On the applicability of CFD microscale models for downscaling the wind measured above an urban area into the canopy layer

The rapid explosion of computation capacities allows nowadays to use Computational Fluid Dynamics (CFD) numerical models, which were usually adopted for building-scale simulations (e.g. ≤ 1 km), for larger-scale atmospheric boundary layer (ABL) simulations (e.g. ≤ 20 km) including wide portions of complex urban environments. Therefore, temporal and spatial scales usually simulated by meteorologists through coarser-resolution mesoscale meteorological models (MMM), rapidly became object of investigations of wind engineers through the development of static and dynamic downscaling methods for coupling MMM and CFD models. However, despite the great achievements of the recent years, such coupling methods are still in their early days and several aspects concerning the wind flow simulation in the urban canopy layer (UCL) deserve further investigations. In particular, two aspects are analyzed in this paper: (i) what the extension of the urban area “explicitly modeled” (i.e. with the real shape of buildings and bridges, etc.) should be and (ii) which point should be selected above the UCL in order to properly downscale the wind field from MMM to CFD models.