The mistral refers to a severe wind blowing over the Gulf of Lions after being channeled in the Rhone valley. It influences the western Mediterranean climate as it brings cold and dry continental air over the warm western Mediterranean, generating intense air‐sea heat exchanges and sea surface cooling, inducing the formation of the western Mediterranean deep water that moves into the Atlantic Ocean. The mistral is frequently observed to extend as far as a few hundred kilometers from the coast, and its fine‐scale dynamics over the sea is still only partially understood as finely resolved observations in time and space are lacking. The boundary layer pressurized balloons (BLPB) developed by the Centre National d'Etudes Spatiales and deployed during HyMeX SOP2 allowed the Lagrangian documentation of the mistral events that occurred between beginning of February to mid‐March 2013. Analyzed in synergy with the AROME‐WMED weather forecast model, all the terms of the Lagrangian formulation of the momentum conservation equation could be quantified showing three different regions: (1) an injection zone where the mistral flow is directed toward the center of the Genoa cyclone due to a strong zonal pressure gradient, enhanced friction, and entrainment in the mountain wake; (2) an ejection zone where the flow is deflected outward of the cyclone due to either the nonnegligible inertia pseudoforce or an inertial oscillation caused by a sudden friction decrease; and (3) a region of geostrophic deceleration due to the weakening of the pressure gradient.

Balloons are one of the key observing platforms for the atmosphere. Radiosounding is the most commonly used technique and provides over a thousand vertical profiles worldwide every day. These data represent an essential cornerstone of data assimilation for numerical weather prediction systems. Although less common (but equally interesting for the in situ investigation of the atmosphere), drifting boundary layer pressurized balloons (BLPBs) offer rare observational skills. These balloons collect meteorological and/or chemical measurements at isopycnal height as they drift in a quasi-Lagrangian way. The BLPB system presented in this paper was developed by the French Space Agency [Centre National d'Etudes Spatiales (CNES)] and has been used in field experiments focusing on precipitation in Africa [African Monsoon Multiscale Analysis (AMMA)] and the Mediterranean [Hydrological Cycle in the Mediterranean Experiment (HyMeX)] as well as on air pollution in India [Indian Ocean Experiment (INDOEX)] and the Mediterranean [Transport a Longue Distance et Qualite de l'Air dans le bassin Mediterraneen (TRAQA) and Chemistry-Aerosol Mediterranean Experiment (ChArMeX)]. One important advantage of BLPBs is their capability to explore the lowest layers of the atmosphere above the oceans, areas that remain difficult to access. BLPB had a leading role in a complex adaptive observation system for the forecast of severe precipitation events. These balloons collected data in the marine environment of convective systems, which were assimilated in real time to improve the knowledge of the state of the atmosphere in the numerical prediction models of Meteo-France.

Since the 1970s, the French space agency CNES has developed boundary-layer pressurised balloons (BLPBs) with the capability to transport lightweight scientific payloads at isopycnic level and offer a quasi-Lagrangian sampling of the lower atmosphere over very long distances and durations (up to several weeks).Electrochemical concentration cell (ECC) ozonesondes are widely used under small sounding balloons. However, their autonomy is limited to a few hours owing to power consumption and electrolyte evaporation. An adaptation of the ECC sonde has been developed specifically for long-duration BLPB flights. Compared to conventional ECC sondes, the main feature is the possibility of programming periodic measurement sequences (with possible remote control during the flight). To increase the ozonesonde autonomy, the strategy has been adopted of short measurement sequences (2–3 min) regularly spaced in time (e.g. every 15 min). The rest of the time, the sonde pump is turned off. Results of preliminary ground-based tests are first presented. In particular, the sonde was able to provide correct ozone concentrations against a reference UV-absorption ozone analyser every 15 min for 4 days. Then we illustrate results from 16 BLBP flights launched over the western Mediterranean during three summer field campaigns of the ChArMEx project (http://charmex.lsce.ipsl.fr): TRAQA in 2012, and ADRIMED and SAFMED in 2013. BLPB drifting altitudes were in the range 0.25–3.2 km. The longest flight lasted more than 32 h and covered more than 1000 km. Satisfactory data were obtained when compared to independent ozone measurements close in space and time. The quasi-Lagrangian measurements allowed a first look at ozone diurnal evolution in the marine boundary layer as well as in the lower free troposphere. During some flight segments, there was indication of photochemical ozone production in the marine boundary layer or even in the free troposphere, at rates ranging from 1 to 2 ppbv h −1, which is slower than previously found in the boundary layer over land in the same region.

