22 SOLA, 2019, Vol. 15, 22−27, doi:10.2151/sola.2019-005 Quantitative Estimation of Strong Winds in an Urban District during Typhoon Jebi (2018) by Merging Mesoscale Meteorological and Large-Eddy Simulations Tetsuya Takemi, Toshiya Yoshida, Shota Yamasaki, and Kentaro Hase Disaster Prevention Research Institute, Kyoto University, Uji, Japan of strong winds by Typhoon Jebi. Abstract Buildings and structures in urban areas affect most sig- nificantly the magnitude of wind gustiness. Kato et al. (1992) An intense tropical cyclone, Typhoon Jebi (2018), landed the demonstrated that gust factors during typhoons exceed 2 within central part of Japan and caused severe damages. Quantitative the urban canopy from measurements in Tokyo and Yokohama. assessment of strong winds in urban districts under typhoon By combining mesoscale meteorological and large eddy simula- conditions is important to understand the underlying risks. As a tion (LES) models, Nakayama et al. (2012) successfully estimated preliminary study, we investigate the influences of densely built the instantaneous wind speeds with gust factors exceeding 2 in a urban environments on the occurrence of wind gusts in an urban business district of Tokyo during a typhoon landfall. As indicated district of Osaka City during Typhoon Jebi by merging mesoscale in Yoshida et al. (2018) and Yoshida and Takemi (2018), the meteorological and building-resolving large-eddy simulations variability of wind speed become more significant with a larger (LES). With the successful reproduction of the track and intensity variability of building height. These studies suggest that densely of the typhoon in meteorological simulations, the simulated winds built urban districts would be susceptible to typhoon-induced at the boundary-layer top of the LES model are used to quantita- stronger winds than previously thought from the past experiences tively estimate the wind gusts in the urban district. The maximum such as Muroto and Daini-Muroto Typhoons. Thus, quantitatively wind gust in the analysis area of Osaka was estimated as 60− estimating the strong winds in real urban districts by Typhoon Jebi 70 m s−1, which is comparable to the wind speed at the height of (2018) is a scientific challenge and is very important for assessing about 300 m. risks of strong winds in urban districts. (Citation: Takemi, T., T. Yoshida, S. Yamasaki, and K. Hase, As a preliminary study, we estimate quantitatively the strong 2018: Quantitative estimation of strong winds in an urban district winds within a business district of Osaka City during Typhoon during Typhoon Jebi (2018) by merging mesoscale meteorological Jebi (2018) by employing a mesoscale meteorological and an LES and large-eddy simulations. SOLA, 15, 22−27, doi:10.2151/sola. model. By explicitly representing real buildings and structures in 2019-005.) LES, this study examines complex/complicated characteristics of winds within the densely built urban environment. 1. Introduction 2. Procedure of numerical analysis Typhoons are one of the major extreme weather in the western North Pacific region, and sometimes become a great threat to This study used both a mesoscale meteorological model and our society. Recently Typhoon Haiyan (2013) made landfall over a building-resolving LES model. The meteorological model was the Philippines and caused devastating damages by storm surges intended to simulate the typhoon and its meteorological back- (Mori et al. 2014). Historically, Japan faced severe damages due ground, while the LES model simulated turbulent airflows within to strong winds by intense typhoons such as Vera (1959), Nancy a business district of Osaka. (1961), Mireille (1991), and Songda (2004). Most recently, Typhoon Jebi (2018) made landfall on the islands of Shikoku and 2.1 Meteorological model settings Honshu, crossing over the Osaka Bay, and spawned storm surges/ The meteorological model used was the Weather Research and high waves around the bay areas as well as strong winds over the Forecasting (WRF) model/the Advanced Research WRF (Skama- inland areas. Kansai International Airport (KIX) established on a rock et al. 2008) version 4.0. We set two-way-nested computational reclaimed island in the Osaka Bay in 1994 was seriously damaged domains (Fig. 1a): Domain 1 covers parts of the western North by storm surge. Furthermore, a large number of points observed Pacific and the Japanese islands at the 4.5-km horizontal grid, extreme winds, which caused severe damages to houses/buildings, and Domain 2 covers mainly the Kinki region at the 0.9-km grid trees/forests, power lines, etc. (Cabinet Office 2018). on the Lambert conformal projection. The both domains have 56 Typhoon Jebi obtained the lifetime minimum central pressure vertical levels, with the interval extended with height up to the top of 915 hPa (according to