Super typhoons Hato (1713) and Mangkhut (1822), part I: analysis of maximum intensity and wind structure

Introduction

A tropical cyclone (TC) is one of the severe weather systems affecting Hong Kong and southern China. On average, there are about six TCs entering the 500-km range of Hong Kong each year. TCs that directly hit or skirt past Hong Kong and its neighbouring region could bring high winds, heavy rain and severe storm surge to Hong Kong depending on their approaching track and intensity at the time. A numbered TC warning system had been introduced in Hong Kong to warn the public about TC-related hazards since 1917, and the current 1-3-8-9-10 numbering scheme for TC warning signals has been adopted since 1973 (HKO, 2012; Lui et al., 2018). Among the warning signals, the highest TC warning signal, No. 10, means that winds of hurricane force (i.e. 118kmh–1 or more) are expected to affect Hong Kong. This signal is not frequently issued, and only 16 typhoons necessitated its issuance since 1946, an average once in every 4–5 years.

In 2017 and 2018, Hong Kong came under successive strikes of TCs Hato (1713) and Mangkhut (1822), both necessitating the issuance of the No. 10 Signal. The number shown in brackets is assigned by the Japan Meteorological Agency, which is the international identification number of a TC given in four digits. It consists of the last two digits of calendar year followed by a 2-digit series number ID of a storm of tropical storm intensity or above. Figure 1 shows the tracks of Hato and Mangkhut. Both storms skirted past the south-southwest of Hong Kong, bringing extremely high winds and severe storm surge to the territory and causing serious damages. Although there was no fatality, at least 129 people and 458 people were injured in Hong Kong, respectively, during the passages of Hato and Mangkhut (HKO, 2017, 2018). There were, altogether, over 65 000 reports of fallen trees and many incidents of blown-down and falling objects and damage to the glass curtain walls of several commercial buildings. In the case of Mangkhut, electricity supply to over 40 000 households in Hong Kong was interrupted, and the power supply to some remote areas was not fully restored even after 4 days. The supply of fresh water in some places was also affected due to power outages.

The destruction caused by heavy rain, storm surge and high waves induced by Hato and Mangkhut were widespread and serious as the tracks of Hato and Mangkhut were typical tracks that trigger severe storm surge in Hong Kong. When they moved to the south-southwest of Hong Kong, the associated ferocious east to southeasterly winds pushed the sea water towards the shore, and it piled up against the coast. In the case of Hato, with the storm surge coinciding with the high water of the astronomical tide, the aggregated effect resulted in the inundation of many low-lying areas in Hong Kong by sea water. For Mangkhut, although it did not hit Hong Kong during the astronomical high tide, its large circulation and fast-moving speed triggered record-breaking storm surges, generally raising the water level in many parts of Hong Kong by more than 2m.

With respect to wind, both Hato and Mangkhut brought storm-to-hurricane force winds to different parts of the territory. In particular, the wind strength over Hong Kong during the passage of Mangkhut was generally stronger than that of the TCs necessitating the issuance of No. 10 Signals in the last three decades, including Typhoon York in 1999, Severe Typhoon Vicente in 2012 and Super Typhoon Hato in 2017 (see Table 1 and Figure 2). The maximum 60-min mean wind speeds recorded at Waglan Island and Cheung Chau (locations of places in Hong Kong shown in Figure 2; same below) were 161kmh–1 and 157kmh–1, respectively. Both are the second highest records at the corresponding stations, just below the record high of Ellen in 1983. Gusts over 150kmh–1 were registered in most parts of the territory on 16 September 2018, and a maximum gust of 256kmh–1 was recorded at Tate's Cairn (anemometer located 587m above mean sea level), ranking after Wanda in 1962 and Ruby in 1964. A maximum 10-min mean wind of 124kmh–1 was also registered on the North Point anemometer located inside the Victoria Harbour, the first time sustained hurricane force winds were recorded at the station since the start of its operation in 1998.

