In the framework of the Fennec international programme, a field campaign was conducted in June 2011 over the western Sahara. It led to the first observational data set ever obtained that documents the dynamics, thermodynamics and composition of the Saharan atmospheric boundary layer (SABL) under the influence of the heat low. In support to the aircraft operation, four dust forecasts were run daily at low and high resolutions with convection-parameterizing and convection-permitting models, respectively. The unique airborne and ground-based data sets allowed the first ever intercomparison of dust forecasts over the western Sahara. At monthly scale, large aerosol optical depths (AODs) were forecast over the Sahara, a feature observed by satellite retrievals but with different magnitudes. The AOD intensity was correctly predicted by the high-resolution models, while it was underestimated by the low-resolution models. This was partly because of the generation of strong near-surface wind associated with thunderstorm-related density currents that could only be reproduced by models representing convection explicitly. Such models yield emissions mainly in the afternoon that dominate the total emission over the western fringes of the Adrar des Iforas and the Aïr Mountains in the high-resolution forecasts. Over the western Sahara, where the harmattan contributes up to 80 % of dust emission, all the models were successful in forecasting the deep well-mixed SABL. Some of them, however, missed the large near-surface dust concentration generated by density currents and low-level winds. This feature, observed repeatedly by the airborne lidar, was partly forecast by one high-resolution model only.

The Sahel region of West Africa experiences decadal swings between periods of drought and abundant rainfall, and a large body of work asserts that these variations in the West African monsoon are a response to changes in the temperatures of the tropical Atlantic and Indian Oceans. However, here it is shown that when forced by SST alone, most state-of-the-art climate models do not reproduce a statistically significant upward trend in Sahelian precipitation over the last 30 years and that those models with a significant upward trend in rainfall seem to achieve this result for disparate reasons. Here the role of the Saharan heat low (SHL) in the recovery from the Sahelian drought of the 1980s is examined. Using observations and reanalyses, it is demonstrated that there has been an upward trend in SHL temperature that is coincident with the drought recovery. A heat and moisture budget analysis of the SHL suggests that the rise in temperature is due to greenhouse warming by water vapor, but that changes in water vapor are strongly dependent upon the temperature of the SHL: a process termed the Saharan water vapor-temperature (SWAT) feedback. It is shown that the structure of the drought recovery is consistent with a warming SHL and is evidence of a fundamental, but not exclusive, role for the SHL in the recent increase in Sahelian monsoon rainfall.

The Fennec research program involves a diverse range of observations and modelling activities which were designed to tackle the data sparse regions of the Sahara desert. This area can be considered as one of the final frontiers in observational challenge for climate scientists. Weather and climate prediction models show significant systematic errors over the Sahara desert manifested as differences in radiation reaching and leaving the surface, surface temperature, winds, and in representation of the boundary layer. The remoteness of this hostile environment means that it has never been the subject of concentrated surface and airborne observations which are required to reduce these uncertainties. During June 2011 and June 2012 over 200 hours of scientific flying was carried by the UK BAe146 FAAM aircraft, and in 2011 the UK aircraft flew a number of coordinated missions with the SAFIRE Falcon D-20 aircraft. These joint missions permitted extended temporal coverage of specific atmospheric features in the region not possible with a single aircraft. The second season of flying in June 2012 was subjected to two dominant weather regimes. Initially the operating region was dominated by maritime flow, which was then replaced by the climatological deep Saharan heat low pressure system which was in place throughout the 2011 IOP. This has provided opportunities to compare these different regimes and the transition between them. We will outline the advantages of using an aircraft as observational platforms, the ability to link together spatial and temporal features across the vast arena of the Sahara which are simply not accessible through fixed ground sites or even satellites. We will provide details of the comprehensive instrumentation payload, overview of the flight schedule & meteorology behind flights in both 2011 and 2012. We report some of the key findings which were only accessible to us through the use of airborne platforms. These include the first observations of heat flux profile in the Saharan Boundary layer, detailed vertical profiles of mineral dust size distribution both near-field and following various periods of atmospheric transport, insitu sampling of an aged haboob, and coordinated remote sensing LIDAR and radiation measurements with insitu observations (only possible with two aircraft).

