Benefits from typhoons – the Hong Kong perspective

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<ul><li><p>16</p><p>Wea</p><p>ther</p><p> Ja</p><p>nuar</p><p>y 20</p><p>12, V</p><p>ol. 6</p><p>7, N</p><p>o. 1</p><p>Benefits from typhoons the Hong Kong perspectiveHilda Lam, Mang Hin Kok and Karen Kit Ying ShumHong Kong Observatory, Hong Kong, China</p><p>IntroductionTropical cyclones have long been consid-ered one of the most devastating weather systems on Earth. They can produce destruc-tive winds, torrential rain causing floods and landslides, storm surges and phenomenal sea waves which take a heavy toll on com-munities and the environment along their paths. For example, Typhoon Morakot in 2009 brought torrential rain to Taiwan, trig-gering floods, unleashing mudslides and causing the most severe damage there in about 50 years according to press reports. More than 460 people were killed, of whom hundreds were buried beneath the rubble in a village in southern Taiwan (HKO, 2010). Tropical cyclone Gonu in June 2007 was the strongest on record in the Arabian Sea, causing 49 deaths and about 4 billion US dollars (USD) in damage in Oman, where it was considered the nations worst natural disaster (WMO, 2007). In the Islamic Republic of Iran, the cyclone caused 23 deaths and </p><p>around 215 million USD in damage (WMO, 2007). From 1983 to 2006, each year on average saw tropical cyclones cause 472 casualties and 28.7 billion yuans in direct economic losses after making landfall over China mainland and Hainan Island (Zhang et al., 2009).</p><p>In Hong Kong, the most devastating storms in the past 120 years occurred in 1906 and 1937, claiming 15 000 and 11 000 lives respectively (Ho, 2003). The territory has over the years developed an effective warning system and set up stringent build-ing codes to protect the community against tropical cyclones. The implementation of various disaster prevention and mitigation measures has led to a steady decline in the number of deaths associated with these storms in the past 50 years (Figure 1). The damage in monetary terms varied from year to year, depending on the incidence and severity of those tropical cyclones passing close to Hong Kong: the maximum in recent years was about 220 million Hong Kong dol-lars (HKD) in 1999 (Figure 2), which may be compared with the gross domestic product of about 1632 billion HKD in the same year.</p><p>Severe though the human and economic losses caused by tropical cyclones are, they may also bring some benefits to the </p><p> communities and the environment. From the hydrological perspective, the tropical cyclone is one of the key weather systems which provide rainfall for many places (Sugg, 1968). Ryan (1993) reported that rain-fall from tropical cyclones is a valuable source of water for inland Australia, as a sig-nificant amount of the annual total rainfall comes from these events: rain from decay-ing tropical cyclones benefited agricultural production in the dry periods. In China, tropical cyclones Rananim, Haitang and Matsa in 2004 acted as a drought breaker, providing important water resources for Beijing, Hebei, Hunan, Hubei, Jiangxi and Hainan (Liu, 2006). Reservoirs in the affected regions were replenished with water to the extent of about 2.0 billion, 1.2 billion and 1.8 billion cubic metres respectively from the three cyclones, mitigating the scarcity of water and helping to increase both grain production and farmers income that year.</p><p>In addition to agriculture, tropical cyclones may also benefit fisheries (Sugg, 1968; Galvin, 2005), as they can enhance marine life when they pass over the ocean. The asso-ciated strong wind enhances vertical mixing and upwelling in the tropical and subtropi-cal oceans, bringing nutrient-rich water from the depths to fuel photosynthetic </p><p>Figure 1. Lives claimed (dead or missing) by tropical cyclones in Hong Kong.