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Benefits from typhoons – the Hong Kong perspectiveHilda Lam, Mang Hin Kok and Karen Kit Ying ShumHong Kong Observatory, Hong Kong, China
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 nation’s worst natural disaster (WMO, 2007). In the Islamic Republic of Iran, the cyclone caused 23 deaths and
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).
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.
Severe though the human and economic losses caused by tropical cyclones are, they may also bring some benefits to the
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.
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
Figure 1. Lives claimed (dead or missing) by tropical cyclones in Hong Kong.
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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).
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).
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
water supply; potential wind energy, and cooling effects in the hot summer.
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&SD) of the Hong Kong Special Administrative Region (CS&D, 2010a). Wind data is taken from the records of four stations: Cheung Chau, Waglan Island, Tai Mo Shan and Tate’s Cairn. Metadata related to the instruments and meteorological measurements at HKOHq and other stations are available in the Observatory’s publications (Lee et al., 2006; HKO, 2009).
The benefits of tropical cyclones to Hong Kong
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.
Figure 2. Cost of damage due to tropical cyclones in Hong Kong, 1983–2009.
Damage in monetary terms (million HKD)
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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).
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
Figure 3. Contribution of rainfall from tropical cyclones (TC) in Hong Kong.
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Figure 4. Monthly rainfall in Hong Kong from 1962 to 1964.
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1962 (1741 mm) 1963 (901.1 mm) 1964 (2432.1 mm)
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Monthly rainfallRunning 12 month rain1961–1990 normal
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were estimated to be 204, 283, 397 and 339Wm−2 respectively (Table 1). The percentage of time with usable wind speed (3–25ms−1) for the four sites was more than 75%.
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 2007–2009 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 (2000–2009). 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.
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
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 (1961–1990)), 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.
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):
WPD = (0.5)(r)( __
u3 ) (1)
where r is the air density and __
u3 the average of the cube of the mean wind speed.
WPD is given in units of watts per square metres (Wm−2). Since wind turbines typi-cally do not operate below the cut-in wind speed of about 3ms−1, and above the cut-out wind speed of about 25ms−1 (Hong Kong Electric, 2005; Song et al., 2006), the WPD for mean wind speeds less than 3ms−1 and exceeding 25ms−1 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 200Wm−2 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), Tate’s Cairn (hilltop), Tai Mo Shan (hilltop) and Waglan Island (offshore)
Table 1
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.
Cheung Chau (offshor e)
Tate’s Cairn (hilltop)
Tai Mo Shan (hilltop)
Waglan Island (offshore)
Average annual mean wind speeda (ms−1)
5.0 6.1 6.7 6.2
Long term mean WPD (2000–2009) (Wm−2) - A
204 283 397 339
% time with usable wind speed (2000–2009)
76 85 83 84
Mean WPD (Wm−2) during TC passageb - B
475 627 923 782
Ratio of WPD during TC vs. normal time (B:A)
2.3 2.2 2.3 2.3
% time with useable wind speed during TC passageb
79 82 86 84
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 2007–2009.
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.
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 (T1−2) to T2 were extracted. The deviations of these daily mean temperatures from their respective reference mean temperature, the 5-day running mean for 2000–2009, were calculated.
Figures 5(a) and (b) show respectively the track of Fengshen and the time series of the daily mean temperature
Table 2
Tropical cyclones coming within 600km of Hong Kong in 2007–2009.
Year Number Name
2007 5 Toraji, Pabuk, Sepat, Francisco, Peipah
2008 7 Neoguri, Fengshan, Kammuri, Nuri, Hagupit, Higos, Maysak
2009 9 Linfa, Nangka, Soudelor, TD in July, Molave, Goni, Mujigae, Koppu, Parma
Total 21 —
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Table 3
Temperature effect in Hong Kong during the passage of 21 tropical cyclones in 2007–2009 from T1−2 up to T2+3.
