Extreme Winds in Urban Districts
during Typhoon Jebi (2018)
By: Dr. Tetsuya Takemi, Associate Professor
Disaster Prevention Research Institute (DPRI), Kyoto University, JapanTyphoon Jebi (2018) made landfall in the Osaka Bay
area, Japan and caused significant damages tobuildings and houses, trees, power lines, and so on.
The maximum instantaneous wind measured at themeteorological observatory in Osaka reached 47.4 m s 1, the third record in the history of the observatory.
This report documents the damages in an urban districtof Osaka City, Japan and the results of the numericalanalysis on extreme winds in the densely built, urbandistrict.

1. Introduction
Typhoon Jebi (2018) developed in the western North
Pacific in August 2018 and obtained the lifetime
minimum central pressure of 915 hPa. After obtaining
this intensity, Typhoon Jebi maintained its intensity
while moving northward. On September 4th, Typhoon
Jebi finally made landfall on the islands of Shikoku andHonshu, crossing over the Osaka Bay, and spawned
storm surges/high waves around the bay areas as well
as strong winds over the inland areas. KansaiInternational Airport (KIX) established on a reclaimed
island in the Osaka Bay in 1994 was seriously
damaged by storm surge. Furthermore, a large numberof points observed extreme winds, which caused
severe damages to houses/buildings, trees/forests,
power lines, etc.
Typhoon Jebi took a track very similar to Typhoon
Nancy (1961) and Muroto Typhoon (i.e., Daini-MurotoTyphoon) in 1934. In Osaka City, the 1st, 2nd, and 3rdhighest record of instantaneous wind speed is 60.0 m s-1-1

in September 1934 (Muroto), 50.6 m s in September
1961 (Daini-Muroto), and 47.4 m s-1 in September 2018
(Jebi), respectively. In Kyoto City, the maximum
instantaneous wind speed during Typhoon Jebi was
39.4 m s-1, which is the 2nd highest record since theobservation started in 1915 (the 1st record in Kyoto Cityis 42.1 m s-1 in September 1934). This suggests that
this typhoon has been the most threatening windstorm
in the area. Considering the time period spanning from1934 to 2018, the influences of both urbanization andglobal warming should be taken into account to assessthe disaster risks by typhoons.

2. Damages by strong winds in theNamba area, Osaka City
In the Namba area, one of the business district inOsaka City, severe damages to houses, structures,trees along streets and in parks, occurred. Figure 1
shows damage to an old wooden house in the Nambaarea.

Figure 1: damage to an old house in Osaka
City.
There were also damages to roofs, walls, signboards,etc. of buildings and houses. A large number of trees
along the streets and in parks in Osaka City sustained
severe damages.
Figure 2 shows the damages to trees along theMidousuji Street, a major street in Osaka City.

Figure 2: Damages to trees on the Midousuji street.
6

district analyzed here were estimated to be 60 - 70 m
s-1, owing to the downward transport of strong winds
at higher levels above the urban canopy (Figure 4).

3. Numerical analysis of strong winds
in Osaka City
After the severe damages incurred in urban
districts, quantitative estimations of winds in theareas were necessary. However, it is very difficult
to quantitatively estimate strong winds in urban
districts, due to absence of observations withinurban districts, and because of the inability tosimulate airflows in urban districts with only anumerical weather prediction model. Explicit
representations of urban buildings and structures
are critically important to reproduce airflows within
urban districts.

The idea in this study is to combine theFigure 4: The spatial distribution of the maximum instantaneous windadvantages of a numerical weather prediction
speed at the 10-m height from the time series of the wind speed at eachgrid point in a business district of Osaka City.
model and a turbulence model (large-eddy
simulation model; LES) to quantitatively estimate thewind speeds in the business district of Osaka City.
The weather prediction model can reproduce actual
meteorological situations including Typhoon Jebi
4. Implications(2018), while the LES model can incorporate theThe turbulent airflows and the maximum wind gusts
actual buildings and structures and hence reproduce
within the actual business district of Osaka City were
turbulent airflows within actual urban districts. The
successfully
estimatedbycombining
theframework of this analysis is presented in Figure 3.
meteorological simulations and buildingresolving turbulent airflow simulations. Wind
speeds were highly variable, depending on
the arrangement and vertical extent ofsurrounding buildings and structures. From
this study, emphasize is placed on the criticalimportance to understand the risk of strong
winds hidden in urban districts by conducting
similar analyses on various metropolitan
areas. Considering the ongoing re-shaping ofmetropolitan areas and the increasing numberof high-rise buildings in Japan and thepossible impacts of future climate change on
the intensification of typhoons, it is necessary
to understand emerging risks of strong winds
in urban districts.
Figure 3: Conceptual framework of the numerical analysis ofstrong winds in urban districts.

References

Takemi, T., T. Yoshida, S. Yamasaki, and K. Hase,
2019: Quantitative estimation of strong winds in an
urban district during Typhoon Jebi (2018) by merging
mesoscalemeteorologicalandlarge-eddy
simulations. SOLA, 15, 22-27, doi:10.2151/sola.2019-005.

The LES model was able to simulate fluctuating,turbulent airflows within the business district ofOsaka City. By referring to the simulated windspeeds by the weather model and quantifying thewind speeds simulated by the LES model, themaximum wind speeds in some areas within the7