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Extreme Rainfall Events in Hong Kong

Extreme Rainfall Events in Hong Kong

WONG Ka-hing, FAN Man-hei, YEUNG Hon-yin
May 2024

Under global warming, the occurrence of extreme weather will become more frequent and Hong Kong will be in no exception. On 7 and 8 September 2023, extreme rainfall occurred in Hong Kong, breaking three rainfall records (Note 1) and causing major damages to the city. In fact, the rainfall intensity during that rain episode far exceeded the general understanding of the public about rainstorms. To address this, the Hong Kong Observatory (HKO) has strengthened the dissemination of information about rainstorm starting from the rain season of 2024, especially when the rainfall intensity far exceeds the rainfall criteria of the Black Rainstorm Warning Signal, i.e. 70 millimetres (mm) in an hour. In such scenario, Special Weather Tips will be issued to the public through HKO's website and the push notification of the "MyObservatory" mobile app about the exceptional circumstances associated with the rainstorm and the need to be on high alert. For this purpose, the Observatory has introduced the following two new terms:
• Severe Rainstorm (SvR): rainfall intensity exceeding 100 mm in an hour; and
• Exceptionally Severe Rainstorm (ESR): rainfall intensity exceeding 140 mm in an hour.
According to the above definitions, there were 103 days in which SvR were recorded over different parts of the territory since the automatic rain gauge network of HKO established in 1984. Among the 103 days, 9 of them even reached the ESR level (Table 1) during the past 40 years. In terms of monthly distribution, SvR (ESR included) occurred more often in May, June and August (Figure 1).
Date Highest 1-hour rainfall record that day (mm) Districts affected by ESR
8 May 1992 144.5 Eastern District
22 July 1994 211.5 Tsuen Wan District
24 April 2006 166.5 Southern District and Central and Western District
7 June 2008 153.5 Lantau Island, Central and Western District and Yau Tsim Mong District
22 May 2013 153.5 Sai Kung District and Kwun Tong District
15 August 2015 150.5 Tai Po District
7 September 2023 159.5 Wan Chai District, Yau Tsim Mong District, Kowloon City District, Northern District and Eastern District
8 September 2023 152.5 Southern District and Po Toi Island
4 May 2024 145.5 Sai Kung District
Table 1.  ESR Records in Hong Kong from 1984 up to 4 May 2024
Monthly distribution of ESR and SvR in Hong Kong from 1984 up to 4 May 2024
Figure 1.  Monthly distribution of ESR and SvR in Hong Kong from 1984 up to 4 May 2024
A revisit to the hourly rainfall records of the Observatory Headquarters since 1884 (Note 2) revealed that there were eight days with SvR, among which two exceeded the threshold for ESR. The first ESR over the Observatory Headquarters occurred on 7 June 2008 when the Black Rainstorm Warning Signal was in force whereas the second occurrence was right on 7 September 2023 when the "Black Rainstorm of the Century" took place.
It was still a hot topic of scientific research concerning the root causes and triggering conditions of ESR events. Amongst the past cases in Hong Kong, the coastal areas of Guangdong were mostly under the influence of a trough of low pressure. As discussed in educational article "The rainstorm development mechanisms in Hong Kong", the low-level flows from the air masses with contrasting characteristics on the northern/southern sides of the trough will converge along the trough axis continuously, triggering convective development. If the low-level convergence couples with other favourable atmospheric conditions, including sufficient moisture transport, unstable atmosphere, mid-level disturbances, as well as upper-level divergence, severe convection or even rainstorm is likely to occur.
The weather charts of 7 June 2008 at 8 a.m. and 7 September 2023 at 8 p.m., and the corresponding 24-hour rainfall distribution maps of Hong Kong.
Figure 2.  The weather charts of 7 June 2008 at 8 a.m. and 7 September 2023 at 8 p.m., and the corresponding 24-hour rainfall distribution maps of Hong Kong. (The rainfall map at the lower-right refers to the 24-hour rainfall ending at 12 noon of 8 September 2023.)