</p></li><li><p>17</p><p>Weather January 2012, Vol. 67, No. 1</p><p>Benefits from typhoons</p><p>activities and enhance the production of phytoplankton, which is the basis of the ocean food chain (Lin et al., 2003). An analy-sis using satellite data showed that tropical cyclone Kai-Tak in 2000 triggered an aver-age 30-fold increase in surface chlorophyll-a concentration during its 3-day passage over the South China Sea, contributing signifi-cantly to its annual production in the region (Lin et al., 2003).</p><p>Globally, tropical cyclones play an impor-tant role in regulating the climate by redis-tributing heat poleward and driving the thermohaline circulation (Emanuel, 2001; Sriver and Huber, 2007; Jansen and Ferrari, 2009). As the cyclones induce upper-ocean mixing, they lead to an increase in the oceans meridional heat flux, thus cooling tropical latitudes and warming higher lati-tudes (Korty et al., 2008). Moreover, the tor-rential rain brought by them can bury tons of carbon from the terrestrial biosphere in the ocean and help sink carbon which would otherwise be released into the atmosphere by the rotting and burning of vegetation (Hilton et al., 2008; Goldsmith et al., 2008).</p><p>The high winds associated with tropical cyclones could also have a positive impact on the production of wind energy, especially for wind farms in coastal areas. A study on their influence on wind-power </p><p>water supply; potential wind energy, and cooling effects in the hot summer.</p><p>DataIn this study, the tropical cyclone best-track dataset of the Hong Kong Observatory is used to assess their impact. Temperature and rainfall data at Hong Kong Observatory Headquarters (HKOHq) are used to assess the cooling effect and hydrological aspects of tropical cyclones. Monthly electricity con-sumption data were obtained from the Census and Statistics Department (C&amp;SD) of the Hong Kong Special Administrative Region (CS&amp;D, 2010a). Wind data is taken from the records of four stations: Cheung Chau, Waglan Island, Tai Mo Shan and Tates Cairn. Metadata related to the instruments and meteorological measurements at HKOHq and other stations are available in the Observatorys publications (Lee et al., 2006; HKO, 2009). </p><p>The benefits of tropical cyclones to Hong Kong</p><p>A source of water supply and drought breakerHong Kong lacks major rivers, so local rain-fall is a significant source of its water supply. An average of about six tropical cyclones come close to Hong Kong every year, normally between May and November, and they contribute about 30% of the territory's mean annual rainfall (Figure 3). The rest is attributable to the southwest monsoon and its associated troughs. The rain brought by a tropical cyclone is defined here as the rainfall registered at the Observatory from the time the tropical cyclone comes within 600km of Hong Kong to 72 hours after it moves more than 600km away or after its dissipation, whichever is earlier. </p><p>Figure 2. Cost of damage due to tropical cyclones in Hong Kong, 19832009.</p><p>Damage in monetary terms (million HKD)</p><p>0</p><p>50</p><p>100</p><p>150</p><p>200</p><p>250</p><p>1983</p><p>1985</p><p>1987</p><p>1989</p><p>1991</p><p>1993</p><p>1995</p><p>1997</p><p>1999</p><p>2001</p><p>2003</p><p>2005</p><p>2007</p><p>2009</p><p>Milli</p><p>on H</p><p>KD</p><p>generation in China showed that while about 30% of tropical cyclones making landfall may damage wind farms, about 55% of them, where speeds are less than Force 10, are beneficial to the efficiency of wind-power production (Song et al., 2006). </p><p>In this paper, an attempt is made to ascer-tain the benefits of tropical cyclones from the Hong Kong perspective. Three areas are studied: tropical cyclones as a source of </p><p>Figure 3. Contribution of rainfall from tropical cyclones (TC) in Hong Kong.</p><p>0</p><p>500</p><p>1000</p><p>1500</p><p>2000</p><p>2500</p><p>3000</p><p>3500</p><p>4000</p><p>1961</p><p>1964</p><p>1967</p><p>1970</p><p>1973</p><p>1976</p><p>1979</p><p>1982</p><p>1985</p><p>1988</p><p>1991</p><p>1994</p><p>1997</p><p>2000</p><p>2003</p><p>2006</p><p>2009</p><p>Rai</p><p>nfal</p><p>l (mm)</p><p>Other rainTC rain</p><p>Figure 4. Monthly rainfall in Hong Kong from 1962 to 1964.