Based on information from the Hong Kong Observatory (HKO) and Hong Kong Federation of Insurers, the economic losses to Hong Kong were 1.2 billion and 4.6 billion Hong Kong dollars (or 0.15 billion and 0.59 billion US$) for Hato and Mangkhut, respectively (HKO, 2017; HKO, 2018). There were no fatalities in Hong Kong in both cases. While both Hato and Mangkhut brought serious impacts to Hong Kong, they possessed very different storm characteristics. Hato was marked by its unusually rapid intensification near the coastal areas of Guangdong. Although Mangkhut showed signs of weakening after it moved across Luzon and took on a track further away from Hong Kong compared with Hato, its extensive circulation, ferocious winds and fast movement, as well as its special wind structure, made it the most damaging storm to Hong Kong in the last three decades. In this paper, Sections 2 and 3, respectively, summarise the detailed analysis of the intensity and wind structure of Hato and Mangkhut when they edged close to the Pearl River Delta region. A conclusion is given in Section 4.

Analysis of storm intensity and structure of Hato

Figure 3(a) shows the 3-hourly intensity (maximum sustained wind speed over the storm) of Hato based on the best track of HKO. While Hato intensified steadily after entering the South China Sea on 22 August 2017, it underwent rapid intensification between 0500 and 1100 HKT (Hong Kong time; utc + 8 h) on the morning of 23 August 2017, when it edged towards the coast of Guangdong. This rapid intensification was also captured by the 10-min scan of the Himawari-8 satellite images and the doppler weather radar images of the HKO. For satellite imagery analysis, Dvorak analysis using the enhanced infrared (EIR) technique (Dvorak, 1975; Dvorak, 1984) was performed. The Dvorak technique is a primary operational tool for estimating the maximum winds speeds of a TC (Velden et al., 2006). It is a statistical technique based on satellite image interpretation from which a T-number and current intensity (CI) value, ranging between 1 and 8, are assigned. Maximum wind speed and minimum central pressure could then be estimated from the assigned T-number and CI. In the case of an intense TC, the technique utilizes the difference between the temperature of the warm eye and the surrounding cold cloud tops to determine intensity and a T-number and a CI value. The T-number of Hato rose rapidly, from 5.0 to 6.5, between 0850 and 1000 HKT when an eye was forming, and dropped to 5.5 at 1050 HKT. The T-number of Hato momentarily reached 6.5 around 1000 HKT (Figure 4a). A CI number of 6.0–6.5 obtained around 1000 HKT represents a 10-min mean wind speed over 185kmh–1 or above for a short period of time.

The doppler weather radars of the HKO also consistently depicted the well-defined and intense eyewall of Hato when it came within the 256-km range of Hong Kong (Figures 5a,c). A maximum 0^o Plan Position Indicator (PPI) doppler wind of 245kmh–1 was recorded at an altitude of about 1000 m at 0954 HKT on 23 August 2017. Using a conversion factor of 0.7 (Lee, 2010), the estimated 10-m wind near sea level would be around 171kmh–1, with a 95% confidence interval between 146kmh–1 and 194kmh–1. This also hints that the maximum sustained wind speed of Hato may have reached 174–185kmh–1 at the time.

