Tornado Damage Estimation of Cities and Buildings Using Meteorological Model/LES Hybrid Method

Estimation of building damage from strong wind gusts of severe local storms is a challenge in the disaster risk mitigation. It is especially true for actual cities and buildings with complex geometry. In order to realize a reliable damage estimation through numerical approach, proper consideration on multi-scale physics from meteorological process to flows around buildings is needed for severe local storms because their scale is so small that the meteorological field and the process have significant effect to the urban area flow structure in time and space. Also, resolving turbulence flows around the geometry is another key point as it is essential for accurate estimation of peak velocity and pressure acting on the buildings.

In this work, we first discuss detail of tornado flow topology in the city and tornado－structure interaction using our novel meteorological model/LES hybrid method for simulating severe local storms [1]. This method employs an LES-based calculation that is able to regenerate high frequency turbulence to meteorological simulation result of interest before coupling it to the urban-area LES.

Movement of tornado was veered toward the recirculation zone of the buildings and detained awhile as shown in flow visualization of Fig.1(a) and the time history of velocity and pressure of Fig.1 (b). In near-ground region, a number of building-scale vortices were generated and merged with the low-pressure of tornado center. Intensification of vorticity led to prominent local pressure regions as shown in Fig.1 (c) and high wind gust near the ground. On the other hand, in some cases, the tornado vortex high above the ground was somewhat weakened because the vortices from the ground connected to the tornado vortex at random and has no contribution to the rotational motion.

We also display results of damage estimation of Tsukuba Tornado in this paper. This F3 supercell tornado struck Tsukuba City on 6 May 2012 and caused one fatality, 37 injuries and completely destroyed 76 houses [2]. The methodology is as follows: we construct a numerical model that each building and house is reproduced using the GIS data, where we can evaluate the intensity of the tornado gust impact on each residential building and diagnose building damage based on simulated velocity and pressure distribution and wind load capacity of building components such as framework, roof and exterior materials following the work of Kikitsu et al. [3] We confirm the applicability of the present hybrid method to estimation of damage by comparison with the results shown in the field surveying report[4] for building damage caused by Tsukuba Tornado 2012.

Acknowledgement:

The study was funded by the MEXT project: “Advancement of meteorological and global environmental predictions utilizing observational ‘Big Data’” of the social and scientific priority issues (Theme 4) to be tackled by using post K computer of the FLAGSHIP2020 Project (hp170246, hp180194).

References:

[1] Kawaguchi, M., Tamura, T., et al, Meteorological Model/Engineering LES Hybrid Approach: Towards the Risk Assessment of Urban Areas in Strong Wind Events, 23rd Symposium on Boundary Layers and Turbulence, 2018

[2] Mashiko, W., A Numerical Study of the 6 May 2012 Tsukuba City Supercell Tornado.Part I: Vorticity Sources of Low-Level and Midlevel Mesocyclones, Monthly Weather Review, vol.144, pp. 1069-1092, 2016

[3] Kikitsu, H., Nakagawa, T., et al., Wind Speed Estimation Based on Degree of Timber Structural Damage for Rating Tornado Intensity, International Workshop on Wind Related Disasters and Mitigation, 2018

[4] Okuda, Y., Kikitsu, H., et al., On Building Damage Caused by the May 6, 2012 Tornado in Tsukuba City, Ibaraki Prefecture, Japan, International Workshop on Wind Related Disasters and Mitigation, 2018