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International audience; Platinum, palladium, and rhodium have been measured in fresh snow samples from 14 locations within the Aspe Valley (Pyrenees Mountains, France) during two winter seasons, February 2003 and March 2004. Ultraclean procedures were employed for the sampling, sample treatment, and analysis in order to reduce sample contamination. Possible spectral interferences on platinum group element (PGE) analysis by inductively coupled plasma mass spectrometry (ICP-MS) were controlled and corrected. The detection limits obtained were 0.05, 0.45, and 0.075 pg g-1 for Pt, Pd, and Rh, respectively. PGE content in fresh snow from the Pyrenees Mountains range from 0.20 to 2.51 pg g-1 for Pt, 1.45-14.04 pg g-1 for Pd, and 0.24-0.66 pg g-1 for Rh. The higher PGE concentration, generally measured in sites located close to road traffic, exhibit potential resuspension of PGE-enriched particles emitted locally from the car exhaust, although no direct relationship could be observed with the number of vehicles. Measured atmospheric synoptic conditions allowed identification of the origin of air masses reaching the Aspe Valley and therefore provided information about the possible sources of platinum group elements. Higher PGE concentrations measured in 2004 samples, compared to the ones collected in 2003, indicate the influence of atmospheric synoptic conditions on the studied area. Fresh snow samples collected in 2003 could not be linked to a specific source, whereas 2004 samples could be influenced by PGE emissions from European vehicle fleet and Russian PGE-containing mining activities.

The Rhine valley, which stretches from the main Alpine crest to the Lake of Constance, was chosen as the target area to study unstationary aspects of foehn during the Special Observing Period (SOP) of the Mesoscale Alpine Programme (MAP). This large valley is up to 10?km wide and has some of the highest foehn frequencies in the European Alps. The MAP subprogram FORM (FOehn in the Rhine valley during MAP) was designed to investigate various aspects of the foehn including the interaction of foehn flow with the boundary layer and the processes that remove the cold air pool. The subprogram was also focused on improving the understanding and forecasting of foehn-related phenomena such as waves and turbulence. A large number of in-situ and remote sensing observing systems were deployed to take measurements during the field phase of MAP. Among them were about 50 surface stations, up to 9 radiosonde stations, 2 wind profilers, 4 Doppler sodars, 2 scintillometers, 1 scanning and 1 backscatter lidar and different research aircraft. This paper gives an overview of the objectives of FORM, describes the target area and its instrumentation, and provides a detailed synoptic description of the 12 foehn cases observed during the MAP SOP.

Surface and remote-sensing instruments deployed during ESCOMPTE experiment over the Marseille area, along the Mediterranean coast, were used to investigate the fine structure of the atmospheric boundary layer (ABL) during sea-breeze circulation in relation to pollutant transport and diffusion. Six sea-breeze events are analyzed with a particular focus on 25 June 2001. Advection of cool and humid marine air over land has a profound influence on the daytime ABL characteristics. This impact decreases rapidly with the inland distance from the sea. Nearby the coast (3 km inland), the mixing height Zi rises up to 750 m and falls down after 15:00 (UT) when the breeze flow reaches its maximum intensity. A more classical evolution of the ABL is observed at only 11-km inland where Zi culminates in the morning and stabilizes in the afternoon at about 1000 m height. Fine inspection of the data revealed an oscillation of the sea-breeze with a period about 2 h 47 min. This feature, clearly discernable for 3 days at least, is present in several atmospheric variables such as wind, temperature, not only at the ground but also aloft in the ABL as observed by sodar/RASS and UHF wind profilers. In particular, the mixing height Zi deduced from UHF profilers observations is affected also by the same periodicity. This pulsated sea-breeze is observed principally above Marseille and, at the northern and eastern shores of the Berre pond. In summary, the periodic intrusion over land of cool marine air modifies the structure of the ABL in the vicinity of the coast from the point of view of stability, turbulent motions and pollutants concentration. An explanation of the source of this pulsated sea-breeze is suggested.

Aircraft measurements made during the Mesoscale Alpine Programme are used to investigate the atmospheric turbulence inside the large Rhine Valley on the north side of the Alps during five fohn events induced by a southerly to south-westerly flow. Prior to that goal, an analysis based on the observations of different instruments provides the mesoscale and large-scale atmospheric conditions encountered during the aircraft flights. The temporal and spatial structure of the fohn event of 20 October 1999 is presented as an example. In this case, the three-dimensional structure of the turbulence is investigated. An estimate of the different components of the turbulent kinetic energy budget is attempted. It shows that the source of the turbulence is mainly mechanical. In spite of the high inhomogeneity of the turbulence in space and between the cases, the study of five cases allows the description of some general features of the fohn events, such as the plume of turbulence extending downwards in the valley and the near homogeneity of the dissipative length-scale. Copyright © 2003 Royal Meteorological Society.

Enhancement of the air refractive index structure parameter C2n often occurs at the top of the convective boundary layer (CBL), where the absolute values of the vertical gradients of virtual potential temperature and mixing ratio have a peak. This well-known behavior of the C2n profiles is often used to locate the height of the mixed layer Zi from UHF wind profiler observations. In the present study, Zi determination with the C2n-based technique was investigated for a case of clear-air CBL and a case of cloud-topped CBL. In certain circumstances, such as multifold C2n peaks or poorly defined peaks, these techniques fail to correctly retrieve CBL height. In order to improve Zi determination, a new method based on the conjoint use of C2n and Doppler spectral width profiles is proposed and discussed.