As Hato's eye moved across the weather station at Huangmao Zhou, the wind speed and pressure data of this station provided a valuable ground truth for the intensity assessment. Huangmao Zhou is an island located at around 50km south of Hong Kong (Figure 2). Two sets of identical cup anemometers and pressure sensors were installed on the island, of which HMZ is the primary senor, and HM2 is the backup. HMZ and HM2 wind sensors were located at heights of 67m and 65m, respectively, above mean sea level. During the passage of Hato on 23 August 2017, the mean sea-level pressure recorded at Huangmao Zhou fell to a minimum of 954.5hPa at 0952 HKT. When the station encountered the eastern part of the eyewall, HMZ recorded a maximum 10-min mean wind of 256kmh–1 at 1109 HKT. However, HM2's maximum mean wind was about 36kmh–1 lower, with some bumpy values afterwards, and maintained a systematic bias with the wind speed of HMZ (Figure 6). Based on the site inspection on 19 October 2017, it was found that the wind vanes of both HMZ and HM2 were missing, and only one of three cups remained at the HM2 anemometer (Figure 7). The primary anemometer, HMZ, was brought back to the HKO for instrument testing. It was confirmed that the field unit and the mechanical parts of the anemometer operated normally, confirming the reliability of the wind data measured by HMZ during the passage of Hato. As the anemometer of HMZ is located at a height of 67 m above sea level, it is necessary to convert the station wind speed to the standard 10-m level to estimate the maximum winds near the centre of Hato. By using the log wind profile (Eqn 1) recommended by the World Meteorological Organization (WMO) and assuming a roughness length (z₀) of between 0.005 and 0.03 for the terrain of Huangmao Zhou (WMO, 2012), the corrected 10-m wind speed near the centre of Hato ranges from 193kmh–1 to 203kmh–1.   

(1)

(1)where z₁ and z₂ are two heights at the same location, and z₀ is the surface roughness length parameters.

By combining the remote-sensing and surface observations, after the Rapid Intensification (RI) over the coast of Guangdong, Hato attained super typhoon intensity for a short period of time over the seas south of Hong Kong around 1000–1100 HKT on 23 August 2017. The estimated maximum sustained 10-min mean wind at the time was about 185kmh–1 near its centre.

Analysis of storm intensity and structure of Mangkhut

Figure 3(b) shows the 3-hourly intensity (maximum sustained wind speed) of Mangkhut based on the best track of the HKO from 14 to 17 September 2018. Mangkhut reached its peak intensity with a maximum 10-min sustained wind speed near its centre of about 250kmh–1, before hitting the northern part of Luzon on 15 September 2018. Mangkhut weakened after entering the northern part of the South China Sea on 16 September 2018. Satellite images showed that the eyewall near the centre of the storm was considerably disrupted by the terrain of Luzon (Figure 4b). Dvorak analysis of satellite images on the morning of 16 September 2018 suggested that the T-number dropped from 7.5 to 4.5 (corresponding to a 10-min mean wind of about 266kmh–1 falling to about 133kmh–1) when it was near Hong Kong. However, the doppler weather radar images showed an unusual wind structure of Mangkhut with a prominent and intense rainband in the outer circulation (Figures 5b,d). The maximum 0.1^o PPI doppler wind at Tate's Cairn Weather radar was 214kmh–1, which was recorded over the outer circulation but not near the centre at 08:00 HKT on 16 September 2018. The estimated 10-m wind speed would be around 150kmh–1, with a 95% confidence interval between 130kmh–1 and 173kmh–1. The change in the structure of Mangkhut before and after crossing Luzon was clearly depicted in the microwave satellite imagery. The intense convection at the eyewall of Mangkhut was significantly weakened after it moved across Luzon because of its interaction with land (Figures 8a,b). Although the eyewall was rebuilt when it traversed the northern part of the South China Sea, the convection of the eyewall was significantly weaker than that before crossing Luzon. On the contrary, the convection of the spiral rainband outside the eyewall remained intense, and the structure was intact.

The discrepancy between Dvorak and doppler wind analysis may be because the Dvorak technique does not adjust for the effects of system translation on surface wind as it was initially derived using a set of cyclones with an average speed of around 6–22kmh–1 (Brown and Franklin, 2004). As the Dvorak sample included only ‘classical’ TCs, the Dvorak technique is also limited by the unusual wind structure (the maximum wind was recorded over the outer circulation but not near the centre) and the influence of landmass. With high moving speed (once reaching 35kmh–1 (6-h average)), the special wind structure of Mangkhut and influence of landmass, the Dvorak technique may have underestimated the maximum sustained wind speed near the centre of Mangkhut over the northern part of the South China Sea.

