Turbulent Heat and Momentum Fluxes in the Urban Boundary Layer Derived from a k-epsilon Turbulence Closure Model

The turbulent fluxes of heat and momentum inside and outside canopy layer in the urban atmospheric boundary layer are studied with an improved k-epsilon turbulence closure model. 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 (this model hereafter named the stratified model). The heating processes at urban surfaces are also considered. Additionally, in the urban canopy layer, effects of buildings and other urban structures on the momentum or heat transfer are accounted for by introducing source/sink of momentum or heat, and a spatial averaging procedure (this model hereafter named 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.

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.