Studying the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST)

At the end of the afternoon, when the surface heat fluxes start to sharply decrease, the CBL turns from a convective well-mixed layer to an intermittently turbulent residual layer overlying a stably-stratified boundary layer. This transition raises several observational and modelling issues. Even the definition of the boundary layer during this period is fuzzy, since there is no consensus on what criteria to use and no simple scaling laws to apply. Yet it plays an important role in such diverse atmospheric phenomena as transport and diffusion of trace constituents or wind energy production. This phase of the diurnal cycle remains largely unexplored, partly due to the difficulty of measuring weak and intermittent turbulence, anisotropy, horizontal heterogeneity, and rapid time changes. The Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) project is gathering about thirty research scientists from the European Union and the United States to work on this issue. A field campaign (BLLAST-FE) is planned for spring or summer 2011 in Europe. BLLAST will utilize these observations, as well as previous datasets, large-eddy and direct numerical simulations, and mesoscale modelling to better understand the processes, suggest new parameterisations, and evaluate forecast models during this transitional period. We will present the issues raised by the late afternoon transition and our strategy to study it.Peer ReviewedPostprint (published version

Studying the Boundary Layer Late Afternoon nd Sunset Turbulence (BLLAST)

At the end of the afternoon, when the surface heat fluxes start to sharply decrease, the CBL turns from a convective well-mixed layer to an intermittently turbulent residual layer overlying a stably-stratified boundary layer. This transition raises several observational and modeling issues. Even the definition of the boundary layer during this period is fuzzy, since there is no consensus on what criteria to use and no simple scaling laws to apply. Yet it plays an important role in such diverse atmospheric phenomena as transport and diffusion of trace constituents or wind energy production. This phase of the diurnal cycle remains largely unexplored, partly due to the difficulty of measuring weak and intermittent turbulence, anisotropy, horizontal heterogeneity, and rapid time changes. The Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) project is gathering about thirty research scientists from the European Union and the United States to work on this issue. A field campaign (BLLAST-FE) is planned for spring or summer 2011 in Europe. BLLAST will utilize these observations, as well as previous datasets, large-eddy and direct numerical simulations, and mesoscale modeling to better understand the processes, suggest new parameterizations, and evaluate forecast models during this transitional period. We will present the issues raised by the late afternoon transition and our strategy to study it.Peer ReviewedPostprint (published version

The BLLAST field experiment: Boundary-Layer late afternoon and sunset turbulence

Due to the major role of the sun in heating the earth's surface, the atmospheric planetary boundary layer over land is inherently marked by a diurnal cycle. The afternoon transition, the period of the day that connects the daytime dry convective boundary layer to the night-time stable boundary layer, still has a number of unanswered scientific questions. This phase of the diurnal cycle is challenging from both modelling and observational perspectives: it is transitory, most of the forcings are small or null and the turbulence regime changes from fully convective, close to homogeneous and isotropic, toward a more heterogeneous and intermittent state. These issues motivated the BLLAST (Boundary-Layer Late Afternoon and Sunset Turbulence) field campaign that was conducted from 14 June to 8 July 2011 in southern France, in an area of complex and heterogeneous terrain. A wide range of instrumented platforms including full-size aircraft, remotely piloted aircraft systems, remote-sensing instruments, radiosoundings, tethered balloons, surface flux stations and various meteorological towers were deployed over different surface types. The boundary layer, from the earth's surface to the free troposphere, was probed during the entire day, with a focus and intense observation periods that were conducted from midday until sunset. The BLLAST field campaign also provided an opportunity to test innovative measurement systems, such as new miniaturized sensors, and a new technique for frequent radiosoundings of the low troposphere. Twelve fair weather days displaying various meteorological conditions were extensively documented during the field experiment. The boundary-layer growth varied from one day to another depending on many contributions including stability, advection, subsidence, the state of the previous day's residual layer, as well as local, meso- or synoptic scale conditions. Ground-based measurements combined with tethered-balloon and airborne observations captured the turbulence decay from the surface throughout the whole boundary layer and documented the evolution of the turbulence characteristic length scales during the transition period. Closely integrated with the field experiment, numerical studies are now underway with a complete hierarchy of models to support the data interpretation and improve the model representations.publishedVersio