</p><p>0</p><p>500</p><p>1000</p><p>1500</p><p>2000</p><p>2500</p><p>1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 10 11 12</p><p>1962 (1741 mm) 1963 (901.1 mm) 1964 (2432.1 mm)</p><p>Rai</p><p>nfal</p><p>l (mm)</p><p>Monthly rainfallRunning 12 month rain19611990 normal</p></li><li><p>18</p><p>Wea</p><p>ther</p><p> Ja</p><p>nuar</p><p>y 20</p><p>12, V</p><p>ol. 6</p><p>7, N</p><p>o. 1</p><p>Bene</p><p>fits f</p><p>rom</p><p> typh</p><p>oons</p><p>were estimated to be 204, 283, 397 and 339Wm2 respectively (Table 1). The percentage of time with usable wind speed (325ms1) for the four sites was more than 75%.</p><p>To assess the wind power density contrib-uted by tropical cyclones, the wind records of the four stations during the 69 days when tropical cyclones come within 600km of Hong Kong in 20072009 were analysed. Altogether, 21 tropical cyclones (Table 2) met this criterion. The results given in Table 1 show that the WPD induced by tropical cyclones has been more than twice the decadal average (20002009). The per-centage of time with useable winds speed during these 69 days is similar to the long-term value at about 80%. Thus, tropical cyclones in general contribute to the wind-energy potential in Hong Kong.</p><p>Cooling effectMost people in Hong Kong who have expe-rienced the passage of tropical cyclones are familiar with the stuffy and very hot weather due to the subsidence warming and hot continental winds ahead of the storm, and </p><p>The rainfall associated with tropical cyclones can also serve as a breaker for severe drought in Hong Kong. Following a rather dry year in 1962 (annual rainfall of 1741mm against the normal of around 2200mm (19611990)), 1963 was even drier with only 901mm for the whole year, the lowest total since records began at the Observatory in 1884. The government pro-gressively reduced the availability of domes-tic water from three hours daily from 2 May to four hours every other day from 16 May. By June, the running 12-month rainfall had fallen to about 1145mm (Figure 4) and res-ervoir storage declined further. The govern-ment was now forced to restrict water supply to four hours every four days. The arid conditions persisted until 27 May 1964 when Typhoon Viola passed close to Hong Kong and brought a total of 300.6mm of rainfall in five days, terminating the year-long four hours every four days rationing measure (WSD, 2001; Hui, 2005). To resi-dents in Hong Kong, Viola was more than welcome and its benefit was undoubtedly felt.</p><p>Contribution to wind energyIn the spirit of environmental conservation, and to mitigate the effects of climate change, renewable energy has gained pop-ularity worldwide and the feasibility of gen-erating wind energy has been studied in Hong Kong (Wong and Kwan, 2002). The wind-energy resource available at a poten-tial site is expressed in terms of the mean wind-power density (WPD) which is defined as the average wind power available per unit area swept by a wind turbine blade over a certain period (Janardan and Nelson, 1994): </p><p>WPD = (0.5)(r)( __</p><p> u3 ) (1)where r is the air density and </p><p>__ u3 the average </p><p>of the cube of the mean wind speed.WPD is given in units of watts per square </p><p>metres (Wm2). Since wind turbines typi-cally do not operate below the cut-in wind speed of about 3ms1, and above the cut-out wind speed of about 25ms1 (Hong Kong Electric, 2005; Song et al., 2006), the WPD for mean wind speeds less than 3ms1 and exceeding 25ms1 is set at zero. By uti-lizing available wind data at 17 weather stations in Hong Kong from 2000 to 2009, Lee and Mok (2010) assessed the potential wind energy in Hong Kong using Equation (1). The study showed that, from a clima-tological perspective, there was high wind-energy potential with WPD exceeding 200Wm2 at 6 sites out of 17, mainly in exposed places such as on hilltops and islands in offshore waters. The annual mean WPDs of the four stations at Cheung Chau (offshore), Tates Cairn (hilltop), Tai Mo Shan (hilltop) and Waglan Island (offshore) </p><p>Table 1</p><p>The ratio of the average potential WPD during tropical cyclone passage to the annual average potential WPD for four selected sites in Hong Kong based on hourly mean wind records. Hilltops and offshore sites in Hong Kong were selected for their potential for large-scale wind farms.