Typhoon York

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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

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Typhoon York