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  • Challenges in forecasting typhoons from the east

  • Monday, 27th July 2015

Hong Kong is situated on the south China coast, with mainland China to its north and the South China Sea to the south. When tropical cyclones come from the sea and make landfall over the south China coast, they will usually continue to move inland, weaken progressively and dissipate. Some tropical cyclones approach Hong Kong from the east and travel close to the coast of southern China. If the direction of their westward movement changes even slightly (Figure 1), not only will there be a difference in how they make landfall, but there can also be substantial difference in their intensity variations. How can such a small change lead to a large difference?

For a tropical cyclone over the sea to the east of Hong Kong, if it adopts a westerly track with a slight northerly component, it will hit the eastern part of the south China coast sooner (Track 1 in Figure 1; Figure 2), and travel inland for some distance before moving to the north of Hong Kong. That means, the tropical cyclone will weaken over the land for some time before approaching Hong Kong. By that time, it will have weakened considerably and its threat to Hong Kong will be relatively low. Conversely, if its westward track bears a slight southerly component, the tropical cyclone can stay very close to the coast without making landfall, or re-enter the sea after hitting the land (Track 2 in Figure 1). Either way, it can maintain its intensity while edging closer to Hong Kong, posing a greater threat to Hong Kong as demonstrated by Typhoon Maggie that hit the territory in 1999 (Figure 3). Back then, the Observatory issued the Tropical Cyclone Warning Signal No. 9 and Maggie made landfall over the Sai Kung peninsula, bringing gale force winds and heavy rain to Hong Kong. Due to sheltering by terrain, the northerly winds brought by tropical cyclone from the east will usually be relatively weak at first, but strengthen abruptly as the tropical cyclone comes very close to Hong Kong. The case of Maggie clearly illustrated that northerlies in Hong Kong strengthened from strong winds to gales within less than 3 hours (Figure 4). It can readily be seen that for tropical cyclones from the east, even if there is a tiny change in their direction of westward movement, their impact on the weather of Hong Kong can be drastically different.

Figure 1

Figure 1      A small change in the direction of westward movement of tropical cyclone can lead to a change in
how it makes landfall and a substantial difference in its intensity variations.

Figure 2

Figure 2      In September 2001, Tropical Cyclone Nari made landfall at Shantou and then weakened gradually.
It became a tropical depression before it was closest to Hong Kong.

Figure 3

Figure 3      Maggie moved westwards along the south China coast and maintained the intensity as a typhoon
prior to its landfall at Hong Kong.

Figure 4

Figure 4      Local winds strengthened from strong winds to gales within less than 3 hours
when Maggie came very close to Hong Kong.

Linfa that affected Hong Kong in early July 2015 is also an example of tropical cyclones coming from the east. In the morning of 9 July, Linfa was centred over the sea at more than 200 km to the east of Hong Kong. Being a typhoon with intact structure, it headed towards Hong Kong on a westward track. The fixed-wing aircraft jointly sent by the Observatory and the Government Flying Service recorded hurricane force winds in the vicinity of Linfa's eye, and gales of Linfa also extended out to about 100 km from its centre. As Linfa was forecast to traverse along the coast instead of moving inland, its intensity was anticipated to sustain. Local winds might also pick up quickly within a short period of time when Linfa moved further close to Hong Kong. It turned out that while Linfa skirted at just about 50 km to the north of the Observatory (Figure 5), it rapidly weakened and its gales did not affect Hong Kong generally. This formed a stark contrast to what happened with Tropical Cyclone Utor in 2001 (Figure 6). Although Utor weakened into a severe tropical storm following its landfall near Shanwei and skirted to the north of the Observatory at around 80 km inland, it was still able to bring gale force winds and heavy rain to Hong Kong for a prolonged period.

Figure 5

Figure 5      Linfa rapidly weakened before arrival and skirted at just about 50 km to the north of the Observatory.

Figure 6

Figure 6      Utor moved over the inland areas at about 80 km to the north of the Observatory and brought
gale force winds and heavy rain to Hong Kong.

Although the weather conditions and scientific factors in the above cases are different, all of them demonstrate that a small change can lead to a large difference. Therefore, forecasting typhoons from the east is full of challenges and their threat to Hong Kong should not be taken lightly. These cases deserve more research studies and they also serve as prime examples for considering the issuance of tropical cyclone warnings in the future.

LS Lee

Last revision date: <19 Aug 2015>