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Friday, 20th July 2012

Tropical Cyclone Signals in June

Lately in June, two tropical cyclones affected Hong Kong in a row. Severe Tropical Storm Talim marked the onset of tropical cyclone season of Hong Kong in 2012, while Tropical Storm Doksuri necessitated the issuance of the first No. 8 Signal (i.e. Gale or Storm Signal) in this year by the Observatory. This is in line with the earlier forecast by the Observatory that tropical cyclones may start to affect the territory in June or earlier.

Whenever a tropical cyclone affects Hong Kong, its development attracts a great deal of attention from the public. Some members of the public may refer to the tropical cyclone track and position information provided by the Observatory, discuss the movement of the storm, and may even infer the issuance time of tropical cyclone signals. During the passage of Doksuri, some members of the public expressed that the issuance of No.8 Signal could be determined by the storm's distance to Hong Kong (for example 100 km or 200 km). Yet, the Tropical Cyclone Warning System actually operates based on the forecast and actual wind strength in Hong Kong. The local wind strength is determined by numerous factors including the tropical cyclone's intensity, its distance from Hong Kong, its wind structure, and the change in local wind direction (please refer to the article Tropical Cyclone Season in 2011 for more details). We will elaborate the considerations behind issuing tropical cyclone signals, taking the two cases in June as examples.

Tropical Storm Doksuri

Tropical cyclone is categorized according to the maximum sustained wind speed near its centre. Hong Kong adopts a system with six categories, ranging from the weakest "Tropical Depression" to the strongest "Super Typhoon" [1]. When Doksuri traversed across the northern part of the South China Sea (Figure 1), it was categorized as a tropical storm with the maximum wind speed near its centre of about 85 km/h (i.e. the second weakest category) for most of the time. In general, tropical storms rarely cause No. 8 Signal. One such example occurred in 2004 (Figure 2) when Kompasu took a west-northwesterly track across the northern part of South China Sea. However, its track exhibited larger variations as compare to that of Doksuri.

Table 1

Table 1 :     The issuance time of No.8 Signal during the passage of tropical cyclones Kompasu, Doksuri and Nesat, and the corresponding intensity, distance and bearing from HK, speed and direction of movement of the tropical cyclones.


With reference to the above table, when the Observatory issued No. 8 Signal during the passage of Kompasu, it was only about 60 km from Hong Kong (the distance for Doksuri was about 100 km). Since the wind strength near the centre of a tropical storm only reaches gale force and wind speed generally decreases with increasing distance from its centre, tropical storm has to move very close to Hong Kong to bring a No. 8 Signal. On the contrary, under the combined effect of the northeast monsoon, a stronger tropical cyclone like Typhoon Nesat on 29 September 2011 can bring gale force winds to Hong Kong even though it was more than 300 km away from the territory. Hence, there is no prescribed distance from Hong Kong, below which a No. 8 Signal must be issued. We need also to consider other factors such as intensity and wind structure of the tropical cyclone, etc.

Severe Tropical Storm Talim

When Tropical Cyclone Talim affected Hong Kong, the Observatory once issued the Strong Wind Signal, No. 3. Although the Talim's impact to Hong Kong was relatively mild, fallen trees were still reported at various places. With its anchor chain broken by waves, a yacht also washed ashore in Sai Kung. Figure 3 depicted the actual track (solid line) of Talim and the forecast track at 8 p.m., 18 June (dotted line). Based on the forecast that night, there were signs that Talim would intensify from a tropical storm to a severe tropical storm, and would adopt a more northeasterly track closer to Hong Kong, skirting at about 180 kilometres and bringing a higher chance of strong winds to the territory. As such, the Observatory issued the Strong Wind Signal, No.3 at 10:40 p.m. that night. Intensified into a severe tropical storm the next morning, Talim indeed took a more northeasterly track and moved closer to Hong Kong. However, Talim changed its course of movement somewhat later than previously expected, leaving it skirting at about 260 kilometres from Hong Kong. The local winds did not pick up that much as Talim's effect diminished.

Although Talim did not impose great impact to Hong Kong, its strength and destructive power should not be taken lightly. According to the press, it caused the death of at least 3 people, and brought flooding and interruptions of electricity supply to many parts of Taiwan during its passage. The shoreline in Fujian was damaged while 400,000 hectares of farmland in Ninbo, Zhejiang were also inundated .

Summary

To summarize, tropical cyclone forecasting is still very challenging. In the spirit of science and professionalism, we will continue to improve tropical cyclone forecasting skills with an aim to protecting the life and property of the public more effectively.


CT Shum, KL Lee

Figure 1
Figure 1 :     Track of Tropical Cyclone Doksuri during its passage across the northern part of the South China Sea in June 2012


Figure 2

Figure 2 :     Track of Tropical Cyclone Kompasu in July 2004


Figure 3
Figure 3 :     Track of Tropical Cyclone Talim (Solid line). The forecast track of Talim at 8 p.m., 18 June 2012 was shown in dotted line.



Reference:

[1]      http://www.hko.gov.hk/informtc/class.htm


Last revision date: <17 Jan 2013>