The study is focused on the retrieval and validation of the turbulent kinetic energy dissipation rate efrom spectral width measurements made by a UHF wind profiler in the convective atmospheric boundary layer. The possibility to deduce e, which is one of the most important parameters for boundary-layer study and monitoring, from Doppler radar spectral width measurements has a firm theoretical basis established by numerous earlier works. However, the major drawback of this approach lies in various meteorological and instrumental sources of non-turbulent spectral width broadening which have to be recognised and accounted for. In the first part of the study, the theoretical background of the e retrieval is detailed and all possible causes of spectral broadening are listed and evaluated. In the second part, the method is applied to four days of UHF diurnal boundary-layer observations, collected during the TRAC-98 experiment, for which in situ aircraft e measurements were available. Comparison between radar-retrieved eand in situ aircraft measurements yields a fairly good agreement with a linear correlation coefficient of about 0.9 and a residual bias less than 2 × 10-4 m2 s-3. The analysis of ederived from vertical and off-zenith observation leads to the recommendation, that in the boundary layer, where the wind is usually moderate, data collected by off-zenith beams should be used. Indeed, the measurement of the vertical spectral width, less affected by the large-scale broadening factor, can still be altered by the ground clutter removal.

A C-band meteorological Doppler radar has been used to investigate the dynamic processes and the coherent organizations within a clear air atmospheric boundary layer (ABL). Depending on the atmospheric conditions, this moderate sensitive radar was able to provide a coherent and continuous velocity field from 0.1 km up to 3 km above ground, and over a horizontal range reaching at least 30 km. Here the focus is on the profiling capability of this Doppler radar in a clear air boundary layer. The velocity volume processing method was used to deduce vertical profiles of the wind field from the panoramic conical scannings. A comparison between the observations of this C-band radar and two UHF wind profilers is presented. Good agreement was obtained in the measurements of the wind velocity and of the vertical and temporal evolution of the reflectivity. In particular, as for UHF wind profilers, the ABL top was found coincident for the C-band radar with a bright band of reflectivity maximum, and both type...

This paper describes the characteristics of the temperature and wind speed fluctuations measurements inside the north Congo forest during the DECAFE experiment in February 1988. The temperature spectra show a maximum at 0.018 Hz with a slope close to − 2 3 and the scattering increases as the frequency decreases while the nS(n) wind speed spectra has a maximum at 0.02 Hz with a negative slope at higher frequencies. The dynamical or thermal turbulence in the canopy layer is characterized by a maximum variance in the middle of the day and by two singularities during the transition time at sunrise and sunset when variances are small. This behavior could be due to the turbulent kinetic energy which increases turbulence in the upper part of the canopy layer and also to strongly anisotropic motions generated by the non-homogeneity of the temperature and wind fields inside the forest.

The main objective of the present paper is the use of a constant volume balloon (CVB) as a tool to (i) study trapped lee waves and (ii) assess the forecasting capability of a nonhydrostatic numerical model. Then, CVB data obtained during the Pyrenees Experiment (PYREX) are compared with nonhydrostatic two-dimensional trapped lee waves simulated by the Meso-NH model. This model is a community research model based on the Lipps–Hemler form of the anelastic system, which has been recently developed by the CNRM of Meteo-France and the Laboratoire d’Aerologie of Universite Paul Sabatier in France. To analyze how the CVB responds to lee waves, a simple CVB model is first applied to academic atmospheric stationary wave flows, analogous to those encountered during PYREX. This model takes into account the vertical velocity of the surrounding air, geometrical parameters of the balloon, and the atmospheric heating processes. Results show that the CVB reacts well to the atmospheric wave period, with a phase d...

This study presents an analysis of turbulence data from aircraft measurements made during the PYREX experiment. The data were gathered in a central region of the Pyrenees, a mountain-range with a more or less west-east orientation which constitutes a major barrier for northerly and southerly flows. The data used come principally from a Merlin IV aircraft which performed about 100 straight-and-level 20 km runs perpendicular or parallel to the main mountain-range axis. The basic turbulence moments are presented. The data from mountain flows with similar upstream conditions were gathered together in order to construct composite two-dimensional turbulence fields in the vertical plane perpendicular to the axis of the range. These fields clearly show the location of the turbulence areas on the lee side of the mountain range. The turbulent kinetic energy field shows that turbulence is principally produced by a wake effect. On the lee side the profiles of the various turbulence parameters indicate a maximum at an altitude close to the average height of the mountain in a region having both a strong wind shear and a weak lapse rate. Finally, we show that the turbulence observed in this study obeys the universal law σ-3ωϵλ≈ constant, where sw is the standard deviation of the vertical velocity, λ the spectral length-scale and ϵ the dissipation rate of turbulent kinetic energy. The average value of 2.3 found for this constant is comparable to the values found by other authors under various conditions of turbulence, in particular homogeneous turbulence.

The marine atmospheric boundary layer (MABL) analysed over the midlatitude ocean during SEMAPHORE (Structure des Echanges Mer-Atmosphere, Proprietes des Heterogeneites Oceaniques: Recherche Experimentale) was topped by a broken stratocumulus layer. Around midday, this layer was decoupled from the surface. the mixed layer which develops under the influence of surface buoyancy flux therefore stays well below the cloud layer. In this paper, we present a parametrization of turbulence moments and spectral characteristics in this mixed layer. the data were gathered in autumn 1993 by two instrumented aircraft in the Azores region. We began by analysing a large number of runs, each over 100 km in length, in order to determine what scales must be taken into account for estimating turbulence moments. the turbulence characteristics were thus calculated on 241 straight and level runs performed at various altitudes in the mixed layer during 17 flights over a period of 10 days, with various winds and surface fluxes. the turbulence profiles were normalized using the convective-scales, computed from the mixed-layer thickness (not the whole MABL thickness), and the surface fluxes. the contribution to buoyancy of temperature and moisture fluctuations are of the same order of magnitude, but they are of the same sign in the lower part of the mixed layer and of the opposite sign in the upper part. the result is that buoyancy flux vanishes at the top of the mixed layer, and entrainment is weak. These features are significantly different from what is observed in continental convective boundary layers. We also examine the usefulness of skewness profiles (mainly of moisture skewness) in understanding mixed-layer dynamics (mainly of the respective contributions of the surface and top processes).

