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Compliment to Hong Kong Observatory on handling of Typhoon Nuri

(Email from the public received on 23 August 2008)

Dear Director of HKO and Senior Scientific Officers,

I am writing to give praise and compliments to the HKO for your excellent handling of the issuance of TC signals for TC Nuri yesterday.

1. The timing for issuance of TC signals of No. 8 and No. 9 were appropriate, impeccable, and considerate for the mass public in Hong Kong, yet it met the objective scientific requirements for No. 8 and 9. If you had issued the No. 8 signal around noon or 1pm, the impact would have been unthinkable for people traveling home in NW/northerly gales and bricks and metal/bamboo scaffolding crashing to the ground. It was evident that Nuri, a Category One Typhoon at that time, was heading towards HK (or vicinity of HK) from various agencies forecast, including HKO, JTWC, JMA, and Beijing.

2. Though there has been criticism on the web why No. 10 was not issued, you have been quite frank and open about No. 10. At 3pm, it was mentioned in the Weather Bulletin that No. 10 would be considered. As Nuri was downgraded to STS later, No. 10 would not have been necessary. Although hurricane force winds were recorded later in the south side of HK for a few hours after Nuri passed through the Kowloon Peninsula and then veered to the NW, the hurricane force winds were not sufficiently sustained (based on 10-min avg) within the city, nor the 8 weather stations. It was a bit of a surprise that Nuri caused hurricane force winds in Waglan Island, Cheung Chau, Green Island, Tai Mei Tuk, Sai Kung, Tate's Cairn, Ngong Ping, and even Kai Tak (gusts). But Tsing Yi and Chek Lap Kok did not reach hurricane force (only storm force recorded). Overall, No. 9 was sufficient as the sustained winds in most parts of the south of HK are gale to storm force only,

3. I understand the biggest anomaly and controversy right now is how to define and agree on the actual path of Nuri and this direct hit is unprecedented in the history of HKO (at least sophisticated radar equipment were not available in the 60s to 80s). A decoupling of the LLCC occurred during the passage of Nuri. After making landfall at 20km east of HKO near Sai Kung shores, 2 vortices were formed after the decoupling and reorganization. One vortex veered SW from E to W across the Kowloon Peninsula, and it appeared that Ma On Shan or the Kowloon Range blocked its movement to the NW. This LLCC crossed King's Park around 5:30p.m. and then veered to northern tip of Lantau and then moving further W and then NW. This was evident from the surface winds observations. Another LLCC was formed which continued to move NW after the Sai Kung landfall, almost diagonally from SE to NW across HK, and then towards western Shenzhen. This was explained in the anomaly in the surface winds. To make things even more complicated, from the satellite fixes, due to the strong shear to the north, the center of the TC at the upper level appeared to have veered to the SW waters west of Macau. It was so exciting to compare the radar with the satellite scans to witness this decoupling! I guess that you must have a very difficult job to analyze this, not to mention how to forecast this system after its landfall. If you apply the usual rule that TC landing east of HK will bring less damaging winds, then this case would not apply. It actually passed to HK from east to west (first LLCC) and brought sustained gale to storm force winds gusting to hurricane force from its powerful SE quadrant. A STS by strictest definition, yet a borderline Typhoon if you just look at its SE quadrant which is unhampered by mountains and shear in the north quadrants. Amazing, isn't it? I know the final verdict for Nuri has yet to be announced by your team, but the 2 vortices at surface level observation seems to hold at this time.

Again, your efforts have been laudable and I sincerely eulogize your team for your great efforts!

Warm regards,
Albert Tsang

Last revision date: <20 Dec 2012>