Long-range transport of Saharan dust and its radiative impacton precipitation forecast: a case study during the Convective andOrographically-induced Precipitation Study (COPS)

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 to an episode of intense convection, a layer of dry, clean air capped by a moist, dusty layer was observed using Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) and airborne and ground-based lidar observations fromNorth Africa to western Europe. The origin of the different layers was investigated using the regional model Meso- NH. For the purpose of modelling evaluation, a lidar simulator was developed for direct comparison of observed and simulated vertical structures of the lidar backscattered signal. Overall, the model reproduced the vertical structure of dust probed several times by the different lidar systems during its long-range transport. From Lagrangian back trajectories it was found that the dust was mobilized from sources in Mauritania six days earlier, while the dry layer subsided over the north Atlantic. Off the Moroccan coasts, the dry layer folded down beneath the dusty air mass and the two-layer structure was advected to the Rhine valley in about two days. By heating the atmosphere, the dust layer changed the static stability of the atmosphere and thus the occurrence of convection. A study of sensitivity to the radiative effect of dust showed a better prediction of precipitation when a dust prognostic scheme was used rather than climatology or when dust effects were ignored. This result suggests that dust episodes that occur prior to convective events might be important for quantitative precipitation forecasts.