Improvement of the Mellor-Yamada turbulence closure model based on large-eddy simulation data

On the basis of data constructed with large-eddy simulation (LES), an attempt is made to improve the Mellor-Yamada (M-Y) turbulence closure model. Firstly, stably-stratified and convective planetary boundary layers without moisture are simulated by a LES model to obtain a database for the improvement. Secondly, based on the LES data, closure constants are re-evaluated and a new diagnostic equation for the master length scale L is proposed. The new equation is characterized by allowing L in the surface layer to vary with stability instead of constant kz, where k is the von Karman constant, and z is height. The non-dimensional eddy-diffusivity coefficients calculated from the modified M-Y model are in satisfactory agreement with those from the LES data. It is found that the modified M-Y model improves the original one largely, and that the improvement is achieved by considering buoyancy effects on the pressure covariances andby using the newly proposed equation for L.