Dynamique et microphysique des systèmes convectifs de l'Afrique de l'Ouest : Une analyse d'une ligne de grains par radar Doppler polarimétrique

Abstract : During the AMMA international project, aimed to observe the variability of the West African Monsoon, the C-band polarimetric Doppler radar RONSARD and a bistatic receiver have been deployed in northern Benin during summer 2006 in order to document dynamics and microphysics of monsoon-related convective systems.
The preparation phase of this campaign gave us an opportunity to examine and to propose a solution to the problem involved in a monostatic-bistatic Doppler radar network: the sidelobe contamination of bistatic measurements by the monostatic radar antenna gain. This solution based on variational analysis, proposes an alternative solution to the forced rejection of bistatic Doppler data so far considered. Tests with synthetic and real data are performed and this method allow us to use the complete sampled dataset in order to retrieve 3D wind fields on more important domains, using classical dual-Doppler methods.
During the campaign, the bistatic receiver did not work and only the RONSARD radar provided exploitable measurements on northern Benin. In particular, the RONSARD data collected within the 28th July 2006 squall line were used to document the dynamics and microphysics at convective scale and mesoscale in this study. Dynamics, comparable to those from previous studies, and hydrometeor distribution allow us to better understand the microphysical processes involved in different regions of interest. Microphysical retrievals are consistent with the airflow description showing an evident hydrometeor organization in the liquid phase with light, moderate and heavy rain in the convective part, and light to moderate rain in the stratiform rain. Near the 0° C isotherm, a melting layer of wet snow could be identified. In the convective region, graupel-hail mixture was found to be embedded in an overall region of dry snow and ice crystal. Melting of solid particles and coalescence of lighter rain particles could contribute to the moderate rain reinforcement of the precipitations. At mesoscale, a composite analysis shows that moderate rain was strongly correlated with the presence of graupel-hail mixture aloft and highlights the role played by evaporation under the trailing stratiform region.
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Lamrani Nabil. Dynamique et microphysique des systèmes convectifs de l'Afrique de l'Ouest : Une analyse d'une ligne de grains par radar Doppler polarimétrique. Planète et Univers [physics]. Université Paul Sabatier - Toulouse III, 2008. Français. ⟨tel-00358206⟩



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