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Initiation et organisation d'un système fortement précipitant méditerranéen dans le cadre de période d'observation spéciale HyMeX : simulations aux échelles hectométriques

Abstract : Heavy precipitating events affecting the whole Mediterranean region are dangerous phenomena. They can cause flash floods leading to significant material and human damages. Numerical Weather Prediction (NWP) model made significant progress, especially through fine scale modeling. However, the prediction of these intense rainfall events remains imprecise. In the past, studies has already highlighted the major meteorological ingredients of these extreme phenomena. However, some fine scale mechanisms (microphysics and turbulence) and their interaction with larger scale are still not well known. The HyMeX research program is dedicated to improve the understanding of these processes and their representation in numerical models based on an unprecedented observations dataset. HyMeX also provides the first observations of heavy precipitating systems and their environment over the sea. The objective of this thesis is to analyze the representation of the processes participating in the initiation and the organization of convection in numerical models, especially over sea. The meteorological context and the precipitating systems of the POI16a convective event (26/10/2012) were first studied with numerous observations and a reference simulation realized at 2.5 km horizontal resolution. The environment, the initiation and the organisation mechanisms for convection have been analyzed, showing a convection initiation triggered by a convergence in humid and unstable low-level flow and by a cold pool under the storm playing a role of deflection of the low-level flow, reinforcing the convergence. This event was then simulated at a finer horizontal resolution of 500 m. This range of resolution, toward which the operational models curently operate, is located in the grey zone of the turbulence. The representation of turbulent processes between an explicit representation and parameterization is delicate. A sensitivity study to the turbulent mixing length was carried out. A strong impact is observed on both the environment (especially on the low-levels wind and humidity), the dynamic processes (vertical speed) and the microphysics in the precipitating system (hydrometeor content, precipitation and cold pool intensity). The mixing length commonly used in this range of resolution produces less subgrid turbulence and gives a more intense convective system. Finally, to extract from the grey zone of turbulence and the associated difficulties, the resolution was refined to the LES scale. The first LES simulation of a real heavy precipitating Mediterranean event was carried out with a horizontal resolution of 150 m on a large grid covering the north-west of the Mediterranean bassin (900 million points). The simulation in the grey zone of turbulence proposed a representation of the environment quite similar, but a convective system more intense compared to the reality of the LES. A rapid analysis of the simulated precipitating system structure showed the opportunity offered by this LES simulation to study the dynamic and the microphysical organization of the system at a finer scale.
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Maxime Martinet. Initiation et organisation d'un système fortement précipitant méditerranéen dans le cadre de période d'observation spéciale HyMeX : simulations aux échelles hectométriques. Météorologie. Université Paul Sabatier - Toulouse III, 2017. Français. ⟨NNT : 2017TOU30312⟩. ⟨tel-02926147⟩

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