Special Report – Lessons from science: peak wind intensity, maximum sea level and extreme high temperature

Macau Business | August 2022 | Special Report | Hato’s “ghost” – 5 years on

According to the SMG’s analysis of the super typhoon after the event, “Hato had exhibited rapid intensification before it made landfall. This phenomenon was caused by a combination of several meteorological conditions at the time. Some of them include the high sea-surface temperature region in the northern part of the South China Sea, the inverted trough accompanied by the increase of wind speed in the upper atmosphere, and the relatively low vertical wind shear in the northern part of the South China Sea.”

Added to this explanation sent to Macau Business by Macau’s weather bureau are the accounts of several others who researched what happened on the day.

On 23 August, 2017, Typhoon Hato rapidly intensified by 10 knots within 3 hours just prior to landfall in the city of Macau (…). Hato’s surface winds in excess of 50 miles per second devastated the city, causing unprecedented damage and social impact. This study reveals that anomalously warm ocean conditions in the nearshore shallow water (depth < 30 m) likely played a key role in Hato’s fast intensification. In particular, cooling of the sea-surface temperature (SST) generated by Hato at the critical landfall point was estimated to be only 0.1–0.5 °C. The results from both a simple ocean mixing scheme and full dynamical ocean model indicate that SST cooling was minimized in the shallow coastal waters due to a lack of cool water at depth. Given the nearly invariant SST in the coastal waters, we estimate a large amount of heat flux, i.e., 1.9 kilowatts per square meter, during the landfall period. Experiments indicate that in the absence of shallow bathymetry, and thus, if nominal cool water had been available for vertical mixing, the SST cooling would have been enhanced from 0.1 °C to 1.4 °C, and sea-to-air heat flux reduced by about a quarter. Numerical simulations with an atmospheric model suggest that the intensity of Hato was very sensitive to air-sea heat flux in the coastal region, indicating the critical importance of coastal ocean hydrography. [Pun I-F, Chan JCL, Lin I-I, Chan KTF, Price JF, Ko DS, Lien C-C, Wu Y-L, Huang H-C. Rapid Intensification of Typhoon Hato (2017) over Shallow Water. Sustainability. 2019; 11(13):3709. https://doi.org/10.3390/su11133709]

In this study, the characteristics and mechanisms of tide-surge interaction in the Pearl River Estuary (PRE) during Typhoon Hato in August 2017 are studied in detail using a 3D nearshore hydrodynamic model (…). Three different types of model runs are conducted in order to separate the water level variations due to the astronomical tide, storm surge, and tide-surge interactions in the Pearl River Estuary. The results show the strong tidal modulation of the surge level, as well as alteration of the phase of surge, which also changes the peak storm tidal level, in addition to the tidal modulation effects. In order to numerically assess the contributions of three non-linear processes in the tide-surge interaction and quantify their relative significance, the widely used “subtraction” approach and a new “addition” approach are tested in this study (…). Detailed analysis using the “addition” approach indicates that the quadratic bottom friction, shallow water effect, and nonlinear advective effect play the first, second, and third most important roles in the tidal-surge interaction in the estuary, respectively. [Zheng P, Li M, Wang C, Wolf J, Chen X, De Dominicis M, Yao P and Hu Z (2020) Tide-Surge Interaction in the Pearl River Estuary: A Case Study of Typhoon Hato. Front. Mar. Sci. 7:236. doi: 10.3389/fmars.2020.00236]

On 23 August 2017 a Category 3 hurricane, Typhoon Hato, struck southern China. Among the hardest hit cities, Macau experienced the worst flooding since 1925. In this paper, we present a high-resolution survey map recording inundation depths and distances at 278 sites in Macau. We show that one-half of the Macau Peninsula was inundated, with the extent largely confined by the hilly topography. The Inner Harbor area suffered the most, with a maximum inundation depth of 3.1 m at the coast. (…) One of the important observations is that regardless of the tidal level during Typhoon Hato’s landfall, the Inner Harbor area would have been inundated at least with a maximum inundation depth of up to 0.5–1.0 m. On the other hand, although Typhoon Hato broke all historical records in terms of storm-surge heights and area flooded, much worse scenarios could have been expected if Typhoon Hato had occurred at a higher tidal level, and thus, caution is required if Typhoon Hato is to be used as the worst-case scenario for designing future coastal defense measures. This is especially true when taking sea level rise (SLR) into consideration, as 0.5 or 1 meter SLR could significantly increase the severity of the resulting inundation for most of the territory in Macau, under both high-tide and low-tide conditions. [Li, L., Yang, J., Lin, C.-Y., Chua, C. T., Wang, Y., Zhao, K., Wu, Y.-T., Liu, P. L.-F., Switzer, A. D., Mok, K. M., Wang, P., and Peng, D.: Field survey of Typhoon Hato (2017) and a comparison with storm surge modeling in Macau, Nat. Hazards Earth Syst. Sci., 18, 3167–3178, https://doi.org/10.5194/nhess-18-3167-2018 , 2018.]

The destructiveness and potential hazards brought to the Pearl River Delta (PRD) by the category-3 typhoon Hato in 2017 have been studied. The results show that wind flow is one of the key parameters influenced by tropical cyclones. The observed wind at Shenzhen station changed from median southwesterly and calm northerly to strong easterly during the evolution of Hato as it approached the PRD and during landfall, respectively. The peak wind intensity at the surface level and a height of 300 m reached over 17 and 30 meters per second, respectively. In Zhuhai, the area closest to the landfall location, the situ observation shows that the maximum wind and the maximum gust on 23 August 2017 reached 29.9 and over 50 meters per second, respectively, which is a record-breaking intensity compared with the highest recorded intensity during tropical cyclone (TC) activity in Typhoon Vicente in 2012. The maximum sea level during 23 August 2017, with an added influence from the storm surge and the astronomical tide, was found to be over 3.9 m, to the west of Hong Kong. Extreme high temperature was also recorded on 22 August 2017 before the landfall, with daily maximum temperatures of 38.4, 38 and 36.9 °C in Shenzhen, Macau and Hong Kong, respectively. Based on the heat index calculated with the temperature record at Shenzhen’s station, the hot temperature hazard reached “danger” levels. On the other hand, a prominent air quality deterioration was observed on 21 August 2017. The concentrations rapidly increased to 1 time greater than those on the previous day in Hong Kong. TC-induced sinking motion, continental advection, and lower amount of cloud cover were observed before the landfall, and would be the possible factors causing the extreme high temperature and the poor air quality. This case study illustrates that the influences of Hato to the PRD were not only limited to their destructiveness during landfall, but also brought the extreme high temperature and poor air quality. [Eric C. H. Chow, Min Wen, Lei Li, Marco Y. T. Leung, Paxson K. Y. Cheung and Wen Zhou: Assessment of the Environmental and Societal Impacts of the Category-3 Typhoon Hato, Atmosphere 2019, 10(6), 296; https://doi.org/10.3390/atmos10060296 ]

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