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Fennec dust forecast intercomparison over the Sahara in June 2011

Abstract : 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-5 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 some satellite retrievals but mislocated by others over the Sahel. The AOD intensity was correctly predicted by the high-resolution models while being underestimated by the low-resolution models. This was partly because 10 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 to emissions mainly in the afternoon that dominate the total emission over the western fringes of the Adrar des Iforas and 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 15 the deep well-mixed SABL. Some of them, however, missed the large near-surface dust extinction 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.
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Jean-Pierre Chaboureau, Cyrille Flamant, Thibaut Dauhut, Cécile Kocha, Jean - Philippe Lafore, et al.. Fennec dust forecast intercomparison over the Sahara in June 2011. Atmospheric Chemistry and Physics Discussions, European Geosciences Union, 2016, pp.(under review). ⟨10.5194/acp-2016-12⟩. ⟨hal-01259696⟩



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