What is New in The Gray Zone ?

The gray zone of turbulence is the range of scales for which the model grid length is closed to the size of the largest boundary-layer structures. With increasing computational power the operational models are now able to run at these resolutions.

In order to characterize the subgrid turbulence in the gray zone, Large-Eddy Simulation (LES) fields are averaged to obtain resolved fields of reference at coarser resolutions. This method allows to quantify the defaults of the model. Turbulence schemes are not adapted to represent the characteristics of the subgrid turbulence in the gray zone. Especially, convective boundary-layer (BL) thermals are partly resolved but remains mainly subgrid scale, which is not correctly simulated be traditional mass-flux scheme. Mass-fux scheme should produce smaller thermals than those at mesoscale. The mass-flux scheme is only used in case of shallow convection. However, in the gray zone, the models have to move from a unidirectional (mesoscale) to a tridimensional (LES) turbulence scheme. So the shear driven production of turbulence has also to be modified. A shear driven neutral BL is studied. The gray zone of turbulence spreads from 25 m to 800m. The horizontal movements cannot be neglected, so a tri-dimensional turbulence scheme is required. However, the turbulence is not consistently isotropic, as assumed in LES. In convective BL, the horizontal turbulent movements must be taken into account at resolutions finer than 1 km resolution.