Coupling of WRF and building-resolving urban CFD models for analysis of strong winds over an urban area

Atmospheric flow is characterized by mainly two factors: a large-scale meteorological disturbance and a small-scale wind velocity fluctuation produced by surface roughness elements. Particularly, for highly roughened ground surface, such as populated urban area, high-rise buildings have a significant influence on a small-scale fluctuation of atmospheric flow. In the case of strong winds occurrence induced by typhoons or hurricanes, the characteristics of wind velocity fluctuations should be evaluated, considering not only the weather conditions but also the aerodynamic effects of urban buildings. There are various methods of predicting atmospheric flows, e.g., Numerical Weather Prediction (NWP) and Computational Fluid Dynamics (CFD) models. The NWP model is commonly used to simulate current weather conditions and the future state of the atmosphere. In these models, the aerodynamic effects of urban surface are represented as simplified parameterizations, such as the roughness-length approach and the drag-force approach. Because individual buildings are not explicitly represented in the NWP model, it is difficult to reproduce wind velocity fluctuations considering urban surface geometries. On the other hand, the CFD model is generally designed to predict a small-scale turbulent motion. Recently, with the rapid development of computational technology, the CFD technique is used to simulate air flow over explicitly resolved individual buildings within urban areas. In CFD simulations, there are two different approaches, the Reynolds-Averaged Navier-Stokes (RANS) and Large-Eddy Simulation (LES) models, which are both effective for predicting turbulent flows. In RANS model, a mean wind flow is computed, delivering an ensemble- or time-averaged solution, and all turbulent motions are modeled with a turbulence model. The LES model explicitly resolves turbulent flow with all scales larger than the grid size, and the turbulent flow with scales smaller than the grid size is parameterized using the eddy viscosity model. Particularly, LES has come to be regarded as an effective prediction method for capturing the unsteady wind flows. Therefore, coupling of the NWP model with the CFD model using LES can be effective means to predict small-scale wind fluctuations over urban areas under real meteorological conditions. In this study, we performed a numerical simulation for strong winds over the central Tokyo during the passage of Typhoon Melor (2009). For a mesoscale simulation, the Weather Research and Forecasting (WRF) model is used. In LES model, the urban surface geometry is explicitly resolved. At the inlet boundary of the LES computational region, the wind profile obtained from the WRF model is imposed and turbulent fluctuations are generated by the existing turbulent inflow generation method (Kataoka et al, 2002). Compared to Mesoscale Analysis data by Japan Meteorological Agency (JMA), the simulation results show that the variability of wind velocity is similar to that observed at a surface site of JMA.