Regional Specialized Meteorological at the 20-hPa level. The model topography was created from the Center Tokyo) and took a track very similar to Typhoon Nancy 30-arc-seconds elevation data from the United States Geological (1961) (i.e., Daini-Muroto Typhoon) and Muroto Typhoon in Survey, while the land-use characteristics were determined by the 1934. In Osaka City, the 1st, 2nd, and 3rd highest record of in- MODIS-based 21-category data. stantaneous wind speed is 60.0 m s−1 in September 1934 (Muroto), We set the physics parameterizations similar to those em- 50.6 m s−1 in September 1961 (Daini-Muroto), and 47.4 m s−1 in ployed in Takemi et al. (2016a, 2016b) and Nayak and Takemi September 2018 (Jebi), respectively, suggesting that a typhoon (2019): e.g, the 6-category single-moment microphysics scheme has been the most threatening windstorm in the area. Considering of Hong and Lim (2006); the Yonsei University boundary-layer the time period spanning from 1934 to 2018, the influences of scheme (Hong et al. 2006), and a Monin-Obukhov similarity- urbanization and global warming should be taken into account to based surface-flux scheme (Jiménez et al. 2012). No cumulus assess the disaster risks by typhoons. This study focuses on the parameterization was used. influences of urban buildings and structures on the development The 6-hourly Global Spectral Model analyses of Japan Mete- orological Agency were used to prescribe the initial and boundary Corresponding author: Tetsuya Takemi, Disaster Prevention Research conditions for the atmospheric and surface states. Merged Institute, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan. E-mail: satellite and in situ data Global Daily Sea Surface Temperatures [email protected]. ©The Author(s) 2019. This is an open access article published by the Meteorological Society of Japan under a Creative Commons Attribution 4.0 International (CC BY 4.0) license (http://creativecommons.org/license/by/4.0). Takemi et al., Strong Winds in an Urban District during Typhoon Jebi (2018) 23 shown). Thus, the assumption of no mean vertical motion, neutral condition, and southerly inflow in LES is validated. The time integration was conducted for 5400 seconds; the out- puts during the last 1800 seconds were used for the analysis. The wind speeds were normalized by the reference wind speed U¥ de- fined as the wind at the boundary-layer top, i.e., the 326-m height (z¥) which was chosen as the height at which the maximum of the mean horizontal winds appeared (see Fig. 5). Note here that this boundary-layer height is different from the actual one under the typhoon condition. The idea for this normalization was to use the WRF outputs as a reference to obtain quantitative values for wind speeds within urban canopy, as described in Section 3.2. Ahmad et al. (2017) demonstrated the usefulness of this normalization to assess gust intensities within urban canopies at different locations. 2.3 Merging meteorological and LES models Fig. 1. The computational domains for (a) the WRF model and (b) the LES There are some approaches in merging meteorological and model. In (a), two nested domains (D01: Domain 1; D02: Domain 2) were LES models (e.g. Mayor et al. 2002; Mochida et al. 2011; Nakaya- exhibited. In (b), the main computational domain is indicated by white sol- ma et al. 2012; Muñoz-Esparza et al. 2014; Li et a. 2017; Huang id box, having a 3 km (south-north) by 2 km (west-east) area within Osaka et al. 2018): the outputs of a meteorological model are somehow City. The color denotes the height of the buildings and structures obtained directly imposed as the inflow conditions of an LES model. In from the 2-m-mesh DSM data. this preliminary study, we employed a simple approach to use winds only at a certain height from the meteorological model as a reference for airflows simulated in LES (Takemi et al. 2006). The idea for this analysis is to combine the advantages of the both (MGDSST) data were used for prescribing SSTs. To accelerate WRF and LES models: the WRF simulation provides the meteo- the development of the typhoon from the analyses, we added an rologically driven wind speeds UWRF at z¥, while the LES defines initial vortex from a typhoon-bogus scheme. With the initial times the ratio of wind speed Uz within urban canopy layer against U¥. changed from 1200 UTC 31 August to 1200 UTC 2 September Thus, wind speed at height z within the urban canopy is deter- 2018 at the 6-hour interval, time integration was done until 1800 mined as follows: UTC 4 September and totally conducted 9 simulations. The sim- U ulated outputs for Domain 2 were done at 10-minute interval and () z Uz= UWRF . (1) used for the following analysis. U∞ 2.2 Building-resolving LES modeling Normalized wind speed from LES is dimensionalized with the aid We used the LES model developed by Yoshida et al.