| Summary: | A Saharan dust event affected the Rhine valley in southwestern Germany and eastern France on 1 August 2007 during the Convective and Orographically-induced Precipitation Study (COPS) experiment. Prior an episode of intense convection, a layer of dry and clean air capped by a warm and dusty layer was observed using Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO), airborne and ground-based lidar observations from North Africa to Western Europe. The origin of the different layers was investigated using a regional Meso-NH model simulation. Overall, the model reproduces the vertical structure of dust and water vapor. From the simulated Lagrangian back trajectories it is found that the dust was mobilized from Mauritania sources several days before while the dry layer subsided over the north Atlantic. Off Morocco, the dry layer fold down beneath the warm and dusty air and the two-layer structure was advected to the Rhine valley in about two days. As the dry layer can delay the triggering of convection for a few hours, such dusty episodes that occur prior to convective events might be of importance for quantitative precipitation forecasts.
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