CERFACS CERFACS Scientific Activity Report Jan. 2010 – Dec. 2011

AVIATION AND ENVIRONMENT
FIG. 2.3: Large-eddy simulations of atmospheric turbulence at Kilometer scale : Snapshots of potential
temperature fluctuations (left) and spectra of turbulent kinetic energy (right)
2.2 Wake simulation in the diffusion regime (O. Thouron, R. Paoli,
D. Cariolle)
2.2.1 Direct simulation of the atmospheric turbulence (O. Thouron, R. Paoli)
The objective of this study was to understand the properties of atmospheric turbulence in the upper
troposphere lower stratosphere (UTLS) at scale of O(1 Km), which are of interest for the dispersion of
contrails in the diffusion regime. Large-eddy simulations of stratified turbulence have been carried out in
cubic computational domains that are representative of portions of the UTLS. The forcing scheme by Paoli
and Shariff (2009) used to sustain turbulence was implemented in MesoNH, the mesoscale model of the
atmospheric research community jointly developed by CNRM and Laboratoire d’Aérologie.
Preliminary studies have been carried out to determine the optimal resolution required by the physics of
the problem : (i) for typical atmospheric density gradient, small-scale turbulence simulations with 1 cm
resolution showed that isotropic turbulence can be achieved up to scales of about 10 m ; (ii) using LES in
100 m 3 , 200 m 3 and 400 m 3 cubic domains it was shown that resolution of O(1 m) is necessary to insure
the turbulence model represents correctly sub-grid scale fluctuations.
Based on these tests, simulation of 2 km and 4 km cubic domain with uniform 10 m, 20 m and 40
m resolution have been performed. Three large-scale turbulence levels have been considered, which are
representative of ‘strong’, ‘mild’ and ‘weak’ turbulence as retrieved from available measurements in the
UTLS (Wroblewski et al., 2010), see Fig. 2.3(left).
This work allowed the establishment of optimal numerical configuration to study stratified turbulence in the
UTLS. As suggested by Waite (2011) the resolution has to be higher than buoyancy scale (which depends on
turbulence intensity) to correctly represent the turbulence. In addition, it was shown that for typical UTLS
turbulence the minimum vertical domain size must be at least 4 Km to ensure that turbulent eddies are not
confined. The outcome from these simulations gave a first contribution to the understanding of stratified
turbulence in the UTLS at the scales of interest to aviation. The analysis showed that at scales of 10 to 100
km the slopes of kinetic and potential energy spectra get closer to -5/3 as turbulence intensity increases, and
that lowering the turbulence intensity leads to steeper slopes and smaller vertical extensions of turbulent
eddies (see Fig. 2.3(right)). Finally, this study allowed setting up MesoNH and preparing the computational
framework for large-scale simulations of atmospheric turbulence (4 Km computational domains with 2 m
resolution, grid size of 8 billions points) that will be carried out in 2012 on massively parallel computers of
PRACE and INCITE infrastructures.
– Waite, M. L. Stratified turbulence at the buoyancy scale, 23, 066602, 2011
CERFACS ACTIVITY REPORT 183