Analyse dynamique de la pollution de l'air dans la troposphère

La caractérisation et la prévision des épisodes de pollution de l air nécessitent une approche à la fois chimique et météorologique. Dans ce travail, nous nous sommes donc attachés à mettre en évidence les relations entre les phénomènes météorologiques et la pollution de l air. Pour cela, nous avons mis en œuvre, dans l agglomération dunkerquoise, une campagne de mesures de six mois, associant des mesures météorologiques, micro-météorologiques et de composés de type BTEX (Benzène, Toluène, Ethyl-Benzène et Xylènes). Des mesures de flux turbulents ont permis d évaluer la capacité dispersive de l atmosphère lors de périodes anticycloniques, de pressionnaires, de passages de front et d épisodes de brises côtières. Cette étude a montré en particulier le rôle majeur de la brise, à l origine des plus importants épisodes de pollution de la campagne. Afin d appréhender l aspect dynamique tridimensionnel de la brise et par conséquent, le devenir des polluants en altitude, nous avons caractérisé la structure d un épisode de brise à partir d instruments de télédétection optique et acoustique et de simulations numériques. L étude de la pollution de fond a été menée à proximité du site de surveillance du Pic du Midi de Bigorre (PDM) dans les Pyrénées. Des études climatologiques précédentes ont révélé un comportement atypique du cycle d ozone en période estivale, se traduisant par une diminution d ozone en milieu de journée. A l aide d un lidar à absorption différentielle, nous avons pu mesurer au voisinage du PDM les profils verticaux de l ozone et du coefficient d extinction afin d étudier in situ le rôle des phénomènes météorologiques locaux de brises de pentes.The characterization and the forecast of air pollution episodes require a combined chemical and weather approach. So, in this work the relations between the meteorological situation and the air pollution. Thereby, a field campaign associating weather, micrometeorological and BTEX (Benzene, Toluene, Ethyl-Benzene and Xylenes) compounds was carried out during six months in Dunkirk. Measurements of turbulent flows were used to evaluate the dispersive capacity of the atmosphere during anticyclonic and depressionary periods, front passages and sea breeze episodes. This study showed in particular that the sea breeze was responsible of the most important pollution episodes of the measurements period. In order to apprehend the three-dimensional dynamic aspect of the breeze and consequently, the pollution behaviour in altitude, the structure of a sea breeze episode was characterized from optical and acoustic teledetection instruments and numerical simulations. The study of the background pollution occured in the neighbouring of the monitoring site of the Pic du Midi de Bigorre (PDM) in the Pyrenees (France). Previous studies revealed an atypical behavior of the ozone cycle in summer period, resulting in a decline of ozone in the middle of day. In order to study experimentally the role of the local weather phenomena, the vertical profiles of the ozone and of the extinction coefficient were measured with a differential absorption lidar in the vicinity of the PDM.DUNKERQUE-BU Lettres Sci.Hum. (591832101) / SudocSudocFranceF

Characterization of aerosol pollution in North of France (relation between mass, optical properties , vertical distribution and meteorology)

Une atmosphère saine est un besoin élémentaire pour le bien être et la santé humaine. La matière particulaire en suspension (Particle Matter, PM) est bien connue pour avoir un impact significatif sur la santé. Les mesures de PM2.5 et PM10 au niveau du sol reflètent l influence de la dynamique de la couche limite et du mélange des aérosols locaux ou advectés sur de grandes distances. Le lien entre épaisseur optique en aerosol (aerosol optical thickness, AOT) et PM dépend de la relation entre propriétés optiques et massiques et de la distribution verticale des particules dans l atmosphère. Nous présentons 3 expériences de terrain dédiées à la caractérisation des aérosols de pollution dans le Nord de la France: la première lors d un évenement de pollution printanier sur Lille, la seconde durant un événement de pollution hivernal sur Dunkerque et la troisième durant des occurrences de brise de mer sur le littoral Dunkerquois. Nous avons utilisé 2 systèmes Lidar différents, le premier dans le visible (532 nm) et le second dans l UV (355 nm); un photomètre solaire automatique et des mesures de PM2.5 et PM10 par TEOM. L altitude supérieure de la couche de mélange (Mixed boundary layer, MBL) est détectée par Lidar et nous avons été capable de suivre le développement classique de la couche limite convective ainsi que des décroissances brutales d altitude de la MBL dues à la brise de mer. Les profils d extinction aérosols ont été estimés en utilisant un rapport Lidar de 67 sr à 532 nm à Lille, 77 sr à 532 nm et 30 sr à 355 nm à Dunkerque. Nous avons analysé l impact du transport grande échelle de masses d air polluée, du développement convectif de la MBL et du développement de la cellule de brise de mer sur les profils verticaux d extinction en aérosols. Le signal Lidar dans les premières centaines de mètres est très bien corrélé (coefficient de corrélation supérieur à 0.9) avec les concentrations massiques mesurées au sol dans tous les cas. Il est également montré que l introduction de la hauteur de la MBL permet une meilleure détermination des PM à partir de l épaisseur optique.Clean air is considered to be a basic requirement for human health and well-being. Particulate matter is known to have a significant impact on health. The variability of Particle Matter (PM2.5 and PM10) concentrations recorded at ground-level is influenced by the boundary layer dynamics, local emissions, and advection and mixing of large scale transported aerosols. The link between columnar aerosol optical thickness (AOT) and ground-level PM depends on the relationship between mass and optical properties and on the vertical distribution of aerosols in the atmosphere. We present three field experiments dedicated to the characterization of pollution aerosols in the North of France: the first one during a spring pollution episode in metropolitan area of Lille (50.61N, 3.14E), the second one during a winter pollution episode in the industrial coastal city of Dunkerque (5104'N; 238'E) and the third one during summer sea breezes on coastal area of Dunkerque. We have used 2 different Lidar systems, one in the UV (355 nm) and the other one in the visible (532 nm), an automatic sun photometer, and PM2.5 and PM10 measurements with TEOM. The mixed layer (MBL) top altitude is detected from the Lidar signal and we were able to monitor the classical diurnal evolution of the convective continental boundary as well as short-time decreases in the MBL height due to sea breeze occurrences. The aerosol extinction profiles were estimated using a Lidar ratio of 67 sr at 532 nm in Lille, and 77 sr at 532 nm and 30 sr at 355 m in Dunkerque. We have analyzed the impact of long range transport of polluted air masses, convective development of the MBL, and sea breeze development on the vertical profile of aerosol extinction coefficient. The Lidar signal in the first few hundred meters is well correlated (correlation coefficient above 0.9) with the PM concentrations in all cases. It is found that introducing the Lidar derived MBL height enable a better estimation of PM from measured AOT. Clean air is considered to be a basic requirement for human health and well-being. Particulate matter is known to have a significant impact on health. The variability of Particle Matter (PM2.5 and PM10) concentrations recorded at ground-level is influenced by the boundary layer dynamics, local emissions, and advection and mixing of large scale transported aerosols. The link between columnar aerosol optical thickness (AOT) and ground-level PM depends on the relationship between mass and optical properties and on the vertical distribution of aerosols in the atmosphere. We present three field experiments dedicated to the characterization of pollution aerosols in the North of France: the first one during a spring pollution episode in metropolitan area of Lille (50.61N, 3.14E), the second one during a winter pollution episode in the industrial coastal city of Dunkerque (5104'N; 238'E) and the third one during summer sea breezes on coastal area of Dunkerque. We have used 2 different Lidar systems, one in the UV (355 nm) and the other one in the visible (532 nm), an automatic sun photometer, and PM2.5 and PM10 measurements with TEOM. The mixed layer (MBL) top altitude is detected from the Lidar signal and we were able to monitor the classical diurnal evolution of the convective continental boundary as well as short-time decreases in the MBL height due to sea breeze occurrences. The aerosol extinction profiles were estimated using a Lidar ratio of 67 sr at 532 nm in Lille, and 77 sr at 532 nm and 30 sr at 355 m in Dunkerque. We have analyzed the impact of long range transport of polluted air masses, convective development of the MBL, and sea breeze development on the vertical profile of aerosol extinction coefficient. The Lidar signal in the first few hundred meters is well correlated (correlation coefficient above 0.9) with the PM concentrations in all cases. It is found that introducing the Lidar derived MBL height enable a better estimation of PM from measured AOT.LILLE1-Bib. Electronique (590099901) / SudocSudocFranceF

