A turbulence scheme allowing for mesoscale and large-eddy simulations

The paper describes the turbulence scheme implemented in the Meso-NH commun ity 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-precipit ating processes. A particularity of the scheme is the use of variable turbu lent Prandtl and Schmidt numbers, consistently derived from the complete se t of second-order turbulent-moment equations. The results of three idealize d boundary-layer simulations allowing detailed comparisons with other large -eddy simulation (LES) models are discussed, and lead to the conclusion tha t 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 f or the mesoscale configuration of the model, if an appropriate parametrizat ion of the eddy length-scale is used. The mixing-length specification is th en the only aspect of the scheme which differs from the LES to the mesoscal e configuration, and the numerical constants used for the closure terms are the same in both configurations. The scheme is run in single-column mode f or 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 resu lt is original, and is due to the proper formulation of the parametrized mi xing length and of the turbulent Prandtl and Schmidt numbers. In fact, a co mparison of the parametrized mixing length with the length-scale of the ene rgy-containing eddies deduced by spectral analysis of the LES shows interes ting similarity.