Articles | Volume 13
19 Apr 2016
 | 19 Apr 2016

Representation of the grey zone of turbulence in the atmospheric boundary layer

Rachel Honnert

Abstract. Numerical weather prediction model forecasts at horizontal grid lengths in the range of 100 to 1 km are now possible. This range of scales is the "grey zone of turbulence". Previous studies, based on large-eddy simulation (LES) analysis from the MésoNH model, showed that some assumptions of some turbulence schemes on boundary-layer structures are not valid. Indeed, boundary-layer thermals are now partly resolved, and the subgrid remaining part of the thermals is possibly largely or completely absent from the model columns. First, some modifications of the equations of the shallow convection scheme have been tested in the MésoNH model and in an idealized version of the operational AROME model at resolutions coarser than 500 m. Secondly, although the turbulence is mainly vertical at mesoscale (>  2 km resolution), it is isotropic in LES (<  100 m resolution). It has been proved by LES analysis that, in convective boundary layers, the horizontal production of turbulence cannot be neglected at resolutions finer than half of the boundary-layer height. Thus, in the grey zone, fully unidirectional turbulence scheme should become tridirectional around 500 m resolution. At Météo-France, the dynamical turbulence is modelled by a K-gradient in LES as well as at mesoscale in both MésoNH and AROME, which needs mixing lengths in the formulation. Vertical and horizontal mixing lengths have been calculated from LES of neutral and convective cases at resolutions in the grey zone.

Short summary
Numerical weather prediction model forecasts at hectometric resolution are now possible. This range of scales is called the "grey zone of turbulence" as turbulence structures are neither mainly resolved nor entirely subgrid. Turbulence parametrizations have to be adapted, and the adaptations at these scales depend on the existing parametrizations at larger or smaller scales. This study explains the modifications in current Météo France models to the grey zone of turbulence.