Assessment of industrial contribution and ship emissions at an industrial and coastal site in Northern France

International audienc

Assessment of industrial contribution and ship emissions at an industrial and coastal site in Northern France

International audienc

Mixing of free-tropospheric air with the lowland boundary layer during anabatic transport to a high altitude station

International audienceHigh altitude stations are the only platforms allowing for continuous measurements of the free-troposphere composition, and monitoring of trends away from pollution sources. However, they are influenced by mountain breezes and convection that bring air from the lowland boundary layer up to the summits. In summer 2005, a field campaign involving in situ measurements and ozone lidars was organized in the Pyrenees to investigate the impact of such processes on in situ measurements at the Pic du Midi (PDM) high altitude station (2875 m a.s.l.). On June 17 and 19, a plain-to-mountain thermal circulation developed during the day. Observations show that direct transport of lowland air masses to PDM cannot account for ozone measurements at the station. Also, according to measurements, the PDM station did not directly sample the free troposphere. These two days were further investigated using a Lagrangian box model combining transport, photochemistry and mixing with the background troposphere. It was possible to reproduce and analyze ozone time series recorded at PDM, and quantify the partial mixing with free tropospheric air during the transport. A large fraction (43 to 86%) of air from the lower free troposphere was found to contribute to the gas melange sampled at PDM, with the best agreement found for fractions 57% (resp. 74%) on June 17 (resp. June 19)

Impact of sea breeze on vertical structure of aerosol optical properties in Dunkerque, France

International audienceDuring July 2008, we used an elastic backscattering LIDAR to monitor the aerosol vertical distribution at the coastal area of Dunkerque, France. Here we report the sea breeze event which was observed with more highlighted effect of aerosol on 25 July. By combining LIDAR measurements with Sun photometer-retrieved aerosol optical thickness, we estimated an average LIDAR ratio of 33 sr (± 14 sr) for the estimation of aerosol extinction profiles during the sea breeze. The LIDAR derived aerosol extinction in the first 200 m is clearly affected by the sea breeze and increases by more than 100% at the time of sea breeze arrival. A sharp convective boundary layer height decrease is observed in the LIDAR data due to the formation of the thermal internal boundary layer in the lowest part of the sea-to-land flow. PM2.5 concentration increases due to the thermal internal boundary layer formation and reaches its maximum between 1 and 2h after the front overpass. Except during the front overpass, the PM2.5 is well correlated to the inverse of the mixing height detected by the LIDAR

One year online chemical speciation of submicron particulate matter (PM1) sampled at a French industrial and coastal site

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