The seventh International Conference on Urban Climate,
29 June - 3 July 2009, Yokohama, Japan
A HIGH-RESOLUTION NUMERICAL ANALYSIS OF THE EFFECTS OF COMPLEX
TOPOGRAPHY TO DISASTROUS RAINSTORM DEVELOPMENT OVER AN
URBAN AREA: A CASE STUDY OF THE 28 JULY 2008 SEVERE LOCAL RAIN
EVENTS IN THE KINKI REGION
Tetsuya Takemi, Kenichi Tatsumi, Hirohiko Ishikawa
Disaster Prevention Research Institute, Kyoto University, Uji, Kyoto, Japan
Abstract
This study investigates the effects of complex and small-scale topographical features to the development of
disastrous rainstorm in urban regions by conducting high-resolution simulations with the Weather Research and
Forecasting (WRF) model. The 28 July 2008 severe local rain events in the Kinki region, the second largest
metropolitan area in Japan, are chosen for the present case study. Under the effects of complex topography,
cumulonimbus clouds continuously develop at the similar locations with time and produce locally concentrated
rainfall. Resolving complex and small-scale terrain features are important in numerically representing high
precipitation for the present case.
Key words: rain storm, urban flooding, mesoscale meteorological modeling, numerical weather prediction
1. INTRODUCTION
Highly populated urban areas in Japan are mostly located in regions either near the mountains or under the
influence of complex topography. Owing to the development of meteorological disturbances such as extratropical
cyclones, fronts, and tropical cyclones, disastrous rain and wind storms frequently occur in such urban areas.
Highly concentrated population and social infrastructure in urban areas are a vulnerable factor for the occurrence
of disasters, which tend to be quite local. In order to mitigate and prevent local-scale disasters, high-resolution
information by numerical weather forecasting is vital. Simulating numerically local-scale severe local wind/rain
storms requires a high-resolution representation of small-scale topographical features, especially for complex
topography.
Takemi (2009) conducted high-resolution simulations of wind storms in coastal regions caused by extra-tropical
cyclones by resolving small-scale terrain features with the use of high-resolution digital elevation model (DEM)
data. He showed that the variability of surface winds is significantly affected by the locality of terrain features.
Such surface wind variability resulted from complex topography has also been investigated by several studies
(e.g., Rife and Davis 2005; Jimenez et al. 2008), which indicated that terrain characteristics control the variability
of surface wind speed and direction. Although it has been recognized that representing steep/complex terrain in
meteorological models with a high spatial resolution becomes more critical for quantitative wind forecasts, most of
high-resolution simulation studies do not make significant efforts in incorporating high-resolution DEM at grids on
the order of 100 m.
In this study, we investigate the physics processes for a heavy rainfall event that occurred in Kobe City, a
coastal city in the Kinki metropolitan in Japan, by conducting a high-resolution simulation with the Weather
Research and Forecasting (WRF) model. The 28 July 2008 severe local rain event in the Kinki region, the second
largest metropolitan area in Japan, is chosen for the present case study. We incorporate 50-m mesh DEM data in
order to produce modeled topography.
2. NUMERICAL MODEL AND SIMULATION SETTINGS
The mesoscale meteorological model used here is the WRF/Advanced Research WRF (WRF/ARW, version
3.0.1.1, Skamarock) which is developed mainly by the U.S. National Center for Atmospheric Research. The
model solves a non-hydrostatic, compressible equation system for the atmosphere. The WRF model has a
nesting capability that can resolve the area of interest with a fine grid spacing. In this study, four nested
computational domains (with the top being at 50 hPa) are set, with the outermost domain (2200 km by 2400 km)
covering most of Japan’s territory with a horizontal grid spacing of 10 km, decreasing the grid spacing as 2.5 km500 m-100 m for inner domains, and thereby the innermost domain (30 km by 25 km) covering the Kobe urban
area and the surrounding mountains. The vertical coordinate system is a terrain-following system based on the
hydrostatic pressure which is normalized by the pressures at the surface and the upper boundary. The number of
vertical grids is 40, with 10 grids in the lowest 500 m.
In determining the initial and boundary conditions, we use the 6-hourly Global Analysis data (GANAL) of Japan
Corresponding author address: Dr. Tetsuya Takemi, Disaster Prevention Research Institute, Kyoto University,
Gokasho, Uji, Kyoto 611-0011, Japan. E-mail: takemi@storm.dpri.kyoto-u.ac.jp
The seventh International Conference on Urban Climate,
29 June - 3 July 2009, Yokohama, Japan
Meteorological Agency (JMA), the 6-hourly Final Analysis data of the U.S. National Centers for Environmental
Prediction, and the daily Merged Sea Surface Temperature (MGDSST) analysis of JMA. For creating the terrain
and land-use/land-cover (LU/LC) data for the simulation, the two outer domains use the global 30-second
topography data (GTOPO30) from the U.S. Geological Survey (USGS). On the other hand, the 50-m mesh DEM
by the Geographical Survey Institute of Japan is used to create the terrains in the 500-m and 100-m grid domains.
The model is initialized at 1200 UTC 27 July 2008 and is integrated for 24 hours. The integration time steps are
45 sec for the outermost domain and 0.15 sec for the innermost domain. The times are referred to as UTC, which
is 9 hours behind Japan Standard Time.
