A k-epsilon Turbulence Closure Model for Urban Atmospheric Boundary Layer (Formerly paper P6B.1)

A turbulent closure model is developed for the simulation of turbulent momentum, heat and water vapor transfer in the urban atmospheric boundary layer, including the urban canopy layer. The model is a k-epsilon type, improved for the computation under density stratification and with the presence of solid objects. In the improved k-epsilon model, the transport of momentum and heat in the vertical direction under thermal stratification is evaluated based on the assumption of a near-equilibrium shear flow where transport effects on the stresses and heat fluxes are negligible (here after this model is called the stratified model). The heating processes at urban surfaces are also simulated by employing a building canopy heating model. Additionally, in the urban canopy layer, effects of buildings and other urban structures on the momentum, heat and water vapor transfer are accounted for by introducing source/sink of momentum or heat, and a spatial averaging procedure (here after this model is called the urban canopy model).

The performances of the stratified model for the computation in the planetary boundary layer under stable stratification, weakly stable, neutral, weakly unstable, and unstable conditions are investigated by comparing computed results with results from literature. Results of the computations show that the model can perform better, compared with the standard k-epsilon model or other turbulence closure models for the atmospheric boundary layer, especially inside the urban canopy layer.

The models are used to study the influence of different closure assumptions on the heat and momentum transfer in the vertical direction with the presence of the urban canopy. Computational results on the heat and momentum fluxes by using the standard k-epsilon model are significantly different from those obtained by using the stratified model. A similar thing can be said about computational results obtained by using the stratified model and the urban canopy model, respectively. At last, the urban canopy model is employed to investigate various characteristics of turbulent fluxes under different urban canopy conditions.