Tropical cyclone day
Total deviation from the mean (degC) 2000–2009
Days Average deviation from the mean (degC/day) 2000–2009
T1−2–T2 −16 93 −0.17
T1−2–T2+1 −26.8 111 −0.24
T1−2–T2+2 −31.2 128 −0.24
T1−2–T2+3 −35.8 145 −0.25
When a TC formed near the 600km circle of Hong Kong, T1−2 may refer to the day of TC formation.
10-year mean (2000–2009) is used as the reference.
Figure 5. (a) Track and (b) time series of daily mean temperature (°C) at HKO for Typhoon Fengshen.
(a) (b)
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daily mean temp
5day mean temp (2000-2009)
T1 T2
(CS&D, 2010b)). Therefore, for 93 tropical cyclone days in 2007–2009, 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 2007–2009 (CS&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 to temperature change is sta-ble throughout the temperature ranges dur-ing tropical cyclone days and the response of electricity consumption to daily tempera-ture changes is comparable to that of the monthly temperature changes. Given the average electricity tariff of HKD 0.94 per kWh in 2011, the savings in electricity consump-tion for 2007–2009 due to tropical cyclones is estimated to be of the order of 80 million HKD.
ConclusionNotwithstanding the fact that tropical cyclones are natural hazards usually leading to casualties, damage and economic losses, the wind and rain associated with them also have positive implications for Hong Kong. Climatologically, tropical cyclones contrib-ute around 30% of the annual rainfall in Hong Kong. In summers with prolonged dry and hot spells, the rainfall associated with them alleviates the drought and heat. The heavy downpour of Typhoon Viola in May 1964 terminated the most severe drought on record in Hong Kong in 1963.
Moreover, the high wind brought by tropical cyclones contributes to the wind energy potential. During 2007–2009, the wind energy potential induced by tropical cyclones was at least twice the long- term average for hilltops and offshore islands.
The study of local temperature during the passage of 21 tropical cyclones over the
0.17 degC per day during the corresponding 93 tropical cyclone days.
If the rainfall in its wake is also attributed to the tropical cyclone, the cooling effect may be re-computed, say for the tropical cyclone days from T1 − 2 to T2 + 3. For all 21 tropical cyclones together, there was then a cooling effect of 0.25 degC per day for 145 tropical cyclone days (Table 3).
According to recent studies (Fung, 2004; Lee et al., 2010), there is a significant correla-tion between the electricity consumption and temperature in Hong Kong. The elec-tricity consumption increases with the increase in the mean temperature during the warmest months, mainly due to the use of air conditioning. As such, the cooling effect of tropical cyclones may not only alle-viate the thermal stress in hot summer weather but also contribute to a reduction in electricity consumption. A rough esti-mate based on the correlations between HKOHq mean temperature and electricity consumption (see Figure 7) suggests that a reduction of 0.17 degC could reduce the electricity demand by about 553 134kWh per day in the domestic sector and 381 244kWh per day in the commercial sec-tor respectively (assuming an average pop-ulation of about seven million in Hong Kong
at HKO against the reference mean temperature during its passage. Using our definition, Fengshen affected Hong Kong for six days (from T1−2 to T2). As it approached Hong Kong from the south, the subsidence warming was not significant. Coupled with the fully fledged typhoon hit-ting the territory almost head-on, Fengshen caused a total net cooling of 6 degC in the daily mean temperature for the 6 days, i.e. an average of 1 degC per tropical cyclone day (Table 3). Figure 6(a) and (b) show the same details for Typhoon Hagupit in 2008: there was net warming during its passage. This was attributed to the subsidence warm-ing ahead of the storm and the fact that it passed at quite a distance (about 180km) from Hong Kong.
Of the 21 tropical cyclones affecting Hong Kong from 2007 to 2009, 13 contributed net cooling during their passage. The remaining eight resulted in a net warming due to vari-ous factors such as (a) inducing strong sub-sidence warming before their passage and (b) producing less rain (or rainfall of shorter duration) as a result of passing relatively far from Hong Kong, tracking past Hong Kong to the northeast or having a compact circulation. On average, the 21 tropical cyclones resulted in a net cooling of
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AcknowledgementsThe authors thank colleagues of the Hong Kong Observatory, Dr B Y Lee, Mr H Y Mok and Dr T C Lee for their useful comments on the manuscript.
