A k-ε turbulence closure model for the atmospheric boundary layer including urban canopy

A numerical model for the computation of the wind field, air temperature and humidity in the atmospheric boundary layer (ABL) including the urban canopy was developed for urban climate simulation. The governing equations of the model are derived by applying ensemble and spatial averages to the Navier-Stokes equation, continuity equation and equations for heat and water vapour transfer in the air. With the spatial averaging procedure, effects of buildings and other urban structures in the urban canopy can be accounted for by introducing an effective volume function, defined as the ratio between the volume of air in a computational mesh over the total volume of the mesh. The improved k - epsilon model accounts for the anisotropy of the turbulence field under density stratification. In the improved k - epsilon model, the transport of momentum and heat in the vertical direction under density stratification is evaluated based on the assumption of a near-equilibrium shear flow where transport effects on the stresses and heat fluxes are negligible. The heating processes at surfaces of buildings and ground are also modelled. The comparison of the computational results obtained with the present model and existing observational data and numerical models shows that the present model is capable of predicting the structure of turbulence in the urban canopy layer under density stratification. Numerical experiments with the new model show that the flow behaviour of the air in the urban canopy layer is strongly affected by the existence of buildings and density stratification.