typhoon york
TRANSCRIPT
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Typhoon York (9915) 12 - 17 September 1999
Typhoon York developed as a tropical depression about 420 km northeast of Manila
on 12 September 1999. Tracking westwards, York swept across the northern part of
Luzon and entered the South China Sea. York intensified into a tropical storm on 13
September. Its movement then became erratic. Heading north at first, York
strengthened into a severe tropical storm on 14 September. It then turned
northwestwards before becoming almost stationary and attaining typhoon strength the
following night. York picked up speed to about 20 km/h and began to head towards
Hong Kong on the early morning of 16 September. The maximum sustained winds
and minimum sea-level pressure near its centre were estimated to be 130 km/h and
965 hPa. After battering Hong Kong, York entered the Zhujiang Kou (Pearl River
Estuary) (see Figure 5.1). It made landfall near Zhuhai and weakened into a severe
tropical storm that evening. Moving further inland, York rapidly became an area of
low pressure the next day.
In Hong Kong the Standby Signal No. 1 was hoisted at 10:45am on 13 September
when York was about 650 km to the southeast. It was hot with a few showers. Local
winds were mainly moderate northerly on 14 September. But as York intensified into
a severe tropical storm and headed towards the coast of Guangdong, local winds
started to strengthen from the north. The Strong Wind Signal No. 3 was hoisted at
10:15am on 15 September.
Under the influence of York's rainbands, the weather began to deteriorate with a few
squally showers in Hong Kong that day. York became almost stationary and soon
intensified into a typhoon that night. An eye was discernible on radar and satellite
imageries. As York approached Hong Kong, local winds reached gale force offshore
and the No. 8 NORTHWEST Gale or Storm Signal was hoisted at 3:15am on 16
September. Winds strengthened rapidly in the next few hours. The Increasing Gale
or Storm Signal No. 9 was hoisted at 5:20am and the Hurricane Signal No. 10 at
6:45am. This was the first time since 1983 that the No. 10 signal was hoisted. The
signal was in force for 11 hours, the longest on record. York was also the second
tropical cyclone necessitating the hoisting of No. 9 or higher signals in the year. The
last time that No. 9 or higher signals had to be hoisted on two separate occasions
between January and September was in 1964.
Winds of hurricane force, firstly northeasterly and then southwesterly, buffeted HongKong on 16 September. Local winds experienced a temporary lull during the eye's
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passage. The eye of York was closest to the Hong Kong Observatory Headquarters at
around 10am when it was about 20 km to the south-southwest.
During the passage of York, a maximum hourly wind of 151 km/h and a maximum
gust of 234 km/h were recorded at Waglan. The gust is the highest recorded at
Waglan. At Cheung Chau, a maximum hourly wind of 112 km/h and a maximum
gust of 182 km/h were recorded. As York entered the Zhujiang Kou, local winds over
most parts of Hong Kong turned southwesterly. (HKO)
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Figure Error! No text of specified style in document. .2 Track of York over
Hong Kong on 16 September, 1999
Fi ure Error! No text of specified st le in document. .1 Track of T hoon York
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Typhoon York in 1999 affected Hong Kong from 10:15am on 15 Sep. 1999 to
00:45am on 17 Sep. 1999. From 03:15am on 16 Sep. 1999 No.8 tropical cyclone
signal was issued until 22:10pm on 16 Sep. 1999. Therefore, 19 hours data from
03:00 to 22:00 on 16 Sep. 1999 were analyzed as an example for typhoon across the
bridge.
During the 19 hour period, the azimuth of 10-m mean wind at tower tops is displayed
in Figure 5.3, the dashed line in the figure shows the direction of alignment of Ting
Kau Bridge. From 05:00 to 22:00 on 16 Sep. 1999, the wind direction changed from
North to South. As the result of the statistical mean wind direction, the wind direction
of Tsing Yi Tower in 1999 is unreliable.
Figure 5.4 shows the azimuth of 10-m mean wind at deck level. As those of the tower
tops, the wind direction was changed from North to South. From 09:00 to 11:00, the
wind direction is almost perpendicular to the alignment of the bridge, which is the
most severe direction for the bridge.
Figure Error! No text of specified style in document. .3 Variation of -
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Figure 5.5 shows the variation of 10-m mean wind speed at tower tops. The patterns
of variation of Ting Kau Tower and Main Tower are similar to each other. The wind
speed of Main Tower is larger than that of Ting Kau Tower as it is higher. The
maximum 10-m mean wind speed occurred at about 09:20 with 38.2m/s.
The variation of 10-m mean wind speed at deck levels is presented in Figure 5.6, the
maximum value is 30.9m/s at about 09:10 at the east side of Tsing Yi mid-span.
Because the wind came from Northeast, the wind speed at the east side of the bridge
is larger than that at the west side. Therefore, during 09:00 to 10:00, it is the largest
wind speed at the most severe direction. The situation of the bridge during this hour
will be analyzed in detail.
Figure Error! No text of specified style in document. .5 Variation of 10-m mean
Figure Error! No text of specified style in document. .4 Variation of azimuth of
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The wind incidence is defined as the angle between the mean wind and the horizontal
plane, and it is positive when wind bellows upwards. Figure 5.7 shows the time-
varying incidences of 10-m mean wind measured at the deck levels. It is seen that the
measured wind incidence varied within 10.
The integral scales of the turbulences during Typhoon York were then obtained using
the curve fitting method as presented in Appendix C based on 10-minute duration.