In the field phases of the European Tracer EXperiment (ETEX), an inert tracer was released for 12 h into the atmosphere and samples taken at several locations downwind. During the same time, several Constant Volume Balloons (CVB) (10 and 6 for ETEX first and second release, respectively) were launched into different altitudes and followed as far as 21–188 km, to indicate the initial dispersion directions of the tracer puff. A model simulating the CVB behaviour in hydrostatic meso-scale model forecasts is applied to ETEX data to demonstrate its capability to predict the tracer puff mean axis over long distances (−2000 km). CVB model results are first compared to air parcels trajectories and 2D (i.e. isentropic, isobaric and isodensity) trajectories. Then they are compared to the measured CVB trajectories and finally to the tracer puff trajectories. As expected, the CVB model and isodensity model trajectories are found to be identical. The 16 CVBs calculated trajectories nearly overlap the real ones over 21–188 km with mean absolute horizontal transport deviations less than 20 km (average value of 8.2 km). The corresponding relative transport deviations are less than 45% with an average value of 20.6%. Better predictions are obtained for the ETEX second release. During the 60 h following ETEX’s first release start, the simulated CVBs are mainly found in the area of the maximum surface concentrations of the released tracer, up to 2000 km. Up to 36 h after ETEX second tracer release start, the simulated CVB trajectories predict well the mean axis of the tracer puff, but failed later.

The sensitivity of the marine atmospheric boundary layer (MABL) subjected to sea surface temperatures (SST) during the Structure des Echanges Mer-Atmosphere, Proprietes des Heterogeneites Oceaniques: Recherche Experimentale (SEMAPHORE) experiment in 1993 has been studied. Atmospheric analyses produced by the Action de Recherche, Petite Echelle, Grande Echelle (ARPEGE) operational model at the French meteorological weather service assimilated data sets collected between October 7 and November 17, 1993, merged with the Global Telecommunication System (GTS) data. Analyses were validated against independent data from aircraft instruments collected along a section crossing the Azores oceanic front, not assimilated into the model. The responses of the mean MABL in the aircraft cross section to changes in SST gradients of about 1°C/100 km were the presence of an atmospheric front with horizontal gradients of 1°C/100 km and an increase of the wind intensity from the cold to the warm side during an anticyclonic synoptic situation. The study of the spatiotemporal characteristics of the MABL shows that during 3 days of an anticyclonic synoptic situation the SST is remarkably stationary because it is principally controlled by the Azores ocean current, which has a timescale of about 10 days. However, the temperature and the wind in the MABL are influenced by the prevailing atmospheric conditions. The ocean does not appear to react to the surface atmospheric forcing on the timescale of 3 days, whereas the atmospheric structures are modified by local and synoptic-scale advection. The MABL response appears to be much quicker than that of the SSTs. The correlation between the wind and the thermal structure in the MABL is dominated by the ageostrophic and not by the geostrophic component. In particular, the enhancement of the wind on either side of the SST front is mainly due to the ageostrophic component. Although the surface heat fluxes are not the only cause of ageostrophy, the surface buoyancy flux Q b appears to be an important local source.

Two major campaigns (Surface of the Oceans, Fluxes and Interactions with the Atmosphere (SOFIA) and Structure des Echanges Mer-Atmosphere, Proprietes des Heterogeneites Oceaniques: Recherche Experimentale (SEMAPHORE)) devoted to the study of ocean-atmosphere interaction were conducted in 1992 and 1993, respectively, in the Azores region. Among the various platforms deployed, instrumented aircraft and ship allowed the measurement of the turbulent flux of sensible heat, latent heat, and momentum. From coordinated missions we can evaluate the sea surface fluxes from (1) bulk relations and mean measurements performed aboard the ship in the atmospheric surface layer and (2) turbulence measurements aboard aircraft, which allowed the flux profiles to be estimated through the whole atmospheric boundary layer and therefore to be extrapolated toward the sea surface level. Continuous ship fluxes were calculated with bulk coefficients deduced from inertial-dissipation measurements in the same experiments, whereas aircraft fluxes were calculated with eddy-correlation technique. We present a comparison between these two estimations. Although momentum flux agrees quite well, aircraft estimations of sensible and latent heat flux are lower than those of the ship. This result is surprising, since aircraft momentum flux estimates are often considered as much less accurate than scalar flux estimates. The various sources of errors on the aircraft and ship flux estimates are discussed. For sensible and latent heat flux, random errors on aircraft estimates, as well as variability of ship flux estimates, are lower than the discrepancy between the two platforms, whereas the momentum flux estimates cannot be considered as significantly different. Furthermore, the consequence of the high-pass filtering of the aircraft signals on the flux values is analyzed; it is weak at the lowest altitudes flown and cannot therefore explain the discrepancies between the two platforms but becomes considerable at upper levels in the boundary layer. From arguments linked to the imbalance of the surface energy budget, established during previous campaigns performed over land surfaces with aircraft, we conclude that aircraft heat fluxes are probably also underestimated over the sea.

The Structure des Echanges Mer-Atmosphere, Proprietes des Heterogeneites Oceaniques: Recherche Experimentale (SEMAPHORE) experiment, the third phase of which took place between October 4 and November 17, 1993, was conducted over the oceanic Azores Current located in the Azores basin and mainly marked at the surface by a thermal front due to the gradient of the sea surface temperature (SST) of about 1° to 2°C per 100 km. The evolution of the marine atmospheric boundary layer (MABL) over the SST front was studied with two aircraft and a ship in different meteorological conditions. For each case, the influence of the incoming air direction with respect to the orientation of the oceanic front was taken into account. During the campaign, advanced very high resolution radiometer pictures did not show any relation between the SST field and the cloud cover. The MABL was systematically thicker on the warm side than on the cold side. The mean MABL structure described from aircraft data collected in a vertical plane crossing the oceanic front was characterized by (1) an atmospheric horizontal gradient of 1° to 2°C per 100 km in the whole depth of the mixed layer and (2) an increase of the wind intensity from the cold to the warm side when the synoptic wind blew from the cold side. The surface sensible heat (latent heat) flux always increased from the cold to the warm sector owing to the increase of the wind and of the temperature (specific humidity) difference between the surface and the air. Turbulence increased from the cold to the warm side in conjunction with the MABL thickening, but the normalized profiles presented the same structure, regardless of the position over the SST front. In agreement with the Action de Recherche Programme te Petite Echelle and Grande Echelle model, the mean temperature and momentum budgets were highly influenced by the horizontal temperature gradient. In particular, the strong ageostrophic influence in the MABL above the SST front seems linked with the secondary circulation due to the SST front.

