Effects of subgrid-scale turbulence parametrization on the representation of clear-air turbulence using kilometre- to hectometre-scale numerical simulations

Rogel, Léo ; Ricard, Didier ; Bazile, Éric ; Sandu, Irina

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<p align=justify>A turbulence event arising in a jet exit region above the Belgium-Luxembourg area, determined from airliner in-situ measurements, is reproduced using the meteorological models Application de la Recherche à l'Opérationnel à Méso-Echelle (AROME) and Meso-NH at horizontal resolutions of 1.3 km and 260 m. The behaviour of the subgrid turbulence scheme at 1.3 km and its sensitivity to various parameters are analysed, with results being evaluated using measurements. An increase of the vertical resolution around the tropopause levels with ∆z ≤ 300 m is shown to greatly enhance the turbulence representation. The use of a non-local formulation of the mixing length in the current parametrization at 1.3 km allows reproduction of a turbulence signal in agreement with the observations. On the contrary, the use of a fully three-dimensional formulation has no impact on the simulation at this resolution (1.3 km). Using the 260 m runs, this turbulence event is linked to hydrodynamical wind shear instabilities characterized by horizontal wavelength of 4.5 km, sub-resolved at the operational resolution. At these small grid-size scales, turbulence evolution and equation budgets reflect an equilibrium between dynamical production and turbulence dissipation, and highlight the importance of horizontal gradients. Subgrid turbulence intensities are assessed to be underestimated by the current parametrization at 1.3 km when compared with this high-resolution reference simulation. Finally, different tests on the turbulence parametrization illustrate a transfer between resolved and subgrid kinetic energy in the model. This transfer stresses the importance of a trade-off between mixing intensity and the representation of wind at resolved scales for the upper troposphere.</p>
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