3. RESULTS
Rain storms developed in various locations in the Kinki district on 28 July 2008; Kobe is one of the locations
which suffered a significant disaster. Kobe is a port city, located to the south of mountains (Fig. 1). The synopticscale background is characterized by Typhoon Fung-Wong over the Taiwan region and a stationary front to the
north of Kinki over the Japan Sea. In the simulation, a banded precipitating system develops near the stationary
front in the morning of 28 July 2008 (fig. 2), which moves southward. At the noontime, convective instability and
moisture content increases in the plains regions where large cities exist (Figs. 3 and 4). Stationary front seems to
move southward covering northern half of the Kinki district (which is identified as a sharp transition line in the
CAPE value). At this time, precipitating convection develops ahead of the stationary front, where convective
instability is quite large.
Fig. 1: Topography of the innermost domain.
Fig. 3: Horizontal distribution of CAPE in the
2.5-km domain at 0300 UTC 28 July.
Fig. 2: Radar-reflectivity (dbZ) in the 2.5-km
mesh domain at 0000 UTC 28 July 2008.
Fig. 4: The same as Fig. 3, except for
precipitable water vapor.
Interestingly, precipitating convection develops, being not only affected by synoptic-scale conditions but also by
topographic features: convective clouds develop at the similar locations with time and produce locally
The seventh International Conference on Urban Climate,
29 June - 3 July 2009, Yokohama, Japan
concentrated rainfall. This indicates that local-scale complex/steep topography will trigger convection if instability
exists. Figures 5, 6 and 7 exhibit the total precipitation amount for 6 hours during 0000 UTC and 0600 UTC 28
May over the three inner domains. High precipitation amounts extending in an east-west direction over the Kyoto,
Osaka, and Kobe urban areas are the reflection of both convective instability, as seen in Figs. 3 and 4, and
stationary convective development. From the region north of Kobe where a significant amount of precipitation is
seen, convective clouds sometimes move southward into the city center of Kobe overriding high mountains (i.e.,
Rokko Mountains). One of such convective clouds produce much rainfall in the mountains, leading to an abrupt
increase in the water level of Toga River within a very-short time period. This event suggests that metropolitan
areas in Japan, most of which are located near the mountains and/or complex terrains, are highly vulnerable to
this type of short-time strong precipitation, although the total amount of precipitation is not so striking.
A benefit for the present high-resolution simulation is that it can represent the locality of high precipitation areas
in a well-resolved sense. Since single-cell convective clouds and associated rain areas have horizontal scales on
the order of 1 km to 10 km, a grid spacing of at least O(100 m) should be necessary in order to represent
convection in a well-resolved way. In addition, a high-resolution representation of complex topography is
obviously desired. This is more applicable to the simulation of surface wind variation.
Fig. 5: Total rainfall during 0000
UTC and 0600 UTC 28 July in
the 2.5-km domain.
Fig. 6: The same as Fig. 5, except
for the 500-m domain.
Fig. 7: The same as Fig. 5,
except for the 100-m domain.
4. CONCLUSIONS AND FUTURE DIRECTIONS
Resolving complex and small-scale terrain features are important in numerically representing the locality of high
precipitation for the present case. A high-resolution representation of complex topography is also preferred. In
general, metropolitan areas in Japan, most of which are located near and/or surrounded by the mountains and/or
complex terrains, are highly vulnerable to short-time strong precipitation, although the total amount of precipitation
is not so striking. Therefore, local-scale information regarding high precipitation/wind is critical for the mitigation
and prevention of disasters due to meteorological disturbances.
A real-time forecasting system for local-scale meteorology over urban and suburban areas is now being
developed. Figure 8 shows a sample image from the local-scale, real-time weather forecasting system under
being developed. Based on map information, outputs from real-time numerical weather forecasting with the WRF
model, as well as current meteorological conditions, can be displayed with the use of a web browser. Users can
get meteorological information at their own requests on the web.
In order to better provide prediction information, a data assimilation technique applied for water vapor fields
would be useful especially for the quantitative evaluation of precipitation. This is now under way in our real-time
forecasting system.
Acknowledgements
This study is supported by a research fund from the Ministry of Land, Infrastructure, Transport and Tourism,
under a project led by Prof. H. Mase of Kyoto University.
The seventh International Conference on Urban Climate,
29 June - 3 July 2009, Yokohama, Japan
References
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Surface wind regionalization in complex terrain, J. Appl. Meteor. Clim., 47, 308-325.
Rife D. L., and Davis, C. A., 2005. Verification of temporal variations in mesoscale numerical wind forecasts, Mon.
Wea. Rev., 133, 3368-3381.
Skamarock, W. C., Klemp, J. B., Dudhia, J., Gill, D. O., Barker, D. M., Duda, M. G., Huang, X.-Y., Wang, W.,
Powers, J. G., 2008. A description of the Advanced Research WRF Version 3, NCAR Tech. Note, NCAR/TN475+STR, 113 pp.
Takemi, T., 2009. High-resolution numerical simulations of surface wind variability by resolving small-scale terrain
features, Theoretical and Applied Mechanics Japan, 57, 421-428.
Figure 8: A sample view of the local-scale, real-time weather forecasting system being developed at Disaster
Prevention Research Institute, Kyoto University.