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 Kármán constant, and z is height. The non-dimensional eddy-diffusivity coefficients calculated from the modified M–Ymodel are in satisfactory agreement with those from the LES data. It is found that the modified M–Ymodel 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 forL.