The Pyrénées experiment (PYREX), launched by the French and Spanish meteorological services, had provided an extensive database that was used in the present work to describe the airflow around the Pyrénées and to evaluate the applicability of the linear theory to stable flows that are blocked by a relatively large mountain range. The direction of incidence of the incoming synoptic wind was used to classify the flow in northerly and southerly categories. In both categories, the directions of incidence bear westward from normal incidence to the main axis of the mountain range. Froude (Fr) and Rossby (Ro) numbers were used to represent the thermodynamic characteristics of the incoming air masses and the scale of interaction in terms of the mountain shape. Here, Fr and Ro were found linearly correlated, and their combination corresponding to blocked flow in all the cases. Average cross correlations between the pressure drag across the mountain and the vertical profiles of the wind components at selected locations demonstrate that the horizontal flow is clearly separated in an upper undisturbed regime and a lower blocked regime in which regional winds are induced. The separating layer corresponding to the mean Froude number encountered during PYREX was found at an altitude of about two-thirds of the mountaintop. Due to the perturbation induced by the deflected wind, the total wind during northerly flows is stronger on the east side than on the west side, while during southerly incoming flows, the total wind distribution is almost symmetric. Good correlation was found among the pressure field perturbation, the pressure drag, and Froude number of the incoming flow. Clear correlation rules can be deduced from the analysis of the experimental data at different altitudes for both categories of incoming flows. Therefore, since the pressure drag can be easily determined using field measurements, it becomes a powerful tool for the prediction of the regional winds around the Pyrénées.

This paper considers a linear hydrostatic model of a stable, uniform, constant rotational airflow over three- dimensional, elliptic, cross-sectional families of mountains in a z system. The surface pressure and the winds that are induced around the mountain chain are deduced using Fourier representation in both horizontal directions. The surface pressure perturbations and the induced wind intensities are linked to 1) the incoming airmass thermodynamic properties through Froude and Rossby numbers, 2) the geometrical aspect ratio of the mountain, 3) the direction of incidence of the incoming flow relative to the mountain orientation, and 4) the Coriolis effect through the Rossby number. The balance between the different factors that contribute to the morphology of the pressure and wind fields was established for northerly and southerly incoming flows that were blocked by an elliptical barrier resembling the Pyrenees mountain chain. Fair agreement was found between the results of the model and the e...

The third intensive observation period (IOP3) of PYREX was a case of strong lee waves generated by a southerly wind crossing the Pyrenees chain. Upstream radiosounds and measurements obtained by aircraft along the chain transect and by constant volume balloons launched near the crest provided spatial characteristics of the lee wave at different times and heights. Values ranging from 7 to 14 km for the horizontal wavelength, and from 3 to 5 m s−1 for the maximum amplitude of the air vertical velocity, were observed. The lee wave horizontal extent, measured from the crest line, reached 30 to 55 km. In addition, two very high frequency stratosphere-troposphere (VHF ST) radars, one on the mountain mean axis and another downstream in the lee wave field, observed the temporal variations of the vertical profiles of the vertical velocity. The analysis of those observed variations and their vertical distribution allowed the stationarity of the wave to be studied. The lee wave was found to be far from stat...

International audience; About six years ago, the planned field experiment Pyrenean Experiment (PYREX) was presented in the Bulletin. After a successful field phase in October and November 1990, and much work to prepare a consistent database, interpret the measurements, and compare model results with observations, it is possible to present today a summary of scientific results. New insight has been obtained in the areas of mountain drag measurement and interpretation, humidity measurement, boundary layer measurement by lidar, the ability of mesoscale models to represent orographic flows, the roughness length of mountains, and parameterization of subgrid-scale gravity waves. The present paper summarizes these results and may be understood as an introduction to this unique dataset on orographic flows. The authors believe that much interesting research can still be achieved with the PYREX data, which are accessible to any interested scientist.

Lʼobjet de cette etude est dʼetablir un etat de reference des contaminants metalliques pour le suivi des emissions du trafic routier dans une vallee pyreneenne (vallee dʼAspe), lie a lʼouverture du tunnel du Somport. Les elements traces metalliques (ETM) (Al, Na, Mg, Ca, K, V, Mn, Cr, Zn, Cu, Rb, Cd, Sn, Sb, Ba, Ce, Pb et U) et les rapports isotopiques du plomb (208Pb/206Pb et 206Pb/207Pb) ont ete determines dans differents recepteurs atmospheriques (neige fraiche, depot total, particules atmospheriques et lichen corticole). La quantification des ETM a necessite la mise en œuvre de protocoles de prelevements et dʼanalyses ultra-traces. Les differents recepteurs atmospheriques etudies ont permis dʼintegrer les apports atmospheriques sur des echelles de temps allant de lʼevenement journalier (neige fraiche) a lʼaccumulation des apports pluriannuels (lichens) le long de lʼaxe nord-sud de la vallee et en fonction de lʼaltitude.Lʼensemble des resultats montre que la vallee dʼAspe presente un niveau de contamination specifique des zones eloignees des emissions anthropiques. Les principaux contaminants metalliques identifies sont Cd, Sb, Zn, Cu, Pb et Sn. En outre, la composition en ETM des recepteurs analyses montrent une contribution des contaminants : (i) liee aux emissions locales des combustions domestiques, a la pratique saisonniere de lʼecobuage et au trafic routier, et (ii) aux sources anthropiques regionales de lʼincineration des dechets menagers, des industries et des centres urbains. La vallee est egalement soumise significativement a lʼerosion eolienne et aux apports du transport a grande echelle de lʼhemisphere Nord. La caracterisation des ETM et des rapports isotopiques du plomb dans le tunnel du Somport indique (i) que le trafic routier emet significativement de l'etain, du cuivre et du zinc, et (ii) bien que le plomb presente de faibles emissions, sa composition isotopique montre une source faiblement radiogenique du trafic routier. Toutefois, le faible niveau de circulation automobile dans la vallee dʼAspe ne permet pas de mettre evidence une contamination significative provenant des emissions du trafic routier.

