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Etude in-situ des écoulements catabatiques sur pente alpine forte en situation anticyclonique

Abstract : Katabatic winds are very frequent gravity flows on the Earth’s surface, but are still not well understood or modeled, especially when developing on steep slopes. In-situ winter measurements, using a 10 m mast equipped with sonic anemometers and thermocouples on a snow-covered alpine slope of 30°, allow to study the wind speed and temperature profiles as well as the turbulent quantities of these gravity flows. Sensors embedded under a tethered balloon complete the velocity and temperature profiles, and give an estimate of the ambient stratification. This measurement campaign, carried out in February 2019, is unique because it gives access for the first time to slope-normal velocity, and to near-surface velocity measurements (using a 3D pitot). These data are complementary to a preliminary measurement campaign conducted in November 2012 on stony soil at the same site, using a 7 m mast. They should allow to progress in the understanding of the turbulent processes in the atmospheric boundary layer in complex relief.A model for the evolution of integral quantities along the slope, inspired by the scientific literature, is developed from the height-integrated heat and momentum equations. The integral velocity increases according to a power law 1/3 of the distance from the top, until it reaches a maximum value and then decreases. The model also allows to estimate the surface heat flux, and its evolution along the slope. Finally, we show an oscillation of the velocity norm over a period of the order of 20 minutes, related to the ambient stratification.Measurements made with a high frequency (1250 Hz) 3D pitot near the surface show a logarithmic velocity law, with a minor correction related to gravity effects. The inner region of the jet is not a constant flux layer, and the use of an analytical model developed in the literature allows to reproduce the behavior of the turbulent momentum flux under the maximum wind speed, consistent with the data. Finally, the inertial subrange of the velocity spectra made from the pitot data is fully resolved.Furthermore, turbulent measurements within the katabatic jet show that buoyancy can generate turbulence kinetic energy (TKE) in the upper part of the profile, whereas it usually acts as a TKE sink, under thermally stable conditions in the turbulent boundary layers. The 2019 measurement campaign provides access to the data in the coordinate system of the topography : the slope-normal velocity is negative in the lower part of the jet, and becomes positive in the middle of the jet. An analytical model, based on the momentum equation, allows to find slope-normal velocity consistent with the measurements in the lower part of the jet. The data in the topographic coordinate system also allow to estimate some terms of the TKE budget, rarely accessible on steep slopes with in-situ measurements, such as the TKE advection on z. In addition to the TKE budget, the determination of the shear and buoyancy terms of the turbulent momentum flux budget equation allows us to redefine the stress Richardson number, never used in the katabatic wind literature. This stress Richardson number is a complementary stability parameter to the flux Richardson number defined from the TKE budget. The turbulent fluxes vary with the gradient Richardson number Ri, with a ratio of turbulent diffusivity to turbulent viscosity varying from almost 1.4 (Ri ≈ 0) to 0.5 (Ri ≥ 0.5). A mixing efficiency parameter is introduced, considering the production or consumption of TKE by the turbulent sensible heat flux parallel to the surface.
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Submitted on : Friday, September 2, 2022 - 1:20:51 AM
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Claudine Charrondière. Etude in-situ des écoulements catabatiques sur pente alpine forte en situation anticyclonique. Météorologie. Université Grenoble Alpes [2020-..], 2021. Français. ⟨NNT : 2021GRALU028⟩. ⟨tel-03767542⟩

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