A turbulence scheme allowing for mesoscale and large‐eddy simulations

Abstract

The paper describes the turbulence scheme implemented in the Meso‐NH community research model, and reports on some validation studies. Since the model is intended to perform both large‐eddy and mesoscale simulations, we have developed a full three‐dimensional scheme, based on the original method of Redelsperger and Sommeria. A prognostic equation for the turbulent kinetic energy is used, together with conservative variables for moist non‐precipitating processes. A particularity of the scheme is the use of variable turbulent Prandtl and Schmidt numbers, consistently derived from the complete set of second‐order turbulent‐moment equations. The results of three idealized boundary‐layer simulations allowing detailed comparisons with other large‐eddy simulation (LES) models are discussed, and lead to the conclusion that the model is performing satisfactorily. The vertical flux and gradient computation can be run in isolation from the rest of the scheme, providing an efficient single‐column parametrization for the mesoscale configuration of the model, if an appropriate parametrization of the eddy length‐scale is used. The mixing‐length specification is then the only aspect of the scheme which differs from the LES to the mesoscale configuration, and the numerical constants used for the closure terms are the same in both configurations. The scheme is run in single‐column mode for the same three cases as above, and a comparison of single‐column and LES results again leads to satisfactory results. It is believed that this result is original, and is due to the proper formulation of the parametrized mixing length and of the turbulent Prandtl and Schmidt numbers. In fact, a comparison of the parametrized mixing length with the length‐scale of the energy‐containing eddies deduced by spectral analysis of the LES shows interesting similarity.