An intense tropical cyclone, Typhoon Jebi, landed the central part of Japan in September 2018 and caused severe damages due to strong winds. Typhoon Jebi obtained the lifetime minimum central pressure of 915 hPa and took a track very similar to past disastrous typhoons: Typhoon Nancy (1961) and Muroto Typhoon (1934). In Osaka City, the 1st, 2nd, and 3rd highest record of instantaneous wind speeds are 60.0 m/s in September 1934, 50.6 m/s in September 1961, and 47.4 m/s in September 2018, respectively, suggesting that a typhoon has been the most threatening windstorm in the area. Buildings and structures in urban areas are known to affect significantly the magnitude of wind gustiness. Because of the growing urbanization globally, quantification of turbulent winds in densely-built, urban districts is important to understand the underlying risks of wind damages. We investigate the influences of densely built urban environments on the occurrence of wind gusts in urban districts of Osaka and Kyoto City during the landfall of Typhoon Jebi by merging mesoscale meteorological and building-resolving large-eddy simulations (LES). By explicitly representing realistic buildings and structures in LES, this study examines complex/complicated characteristics of winds within the densely built urban environment. With the successful reproduction of the track and intensity of the typhoon in meteorological simulations, the simulated winds at the boundary-layer top of the LES model are used to quantify the wind gusts in the urban district. The maximum wind gust in the analysis area of Osaka is around 60-70 m/s, which is comparable to the wind speed at the height of about 300 m. Such wind gusts are generated by instantaneous downward momentum transfer in areas of a cluster of buildings with variable heights. Instantaneous wind gusts are further examined in terms of building density and are found to be strongest when the building packing density is moderate. The results suggest that the risks of wind damages would be maximized in urban districts where the building height is inhomogeneous and the packing density is moderate.

Airflows over complex geometrical surfaces such as complex terrain and densely built urban districts are highly turbulent and sometimes become a threat under disturbed weather conditions. Among the meteorological disturbances, the landfalls of tropical cyclones cause strong winds and may lead to severe damages in populated urban areas and in mountainous areas. Because gusty winds are primarily a cause for damages, diagnosing and predicting quantitatively wind and turbulent characteristics are critically important. In order to assess strong wind hazards in complex urban areas, this study uses a large-eddy-simulation (LES) model with buildings and structures explicitly resolved at an O(1 m) grid spacing. With such a building-resolving LES model, we have analyzed turbulent airflows in urban districts of Japanese major cities and have found that the LES model is capable of estimating the magnitude of gusty winds and turbulent fluctuations. In order to improve the accuracy in representing the properties of airflows, we have developed a data assimilation method which incorporates observed turbulence. The proposed data assimilation method used a vibration equation which can incorporate turbulence winds toward target mean winds while maintaining small-scale turbulent fluctuations and was applied for airflows in actual urban districts of Kyoto City by incorporating data obtained from meteorological observations located in Kyoto. We have concluded that the data assimilation method using the vibration equation successfully nudges toward the target mean winds while maintaining small-scale turbulent fluctuations well. Our recent LES analyses of airflows in urban districts have been extended to studies related to the impact assessment of and the adaptation to climate change in urban areas. A building-resolving LES model has become a practical tool to more applied side of turbulent airflow analysis.

A gusty wind by typhoons is one of the major natural hazards and has been the most threatening windstorm in urban districts. In recent year some major cities in Japan have experienced extreme winds during typhoon landfalls. For example, Typhoon Jebi (2018) caused extreme wind gusts in Osaka and Kyoto, while Typhoon Faxai (2019) and Hagibis (2019) produced high winds in Tokyo and neighboring cities. Urban roughness obstacles exert significant influences on the magnitude of wind gustiness. With the growing urbanization globally, the quantification of turbulent winds in densely built, urban districts is important to the assessment and prediction of risks of wind damages and the understanding of the underlying physical mechanisms. Influences of densely built urban environments on the occurrence of wind gusts in urban districts during the typhoon landfalls are studied by merging mesoscale meteorological and building-resolving large-eddy simulations (LES), which allows an explicit representation of the complicated building structures while retaining the strong mesoscale perturbations from the typhoon. The actual building data of Osaka, Kyoto, and Tokyo are used in the building-resolving LES computational domains. With the successful reproduction of the track and intensity of the typhoon in meteorological simulations, the simulated winds at the simulated boundary-layer top are used to quantify the wind gusts in the urban district. The maximum wind gust in the analysis area of Osaka during the landfall of Typhoon Jebi is found to exceed 60 m/s, which is comparable to the wind speed at the height of about 300 m. Such wind gusts are generated by the instantaneous downward momentum transfer in areas, where buildings of great height variability are clustered. The instantaneous wind gusts are found to be the strongest for moderate building packing density. The results suggest that the risks of wind damages are mostly likely to be maximized in urban districts of high building-height variability and moderate packing density.