A Mesoscale-to-LES Modeling of Tornado-Like Vortex and Associated Local Strong Winds in Urban Area

Tornadoes in urban areas pose a great threat to local residents and buildings. The tornado's fine-scale dynamic structures in turn can be disturbed by these buildings. This study employs an advanced modeling system of mesoscale-to-large eddy simulation (LES) to explicitly resolve both the tornado-like vortex (TLV) and building-induced flow disturbances. Our modeling reasonably reproduced a TLV passing over building clusters, while the evolutionary characteristics of TLV are comparable to that of the video-captured tornado funnels of an actual event in South China. The structures of the TLV are significantly disturbed as it approaches high-rise building clusters. After arriving at the downstream of the building cluster, the disturbed TLV regains a well-defined structure. Such a reconstruction of TLV structures is characterized by the vertical alignment of the original aloft TLV structures and low-level building-induced lee vortex. As the TLV detaches the building cluster, it leaves open space for a wind corridor, a band area of high wind speeds along the direction of ambient flow in the open area among building clusters. This corridor delivers momentum that, when coupled with the reconstruction of TLV, favors local strong winds. Such a type of local strong winds are highly related to the heights of building clusters but are somewhat less sensitive to the detailed TLV track relative to building clusters, as indicated by numerical sensitivity experiments. These results provide insight into possible wind speed changes when tornadoes encounter buildings, which may have valuable implications for urban wind hazard mitigation.