Coupled CFD framework with mesoscale urban climate model: Application to microscale urban flows with weak synoptic forcing

A computational fluid dynamics (CFD) model was developed in the open-source CFD toolbox OpenFOAM for studying microscale urban flows during periods of weak synoptic forcing. The OpenFOAM model is coupled to the regional atmospheric model COSMO to provide boundary conditions for atmospheric variables. The urban canopy model DCEP is used to calculate surface energy balance and estimate the surface temperature boundary conditions. The proposed coupled model was tested for simulations of urban flows in a dense urban area of Zurich, Switzerland during a heatwave day and a day with strong background regional winds. It is shown that the coupled OpenFOAM model can qualitatively resolve small scale unsteady buoyant flows induced by heated buildings and water bodies. Comparing with the observation measurement, the coupled OpenFOAM model and COSMO can predict diurnal temperature with small errors for both sample days. For the wind speed and the wind direction, the errors of both models are higher on the heatwave day, due to the presence of unsteady buoyant flows. It is concluded that to obtain quantitatively accurate results, several improvements in transport models, surface fluxes, coupling strategies, etc. are needed. In addition, the results should be compared comprehensively with a well-located network of sensors.