Understanding the Extreme Winds over Built Surfaces

Urban surface complexity is known to exacerbate the spatio-temporal variability of flow and transport in cities, resulting in highly heterogeneous wind fields. As a result, the probability density function of wind speed and direction is broader compared to that over homogeneous terrain, and the likelihood of extreme high or low winds, which is related to resident comfort and safety, increases accordingly. This study investigates these urban wind extrema by conducting large-eddy simulations (LESs),where the buildings are resolved explicitly using the immersed boundary method. The building geometries were obtained from the high-resolution digital surface maps (DSMs). An analysis of a few selected cities indicates that gusts tend to occur along wind-oriented streets and on both sides of high-rise buildings, while low wind regions tend to be mostly located in the lee of tall and/or wide buildings. Possible geometric parameters explaining these winds extrema are investigated, such as the street orientation and the building densities. These findings provide insight for improving both urban planning and sensing.