue distance dans les modèles numériques.

International audience; In this study we document the evolution, the synoptic trigger and the characteristics of an intense dust event which occurred over the Sahara desert on August 3-5, 2006. The motivation for this study is to highlight the large scale dust production over the Sahara associated with gust fronts of squall lines. The dust emission during this event was initiated by a large-scale cold pool emanated from a squall line that developed over Niger and Mali on August 3. We examine the development of this squall line and its subsequent dust lifting using high temporal resolution false color dust product images from the Meteosat Second Generation Spinning Enhanced Visible and Infrared Imager (MSG-SEVIRI). Observations from Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) are used to characterize the vertical structure of the dust plume as it spreads over the Sahara and near the Atlantic coast line. The European Centre for Medium-Range Weather Forecasting African Monsoon Multidisciplinary Analysis (ECMWF-AMMA) special reanalysis was used to provide the synoptic-scale conditions that favored the occurrence of this event. Particular attention was paid to the intrusion of a mid-level extratropical streamer of potential vorticity that interacts with an African Easterly Wave (AEW) and favored the growth of a low-level cyclonic circulation along the intertropical discontinuity zone over the course of the event. The subsequent coupling of the ITD low-level circulation to an AEW aided both on the formation of the squall line and on the pronounced northward transport of the uplifted dust over the Sahara. The dusty cold pool extended over 2-3 km in altitude and exhibited an aerosol optical depth on the order of 1.5 and a dust load of about 1 Tg on average. Large amount of the dust produced during this event was subject to westward transport over the Atlantic Ocean after being mixed up by the diurnal heating over the Sahara to altitudes as high as 5 km. The pump of moisture by the cold pool into the dry desert favored the development of new convection over the Sahara which resulted on weak precipitation of about 3 mm per hour.