Jean-Pierre Vandervaere (7) Les affiliations des auteurs se trouvent à la fin de l'article. RésuméD'initiative française, le projet international HyMeX a pour objectif d'améliorer la compréhension du cycle de l'eau en Méditerranée, de sa variabilité, de l'échelle de l'événement météorologique aux échelles saisonnières et interannuelles, et de ses caractéristiques sur une décennie, dans un contexte de changement global.Le projet est motivé par le rôle déterminant des processus de mésoéchelle, couplés entre l'atmosphère, la mer et la terre, sur la variabilité du système climatique et sur le déclenchement d'événements hydrométéorologiques extrêmes (précipitations et inondations, vents forts et convection océanique, canicules et sécheresses).Le projet vise enfin à évaluer les conséquences de ces événements extrêmes sur la vulnérabilité sociale et économique de cette région et sa capacité d'adaptation.

Boundary Layer Pressurized Balloons (Buss) from the CNES are small and light balloons equipped with sensors to monitor air thermodynamics and chemistry. Between June 2012 and August 2013, these balloons have been deployed in the North-West Mediterranean, during three successive campaigns: TRAQA, HYMEX and CHARMEX, split in five observing periods, successively. All together, these campaigns comprise 70 balloons (5, 35 and 30 planned by time of writing). Various configurations of BLPB have been implemented. The simplest instrumental set consists in sensors for pressure, relative humidity and temperature, all in-situ and fitted inside a specifically designed shelter on top of the balloon. For CHARMEX, this basic instrumentation was enhanced with ozone sensors (also in TRAQA), solar flux measurements and aerosols counters (LOAC). The data were collected at high rate and were transmitted via an Iridium communication system. The collected data were exploited in near real-time and also assimilated in weather prediction models such as AROME-WESTMED. During these campaigns, the scientific missions of the BLPB always consisted in the monitoring of air masses, the physical characteristics of which were measured all along their travel above the Mediterranean Sea. However, the very different scientific objectives of the campaigns lead to a variety of flight conditions. Also, the Buss were part of a larger observing system that was activated on alert during intensive observing period and the launch/flights were coordinated with other observing platforms such as research aircraft. In this short paper, we describe the BLPB platform, the programmatic context for the deployments and the types of missions, that range from pollution monitoring (ozone and aerosols) to humidity transport and evaporation. Then we present preliminary results about the use of balloon derived meteorological observations in numerical weather prediction (NWP) models.

teaching and research institutions in France or abroad, or from public or private research centers.L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

C ONTROL of vortical flow over delta wings in the subsonic regime has been the subject of many investigations in recent years. In particular, it has been demonstrated that suction at the leading edge of the wing can improve the aerodynamic performance of the wing [1,2]. However, the capability of such an approach to delay the vortex breakdown occurrence in the transonic regime remains poorly assessed in the literature. Thus, the purpose of this study is to numerically evaluate continuous suction effects on the dynamics of the leading-edge vortex as well as on the aerodynamic performance of the wing in the transonic regime. The considered geometry is the VFE-2 wing used in the RTO-AVT-113 project [3]. The sweep angle equals 65 deg and the root chord c is equal to 0.4905 m. The freestreamMach numberM1 equals 0.8, the angle of attack is set to 25.5 deg and the Reynolds number based on c isRec 2 10. This study constitutes a follow-up of [4], in which reference computations without control have been validated against experimental data and in which compressibility effects on the vortical flow have been discussed. The vortical flows in subsonic and transonic regimes over the wing have been experimentally investigated at the DLR, German Aerospace Center in Gottingen by Konrath et al. [5–7] and Schroder et al. [8]. The computations are performed using the FLU3M solver [9]. An implicit time integration is employed. The time integration is carried out by means of the second-order-accurate backward scheme of Gear and the time step is set equal to 5 10 7 s. The spatial scheme is based on the second-order-accurate AUSM P scheme [10]. The turbulent modeling, based on a coupling of the delayed detached-eddy simulationmethod [11] with the zonal detached-eddy simulation approach [12] is presented in [13]. The grid is composed of 23 10 points. In the vortical flow region, ratio drans=bloc (where drans is the Reynolds-averaged Navier–Stokes region thickness) does not exceed 0.01. It ensures a large eddy simulation of the whole part of the vortical flow. The averaging process has been performed over a duration of 15Tc, with Tc being the time scale defined by Tc c=U1. It has been shown in [4] that such a duration allows an accurate prediction of the averaged flow.