Figure 6. (a) Track and (b) time series of daily mean temperature at HKO (°C) for Typhoon Hagupit.
(a) (b)
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Hagupit
Hagupit
13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
9 9 9 9 9 9 9 9 9 9 9 9 9 9 9
2008 2008 2008 2008 2008 2008 2008 2008 2008 2008 2008 2008 2008 2008 2008
daily mean temp
5day mean temp (2000-2009)
T1 T2
T1-1
T1-2
Figure 7. Monthly electricity consumption per capita in April to November against monthly mean temperature in (a) domestic and (b) commercial sectors in Hong Kong.
y = 5.02x107 x - 8.33x108
R2 = 0.77
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northern part of the South China Sea from 2007 to 2009 reveals that the weather asso-ciated with them brought a net cooling effect of about 0.17 degC to 0.25 degC per day during their passage. Tropical cyclones also reduce the demand for electricity.
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© Royal Meteorological Society, 2012
DOI: 10.1002/wea.836
Cleaner air brings better views, more sunshine and warmer
summer days in the NetherlandsAldert J. van Beelen and Aarnout J. van DeldenInstitute for Marine and Atmospheric Research Utrecht, Utrecht University, The Netherlands
IntroductionIn the late 1980s Atsumu Ohmura and sev-eral others (e.g. Ohmura and Lang, 1989) discovered that the amount of solar radia-tion reaching the surface had decreased at many radiation measurement sites between 1960 and 1990. Near densely-populated areas and industries, 30% less solar radiation was reaching the ground in the 1980s than a few decades earlier (Wild, 2009). However, since the mid 1980s a significant increase in visibility has been noted in western Europe (e.g. Doyle and Dorling, 2002), and there are strong indications that a reduction in aero-sol load from anthropogenic emissions (in other words, air pollution) has been the dominant contributor to this effect, which is also referred to as ‘brightening’. In the Netherlands visibility, sunshine duration, surface global short-wave radiation and temperature have shown a significant rise during this period, consistent with direct and indirect aerosol effects, implying large regional aerosol effects on climate. The
brightening has been stronger during con-tinental windflow than during maritime epi-sodes. This article discusses the evidence for brightening in the Netherlands and its pos-sible connection to the accelerated warm-ing since 1985.
Visibility, aerosols and climate The World Meteorological Organization (WMO) gives the following definition of visibility:
Visibility is defined as the greatest distance at which a black object of suitable dimen-sions (located on the ground) can be seen and recognized when observed against the horizon sky during daylight or could be seen and recognized during the night if the general illumination were raised to the normal daylight level.
In the absence of rain or snow, visibility is largely determined by the aerosol concen-tration and humidity near the surface. Aerosols are small suspended particles from both natural (e.g. sea salt) and anthropo-genic (burning of fossil fuel) sources, and their presence always causes a reduction in visibility due to the scattering of light.
When looking at a distant target, its appearance is altered in such a way that the contrast between the target and the back-
ground atmosphere decreases with increas-ing distance. The distance at which the contrast drops below the contrast threshold of the human eye, and the target becomes barely visible, is defined as the visibility (Horvath, 1981). The visibility of an object varies from observer to observer and with ambient conditions. However, a professional observer can estimate a ‘standard’ visibility with reasonable accuracy. Recently, visibility has also been determined less subjectively by transmissometers or scatterometers.
The ability of aerosols to scatter light and alter the visibility is strongly dependent on the ambient relative humidity. Aerosols are normally hygroscopic and they take up water when relative humidity is high, increasing their diameters in the process by as much as a factor of four. For example, light scattering per unit mass for an ammo-nium sulphate aerosol remains fairly con-stant up a relative humidity of 80%, at which point the particle grows rapidly with an attendant sharp rise in light scattering with increasing relative humidity (Cass, 1979). At relative humidities higher than about 80% visibility is decreased to what is often called a ‘haze’. Relative humidity near 100% will lead to condensation of water vapour, reducing visibility even further and often leading to fog. Precipitation will also strongly reduce visibility. Therefore, visibility
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