Back on 7 June 2008, the rainstorm began to affect Hong Kong in the morning and the Observatory had issued the Black Rainstorm Warning Signal for a total of 4 hours and 20 minutes. At the Observatory Headquarters, 145.5 mm of rainfall was recorded during the one-hour period from 8 to 9 a.m., breaking the highest one-hour rainfall record at that time. Over other parts of the territory, 200 mm of rainfall was generally recorded that day with more than 300 mm over Lantau Island and the urban areas, leading up to widespread floodings, 39 landslip reports and road blockage (Figure 3). The latter severely affected the traffic conditions with the road transportation between the urban areas and the airport once interrupted. A large number of flights were delayed as a result. Overall, more than 10 people were injured during to the passage of the rainstorm.
The North Lantau Highway flooded by mud and rain water on 7 June 2008.
Figure 3.  The North Lantau Highway flooded by mud and rain water on 7 June 2008.
The rainstorm of 7 September 2023 began to affect Hong Kong in the evening and the Observatory issued the Black Rainstorm Warning Signal for the longest duration in history totalling 16 hours and 35 minutes, until the afternoon on the next day. At the Observatory Headquarters, 158.1 mm of rainfall was recorded during the one-hour period from 11 p.m. to 12 midnight on 7 September, breaking the previous record that was kept for just 15 years (see the 2008 case above). From 7 to 8 September, many parts of the territory recorded more than 400 mm of rainfall with the Eastern District and Southern District of Hong Kong Island even exceeded 800 mm. There were many landslip reports and 60 flood reports received over the territory (Figure 4). Power and water outages occurred in some places. During the passage of that rainstorm, there were at least two casualties and over 140 people injured.
Flooding situation in Wong Tai Sin (Long Cheung Road section) on 8 September 2023.
Figure 4.  Flooding situation in Wong Tai Sin (Long Cheung Road section) on 8 September 2023. (Source: CRHK News)
Since the Industrial Revolution, excessive resource consumption led to the continuous rise in the concentration of greenhouse gases, exacerbating global warming which in turn increases the water-holding capacity of the atmosphere. The latter provides more favourable conditions for the development of rainstorms, leading up to the possibility of more frequent occurrence of extreme rainfall events. Rainstorms can initiate and dissipate very rapidly with high randomness in movement and downpouring location. As such, early warning of rainstorms remained a major challenge. At present, a more practical and effective strategy to assess the rainfall distribution in Hong Kong is to conduct high-density and high-frequency detections by weather radar and rain gauge networks, followed by rainfall nowcast for the next few hours using technologies such as Artificial Intelligence. When the rainfall intensity reaches the SvR or ESR levels, the Observatory will alert the public through Special Weather Tips to pay attention to the exceptional weather change and take preventive measures accordingly. The Special Weather Tips issued at 8:40 a.m. on 4 May 2024 presents a real example: "... In the past one hour, exceptionally severe rainstorm affected Tseung Kwan O, over 140 millimetres of rainfall have been recorded. It is expected that exceptionally severe rainstorm will affect some areas in the eastern part of the territory. Members of the public should stay on high alert."
To alleviate global warming and its associated impacts, everybody are reminded to conserve resources, reduce waste, and take action in daily life for the betterment of the earth.
Reference:
[1] Hong Kong Observatory: The weather review of September 2023
[2] Hong Kong Observatory: The weather review of June 2008
[3] Press Releases of the Government: Rainstorm roundup (7 June 2008)
[4] Civil Engineering and Development Department: Landslips in 2008
[5] Press Releases of the Government: A September with Super Typhoon Saola and record-breaking rainstorm
[6] Hong Kong Observatory: The rainstorm development mechanisms in Hong Kong
[7] Hong Kong Observatory: Global climate change – greenhouse gases
[8] Hong Kong Observatory: Ranking of highest hourly rainfall of the Hong Kong Observatory
Note 1: The rainstorm from 7 to 8 September 2023 broke the one-hour, two-hour, twelve-hour rainfall records of the Hong Kong Observatory since records begun in 1884. The three new records were 158.1, 201.0 and 605.8 millimetres respectively.
Note 2: The one-hour rainfall records of the Observatory Headquarters refer to the hourly rainfall measured at the clock hours. The one-hour rainfall records of other automatic rain gauges refer to the maximum running 60-minute rainfall record in a day.