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Circulations à fine échelle et qualité de l'air hivernal dans une vallée alpine urbanisée

Abstract : Air quality issues are frequent in urbanized valleys, particularly in wintertime under anticyclonic conditions. Pollution episodes occur due to the combination of increased emissions and atmospheric stratification that inhibits vertical mixing and isolates the valley atmosphere from large-scale dynamics. The transport of pollutants then becomes mainly driven by local thermally driven flows that largely depend on local characteristics and are difficult to represent in numerical weather prediction models. Improving the forecasting of winter pollution situations in mountain areas therefore requires a better understanding of local dynamics under stable conditions. This thesis fall within this objective and aims at improving the understanding of local wind dynamics at valley scale. It is based on high-resolution numerical simulations performed with Méso-NH and data from the Passy-2015 field experiment that took place during the winter of 2014-2015 within the Passy basin, located near Mont-Blanc and at the confluence of three valleys. The PM10 concentrations observed in this basin regularly exceed the regulatory thresholds and show marked heterogeneities within the basin and with adjacent valleys. The wind dynamics study highlights local flow characteristics that are consistent with the PM10 heterogeneities observed within the valley. In particular, flows within the basin show a stratified structure and give rise to heterogeneous ventilation levels. During the day, mass exchanges preferentially occur between the sunniest valley sections. At night, the convergence of flows from tributary valleys, along with the local orography, induces a very heterogeneous flow structure on the vertical and horizontal in the Passy basin. These characteristics tend to reduce ventilation in the basin especially in the eastern sector, which is also the most polluted sector during wintertime episodes. As spring approaches, the increase of solar radiation balances inter-valley mass exchanges, thus reducing pollutant accumulation within the basin. The analysis of mechanisms controlling local circulations underlines the importance of fine scale characteristics of topography and surface (snow cover) that determine the distribution of energy received at the surface.
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Tiphaine Sabatier. Circulations à fine échelle et qualité de l'air hivernal dans une vallée alpine urbanisée. Météorologie. Université Paul Sabatier - Toulouse III, 2018. Français. ⟨NNT : 2018TOU30340⟩. ⟨tel-02417418⟩

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