Figure 5.8 shows the variations of the measured integral scales of three turbulence
velocity components with time, and the corresponding average values are listed in
Table 5.2. It is seen that the measured integral scales generally scattered to a great
degree.
Figure Error! No text of specified style in document. .7 Variation of wind incidences at deck levels
Figure Error! No text of specified style in document. .6 Variation of 10-m mean
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(a) Ting Kau mid-span east side
(b) Ting Kau mid-span west side
c Tsin Yi mid-s an east side
(d) Tsing Yi mid-span west side
Figure Error! No text of specified style in document. .8Variation of Integral scales of turbulence
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Table Error! No text of specified style in document. .1 Average values of integralscales of turbulence
16-Sep-
1999
Ting Kau mid-span
east side
Ting Kau mid-span
west side
Tsing Yi mid-span
east side
Tsing Yi mid-span
west side
Time
(hh:mm)
lu
(m)
Lv
(m)
lw
(m)
lu
(m)
Lv
(m)
lw
(m)
lu
(m)
Lv
(m)
lw
(m)
lu
(m)
Lv
(m)
lw
(m)
03:00 83.6 79.3 56.6 115.7 55.9 47.4 174.4 124.7 94.1 94.9 61.7 45.8
04:00 105.1 58.0 48.7 115.8 59.4 49.5 133.1 83.7 75.3 130.4 68.6 55.1
05:00 204.6 120.3 110.9 145.1 108.3 46.2 257.6 248.4 238.2 91.6 169.0 60.9
06:00 242.5 135.5 153.3 187.4 179.5 59.4 268.6 273.2 162.4 126.8 128.4 63.4
07:00 250.4 259.6 155.3 215.1 255.4 169.8 286.8 204.8 227.5 151.9 105.5 72.1
08:00 188.7 210.1 158.8 199.3 200.2 126.1 267.6 349.4 243.2 158.1 191.6 78.4
09:00 231.8 365.4 208 209.3 301.8 178.4 303.7 395.5 224.9 192.7 205.9 118.8
10:00 209.4 331.6 134.3 138.4 163.7 59.1 280.0 426.8 305.6 132.7 168.8 87.0
11:00 158.6 254.9 123.8 77.8 109.7 40.8 166.2 348.0 156.4 77.4 163.5 36.5
12:00 159.2 222.3 104.8 88.2 75.7 23.6 141.1 209.4 134.5 73.5 123.3 26.1
13:00 165.1 291.9 134.1 99.7 123.0 46.1 167.3 113.4 117.3 94.7 85.1 36.4
14:00 190.8 204.9 120.4 97.9 107.1 78.8 171.0 110.7 132.3 117.1 125.1 57.1
15:00 95.4 87.7 70.8 130.0 82.7 55.9 111.8 110.5 88.3 95.6 85.1 63.7
16:00 91.1 77.6 62.9 128.5 64.6 84.2 86.7 70.1 60.2 102.9 74.9 59.7
17:00 68.3 70.5 26.7 254.0 224.9 77.8 155.7 112.7 71.9 109.8 102.1 61.9
18:00 45.3 49.1 10.6 196.1 240.8 116.8 66.8 75.1 31.1 130.6 150.8 89.719:00 43.2 67.7 8.7 228.7 235.4 141.3 58.7 64.2 34.4 97.4 124.6 75.5
20:00 42.4 37.1 9.9 142.2 224.0 77.9 90.8 39.9 25.3 67.2 62.6 39.1
21:00 63.9 45.6 12.1 259.6 230.4 91.7 112.6 60.0 30.0 53.2 59.8 42.9
The hourly mean integral scales of turbulences during Typhoon York are presented in
Figure 5.9. Around 09:00, because the wind direction was almost perpendicular to the
alignment of the bridge, the integral scale of the cross-bridge wind component (Lv)
became much larger.
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(a) Ting Kau mid-span east side
(b) Ting Kau mid-span west side
(c) Tsing Yi mid-span east side
(d) Tsing Yi mid-span west sideFigure Error! No text of specified style in document. .9
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The time histories of wind speed components recorded at WIGEE01 from 09:00 to
10:00 on 16 Sep. 1999 (Typhoon York) were presented in the Figure 5.10. The
hourly mean wind speed was 24.1m/s and the wind blew from northeast as the most
severe direction for the bridge. The wind incidence is -0.99.
The time histories of fluctuating wind speeds u(t), v(t) and w(t) in the longitudinal,
lateral and upward bridge directions are also extracted from the measured three
components of wind velocity (see Figure 5.11). From the fluctuating wind speed time
histories shown in Figure 5.9, it is found that the turbulence intensities were about
25.2%, 28.5% and 22.7%, for the velocity fluctuations u(t) , v(t) and w(t) , respectively.
The corresponding integral scales of turbulence were 260m, 547m and 195m,
estimated according to the Taylors hypothesis.
Figure Error! No text of specified style in document. .10 Recorded wind speed
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The segment smooth method and the hamming window were applied in the spectral
analysis. One-hour data was divided into 11 sub-segments of 10-minute duration,
with an overlapped length of 5 minutes between two neighbouring segments. The
1536 data points in the 10-minute sub-segment were zero-padded to 2048 points to
meet the requirement of Fast Fourier Transformation (FFT). Figure 5.12 shows the
one-side normalized auto spectra of velocity fluctuations u(t) , v(t) and w(t) .
Figure Error! No text of specified style in document. .11 Three components of
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2u
nSu
2v
nSv
2w
nSw
Figure Error! No text of specified style in document. .12 Auto