Report on Severe Typhoon Koinu (2314)

Severe Typhoon Koinu (2314)
28 September – 9 October 2023

Koinu was the fifth tropical cyclone affecting Hong Kong in 2023. Koniu necessitated the issuance of the Increasing Gale or Storm Signal No. 9 again since Saola hitting Hong Kong in the same year. Koinu brought squally heavy showers to Hong Kong on 8 – 9 October. The rainfall recorded at the Observatory on 9 October reached 369.7 millimetres, more than three times of October’s monthly total normal figure of 120.3 millimetres and was the highest daily rainfall on record for October. Moreover, the 24-hour rainfall from 3 p.m. on 8 October to 3 p.m. next day reached 439.8 millimetres, breaking the highest record for October.

Koinu formed as a tropical depression over the western North Pacific about 1 920 km east of Manila on the night of 28 September. It then moved westwards and intensified gradually. It turned to move northwestwards across the seas east of the Philippines on the afternoon of 30 September and in the following three days. During this period, Koinu intensified into a severe typhoon on the afternoon of 2 October and attained its peak intensity with an estimated maximum sustained wind of 175 km/h near its centre that night. Koinu tracked westwards towards the vicinity of the southern part of Taiwan on 4 October. After moving across the southern part of Taiwan the next day, Koinu weakened into a typhoon and then moved west-southwestwards across the northern part of the South China Sea. Koinu intensified into a severe typhoon again on the night of 6 October and turned gradually to move west-northwestwards, edging slowly towards the coast of Guangdong. It further moved slowly towards the vicinity of the Pearl River Estuary with severe typhoon to typhoon intensity in the following two days. However, under the influence of the northeast monsoon, Koinu weakened rapidly and turned gradually to move southwestwards on 9 October. Finally, it degenerated progressively into an area of low pressure over the coastal waters of Yangjiang that night.

According to press reports, one person was killed and 399 people were injured when Koinu affected Taiwan. Water and electricity supply to more than 6 000 and 460 000 households were disrupted respectively. Around 3 000 people were displaced. Economic loss exceeded USD 18 million. In Macao, two people were injured during the passage of Koinu. There were also 13 incident reports, including fallen trees and landslides.

The Standby Signal No. 1 was issued at 9:40 p.m. on 4 October, when Koinu was about 790 km east of Hong Kong. Under the combined effect of the northeast monsoon and Koinu, local winds were generally moderate to fresh northerlies the next day, occasionally strong offshore and on high ground. With Koinu edging closer to the coast of Guangdong, the No. 3 Strong Wind Signal was issued at 5:40 p.m. on 6 October, when Koinu was about 260 km east-southeast of Hong Kong. Local winds strengthened gradually from 7 October to the next morning, with strong northerlies prevailing over many places and gale winds occasionally affecting offshore and high ground.

As Koinu steadily approached the vicinity of the Pearl River Estuary and its associated gale force winds were expected to affect Hong Kong, the No. 8 Northeast Gale or Storm Signal was issued at 12:40 p.m. on 8 October when Koinu was about 90 km south-southeast of the Observatory Headquarters. Local winds then strengthened significantly with gale winds prevailing over many places in the southern part of the territory, and even reached storm force offshore and on high ground. Koinu came further closer to Hong Kong later in the afternoon, posing a threat to Hong Kong. When the eyewall of Koinu affected Huangmaozhou, around 50 km to the south of Hong Kong, the wind speed at that station increased sharply within a short period of time, reaching hurricane force winds up to 120 km/h. The Increasing Gale or Storm Signal No. 9 was issued at 7:00 p.m. on 8 October when Koinu was only about 70 km south of the Observatory Headquarters. Subsequently, winds at Huangmaozhou further strengthened and the intense rainbands associated with Koinu also affected Hong Kong. Koinu came closest to Hong Kong that night, skirting past about 70 km to the south of the Observatory Headquarters with severe typhoon intensity.

As Koinu weakened and departed from Hong Kong gradually, hurricane force winds no longer posed threat to the territory and the No. 8 Northeast Gale or Storm Signal was issued at 11:50 p.m. on 8 October to replace the Increasing Gale or Storm Signal No. 9. With local winds continuing to moderate, the No. 3 Strong Wind Signal was issued at 11:40 a.m. on 9 October, followed by the issuance of No. 1 Standby Signal at 2:40 p.m. As Koinu further departed from the territory and weakened, all tropical cyclone warning signals were cancelled at 4:20 p.m on that day.

As regards the wind structure, the circulation of Koinu over the South China Sea was rather compact. Satellite imageries (Figure 3c) showed that the coverage of its spiral rainbands was much smaller than the extensive circulation of Mangkhut and even smaller than that of Saola in the same year. Radar imagery (Figure 4b) depicted that, when Koinu passed closely south of Hong Kong, the tight and hurricane force wind bearing eyewall of Koinu was only about 30 kilometers away from Cheung Chau.

Under the influence of Koinu, maximum hourly mean winds of 89, 71 and 71 km/h and gusts of 111, 103 and 94 km/h were recorded at Waglan Island, Green Island and Cheung Chau respectively. A maximum sea level (above chart datum) of 2.84 m was recorded at Waglan Island and a maximum storm surge (above astronomical tide) of 0.65 m was recorded at Tsim Bei Tsui. The lowest instantaneous mean sea-level pressures recorded at some selected stations are as follows:

Under the influence of the outer subsiding air of Koinu, it was generally fine and very hot in Hong Kong during the day on 4 – 5 October. The temperature at the Observatory soared to a maximum of 34.6 degrees on the afternoon of 4 October, the highest monthly absolute maximum temperature on record for October. Moreover, the daily mean temperature on that day reached 30.8 degrees, also the highest on record for October. With Koinu edging closer to the coast of Guangdong, while there were sunny periods during the day on 6 October, the local weather turned cloudy with a few squally showers from that night to the next day. Affected by the intense rainbands associated with Koinu, there were squally heavy showers in Hong Kong on 8 – 9 October, necessitating the issuance of the Black Rainstorm Warning on the morning of 9 October. More than 300 millimetres of rainfall were recorded over most parts of Hong Kong and rainfall even exceeded 600 millimetres in Central and Western, Wan Chai, Wong Tai Sin and Kwun Tong Districts on 8 – 9 October.

In Hong Kong, there were at least 205 reports of fallen trees, 2 reports of landslides and 7 reports of flooding during the passage of Koinu. A tree fell and hit two cleaning workers in Happy Valley. A total of 29 people were injured during the passage of Koinu. A landslide occurred on a mountain slope in Tseung Kwan O. Public transportation services in Hong Kong were also seriously affected by Koinu. Train services on all open sections of the MTR railway were once suspended. At least 90 flights were cancelled at the Hong Kong International Airport. Besides, some passengers had to stay at the airport due to suspension of land transport.