The step-mountain coordinate: physical package

Abstract

The Level 2.5 turbulence closure model in the Mellor-Yamada hierarchy was chosen to represent the turbulence above the surface layer. A severe instability encountered in the early experiments in the turbulent kinetic energy (TKE) equation was found to be of a numerical origin. The Mellor-Yamada Level 2 turbulence closure scheme is used for the surface layer. For additional flexibility, a shallow logarithmic, dynamic turbulence layer, is introduced at the bottom of the Level 2 surface layer. A rather conventional formulation has been chosen for the ground surface processes and surface hydrology. The nonlinear fourth order lateral diffusion scheme was implemented in the model. The Betts and Miller approach has been adopted for deep and shallow cumulus convection. A review of the available results of numerical experiments suggests that the eta model is competitive with other sophisticated models using similar resolutions, and requiring similar computational effort. -from Author