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Monday, 30th June 2008

Typhoon Fengshen

Typhoon Fengshen brought the first number 8 signal this year. It also gave people an unexpected half-day holiday in the morning of 25 June.

Fengshen had a very dangerous look as it approached Hong Kong. But we were lucky. Several factors worked together to reduce its impact on Hong Kong to a very low level. Firstly, as the typhoon moved northwards across the South China Sea, it weakened into a severe tropical storm. Secondly, a couple of hours before it was about to land in Hong Kong, it swerved to the right and narrowly missed Hong Kong, traversing the waters offshore Sai Kung Country Park (figure 1). Thus we were spared a direct hit. Thirdly, at its closest point of approach, Fengshen brought generally northerly winds in Hong Kong. Most places were therefore sheltered by the hills. Finally and most importantly, when Fengshen got close, most people had already left work and were back in the safety of home. There was no need to commute in wind and rain.

Had Fengshen arrived a few hours earlier, I am afraid people would have to face traffic jam in the homeward rush and to go back to work the next morning in unpleasant wind and rain. In that case, the Observatory would be the target of complaints and outright scolding. It would have been a totally different situation. The wind and the rain would be the same, but a difference of a few hours would have resulted in very different outcomes. The reason of course is the human perception. This is the reality which front-line meteorological workers have to live with.

Let us have a look at the science. After the event, the track of Typhoon Fengshen looked rather simple and normal, plodding steadily north-westwards over a period of several days (figure 2). But we at the Observatory (and our counterparts elsewhere in the world) saw in the upper-air weather charts that it was heading into an area of high pressure. Going straight ahead would indeed violate established wisdom. Even the world's most advanced numerical models shared the same sentiment. For consecutive days, the models together predicted Fengshen would turn northward and then north-eastward. At first, it was forecast to turn north east of the Philippines. After crossing the Philippines and entering the South China Sea, it was forecast to move towards Taiwan Strait. Even shortly before reaching Hong Kong, it was forecast to turn and hit eastern Guangdong (see figure 2).

Seen from this perspective, the peculiar thing about the typhoon track is that people who know nothing about computer and meteorology could have predicted it better than scientists aided by computer. What we call "high technology" does fail occasionally. The real high technology actually resides in Nature itself.

Some people might turn it around and say that meteorologists and computers are useless. The man in the street who simply pushes the typhoon centre ahead would have won the competition. I have to point out that this is using a single case to create a "general principle", which is not a good practice. Since computer models became part of the trade, typhoon track forecasting has improved a lot. It is particularly noticeable since 2000 (see figure 3). Of course, there is no room for complacency for scientists. The failure this time marks the beginning of a new round of research efforts.

In serving the public, our work has to sit on a rational base. Providing guidance to forecasters through the use of the computation outputs of computer models is a realization of this principle. It is also the only means of enabling continual improvement. To be able to forecast weather better in the future, there is no other way.

C Y Lam

Figure 1

Figure 2

Figure 3

Last revision date: <17 Jan 2013>