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

Abstract

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 AROME and Meso‐NH at horizontal resolutions of 1.3 km and 260m. The behaviour of the subgrid turbulence scheme at 1.3km and its sensitivity to various parameters are analyzed, with results being evaluated using measurements. An increase of the vertical resolution around the tropopause levels with Δz ≤ 300m is shown to greatly enhance the turbulence representation. The use of a nonlocal formulation of the mixing length in the current parametrization at 1.3km allows to reproduce a turbulence signal in agreement with the observations. On the contrary, the use of a fully 3D formulation has no impact on the simulation at this resolution (1.3km). Using the 260m runs, this turbulence event is linked to hydrodynamical wind shear instabilities characterized by horizontal wavelength of 4.5km, sub‐resolved at the operational resolution. At these small gridsize 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 1.3km when compared to 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 tradeoff between mixing intensity and the representation of wind at resolved scales for the upper troposphere.This article is protected by copyright. All rights reserved.