This paper presents computational simulations of the flow over a 50-deg sweep Missile fin for an angle of attack equal to 25 deg. For such an angle of attack, the flow is expected to fully separate. Nevertheless, Reynolds-averaged Navier-Stokes; computations still predict the presence of a leading-edge vortex. Then, hybrid Reynolds-averaged Navier-Stokes/large eddy simulation methods are assessed. In particular, this study focuses oil the delayed-detached eddy simulation and on a proposed extension of this method (EDDES), in which the objective is to accelerate the destruction of the eddy viscosity in large eddy simulation regions. These two methods are assessed in file case of a boundary-layer flow, and it is shown that the extended delayed-detached eddy simulation behaves as the delayed-detached eddy simulation method. Nevertheless, fit the case of a fully separated flow downstream from it backward facing step, the resolved fluctuations obtained with file extended delayed-detached eddy simulation method are in better agreement with the experimental data titan those of the delayed-detached eddy simulation computation. Finally, these two methods improve the description of the How over the missile fin, predicting a fully separated flow. However, the extended delayed-detached eddy simulation ensures a faster development of instabilities than the delayed-detached eddy simulation and the agreement with the pressure distribution obtained with pressure sensitive paint is much better with the proposed modification of delayed-detached eddy simulation.

This paper presents a methodology for developing finite differences or finite volumes CFD codes on Graphical Processing Units (GPUs) through general purpose guidelines. These guidelines are applied to the implementation on a GPU of a 2D Euler equations solver on a structured grid and its tridimensional extension on multiple GPUs. Several numerical schemes are used. All of them are first-order in time and use a Roe flux differencing scheme in space, which is considered either in its native formulation or using a second-order MUSCL scheme. The 2D problem leads to a discussion about various API, algorithmic and computational optimizations on NVIDA GPUs with 1.3 compute capability, whereas the 3D problem allows to complete the 2D study with the introduction of Fermi GPUs and the definition of a communication system allowing to use efficiently several GPUs on a node.

The international and multidisciplinary HyMeX (Hydrological cYcle in the Mediterranean EXperiment) project aims at a better understanding and quantification of the hydrological cycle and related processes in the Mediterranean, with emphasis on high-impact weather events, inter-annual to decadal variability of the Mediterranean coupled system, and associated trends in the context of global change. HyMeX will focus on high impact weather events such as heavy precipitation, wind-storms or regional strong winds (e.g. Mistral) during special observing periods. The purpose of BAMED (BAlloons in the MEDiterranean) is to perform the deployment of drifting observing platforms on-board of pressurized balloons, during HyMeX observing periods. Two platforms are developed. The Boundary-Layer Pressurized Balloons (BLPBs) drifting above the sea and the surface drifting Aeroclipper that is a tethered balloon with a marine gondola dedicated to air-sea flux estimates. Both aerostats will be deployed upstream to heavy precipitation and during regional wind events, collecting crucial data that are currently lacking in operational weather and marine prediction systems. These aerostats are planned to disseminate, in near-real time, the in-situ measurements collected in the boundary layer. The data, when assimilated in numerical weather prediction systems, are expected to improve the knowledge and the prediction of the events of interest. Indeed, these aerostats could help controlling forecast uncertainties. For instance, some aspects of the weather phenomena are poorly predictable (e.g. location and intensity of highest precipitations). This predictability issue implies that it is crucial to consider aerostats as adaptive observing platforms: for each event, targets will be defined. Moreover, for both drifting platforms, the Mediterranean basin is closed; as a consequence, the trajectories will be short. The coastal location and, above all, the date/time of the launch, are critical parameters to guarantee the balloons' trajectories towards the areas of interest. Some possible launch sites in the North-West Mediterranean have been evaluated on a sample of typical HyMeX meteorological cases. To optimally schedule the launches, a balloon's observation simulator developed at LMD/IPSL and a targeting guidance tool, developed at the CNRM, will be used. The targeting tool allows, in particular, the calculation (today) of an area called sensitive where adding observations (in 1 or 2 days) would improve the prediction of a meteorological event (3 or 4 days ahead).

This aim of this numerical study is to control the separated flow over a generic missile tin characterized by a sweep angle of 50 deg and a sharp leading edge. The flow without control separates at the leading edge and the fin is stalled. To control the flow separation, a blowing slot is placed all along the fin leading edge, and both continuous and pulsed blowing are applied. The nondimensional frequencies F+ based on the freestream velocity and the root chord of the fin are equal to 1, 1.5, 3, 6, and 10. Applying continuous blowing results in a larger reverse-flow region than in the reference case. As a consequence, the aerodynamic performance of the fin is deteriorated. Conversely, using pulsed blowing tends to decrease the pressure over the fin and the extent of the reverse-flow region. The performance of the fin is then enhanced, particularly for a pulsing frequency equal to that of natural vortex-shedding (F+ = 1.5). Finally, the last part of the paper discusses the effects of the suction phase by comparing synthetic-jet actuation and pulsed blowing at the optimal frequency.