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2.1 Review of tropical cyclones in 2009
2.1.1 Tropical cyclones over the western North Pacific (including the South China Sea)
In 2009, a total of 30 tropical cyclones affected the western North Pacific and the South China Sea (i.e. the area bounded by the Equator, 45°N, 100°E and 180°), close to the 30-year (1971-2000) average of 29.6. During the year, 13 tropical cyclones attained typhoon intensity or above, about two below the normal figure. Five of them, namely Choi-Wan (0914), Parma (0917), Melor (0918), Lupit (0920) and Nida (0922) attained super typhoon intensity (maximum 10-minute wind speed of 185 km/h or above near the centre) over the western North Pacific.
The first tropical cyclone of the year formed in May and the last one in November. Figure 2.1 shows the monthly frequencies of the occurrence of tropical cyclones in the western North Pacific and the South China Sea in 2009.
During the year, eleven tropical cyclones hit Mainland China, two crossed over Taiwan, four affected Japan, six traversed or passed close to the Philippines and five made landfall over Vietnam.
The most intense tropical cyclones in 2009 were super typhoons Choi-Wan (0914) and Nida (0922). Both super typhoons had a maximum sustained wind speed of about 210 km/h and a minimum sea-level pressure of about 910 hPa (Table 4.1) when they were located over the western North Pacific about 520 km north of Guam (Fig. 2.3) and 370 km west of Guam respectively (Fig.2.4).
Super Typhoon Parma (0917), which formed over the western North Pacific in late September, persisted for almost 16 days and was the tropical cyclone with the longest life span in the region since Typhoon Verne in 1994. Verne persisted for about 16 days.
The tracks of Severe Tropical Storm Goni (0907) and Super Typhoon Parma (0917) were the most peculiar in 2009. Under the influence of another tropical cyclone Morakot (0908) over the western North Pacific, Goni moved southwestwards skirting the western part of Hainan Island after making landfall over the western part of Guangdong (Fig. 2.5). It then turned to move eastwards to cross the South China Sea before dissipation. The track of Parma was affected by Super Typhoon Melor (0918) over the western North Pacific (Fig. 2.6). It lingered near northern Luzon for four days before moving west-northwestwards across the northern part of the South China Sea.
2.1.2 Tropical cyclones in Hong Kong’s area of responsibility
Amongst the 30 tropical cyclones in 2009, 17 occurred inside Hong Kong’s area of responsibility (i.e. the area bounded by 10°N, 30°N, 105°E and 125°E), more than the 30-year (1971-2000) annual average of 15.4 (Table 2.1). Nine of these 17 tropical cyclones developed within Hong Kong’s area of responsibility. Altogether, 523 tropical cyclone warnings to ships and vessels were issued by the Hong Kong Observatory in 2009 (Table 4.2).
2.1.3 Tropical cyclones over the South China Sea
Thirteen tropical cyclones affected the South China Sea (i.e. the area bounded by 10°N, 25°N, 105°E and 120°E) in 2009. Five of them formed over the area. Eight moved into the area from the western North Pacific.
2.1.4 Tropical cyclones affecting Hong Kong
In 2009, the typhoon season in Hong Kong started on 20 June when Severe Tropical Storm Linfa (0903) moved towards the south China coast and and necessitated the issuance of the Standby Signal No.1. The typhoon season ended on 28 September as Typhoon Ketsana moved away.
Eight tropical cyclones affected Hong Kong (Figure 2.2), more than the average number of 6.5 (Table 2.2). These eight tropical cyclones were Severe Tropical Storm Linfa (0903) and Tropical Storm Nangka (0904) in June, Tropical Storm Soudelor (0905) and Typhoon Molave (0906) in July, Severe Tropical Storm Goni (0907) in August, Tropical Storm Mujigae (0913), Typhoon Koppu (0915) and Typhoon Ketsana (0916) in September. Three of them, Molave, Goni and Koppu necessitated the issuance of the No. 8 Gale or Storm Signal or higher signals in Hong Kong, also more than the normal of one to two per year. Typhoon Molave passed about 40 km to the north-northeast of the Hong Kong Observatory Headquarters and necessitated the issuance of the Increasing Gale or Storm Signal No. 9, the highest tropical cyclone warning signal in 2009. The Strong Wind Signal No. 3 was issued during the passages of Nangka, Soudelor and Mujigae while Linfa and Ketsana necessitated the issuance of the Standby Signal No. 1 in Hong Kong.
During the passage of Koppu, the combined effect of storm surge and high tide raised the sea level to a maximum of 3.43 metres at Tai Po Kau in the northeastern part of Hong Kong, one of the highest sea levels recorded in the past decade.
2.1.5 Tropical cyclone rainfall
Tropical cyclone rainfall (the total rainfall recorded at the Hong Kong Observatory from the time when a tropical cyclone is centred within 600 km of Hong Kong to 72 hours after it has dissipated or moved farther than 600 km away from Hong Kong) in 2009 was 822.4 mm (Table 4.8.1). This was 10% above the normal of 745.5 mm and accounted for some 38% of the year’s total rainfall of 2182.3 mm.
Typhoon Koppu, which affected Hong Kong on 13-15 September, brought 273.0 mm of rainfall to the Hong Kong Observatory Headquarters, and was the wettest tropical cyclone in 2009.