</p><p>Cheung Chau (offshor e)</p><p>Tates Cairn (hilltop)</p><p>Tai Mo Shan (hilltop)</p><p>Waglan Island (offshore)</p><p>Average annual mean wind speeda (ms1)</p><p>5.0 6.1 6.7 6.2</p><p>Long term mean WPD (20002009) (Wm2) - A</p><p>204 283 397 339</p><p>% time with usable wind speed (20002009)</p><p>76 85 83 84</p><p>Mean WPD (Wm2) during TC passageb - B</p><p>475 627 923 782</p><p>Ratio of WPD during TC vs. normal time (B:A)</p><p>2.3 2.2 2.3 2.3</p><p>% time with useable wind speed during TC passageb</p><p>79 82 86 84</p><p>aThe averages are computed based on available wind data at each site and the data period may be different for different sites.bTC passage refers to the 69 days when a TC came within 600km of Hong Kong in 20072009.</p><p>the subsequent cooling due to the rainy and windy conditions during and after its passage. In this study, an attempt is made to quantify the net temperature effect as a result of the passage of tropi-cal cyclones.</p><p>The records of daily mean tempera-ture at the HKOHq when tropical cyclones were in its vicinity from 2007 to 2009 were studied. Using the best-track dataset of the Hong Kong Observatory, the dates when a tropical cyclone came within 600km of Hong Kong (T1) and when it moved outside 600km of Hong Kong (T2) were identi-fied. To take account of the possible subsidence warming ahead of a tropical cyclone, the daily mean temperature from two days before T1 (T12) to T2 were extracted. The deviations of these daily mean temperatures from their respective reference mean temperature, the 5-day running mean for 20002009, were calculated. </p><p>Figures 5(a) and (b) show respectively the track of Fengshen and the time series of the daily mean temperature </p><p>Table 2</p><p>Tropical cyclones coming within 600km of Hong Kong in 20072009.</p><p>Year Number Name </p><p>2007 5 Toraji, Pabuk, Sepat, Francisco, Peipah</p><p>2008 7 Neoguri, Fengshan, Kammuri, Nuri, Hagupit, Higos, Maysak</p><p>2009 9 Linfa, Nangka, Soudelor, TD in July, Molave, Goni, Mujigae, Koppu, Parma</p><p>Total 21 </p></li><li><p>19</p><p>Weather January 2012, Vol. 67, No. 1</p><p>Benefits from typhoons</p><p>Table 3</p><p>Temperature effect in Hong Kong during the passage of 21 tropical cyclones in 20072009 from T12 up to T2+3.</p><p>Tropical cyclone day</p><p>Total deviation from the mean (degC) 20002009</p><p>Days Average deviation from the mean (degC/day) 20002009 </p><p>T12T2 16 93 0.17</p><p>T12T2+1 26.8 111 0.24</p><p>T12T2+2 31.2 128 0.24</p><p>T12T2+3 35.8 145 0.25</p><p>When a TC formed near the 600km circle of Hong Kong, T12 may refer to the day of TC formation.</p><p>10-year mean (20002009) is used as the reference.</p><p>Figure 5. (a) Track and (b) time series of daily mean temperature (C) at HKO for Typhoon Fengshen.</p><p>(a) (b)</p><p>20</p><p>22</p><p>24</p><p>26</p><p>28</p><p>30</p><p>32</p><p>Fengshen</p><p>Fengshen</p><p>Fengshen</p><p>Fengshen</p><p>13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 296 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6</p><p>2008 2008 2008 2008 2008 2008 2008 2008 2008 2008 2008 2008 2008 2008 2008 2008 2008</p><p>daily mean temp5day mean temp (2000-2009)</p><p>T1 T2</p><p>(CS&amp;D, 2010b)). Therefore, for 93 tropical cyclone days in 20072009, this may imply a total saving of about 8.7 107kWh in the two sectors in these three years. This is about one tenth of the consumption in a typical summer month in the domestic sec-tor in Hong Kong for the period 20072009 (CS&amp;D, 2010a). Although the amount is not large, the net saving is still noticeable. However, it should be noted that this esti-mate was based on a couple of assump-tions: that the sensitivity of electricity consumption...</p></li></ul>

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