International audience; ESCOMPTE programme aims at studying the emissions of primary pollutants in industrial and urban areas, their transport, diffusion and transformation in the atmosphere. This experiment, carried out in southeast France, can be used to validate and to improve meteorological and chemical mesoscale models. One major goal of this experiment was to follow the pollutant plumes, and to investigate its thermodynamic and physico-chemical time evolution. This was realized by means of constant volume balloons, located by global position satellite (GPS) and equipped with thermodynamic and ozone sensors, flying at constant density levels. During the two ESCOMPTE campaigns that took place in June and July 2000 and 2001, 40 balloons were launched, 17 of them equipped with ozone sensors during the day from 0800 to 1800 UTC. Balloons' altitudes flight levels ranged between 400 and 1200 m altitude with Mistral (northerly synoptic flow) and Sea Breeze (southerly breeze) conditions. The atmospheric boundary layer (ABL) topography of the experimental domain is complex and varies strongly from day to day. Its depth presents a large gradient from the sea coast to the north part of the ESCOMPTE domain, and also more complex variability within the domain. The balloons' trajectories describe the evolution of the pollutant plume emitted from the industrial area of Fos-Berre or from the Marseille urban area. Constant volume balloons give a good description of the trajectories of these two plumes. The balloons, which fly at an isopicnic level, cross different atmospheric layers chiefly depending on the ABL height in relation with the constant volume balloons flight level. Thus, each balloon flight is decomposed into different segments that correspond to the same atmospheric layer. In each segment, the ozone content variation is analyzed in relation to other thermodynamical parameters measured by the balloon and mainly to the vapor mixing ratio content. During ESCOMPTE campaign, the mean linear rate of chemical net ozone production at the top of the atmospheric boundary layer was found to be around 6 ppb h−1.

This paper summarizes the findings of seven years of research on fohn conducted within the project ‘Fohn in the Rhine Valley during MAP’ (FORM) of the Mesoscale Alpine Programme (MAP). It starts with a brief historical review of fohn research in the Alps, reaching back to the middle of the 19th century. Afterwards, it provides an overview of the experimental and numerical challenges identified before the MAP field experiment and summarizes the key findings made during MAP in observation, simulation and theory. We specifically address the role of the upstream and cross-Alpine flow structure on fohn at a local scale and the processes driving fohn propagation in the Rhine Valley. The crucial importance of interactions between the fohn and cold-air pools frequently filling the lower Rhine Valley is highlighted. In addition, the dynamics of a low-level flow splitting occurring at a valley bifurcation between the Rhine Valley and the Seez Valley are examined. The advances in numerical modelling and forecasting of fohn events in the Rhine Valley are also underlined. Finally, we discuss the main differences between fohn dynamics in the Rhine Valley area and in the Wipp/Inn Valley region and point out some open research questions needing further investigation. Copyright © 2007 Royal Meteorological Society

Ground-based remote sensing systems have been used during the ESCOMPTE campaign, to continuously characterize the boundary-layer behaviour through many atmospheric parameters (wind, extinction and ozone concentration distribution, reflectivity, turbulence). This analysis is focused on the comparison of the atmospheric stratification retrieved from a UV angular ozone lidar, an Ultra High Frequency wind profiler and a sodar, above the area of Marseille, on June 26th 2001 (Intensive Observation Period 2b). The atmospheric stratification is shown to be very complex including two superimposed sea breezes, with an important contribution of advection. The temporal and spatial evolution of the stratification observed by the UV lidar and by the UHF radar are in good agreement although the origin of the echoes of these systems is quite different. The complexity of the dynamic situation has only partially been retrieved by a non-hydrostatic mesoscale model used with a 3 km resolution.

The UBL/CLU (urban boundary layer/couche limite urbaine) observation and modelling campaign is a side-project of the regional photochemistry campaign ESCOMPTE. UBL/CLU focuses on the dynamics and thermodynamics of the urban boundary layer of Marseille, on the Mediterranean coast of France. The objective of UBL/CLU is to document the four-dimensional structure of the urban boundary layer and its relation to the heat and moisture exchanges between the urban canopy and the atmosphere during periods of low wind conditions, from June 4 to July 16, 2001. The project took advantage of the comprehensive observational set-up of the ESCOMPTE campaign over the Berre-Marseille area, especially the ground-based remote sensing, airborne measurements, and the intensive documentation of the regional meteorology. Additional instrumentation was installed as part of UBL/CLU. Analysis objectives focus on (i) validation of several energy balance computational schemes such as LUMPS, TEB and SM2-U, (ii) ground truth and urban canopy signatures suitable for the estimation of urban albedos and aerodynamic surface temperatures from satellite data, (iii) high resolution mapping of urban land cover, land-use and aerodynamic parameters used in UBL models, and (iv) testing the ability of high resolution atmospheric models to simulate the structure of the UBL during land and sea breezes, and the related transport and diffusion of pollutants over different districts of the city. This paper presents initial results from such analyses and details of the overall experimental set-up.