The central Sahara forms an important part of the global climate system. During the northern summer months, the Saharan Heat Low (SHL), caused by intense solar heating, develops over a huge, largely uninhabited expanse of northern Mali, southern Algeria and eastern Mauritania. Dry convection through more than 5000m of the atmosphere, is routine in what is thought to be the deepest such layer on the planet. The SHL also co-locates with the largest loadings of dust anywhere in the Earth's atmosphere, making for a complex yet crucial component of WAM. Much of what is known about the SHL derives from numerical models rather than observations although it is widely accepted that such models show significant systematic errors over the Sahara desert manifested as differences in radiation reaching and leaving the surface, surface temperature, winds, dust and in representation of the boundary layer. In an effort to address the observational deficit in the region, as well as to improve model performance, the Fennec project is a large scale, multi-platform, extended duration observational campaign in the Saharan Heat Low (SHL) region. During the summers of 2011and 2012 a major campaign set about addressing the data deficiency of this important region. The campaign, which involved many more people than are indicated by the authorship of this abstract, featured the use of the instrumented BAe-146 and Falcon aircraft as well as supersite ground-stations both Algeria and Mauritania. The purpose of this overview is to describe project aims, institutional involvement and key elements of the observational campaign, particularly as it relates to the effort to understand dust over the region.

The Saharan Atmospheric Boundary Layer (SABL) structure, dynamics, thermodynamics and composition over the Central Sahara, associated with several concomittant dust lifting processes observed/modelled on 21 June 2011, in the framework of the FENNEC 2011 Special Observing period, are analysed. The aerosol optical depth on that day was in excess of 3. On the morning of June 21, dust lifting occurred at the passing of the African Easterly Waves over Mauritania with dust being raised by cold-pools issued from convective systems having developped the previous day. Behind this wave, the harmattan flow was channeled between the Hoggar and the Atlas and also generated dust. The dust production was amplified when this flow experienced further constriction between dusty density currents flowing down the Atlas slopes and an intense monsoon pulse from the west of the Hoggar also generating much dust. Two aircraft (the SAFIRE Falcon and the FAAM BAe 146)operated over Mauritania and Mali on that day enabled to document the complex interactions between the monsoon flow, the intertropical front, the density currents from the Atlas, in the SABL. AROME operational simulations werealso used to analyse how the different air masses have interacted to form the observed complex multi-layer dust structure in the SABL. Afternoon Falcon 20 and BAe 146 flights sampled the growth of the SABL. A clear influence of the cold pool and the dusty layers above can be observed on the developpement of the boundary layer. Finally, two AROME simulations (one with and one without prognostic dust) were used to investigate the influence of the complex dust layers on the dynamics/thermodynamics of the developping convective boundary layer over the Central Sahara.

Toggle navigation CCSD. HAL: HAL; HALSHS; TEL; MédiHAL; Liste des portails; AURéHAL; API; Data; Documentation. Episciences.org: Episciences.org; Revues; Documentation. Sciencesconf.org; Support. Connexion: Connexion; Connexion avec ORCID; Créer un compte; Mot de passe oublié ? Login oublié ? fr; en. Accueil; Consultation; Rechercher; Derniers dépôts; Dernières publications; Site web LATMOS. Par domaine. Par auteurs. Par année. Par structure. hal-00723562, version 1. The evolution of the Saharan Boundary layer thermodynamics and composition in connection with intra-seasonal pulsations of the West African Heat Low. Cyrille Flamant 1 Christophe Lavaysse 2 J.-P. Chaboureau 3 S. Engelstaedter 4 Cécile Kocha 5 J.-P. Lafore 5 J. Marsham …

