Large Eddy Simulations of Stably Stratified Turbulence : toward a One-Dimensional Turbulent Potential and Kinetic Energy Parameterization

Stably stratified turbulence is observed in stable boundary layer and in the free atmosphere. An appropriate representation of the atmospheric turbulence in stably stratified regime is then crucial to accurately forecast daily weather conditions during night-time and winter. It is also important to better represent momentum transfer in the free tropical troposphere and in highly stably stratified polar boundary layer in climate models. However most numerical weather prediction models currently encounter issues to simulate turbulence at relatively high Richardson number : beyond a critical Richardson number Ric ≈ 0.25, the flow regime turns into a very weak turbulent regime which is not represented by NWP models.

Large eddy simulations of a free atmosphere layer are carried out using the non-hydrostatic mesoscale model Meso-NH developed by the CNRM (Météo-France and CNRS) and the Laboratoire d'Aérologie. For each LES, the free atmosphere layer is initialized by a constant vertical potential temperature gradient and a wind shear. A set of LES is realized with different Ri varying from 0 to 2. An initial potential temperature perturbation is applied. It leads to a stationary stably stratified turbulent regime from which an analysis of the TKE budget and the turbulent structure with respect to the Richardson number is conducted. Convergence issues for extremely stably stratified simulated flow are also discussed.

Starting from the LES, we estimate typical mixing length scales with respect to the static stability using correlation functions of potential temperature and wind variances. These LES and mixing length scales are then used as a reference for the development of a one-dimensional turbulence parameterization using the Energy and Flux Budget (EFB) model described by Zilitinkevich et al. 2013. A preliminary evaluation of a version of the EFB coded in Meso-NH is conducted using nocturnal boundary layer vertical profiles observed with the instrumented Cabauw mast (Van Ulden and Wieringa. 1996)

Further evaluation of the parameterization to the stable boundary layer over complex non-flat terrain will be realized and evaluated using data from the Passy-2015 field campaign conducted in the Northern Alps during the winter of 2014-2015.