The general purpose of this paper is to experimentally study mesoscale dynamical aspects of the Mistral in the coastal area located at the exit of the Rhône-valley. The Mistral is a northerly low-level flow blowing in southern France along the Rhône-valley axis, located between the French Alps and the Massif Central, towards the Mediterranean Sea. The experimental data are obtained by UHF wind profilers deployed during two major field campaigns, MAP (Mesoscale Alpine Program) in autumn 1999, and ESCOMPTE (Expérience sur Site pour COntraindre les Modèles de Pollution atmosphériques et de Transports d'Emission) in summer 2001. Thanks to the use of the time evolution of the vertical profile of the horizontal wind vector, recent works have shown that the dynamics of the Mistral is highly dependent on the season because of the occurrence of specific synoptic patterns. In addition, during summer, thermal forcing leads to a combination of sea breeze with Mistral and weaker Mistral due to the enhanced friction while, during autumn, absence of convective turbulence leads to substantial acceleration as low-level jets are generated in the stably stratified planetary boundary layer. At the exit of the Rhône valley, the gap flow dynamics dominates, whereas at the lee of the Alps, the dynamics is driven by the relative contribution of "flow around" and "flow over" mechanisms, upstream of the Alps. This paper analyses vertical velocity and turbulence, i.e. turbulent dissipation rate, with data obtained by the same UHF wind profilers during the same Mistral events. In autumn, the motions are found to be globally and significantly subsident, which is coherent for a dry, cold and stable flow approaching the sea, and the turbulence is found to be of pure dynamical origin (wind shears and mountain/lee wave breaking), which is coherent with non-convective situations. In summer, due to the ground heating and to the interactions with thermal circulation, the vertical motions are less pronounced and no longer have systematic subsident charateristics. In addition, those vertical motions are found to be much less developed during the nighttimes because of the stabilization of the nocturnal planetary boundary layer due to a ground cooling. The enhanced turbulent dissipation-rate values found at lower levels during the afternoons of weak Mistral cases are consistent with the installation of the summer convective boundary layer and show that, as expected in weaker Mistral events, the convection is the preponderant factor for the turbulence generation. On the other hand, for stronger cases, such a convective boundary layer installation is perturbed by the Mistral.

International audience; The UBL/CLU experiment is a side project of the regional photochemistry experiment ESCOMPTE over the Berre-Marseille area. It took advantage of the large experimental set-up, with chemistry and dynamics, ground-based and airborne instrumentations, including a large array of vertical and 3D sounders. With additional specific instrumentation in the lower urban atmosphere, the UBL/CLU project aimed at documenting the dynamics and thermodynamics of the urban boundary layer (UBL) of the Marseille area during sunny periods. To date the initial data analyses include (a) the quality control inter-comparison of energy flux instrumentations, (b) the quality evaluation of the measurements at the urban stations, (c) the comparison of the Town Energy Balance model with the data obtained over the city center, (d) the evaluation of heat flux measurements with the scintillometers, (e) the temperature and moisture variability at the canopy level, (f) the 3D structure of the UBL during sea breeze episodes, (g) the visible and thermal infrared signatures of the urban canopy, (h) the numerical urban data base and satellite data analysis to generate high resolution land use and aerodynamic roughness maps.

In this paper, the pressure anomaly created by the Pyrenees during the Pyrenees Experiment (PYREX) in the Ebro Valley is studied and analysed. In the first part, pressure disturbance is obtained during some selected cases of PYREX by subtracting the synoptic component from the pressure registered by microbarographs and regular meteorological stations located in the area. As expected from a linear model, a low pressure is found for northerly synoptic winds and a high pressure for southerly synoptic winds. Data shows that the maximum pressure anomaly observed may reach values of the order of 7 hPa. Pressure-anomaly spatial patterns found inside the valley are classified by using principal component analysis (PCA). It is found that the first axes deduced from the PCA carried out represent 75% of the variance in pressure data, indicating that all the stations analysed observed pressure anomalies in a correlated manner. Pressure anomalies are then related to incident-air characteristics expressed as Froude and Rossby numbers. Significant correlation is generally found with these numbers in all cases, demonstrating the dynamic origin of the pressure anomaly computed.

On 11 August 1999, a near-total solar eclipse (80%) was observed in Campistrous, France. The influence of this particular event on the atmospheric boundary layer was observed with a UHF-RASS radar, a sodar and an instrumented mast. The changes in turbulence intensity, radar reflectivity, and temperature on the radiative budget are described in relation to collocated ground meteorological data. The impact of the eclipse induces a clear response of the atmosphere, with a time lag of 15 to 30 min, perceptible in several mean and turbulent meteorological variables up to the top of the atmospheric boundary layer.

The spatial variability of sensible and latent heat flux measured by aircraft over a 90-km × 75-km area, near the Central Supersites of HAPEX-Sahel (Hydrologic and Atmospheric Pilot Experiment, Sahel) is discussed. The data from six flights are presented four of which were obtained during the rainy season; the others being obtained at the beginning of the dry season. A basic difference in the behaviour of the latent heat transfer is revealed when the measurements under dry and wet conditions are compared: the latent heat flux is far more heterogeneous under drying conditions so that the estimation accuracy is reduced if the same integration length is used. It was found that under dry conditions the contribution of low frequency eddies is more important than that of local turbulence: the surface moisture is probably less homogeneous than in the wet period, but it is mostly the interaction between the marine and continental air masses linked to the closeness of the inter-tropical convergence zone that seems to drive the transfers. Two-dimensional fields of fluxes are constructed to study their spatial variability according to the hydrological conditions. These fields are systematically compared with those of the mean parameters likely to drive the transfers. Some characteristics, common to several fields, are revealed that can be considered specific to the climate in this region, at this time of year: an east-west gradient of albedo, a south-north gradient of the sensible heat flux, surface temperature and air temperature and a north-south gradient of specific humidity. However, the aerodynamic formula that relates flux to mean parameters, fails at a 25-km × 25-km scale but gives good results at a larger scale (90 km × 75 km): at this scale, the Dalton number is around 2.5 × 10 −3 .