The central Sahara has one of the most extreme climates on Earth. During the northern summer months, a large low pressure system caused by intense solar heating develops over a huge, largely uninhabited expanse of northern Mali, southern Algeria and eastern Mauritania. This Saharan heat low plays a pivotal role in the West African Monsoon. Based on this, the interested French, British and German communities have decided to propose the FENNEC project which aims at (i) characterizing the Saharan atmospheric boundary layer, (ii) evaluating its representation in regional and global models, and (iii) improving products issued from space-borne observations. A key element of this programme was the organization of an international field campaign in June 2011 over the Saharan heat low region, which will include both ground-based and airborne detachments. The Special Observing Period component of FENNEC-France included the implementation of the SAFIRE Falcon 20 to conduct research on the atmospheric boundary layer and the dust cycle of the Sahara, the installation of a remote sensing station in southern Spain, equipped with a backscatter lidar and a sunphotometer, to study the transport of desert dust to Europe, as well as a couple of GPS stations installed in southern Morocco to investigate the moisture from the Ocean into the Sahara. For the first time, the ALADIN and AROME models (5 and 24 km grid spacing, respectively) have been implemented operationally to provide forecasts of dust events over the Sahara and parts of the Sahel in June 2011 to assist in planning for airborne operations. This effort was complemented by the forecasts made with the Meso-NH model (5 and 20 km resolution). During the SOP period, the ground-based, airborne and space-borne observations have documented the evolution of dynamic properties of thermodynamic and the atmospheric boundary layer Saharan Africa (Mauritania and Mali) during the installation phase of the Saharan heat low west of the continent as well as the increase in aerosol loading associated with the phase shift of the heat low from east to west. During this period, episodes of intense uplift of desert aerosols associated with various dynamic phenomena (fronts, Mediterannean surges, Atlantic inflow of low-level jets, etc ...) have also been documented as well as the export of dust over the Ocean. An overview of implementation plan and of the first observational and modelling results acquired during the time of the SOP will be presented.

Depuis l'expérience internationale pionnière GATE en 1974, notre documentation et notre compréhension du système de mousson africaine avaient évolué relativement lentement. Comment notre vision de cette mousson ressort-elle après les 10 ans du projet AMMA ? Si les principaux acteurs de ce système couplé atmosphère-océan continent étaient déjà bien connus, AMMA a permis d'approfondir le détail des processus en jeu et de leur couplage. D'une vision statique de ce système émerge petit à petit une vision plus dynamique où intervient désormais l'ensemble du globe : l'Afrique en entier, les latitudes tempérées, la mousson indienne et même toute la ceinture tropicale. Une voie est ainsi ouverte pour comprendre sa variabilité intra-saisonnière.

The near-surface low pressure system that develops over western Africa in Boreal summer (know as the Saharan Heat Low) is thought to have a significant influence on regional and global climate due to its links with the Monsoon, the Northern Atlantic and the Mediterranean climate system. The SHL is associated with the deepest atmospheric boundary layer on the planet and is co-located with the highest dust loadings in the world. The processes that link the heat low and dust distribution are only poorly understood. Improving the representation of the heat low and the processes that control the emission and atmospheric distribution of dust in climate and NWP models is crucial if we are to reduce known systematic errors in climate predictions and weather forecasts. In collaboration with European partners, the UK-based consortium project "Fennec - The Saharan Climate System" aims at improving our understanding of this complex climate system by integrating for the first time coordinated ground and aircraft observations from the central Sahara, newly developed satellite products, and the application of regional and global models. On 22 June 2011, two research aircraft operating out of Fuerteventura (Spain) surveyed the Saharan Heat Low centred over Mauritania-Mali border. The aircraft flew simultaneously in the morning and in the afternoon on two different tracks thereby sampling each track four times on that day. Both aircraft were equipped with a downward looking LIDAR for aerosol detection. In total, 51 sondes were dropped during the flights making this the most comprehensive dataset to study the spatio-temporal diurnal evolution of the heat low including the interactions between the atmospheric boundary layer and dust distributions. Combining LIDAR observations, satellite imagery and back-trajectory modelling we show that an aged dust layer was present in the heat low region resulting from previous day's dust activity associated with a south-moving density current from the Atlas mountains and westward-moving Haboob fronts originating along the Algeria-Mali border. We show how the dust is distributed within the atmosphere and how it is modified during the course of the day by various processes including the development of the atmospheric boundary layer and associated dry convection as well as the inflow of moisture-rich monsoon air from the south.