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Interactions entre le champ de vapeur d'eau et les systèmes précipitants

Abstract : This thesis focuses on the links between the evolution of atmospheric water vapour content and precipitation formation. The general goal of the works presented is to improve the understanding of the precipitation formation mechanisms using water vapour measurements, primarily made by GPS. An original statistical study based on 5 years of data (from GPS, rain gauge, and other meteorological probes collocated on a platform in Clermont-Ferrand, France) shows that the increase of integrated water vapour amount (IWV) is, on average, a precursor for rain formation. We also show that the IWV evolution is primarily disconnected from the variations in water vapour mixing ratio measured at the surface. Indeed, unlike moisture at the surface, the IWV reaches its maximum on average 20 minutes before the precipitation peak. This could indicate that the condensation dominates in the whole column, while at the surface there is a strong evaporation. The detailed study of convective precipitations which occurred on 18th of July 2007 (Intensive Operation Period 9a of the Convective and Orographically- induced Precipitation Study COPS) on the lee side of the Vosges Mountains shows the significant contribution of GPS measurement for the study of convective precipitations in mountainous areas. Indeed, Thanks to a synergic use of radars, surface meteorological analysis and GPS receivers (which allow the observation of small scale water vapour field features, with a high temporal resolution), we show that the moisture accumulation occurs several hours before convective initiation and we also show that the triggering of the convection is favoured by moisture flux convergence (MFC). This MFC is associated with surface wind convergence leading to a substantial vertical transport of water vapour, which is observed by the GPS tomography. The wind direction on the windward side of the mountains appears to control the location of this convergence zone. The role of local forcing due to small scale orography (< 5km) is also shown, complementing the observations by the results from high resolution numerical model simulations.
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Laurent Labbouz. Interactions entre le champ de vapeur d'eau et les systèmes précipitants. Sciences de la Terre. Université Blaise Pascal - Clermont-Ferrand II, 2013. Français. ⟨NNT : 2013CLF22358⟩. ⟨tel-00869951⟩

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