The SEMAPHORE (Structure des Echanges Mer-Atmosphère, Propriétés des Hétérogénéités Océaniques: Recherche Expérimentale) experiment has been conducted from June to November 1993 in the Northeast Atlantic between the Azores and Madeira. It was centered on the study of the mesoscale ocean circulation and air-sea interactions. The experimental investigation was achieved at the mesoscale using moorings, floats, and ship hydrological survey, and at a smaller scale by one dedicated ship, two instrumented aircraft, and surface drifting buoys, for one and a half month in October-November (IOP: intense observing period). Observations from meteorological operational satellites as well as spaceborne microwave sensors were used in complement. The main studies undertaken concern the mesoscale ocean, the upper ocean, the atmospheric boundary layer, and the sea surface, and first results are presented for the various topics. From data analysis and model simulations, the main characteristics of the ocean circulation were deduced, showing the close relationship between the Azores front meander and the occurrence of Mediterranean water lenses (meddies), and the shift between the Azores current frontal signature at the surface and within the thermocline. Using drifting buoys and ship data in the upper ocean, the gap between the scales of the atmospheric forcing and the oceanic variability was made evident. A 2 °C decrease and a 40-m deepening of the mixed layer were measured within the IOP, associated with a heating loss of about 100 W m-2. This evolution was shown to be strongly connected to the occurrence of storms at the beginning and the end of October. Above the surface, turbulent measurements from ship and aircraft were analyzed across the surface thermal front, showing a 30% difference in heat fluxes between both sides during a 4-day period, and the respective contributions of the wind and the surface temperature were evaluated. The classical momentum flux bulk parameterization was found to fail in low wind and unstable conditions. Finally, the sea surface was investigated using airborne and satellite radars and wave buoys. A wave model, operationally used, was found to get better results compared with radar and wave-buoy measurements, when initialized using an improved wind field, obtained by assimilating satellite and buoy wind data in a meteorological model. A detailed analysis of a 2-day period showed that the swell component, propagating from a far source area, is underestimated in the wave model. A data base has been created, containing all experimental measurements. It will allow us to pursue the interpretation of observations and to test model simulations in the ocean, at the surface and in the atmospheric boundary layer, and to investigate the ocean-atmosphere coupling at the local and mesoscales.

In this paper we use wavelet transform to analyze aircraft data in the case of inhomogeneous turbulence. The data were gathered during a meteorological experiment (PYREX) to study the airflow around a mountain ridge: the Pyrenees. We present the spatial distribution and the various lengthscales of the energy fluxes computed from turbulence data (thermodynamics) for four specific flows : 1) Mechanical turbulence in the lee side of the mountain; 2) Mountain lee waves and small scale turbulence; 3) Coherent structures within Tramontana wind over the Mediterranean sea; and 4) Inhomogeneous and strongly turbulent valley wind (Cierzo wind). The last two local winds are generated by the Pyrenees mountain range.

International audience; Although the qualitative influence of mountains over the atmosphere has been known for a long time, numerous deficiencies, linked to orography, are still noted, either in forecasts by regional models, or in the long-term behavior of climate models. This is why the French and Spanish weather services are undertaking an important field campaign to document the dynamic modifications to the atmospheric flow generated by the Pyrenean range during a 2-month period (October and November 1990) with six intensive observation periods (IOPs) of 2 to 3 days. The experimental strategy is based largely on mesoscale numerical-model results and will help to validate these models. The main focus is on the documentation of clear-air turbulence generated either by breaking mountain waves, by surface roughness, or by the wind shear induced by the lateral-flow deviation around the mountain. Experimental means include several networks of surface stations, radio soundings, constant-level balloons, four wind profilers, and several research aircraft.

Ozone concentrations have been measured in the northern Congo, near Impfondo (1°37 N, 18°04 E) during the DECAFE experiment in February 1988. This period corresponds to the dry season in the northern part of the equatorial region. The measurements were carried out simultaneously (1) at ground level in a large clearing, (2) inside the forest between 0 and 30 m, and (3) above the forest with a captive balloon flying up to 400 m. The results presented here are compared with the data obtained in the Mayombe forest in the southern part of Congo, near Dimonika in June 1988, during the dry season. For both northern and southern forested areas the ozone concentrations measured at ground level in a large clearing exhibit daily variations with maxima in the afternoon range between 10 and 30 parts per billion by volume (ppbv) and minima at the end of the night between 4 and 15 ppbv. The characterics of each surface ozone cycle are analyzed. Inside the forest, ozone concentrations are found very low near the ground and rarely exceed 15 ppbv above the canopy. At night the ozone concentrations are always close to zero at any level from the ground to the top of the canopy. Ozone removal processes are mainly located in the canopy. The ozone concentration above the forest is seen to increase in the early morning. Likely, this ozone production has a photochemical origin, the precursors coming from soils and biomass burning. The relationships among the vertical profiles of ozone, temperature, and water vapor are discussed.

The objective of the Dynamique et Chimie de l'Atmosphere en Foret Equatoriale (DECAFE) program is to determine the influence of African tropics on atmospheric chemistry. Savannas and forests constitute the main ecosystems in this area. Because of the occurrence of biomass burning during the dry season, savannas of the northern and southern hemispheres are active sources of many constituents which are of interest to atmospheric chemistry. The rain forest, with its important biomass activity is also a source or sink for gases and aerosols. Man's impact on this ecosystem is important since the rate of deforestation is about 0.5% per year. Furthermore, a general air motion from savanna regions toward the tropical rain forest is observable in most parts of Africa, leading to important interactions between the respective emissions of these two ecosystems. The DECAFE experiments presented in this issue were located in the Mayombe forest (southern Congo) and in the forest of the northen Congo. A study of rain chemistry was undertaken in 1986 in the Mayombe forest, then continued in the northern forest. In February 1988 a coordinated experiment was held in the tropical rain forest of the north, near Impfondo. The paper presents an overview of the general meteorological and climatological situation in the African tropics, a study of the climatological features of the sites, and a short description of the Impfondo experiment, which is the subject of most of the contributions to this special issue. The local meteorological situation during the time of the experiment is analyzed.

