The Nature of the Mistral: Observations and Modelling of two MAP Events
NAVAL RESEARCH LAB MONTEREY CA
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Two mistral events observed in 1999 during the Mesoscale Alpine Programme MAP are studied using observational data and high-resolution mesoscale-model simulations from the US Navys Coupled Ocean Atmosphere Mesoscale Prediction System COAMPS. Radiosondes suggested that both mistral wind events were associated with the passage of cold fronts and post-frontal air descent. EuroSat Rapidscan images indicated a stationary and persistent cloud edge along the lee of the Massif Central. The cloud edge marks the beginning of mistral flow acceleration and descent according to COAMPS simulations. Narrow persistent cloud banners were identified trailing from the Mont Loz ere in the Massif Central during both mistral events. COAMPS indicated that these cloud banners were associated with a deeper turbulent boundary layer in wakes induced by hydraulic jumps. The cloud banner and wake defined the western boundary of the mistral. Analysis of dropsonde data and in situ flight data indicated the presence of a sharp shear line separating the cold severe mistral wind from the warm calm wake flow to the lee of the French Alps. Trajectory analysis of model data suggested that air parcels on the two sides of the shear line had different histories.