A 'sticky' rainstorm
- Friday, 31st July 2015
While in summertime, rainstorms do occur occasionally. Talking about rainstorms, one may recall the widespread heavy rain in Hong Kong on 8 May 1992. It caused more than 200 cases of flooding and numerous landslips, paralysing road traffic. Some roads in Admiralty were turned into raging rapids and many places were heavily flooded. Some people were washed by the rapids to low-lying areas and a few people were even found dead. This rainstorm should be well within the memory of the seniors. It was indeed after this rainstorm that the Observatory introduced the rainstorm warning system.
On 22 July this year, under the influence of an active southwesterly airstream, the weather in Hong Kong was unsettled with heavy rain and squally thunderstorms. The western part of Hong Kong Island was affected most, with over 300 millimetres of rainfall recorded throughout the day (Figure 1). The Amber Rainstorm Warning Signal issued by the Observatory lasted for nearly 12 hours and the Landslip Warning was also in force for more than 6 hours.
Figure 1 Rainfall distribution in Hong Kong on 22 July.
What was special about the rainband affecting Hong Kong in this episode was that the area of intense rain was relatively small but stayed at the same location for quite a long time. This was like an adhesive bandage sticking to the southern part of Hong Kong, leading to localized heavy rain over there but not much rainfall in the New Territories. The radar image in Figure 2 shows that the area of intense rain remained almost stationary over the southern part of Hong Kong for a prolonged period in the morning, whereas rain was less intense in the New Territories. Let us take 7:45 to 8:45 a.m., which was one of the periods with heavier rain, as an example. In this hour, 6 to 89 millimetres of rainfall were recorded by various raingauges in Southern District, while 28 to 72 millimetres were recorded by the raingauges in Central and Western District. In Wanchai, the rainfall ranged from 42 to 63 millimetres. Other districts to follow were Islands District, Eastern District, Yau Tsim Mong and Kowloon City, where the hourly rainfall was less than 50 millimetres. Rain was even much less at some places in Kowloon and most parts of the New Territories, over which less than 10 millimetres of rainfall were recorded in one hour. It can be seen from these figures that rain varied greatly from one place to another and torrential rain was confined to some places in the southern part of Hong Kong.
Figure 2 Weather radar images on 22 July at (a) 5:36 a.m. and (b) 9:00 a.m.
Heavy rain is indicated by orange and yellow colours.
The issuance of rainstorm warning signal by the Observatory is mainly based on the amount of rain which has fallen or is expected to fall generally over Hong Kong in an hour. The hourly rainfall criteria for the Amber, Red and Black signals are 30, 50 and 70 millimetres respectively. On 22 July morning, the overall rain intensity in Hong Kong met the criterion of the Amber Rainstorm Warning. Although the hourly rainfall over certain areas of the southern part of Hong Kong was more than 50 millimetres, the Observatory did not further upgrade the rainstorm warning level because heavy rain of such intensity did not extend to greater parts of Hong Kong. Nevertheless, in view of the risk of landslips arising from heavy rain, the Landslip Warning was issued at 10:45 a.m. The Observatory also maintained close liaison with relevant government departments and provided them with rainfall information to facilitate their emergency response and rescue work.
Heavy rain can vary abruptly and randomly. To give an accurate forecast of heavy rain is indeed a great challenge. The rainstorm warning system is designed to alert the public about the potential impact of heavy rain, and to ensure a state of readiness within the essential services to deal with emergencies. Besides knowing the level of rainstorm warning in force, members of the public should also be aware of the situation where they are, be vigilant and take suitable precautions to ensure their safety.