During the Dynamique et Chimie de l'Atmosphere en Foret Equatoriale (DECAFE) campaign, dynamical and thermodynamical measurements were made at Impfondo (1°37′N, 18°04′W), over the dense rain forest of northern Congo during the dry season (February 1988). During the measurement period the experimental site was located south of the intertropical convergence zone ground track which manages the dynamics of the large scale. Above the experimental site, the atmospheric low layers are supplied by monsoon air coming from the Guinean gulf; the upper layers (>1500 m) are supplied by warm and dry air (trade winds) coming from the northern desert region and the savanna. Our experimental approach consists of analyzing the heat and moisture content in the low troposphere from vertical soundings made by a tethered balloon (0–400 m) and an aircraft (0–4000 m). The analysis of the evolution of the observed planetary boundary layer (PBL) is made with a mixed layer one-dimensional model which is forced to represent correctly the observed PBL height growth. The simulated and observed budgets of the heat and moisture in the PBL are balanced by adding dry air to the simulated PBL in the afternoon. This drying out can be maintained only by high levels of entrainment flux at the PBL top. An entrainment velocity of 3 cm s−1 enables the balancing of the moisture budget. This entrainment velocity seems compatible with physicochemical transfers as those of methane and ozone.

Some aspects of the mean and turbulent structures of artificial thermal plumes in the boundary layer (BL) are presented. This analysis is based mainly on measurements with an instrumented aircraft. As initial conditions for plume rise, the characteristics of the BL during the 10 experiments are summarized. Under neutral conditions, plume rise in the BL follows approximately the classical prediction. Plume structure aspects are analyzed inside the upwind active zone corresponding to the region of strongest gradients within the convective column. In an attempt to compare the various experiments, scaling parameters are derived from energetic considerations for both dissipative and first-order parameters. Individual and normalized vertical profiles are given. The statistics presented discriminate between dry and condensed plumes. For both populations, vertical profile tendencies are rather similar in the BL. However, condensed plumes show higher values of the perturbed variables inside the whole colu...

This paper presents a new system of constant volume balloons used to study the dynamical and thermodynamical properties of the tropospheric boundary layer. The system allows to simultaneously localize up to 30 balloons and to observe thermodynamical characteristics of the air during their flight inside the boundary layer. Each balloon is equipped with a radar reflector and a sounding system giving information each second on pressure, humidity and temperature by sequential emissions which are received at a ground station. The trajectories of balloons are obtained by a non-tracking S-band radar where a special hardware processing unit allows to real time cancellation of most ground clutter. A complete set of the balloons positions is obtained every two or three minutes. The system was tested in October 1984 during a field experiment in the south of France. Results of the experimental procedure and of the quality of the balloon, radar and sounding capabilities are given. The scientific use of constant volume balloons in order to study the atmospheric boundary layer is examined.

The characteristics of the boundary layer over complex terrain (Lannemezan - lat.: 43.7° N and, long.: 0.7 ° E) are analyzed for various scales, using measurements obtained during the COCAGNE Experiment. In this first part, the dynamic characteristics of the flow are studied with respect to atmospheric stability and the relief at small (~20 km) and medium scales (~100 km). These relief scales depend on the topographical profile of the Lannemezan Plateau along the dominant axis of the wind (E-W) and the Pyrenees Mountains located at the south of the experimental site. The terrain heterogeneities have a standard deviation of ~48 m and a wavelength of ~2 km.

The work reported here describes the environmental impact of emitting about 1000 MW of dry heat from a concentrated source into the atmosphere. It is based on a large field program conducted jointly by the Centre de Recherches Atmospheriques and Electricite de France. This program provided an opportunity to evaluate the actual environmental impacts of large-scale heat release and to obtain data required to develop parameterization schemes for use in modeling heat releases by intense sources such as dry cooling towers. The heat source is an array of 105 fuel-oil burners distributed over 15 000 m2. An aerial assemblage suspended at two levels (25 and 50 m) over the burner array has been used to collect data (temperature and velocity fields) for analyzing aspects of both the mean and the turbulent components of the flow near the heat source. The flow field near the heat source comprises a cold downdraft upwind zone which supplies the burner area with ambient air, a convective zone containing a hot v...

This study presents an experimental analysis from aircraft measurements above the Pyrenees chain during the PYREX experiment. The Pyrenees chain, roughly WE oriented, is a major barrier for northerly and southerly airflows. We present a case of southerly flow (15 October 1990) and three successive cases of northerly flows above the Pyrenees (14, 15 and 16 November 1990) documented by two aircraft. The aircraft have described a vertical cross section perpendicular to the Pyrenean ridge. This area is described via the thermodynamical and dynamical fields which have a horizontal resolution of 10 km. Three methods for computing the vertical velocity of the air are presented. The horizontal advection terms which play a role in the budget equations are also evaluated. The altitude turbulence zone of 15 October are shown via turbulent fluxes, turbulent kinetic energy (TKE), dissipation rate of TKE and inertial length-scale. A comparison of results obtained by eddy-correlation and inertial-dissipation method is presented. The experimental results show a warm and dry downdraft for the southerly flow with large values for advection terms. All the mountain wave cases are also shown to present an important dynamical perturbation just above the Pyrenees at upper altitudes.

The SOFIA (Surface of the Ocean: Flux and Interaction with the Atmosphere) experiment, included in the ASTEX (Atlantic Stratocumulus Transition Experiment) field program, was conducted in June 1992 in the Azores region in order to investigate air-sea exchanges, as well as the structure of the atmospheric boundary layer and its capping low-level cloud cover. We present an analysis of the vertical structure of the marine atmospheric boundary layer (MABL), and especially of its turbulence characteristics, deduced from the aircraft missions performed during SOFIA. The meteorological situations were characteristic of a temperate latitude under anticyclonic conditions, i.e., with weak to moderate winds, weak surface sensible heat flux, and broken capping low-altitude cloud cover topped by a strong trade inversion. We show that the mixed layer, driven by the surface fluxes, is decoupled from the above cloud layer. Although weak, the surface buoyancy flux, and the convective velocity scale deduced from it, are relevant for scaling the turbulence moments. The mixed layer then follows the behaviour of a continental convective boundary layer, with the exception of the entrainment process, which is weak in the SOFIA data. These results are confirmed by conditional sampling analysis, which shows that the major turbulence source lies in the buoyant moist updrafts at the surface.