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Précipitations méditerranéennes intenses -caractérisation microphysique et dynamique dans l'atmosphère et impacts au sol

Abstract : This study offers a unified formulation for the single- and multi-moment raindrop size distributions (DSD), which were proposed in the framework of scaling analysis in the literature. The key point is to consider the DSD scaled by drop concentration (Nt, 0th order DSD moment), as a probability density function (pdf) of raindrop diameter scaled by characteristic diameter (D/Dc). The Dc is defined as the ratio of the 4th to the 3rd DSD moment. A two-parameter gamma pdf model, with a self-consistency relationship, is found to be suitable for representing the scaling DSD formulation. For the purpose of parameter estimation, two different methods, based on three DSD moments (0th, 3rd and 4th moments) and multiple DSD moments (from 0th to 6th moments), are proposed and then evaluated through the 3-year DSD observations, collected at Alés within the activities of the Cévennes-Vivarais Mediterranean Hydrometeorological Observatory (CVMHO). The results reveal that: 1) the scaled DSD model parameterized by three moments (0th, 3rd and 4th moments) possesses a similar performance compared to that constructed by multiple DSD moments; 2) regardless the application of scaled technique, large variation is still exhibited in this climatological DSD scaled dataset. The scaled DSD formulation is, in a second step, adapted to the one- and two-moment scaling DSD formulations by introducing single and dual power-law models between the reference moments (e.g. rain rate R and/or radar reflectivity factor Z) and the explained moments (total concentration Nt, characteristic diameter Dc). Compared with previous DSD formulations presented in the literature, the presented approach explicitly accounts for the prefactors of the power-law models to produce a uniform and dimensionless scaled distribution, whatever the reference moment(s) considered. In the same manner, two methods based on 1) single or dual power-law models and 2) multiple DSD moments (from 0th to 6th moments), are proposed to estimate the climatological parameters in the one- and two-moment scaling DSD formulations. The results show that: 1) the estimation method has a significant impact on the climatological DSD formulation scaled by one moment; 2) the choice of the reference moment to scale DSD depends on the objectives of the research: e.g. the DSD model scaled by high order moment produces a good performance for large drops at the cost of a poor performance for the small ones; 3) using two scaling moments improves significantly the model performance to represent the natural DSD, compared to the one-moment DSD formulation. In terms of applications of scaling DSD model, the analysis of the inter-event variability is performed on the basis of the scaling formulation containing three parameters (Nt, Dc and µ describing the shape of the gamma function). Different rain phases can be identified by the sudden shifts of moments and parameters in DSD time series. It is found that these rain phases are well linked to different weather processes. And a preliminary relationship is established between the radar observations and DSD parameters. The climatological scaling DSD formulations are also used for the DSD reconstitutions and for rainfall kinetic energy flux density estimations by rain intensity and/or radar reflectivity factor. The results confirm that the application of two scaling moments (R and Z) improves significantly the performance of these models, regardless the different sampling characteristics between radar and raingauge.
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Nan Yu. Précipitations méditerranéennes intenses -caractérisation microphysique et dynamique dans l'atmosphère et impacts au sol. Sciences de la Terre. Université de Grenoble, 2012. Français. ⟨NNT : 2012GRENU013⟩. ⟨tel-00773116⟩



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