Junji Maeda

Nobuyuki Ishida

Taiichi Hayashi

Takeshi Fujii

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An Analysis of a Pressure Pattern in Severe Typhoon Bart Hitting the Japanese Islands 
in 1999 and a Comparison of the Gradient Wind with the Observed Surface Wind 
 
Takeshi FUJII1), Junji MAEDA2), Nobuyuki ISHIDA3) and Taiichi HAYASHI4) 
 
1) General Education and Research Center, Kyoto Sangyo University, Kita-ku, Kyoto, 603-8555, Japan 
2) Graduate School of Human-Environment Studies, Kyushu University, Higashi-ku, Fukuoka, 812-8581, Japan 
3) Japan Steel Tower Co., Ltd.,Wakamatsu-ku, Kitakyushu, 808-0146, Japan 
4) Disaster Prevention Research Institute, Kyoto University, Gokanosho, Uji, Kyoto, 611-0011, Japan 
 
 
 
1. INTRODUCTION 
  Typhoon Bart hit the Kyushu District of Japan on 
Sept. 24, 1999 with very strong intensity and caused 
disasters by high wind and storm surge. One of them 
was a storm surge with 12 deaths at Matsuai-area of 
Shiranui (Fig.1) in Kumamoto Prefecture. 
The track of Typhoon Bart determined by Japan 
Meteorological Agency (JMA) is shown in Fig.1. 
The typhoon formed as a tropical storm over the sea 
south of the Okinawa Island on Sept. 19. It translated 
toward north-northeast with intensifying, and made 
landfall on the Kumamoto Prefecture in Kyushu 
District in the morning on Sept. 24. The typhoon 
translated over the warm sea water exceeding 28 ℃, 
and it maintained a mature stage of 935 hPa in 
central pressure and 45m/s in the maximum wind 
speed at 3 JST, Sept.24, when was 3 hours before 
landfall. 
We have developed an objective method for 
analysis of the pressure pattern in a typhoon in the 
previous research (Mitsuta et al.1)). We analyzed the 
pressure pattern in Typhoon Bart by using this 
method, and computed the gradient wind from it. 
In Kyushu, the wide area wind measurement 
system of high density called NeWMeK (Network 
for Wind Measurement in Kyushu) has been 
organized. Anemometers at sites of NeWMeK are 
installed at the top of the power transmission towers, 
and wind is measured at high altitude of about 30m 
to 50m above the ground surface. The sites are 
distributed at a distance of about 20 km, and total 
number of sites is 121. 
In this study, the gradient wind is computed from 
the pressure pattern analyzed by the objective 
method, and it is compared with the ten-minute 
mean wind observed at the sites of the NeWMeK 
system along the coast line in Southwest Kyushu. It 
is also compared with the surface wind observed at 
the weather stations of JMA. 
 
Fig. 1 The track, the central pressure and the 
maximum wind speed of Typhoon Bart of 
1999. A broken line indicates a stage of a 
tropical depression. Date and time are 
represented by JST. 
 
2. AN ANALYSIS OF PRESSURE PATTERN 
2.1 Pressure formula 
A tropical cyclone in a mature stage has a 
concentric pressure patterns, and the pressure 
distribution can be given by only a radial pressure 
profile. In order to represent a radial pressure profile 
for severe typhoons hitting the Japanese Islands, an 
 1 
 
Fig.2 A pressure distribution obtained by the 
objective analysis. An open circle is a 
weather station of JMA used for the 
pressure analysis. 
  
Fig.3  Tracks of typhoon centers determined 
by the objective pressure analysis and 
JMA. A small circle denotes an 
anemometer site of NeWMeK and a small 
square a weather station of JMA.  
experimental formula presented by Schloemer3) has 
been used in the previous researches by us (Mitsuta 
et al.1): Fujii2)) 
)/1exp( xppp c −∆+=    (1) 
where p is the sea level pressure at the radial 
distance r, pｃ the pressure at a typhoon center, Δp 
the central pressure depth which equals to the 
pressure difference between the typhoon center and 
the periphery, and x a relative radius normalized by a 
radius of the maximum wind rm, x=r/rm. 
  Holland4) extended this equation by adding 
another parameter, in order to represent the pressure 
patterns in tropical cyclones in a small scale in 
Australia. Fujii and Mitsuta5) analyzed pressure 
patterns in three typhoons which made landfall on 
the Japanese Main Islands with a strong intensity in 
the recent years. The result by them showed that the 
pressure patterns can be approximated reasonably 
with by Eq.(1). So, Eq.(1) is used in the present 
study. 
 
