AN EVALUATION OF SEVERAL TURBULENCE SCHEMES FOR THE PREDICTION OF MEAN AND TURBULENT FIELDS IN COMPLEX TERRAIN

A prognostic three-dimensional mesoscale model has been developed and used in one- and two-dimensional modes to evaluate ten local turbulenc e closure schemes. The schemes ranged from first-order to the two-equa tion prognostic schemes. Predictions by the models were compared for a one-dimensional convective boundary layer using mixed layer scaling a nd measurements to interpret the results, Two-dimensional simulations were also performed for a sea-breeze flow and for flow over a hill. Th e results showed that for all of the models considered, minor differen ces were produced in the mean meteorological fields and in the vertica l scalar fluxes, but major differences were apparent in the velocity v ariances and dissipation rate. Predicted tracer concentrations were ve ry sensitive to the turbulence model formulation for dispersion from a point source in the convective boundary layer, particularly for the p rediction of maximum concentrations. Predicted tracer concentrations f rom a surface volume source for the two-dimensional simulations were s imilar for all models, although the degree of mixing in the morning gr owth period produced some differences. Generally, good results for the mean meteorological fields can be obtained with first-order schemes, even if they underpredict the magnitude of turbulence in the convectiv e boundary layer, and reasonable tracer concentrations can also be obt ained with these models provided near-source effects are not important . The two-equation prognostic models performed best for the prediction of turbulence in the convective boundary layer.