On a New k-ε Parametrization Closure for Building-Induced Turbulence

To take into account the modications induced by buildings on flows and turbulent variables, appropriate urban canopy parameterizations (UCPs) have been developed in the last years, to be coupled with mesoscale meteorological models. However, despite several UCPs were successfully able to reproduce wind ows in the urban environment, they lack in correctly defining the mixing length scale for dissipation rate and eddy viscosity. The main objective of this work is to develop and test a new building-induced turbulence closure independent of turbulence length scales, directly solving not only the equation for turbulent kinetic
energy, but also the equation for its dissipation rate (k-ε model). This new turbulence closure has been coupled with the BEP UCP, adding a new term to consider the dissipation rate production induced by buildings drag. The dependence of the drag coeffcient on building height and packing density is calculated from building-resolving CFD RANS simulations. The performance of the new k-ε model is rst tested by means of single-column Reynolds-averaged Navier{Stokes (RANS) simulations in idealized urban areas, using different building packing densities. Results are in good agreement with spatially averaged building-resolving CFD simulations. In particular, vertical profiles of mean and turbulent variables show better results with respect to simulations using turbulence closures adopting a mixing length scale parameterization. Moreover, results highlight the importance of adopting a drag coeffcient dependent on packing density.
The new k-ε parameterization closure for urban areas has been also implemented in the Weather Research and Forecasting (WRF)
model, to evaluate its performance in dynamically reproducing real atmospheric conditions in the city of Trento, situated in the Adige Valley, in the Italian Alps. Results are evaluated against measurements coming from different ground weather stations, and compared with simulations performed adopting other state-of-the-art turbulence closures, i.e. MYJ and BouLac. The k-ε turbulence closure performs similarly to MYJ and BouLac for wind speed and direction, while it generally shows a lower temperature during nighttime in stable conditions, in better accord with experimental data.