2.2 A method of pressure analysis 
  Eq.(1) is fitted to sea level atmospheric pressure 
observed at weather stations of JMA by the method 
of least squares (Fujii2)). A weight in inverse 
proportion to a radial distance from the typhoon 
center is multiplied on pressure data, in order to get a 
close fitting to observed data near the typhoon 
center. 
 The fitting process starts from given initial values 
of a typhoon center position and rm. Then, the first 
estimations of Δp and pc are obtained by the 
method of least squares. A root mean square error is 
computed from the difference between computed 
and observed sea-level pressures at the weather 
stations of JMA. 
  With successive changes of rm at the interval of 
0.5 km, the root mean square error is computed, and 
values of Δp, pc and rm are chosen as a set of values 
showing the minimum error. 
  Then, the initial position of the typhoon center is 
shifted by 0.01o in latitude or in longitude, and 
values of Δp, pc and rm are estimated. By repeating 
this procedure, the best estimated values of the 
center position (latitude and longitude), Δp, pc and 
rm are determined. 
 
2.3 Analyzed pressure pattern 
The analysis of a pressure pattern started at 01 JST, 
Sept.24, and a position (latitude and longitude) of 
the typhoon center, pc, Δp and rm were determined. 
It was repeated at the interval of 10 minutes. The 
pressure pattern obtained by the objective analysis at 
04:30 JST, when it was about one hour and half 
before landfall, is shown in Fig.2. The value of pc is 
 2 
936 hPa, and the typhoon maintains very severe 
intensity. The value of rm is 42 km, and the area 
including the maximum wind passes the coast area 
along the Yatsushiro Sea. 
A track of the typhoon centers determined by the 
objective pressure analysis is shown with the track 
determined preliminarily by JMA in Fig.3. These 
two typhoon tracks are 30 km apart in maximum. 
 
3. THE GRADIENT WIND 
  Under an assumption of neglecting the friction 
force, the gradient wind speed, Vgr, can be computed 
from the pressure pattern approximated by Eq.(1). 
dr
dpfV
r
V
gr
t
gr
ρ
12
=+        (2) 
where rt is the radius of curvature of air parcel 
trajectory, f the Coriolis parameter, ρair density. 
The radius of curvature in air parcel trajectory in 
Eq. (2) for a moving typhoon is replaced by that of 
the streamline, rs, by the Blaton's formula (e.g. 
Holmboe et al.5)).  








−= θsin111
grst V
C
rr
        (3) 
where C is the translation speed of a pressure pattern, 
θ an angle measured from the translating direction 
of the pressure pattern clockwisely. 
Then, an unsymmetrical pattern of the gradient 
wind speed is obtained.  At the right-hand side of 
the typhoon translation direction, the wind is 
evaluated stronger than that of a stationary typhoon, 
and at the left-hand side, it is weaker. 
A distribution of the gradient wind speed 
computed from the pressure pattern in Fig.2 is 
shown in Fig.4. In the coast area along the 
Yatsushiro Sea, the gradient wind exceeds 50 m/s, 
and the maximum wind is 53 m/s. 
 
4. A RATIO OF THE OBSERVED AND 
GRADIENT WIND SPEEDS 
  The gradient wind speeds computed from the 
analyzed pressure pattern are compared with the 
ten-minute mean wind speeds recorded at the six 
sites of NeWMeK in the southwest coast area of 
Kyushu, where violent wind was observed. These 
sites are Nos. 71, 72, 77, 97, 98, and 100, and their 
locations are shown in Fig.3. The recorded wind 
speed is represented as a ratio to the gradient wind. 
This ratio is shown in relation to the relative radius, 
x, in Fig. 5. The ratio is about 0.5 in average and has 
no systematic variation with the relative radius in the 
outside of the maximum wind radius, x>1.0. 
However, in the inside of the maximum wind radius, 
x<1.0, the ratio increases rapidly with decrease of x.  
This increase of the wind ratio with approaching 
to the typhoon center is considered to be caused by 
inflow in the atmospheric boundary layer. In the 
boundary layer, angular momentum is transported 
inward across isobars by inflow produced by the 
ground surface friction. This transport accelerates air 
parcels inward and, as a result, causes the increase of 
the wind ratio. 
It is well known that the surface wind sometimes 
exceeds the gradient wind in the inside of the 
 
Fig.4 A distribution of the gradient wind speed
at 04:30 JST, Sept. 24. This distribution
was computed from the pressure pattern
shown in Fig.2. 
 