For in-situ observations, Huangmao Zhou encountered the northern part of the eyewall of Mangkhut between 1200 and 1300 HKT on 16 September 2018, and HMZ and HM2 both registered maximum 10-minute mean winds of around 161kmh–1. The minimum mean sea-level pressure dropped to 960.6hPa. After adjusting for elevation, the corresponding corrected 10-m winds near the centre of Mangkhut ranged from 122 to 130kmh–1. We have also reviewed a number of weather stations over the Pearl River Estuary during the passage of Mangkhut. The maximum 10-minute mean wind speeds recorded at Waglan Island and Cheung Chau in Hong Kong were 180kmh–1 and 173kmh–1, respectively. Besides, the automatic weather station under testing at Clear Water Bay even recorded a maximum 10-min average wind speed of 191kmh–1, which is believed to be the highest record near the surface since the HKO's commencement of automatic weather station installation in Hong Kong in the 1980s. The automatic weather station at Clear Water Bay is located on a complex terrain, with the anemometer at an elevation over 70m above sea level. The corresponding wind speed near sea level is estimated to be lower than 185kmh–1. The wind speeds recorded over Macao, Zhuhai and Taishan, which were closer to the centre of Mangkhut, were weaker than those over Hong Kong. In fact, the wind strength over Hong Kong was the strongest among the Pearl River Delta region. This is generally in line with the above-mentioned more intense outer circulation structure of Mangkhut as observed in the doppler weather radars and microwave satellite imagery.

Reconnaissance flights for Mangkhut were conducted by Hong Kong (the HKO in collaboration with the Hong Kong Government Flying Service) and a nearby meteorological organisation on 15 September 2018. The invaluable dropsonde data showed the high wind areas over the spiral rainband in the northern semicircle of the storm (Figure 9), which was later confirmed by the doppler radar velocities (Figure 5d). Vertical wind profile sampled by a dropsonde at the outer periphery of Mangkhut (Figure 10) suggested a wind speed maximum of up to 144kmh–1 around an altitude of 1000m, while below that, there is a steep, approximately linear increase in wind speed with height from the surface. The ‘linear’ increase of wind speed is somehow different from the shape of wind profiles sampled by previous dropsonde missions of the HKO for different TCs (Chan et al., 2018). Such a steep increase of boundary layer wind speed with height might contribute to the extreme wind speeds recorded on the mountain tops in Hong Kong (which are about 900m tall).

Analysis of microwave satellite imagery, doppler radar imagery, surface observations and flight reconnaissance data revealed that the winds associated with the spiral rainband outside the eyewall of Mangkhut were stronger than those near the eyewall, leading to a high confidence that hurricane force winds affected Hong Kong even though Mangkhut skirted about 100km to the southwest of Hong Kong. Moreover, Mangkhut moved rapidly west-northwestward at a speed of 35kmh–1 across the northern part of the South China Sea. With Hong Kong staying in the dangerous semicircle for a long time, when this spiral rainband swept across Hong Kong, the superposition of wind speed and translational speed of the storm brought destructive winds to Hong Kong. The wind structure and size of the TC played an important role in the wind strength experienced in Hong Kong.

Conclusions

In summary, based on the available information, Hato attained super typhoon intensity for a short period of time over the sea areas south of Hong Kong on the morning of 23 August 2017 just before landfall, with an estimated maximum sustained 10-minute mean wind of 185kmh–1 near its centre. As a challenge to forecast, the rapid intensification of Hato was well captured by satellite images. In the case of Mangkhut, satellite observation, radar observation and surface observation showed that the maximum winds near the centre were weaker than that of Hato when the storm edged towards the coast of Guangdong. However, as the winds over the outer spiral rainband of Mangkhut were stronger than those near its centre, the wind strength over Hong Kong under the influence of Mangkhut was generally stronger than that of Hato. The wind structure and size of a TC played an important role in the wind strength experienced by Hong Kong. Challenge in forecasting and early warnings of Hato and Mangkhut will be examined in Part II of the paper.