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Microphysique glacée des systèmes convectifs observés dans le cadre de Megha-Tropiques en Afrique de l'Ouest : comparaison des mesures aéroportées avec des radars sol et un modèle numérique

Abstract : Tropical meteorology is a major issue for atmospheric physics and earth’s climate. The Megha-Tropiques satellite combines several teledetection instruments which need complex restitution algorithms. This work contributes to the development of validation’s strategies for these algorithms. This requires a better knowledge of the tropical mesoscale convective systems’ (MCS) ice phase. In this thesis, we use data from the Niamey’s (Niger) campaign, which took place during summer 2010. Numerous MCS with high Ice Water Content (IWC) were analyzed with an airborne instrumentation and ground radars. Reflectivity Zin-situ is calculated using airborne microphysic probes’ information and a mass-diameter relationship. A spatial and temporal interpolation technique is developed to colocalize the aircraft position with ground radar measurements. This method leads to the validation of Zin-situ calculation. The relationship between reflectivity and IWC is not satisfactory for the MIT precipitation radar. Moreover, the cloud radar gives better informations about the IWC than the precipitation radar. Indeed, precipitation radars are too sensitive to large ice crystals. Therefore, in-situ measurements are essential to get microphysic information in order to validate restitution algorithms used by satellites. They can’t be replaced by ground based reflectivity measurements. The WRF (Weather Research and Forecasting) model was used in order to get a better knowledge of MCS. In this work, we analyzed one case study. For this case, WRF generates a typical squall line but it doesn’t correctly reproduce every observed characteristics. Several dynamical and microphysical differences appear between simulation and observations. The simulated reflectivity field is analyzed by CFAD (Contoured Frequency by Altitude Diagrams) and it shows a general underestimated reflectivity compared to the observations. The Thompson microphysic scheme is replaced by the more complex Morrison scheme, but this modification doesn’t improve the results of the simulation. Consequently, the WRF model isn’t yet efficient enough to help with the restitution algorithms’ validation.
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Elise Drigeard. Microphysique glacée des systèmes convectifs observés dans le cadre de Megha-Tropiques en Afrique de l'Ouest : comparaison des mesures aéroportées avec des radars sol et un modèle numérique. Sciences de la Terre. Université Blaise Pascal - Clermont-Ferrand II, 2014. Français. ⟨NNT : 2014CLF22531⟩. ⟨tel-01158109⟩

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