Fig.5 A change of the ratio of the wind speeds with
relative radius, x. The line represents an
averaged variation at the sites of NeWMeK. 
 3 
maximum wind radius, and it is called as the super 
gradient wind (Gray 7); Mituta et al. 8)). 
The surface wind speed observed at the six 
weather stations of JMA along the coast line of the 
Yatsushiro Sea and the East China Sea is also 
represented as the ratio to the gradient wind speed, 
and it is added in Fig.5. 
Generally, the ratio at stations of JMA is smaller 
than those of NeWMeK. It is considered to have its 
cause in difference among anemometer heights. So, 
the ratios in a radius of 1.0<x<2.0 having no 
systematic variation with x is shown in relation to 
the anemometer height in Fig.6. There is a wide 
scattering in ratios at the same station, and it is 
considered to be caused by topography which is 
different by a wind direction. A vertical profile 
obtained by synthesis of the wind ratios at different 
places may have little important significance, but a 
regression line is drawn on an assumption of an 
increase of the ratio in proportion to a power of 
height. Then, the ratio increases in proportion to 0.33 
power of height. 
 
5. SUMMARY 
  We analyzed the pressure pattern in Typhoon Bart 
having made landfall with very strong intensity on 
the Kyushu District of Japan on Sept. 24, 1999. The 
gradient wind was computed from the pressure 
pattern and was compared with the wind observed at 
the sites of NeWMeK. The ratio of the observed 
wind speed to the gradient wind speed was about 0.5 
in the outside of the maximum wind radius, but 
increased rapidly with decrease of relative radius in 
the inside of it. In addition of the wind data at six 
stations of JMA, the wind ratio in the outside of the 
maximum wind increased with height in proportion 
to 0.33 power of the anemometer height. 
 
Acknowledgements 
  The authors would like to express their thanks to 
Kyushu Electric Power Co., Inc for maintenance and 
providing of wind data of NeWMeK. They also 
would like to express their thanks to Japan 
Meteorological Agency for providing meteorological 
records. 
 
References 
1) Mitsuta, Y. T. Fujii and K. Kawahira, 1979 : 
Analysis of typhoon pressure patterns over 
Japanese Islands. Natural Disaster Science, 
Vol.1, No.1, pp.3-19 
2) Fujii, T., 1998 : Statistical analysis of the 
characteristics of severe typhoons hitting the 
Japanese Main Islands. Mon. Wea. Rev., Vol.126, 
pp. 1091-1097． 
3) Schloemer, R. W., 1954 : Analysis and synthesis 
of hurricane wind patterns over Lake 
Okeechobee, Florida. Hydrometeorological 
Report, USWB, No.31, 49pp.， 
 
Fig.6 A relation between the ratio of wind speed
ratio and anemometer height 
4) Holland, G.J., 1980 : An analytic model of the 
wind and pressure profiles in hurricanes, Mon. 
Wea. Rev., Vol.108, pp.112-1218 
5) Fujii, T., and Y. Mitsuta, 1995 : On the radial 
pressure profiles of typhoons. Ann. Disas. Prev. 
Res. Ins., Kyoto Univ., No.38 B-1, pp.101-116. 
(in Japanese) 
6) Holmboe, J., G.E. Forsythe and W. Gustin, 
1945 : Dynamic meteorology. John Willy and 
Sons, Inc., pp.207-209 
7) Gray, W. M. and D. J. Shea,, 1973 : The 
hurricane's inner core region, Ⅱ. J. Atmos. Sci., 
Vol. 30, pp.1565-1576 
8) Mitsuta,Y., T. Suenobu, and T. Fujii, 1988 : 
Supergradient surface wind in the eye of a 
typhoon. J. Meteor. Soc. Japan, Vol.66, 
pp.505-508 
 
Key Words: typhoon; pressure profile; gradient wind; 
surface wind, NeWMeK 
 
 4 


An analysis of pressure pattern in severe Typhoon Bart hitting the Japanese Islands in 1999 and a comparison of the gradient wind with the observed surface wind

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An analysis of pressure pattern in severe Typhoon Bart hitting the Japanese Islands in 1999 and a comparison of the gradient wind with the observed surface wind

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