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I will present some use of Large-Eddy simulations that correspond to numerical simulations of shallow clouds or clear boundary layer where most of the processes are resolved. After, a first description of such simulation and their main advantages and drawbacks, I will detail how they can be used both for the understanding of boundary-layer processes and for the improvement of their representation in models. I will in particular detail a simulation of initiation of deep convection observed during the AMMA field campaign that occurs over West Africa. With such a simulation, we show the different processes leading to initiation of deep convection in this very dry environment not favorable to convection. I will also show how these simulations can be used to derive parametrization-oriented diagnostics that have been used to improve boundary-layer parametrizations.; Je présenterai quelques utilisations des simulations explicites type Large-Eddy Simulations qui correspondent à des simulations numériques de nuages ​​peu profonds ou de couche limite en ciel clair où la plupart des processus sont résolus. Après, une première description de ces simulations et de leurs principaux avantages et inconvénients, je détaillerai comment elles peuvent être utilisées à la fois pour la compréhension des processus de couche limite et pour l'amélioration de leur représentation dans les modèles. Je détaillerai notamment une simulation d'initiation de convection profonde observée lors de la campagne de terrain AMMA qui se déroule sur l'Afrique de l'Ouest. Avec une telle simulation, nous montrons les différents processus conduisant à l'initiation de la convection profonde dans cet environnement très sec peu favorable à la convection. Je montrerai également comment ces simulations peuvent être utilisées pour dériver des diagnostics orientés paramétrisation qui ont été utilisés pour améliorer les paramétrisations de la couche limite.

International audience; So-called ‘dry’ microburst outflows are well known phenomena in desert environments when rain from moist convection aloft evaporates into deep, dry-adiabatic boundary layers. Extreme synoptic scale versions of this convective scale phenomenon have been documented in this study, in which the collective episodes of convective downdraft feed a common cold pool that expands as a gust front density current, raising large amount of dust in the boundary layer, and initiating new moist convection over the Sahara. Satellite observations from the Spinning Enhanced Visible and Infra-Red Imager (SEVIRI) and the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) combined with selected West African surface station observations have been integrated to study the gust front and its associated dust activity in the period of August 3-6, 2006. The meteorological conditions accompanying this event have been described using the European Centre for Medium-range Weather Forecasts (ECMWF) analyses. The gust front was initiated by a cluster of isolated cumulonimbus clouds over central Niger at 1400 UT on August 3 that lengthened to MCS size over Mali by the end of the day. At maximum expansion on August 5, the extending gust front exceeded 1500 km in length, with a transited area of lofted dust reaching a million square kilometers, mostly over southern Algeria and northern Mali. The northward gust front speed, estimated with SEVERI imagery, is rapid in initial stages but declines with time as the cold air absorbs heat from the hot desert surface and the gust front density contrast is diluted. The synoptic character of this event (both the length and the duration) allows for four intersections with CALIPSO orbits, thereby providing information on the evolution of the characteristics of the dusty gust front during its lifetime. Young dusty gust fronts (i.e. during the first 24 hours of the event) were characterized by lidar reflectivity at 532 nm in excess of 3 x 10-3 km-1 sr-1, temperature drops exceeding 10°C, 1 km visibility and their associated dense dust cloud reached 2 km in altitude. Older dusty gust fronts (i.e. from August 5 on) were associated with weaker lidar reflectivities (below 3 x 10-3 km-1 sr-1), same visibility conditions (~1km), temperature drops at the surface of about 5°C and their associated dust clouds reached higher altitudes (3-4 km). The northward transport of moisture over the Sahara desert associated with the northward excursion of the gust front was evident during this event. All gust front crossings were characterized by moisture increases at the surface. A large area (~400 km over longitude and 100 km over latitude) of high Water Vapor Mixing Ratios (up to 16 g kg-1) covered northeastern Mali and southwestern Algeria, confirming an impact at the synoptic scale. The pronounced northward propagation of the dusty gust front and its associated moisture is shown to have been favored by the presence of an active African Easterly Wave.

International audience; Three-dimensional mesoscale numerical simulations were performed over Niger in order to investigate dry cyclogenesis in the West African intertropical discontinuity (ITD) during the summer, when it is located over the Sahel. The implications of dry cyclogenesis on dust emission and transport over West Africa are also addressed using the model results, together with spaceborne observations from the Spinning Enhanced Visible and Infra-Red Imager (SEVIRI) and the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP). The study focuses on the case of 7-8 July 2006, during the African Monsoon Multidisciplinary Analysis (AMMA) Special Observing Period 2a1. Model results show the formation of three dry cyclones in the ITD during a 24-h period. Simulations are used to investigate the formation and the development of one of these cyclones over Niger in the lee of the Hoggar-Aïr Mountains. They show the development of the vortex to be associated with (1) strong horizontal shear and low-level convergence existing along the monsoon shearline and (2) enhanced northeasterly winds associated with orographic blocking of air masses from the Mediterranean Sea. The dry cyclone was apparent between 0700 and 1300 UTC in the simulation, and it was approximately 400 km wide and 1500 m deep. Potential vorticity in the center of vortex reached nearly 6 PVU at the end of the cyclogenesis period (1000 UTC). The role of the orography on cyclogenesis along the ITD was evaluated through model simulations without orography. The comparison of the characteristics of the vortex in the simulations with and without orography suggests that the orography plays a secondary but still important role in the formation of the cyclone. Orography and related flow splitting tend to create low-level jets in the lee of the Hoggar and Aïr mountains which, in turn, create conditions favorable for the onset of a better defined and more intense vortex in the ITD region. Moreover, orography blocking appears to favor the occurrence of a longer-lived cyclone. Furthermore, model results suggested that strong surface winds (∼11 m s−1) enhanced by the intensification of the vortex led to the emission of dust mass fluxes as large as 3 μg m−2 s−1. The mobilized dust was mixed upward to a height of 4-5 km to be made available for long-range transport. This study suggests that the occurrence of dry vortices in the ITD region may contribute significantly to the total dust activity over West Africa during summer. The distribution of dust over the Sahara-Sahel may be affected over areas and at time scales much larger than those associated with the cyclone itself.

The “flux, etat de la mer, et télédétection en conditions de fetch variable” (FETCH) was aimed at studying the physical processes associated with air–sea exchanges and mesoscale oceanic circulation in a coastal region dominated by frequent strong offshore winds. The experiment took place in March–April 1998 in the northwestern Mediterranean Sea (Gulf of Lion). Observations were collected with the R/V L'Atalante, with an air–sea interaction spar (ASIS) buoy, with waverider buoys, and with research aircraft equipped for in situ and remote sensing measurements. The present paper is an introduction to the following special section, which groups 12 papers (including this one) presenting results on turbulent flux measurements at the ocean surface, on the behavior of the marine atmospheric boundary layer, on the ocean waves characteristics, on the ocean circulation, and on remote sensing of surface parameters. This overview presents the background and objectives of FETCH, the experimental setup and operations, and the dominant atmospheric and oceanic conditions and introduces the different papers of the special section.

International audience; Data from a recent field campaign are used to analyze the nonstationary aspects of air-sea heat exchanges and marine atmospheric boundary layer (MABL) thermodynamics over the Gulf of Lion (GoL) in connection with synoptic forcing. The data set includes measurements made from a wide range of platforms (sea-borne, airborne, and space-borne) as well as three-dimensional atmospheric modeling. The analysis focuses on the 24 March 1998 Tramontane/Mistral event. It is shown that the nonstationary nature of the wind regime over the GoL was controlled by the multistage evolution of an Alpine lee cyclone over the Tyrrhenian Sea (between Sardinia and continental Italy). In the early stage (low at 1014 hPa) the Tramontane flow prevailed over the GoL. As the low deepened (1010 hPa), the prevailing wind regime shifted to a well-established Mistral that peaked around 1200 UTC. In the afternoon the Mistral was progressively disrupted by a strengthening outflow coming from the Ligurian Sea in response to the deepening low over the Tyrrhenian Sea (1008 hPa) and the channelling induced by the presence of the Apennine range (Italy) and the Alps. In the evening the Mistral was again well established over the GoL as the depression continued to deepen (1002 hPa) but moved to the southeast, reducing the influence of outflow from the Ligurian Sea on the flow over the GoL. The air-sea heat exchanges and the structure of the MABL over the GoL were observed to differ significantly between the established Mistral period and the disrupted Mistral period. In the latter period, surface latent and sensible heat fluxes were reduced by a factor of 2, on average. During that latter period, air-sea moisture exchanges were mainly driven by dynamics, whereas during the former period, both winds and vertical moisture gradients controlled moisture exchanges. The MABL was shallower during the latter period (0.7 km instead of 1.2 km) because of reduced surface turbulent heat fluxes and increased wind shear at the top of the MABL in connection with the outflow from the Ligurian Sea. In the period of established Mistral, gravity waves above the MABL were observed to have an influence on the MABL structure. In the perturbed Mistral case this influence was not observed. Over the GoL the ubiquitous presence of sheltered regions (i.e., regions of reduced wind speed in the MABL) in the lee of the three major mountain ranges surrounding the GoL (namely, the Pyrénées, the Massif Central, and the Alps) was shown to have an impact on surface turbulent heat fluxes. The position of these sheltered regions, which evolved with the synoptic conditions, was the key to a correct interpretation of multiplatform surface turbulent flux measurements made over the GoL on 24 March 1998.