Occurrence And Significance Of Thunderstorms In Hong Kong .
Reprint 726Occurrence and Significance of Thunderstorms in Hong KongE.C.C. Choi*, H.Y. Mok & T.C. Lee12th International Conference on Wind Engineering,Cairns, Australia, 1-6 July 2007* Department of Building and Construction, City University of Hong Kong
Occurrence and significance of thunderstorms in Hong KongChoi E.C.C.a,aMok H. Y. b. &Lee T. C. bDepartment of Building and Construction, City University of Hong Kong, Hong KongbHong Kong Observatory, Hong KongABSTRACT: Hong Kong is situated on the south China coast and is located right in thewestern Pacific typhoon belt. It is frequently subjected to the devastating effect of cyclonicwind. To address this issue, the Hong Kong wind code is written to cater for the tropicalcyclone wind. Buildings and structures in Hong Kong are well designed for loadings inducedby such winds. There is also the tropical cyclone warning issued by the Hong KongObservatory prior to the arrival of a tropical cyclone and in general people are familiar with thedamaging effects of tropical cyclones and due precautions are taken. Thunderstorm is alsocommon in Hong Kong. In the summer months there are on the average more than fivethunderstorm days. The gust during thunderstorms can be very strong. Over the yearsdamages to property and life caused by thunderstorm wind have been reported. Thunderstormwarning is also issued by the Hong Kong Observatory; however members of public in general donot often pay attention to the potential hazards of gusts and squalls associated withthunderstorms. The objective of this study is to investigate the significance of thunderstormwinds in terms of its occurrence and magnitude. Result of the study indicates that thunderstormplays an important role in the wind climate of Hong Kong. The ranking and number ofthunderstorm gusts in the higher gusts in a year are significant. The geographic influence onthunderstorm wind is also studied.KEYWORDS: thunderstorm, probability distribution, wind type, squall line, wind speed,thunderstorm-occurrence.1. INTRODUCTIONSituated on the southern edge of the Asiatic continent, Hong Kong's climate is sub-tropical. Itis subjected to two monsoon seasons, the northeast in winter and the southwest in summer.Severe weather phenomena that can affect Hong Kong include tropical cyclones, strong winterand summer monsoon winds as well as thunderstorms with associated squalls that are mostfrequent from April to September. On average, Hong Kong is affected by about 6 to 7 tropicalcyclones per year. July, August and September are the months during which Hong Kong ismost likely to be affected by tropical cyclones. Monsoon winds in Hong Kong can occasionallyreach gales, especially over offshore and on high ground. Besides the monsoon and the tropicalcyclone winds, Hong Kong is also subjected to the squalls of thunderstorms which occur moreoften in the spring and summer months. Figure 1 shows a histogram of the mean number ofthunderstorm days as observed in Hong Kong in different months (from 1961 to 1990). Onaverage, the number of thunderstorm days during the summer months from May to August is inexcess of five.As such, it is evident that Hong Kong is having a mixed weather climate and is affectedby three distinct types of wind systems. Previous studies using wind data up to mid-1980s
suggested that the damaging winds and gusts in Hong Kong are mainly associated with tropicalcyclones and intense monsoons. Thunderstorm winds which are more transient and localized innature are considered to be statistically less significant and neglected [Tam, 1987; Koo, 1988].However in recent years, damages due to thunderstorm wind have been reported. With theestablishment and continuous expansion of the automatic weather station (AWS) network inHong Kong since mid-1980s by the Hong Kong Observatory, more high frequency wind data(1-minute resolution) have been collected at different places in Hong Kong in the past twodecades. These data are useful for monitoring wind conditions in different parts of the territoryin thunderstorm situations.This paper attempts to study the significance of thunderstorm winds in Hong Kongusing the gust data of five AWSs in the last 20 years or so. Results suggest that significantpercentages of the highest gust records of these stations are due to the fierce gusts associatedwith thunderstorms.2 THUNDERSTORMS AND SQUALL LINESThunderstorms are common weather phenomena around the world. A thunderstorm usuallylasts no more than a few hours. During the mature stage of the thunderstorm, there are gustywinds and heavy precipitation. The gust front is the leading edge of the cold air resulting fromthe downdraft and is characterized by an abrupt change of temperature, wind speed, and anincrease in pressure. In severe thunderstorms, the strong downdraft develops into a downburst,which diverges horizontally as an outburst of destructive winds upon reaching the ground[Glickman, 2000].Unlike the monsoon and tropical cyclone winds which move in the horizontal directionover a substantial stretch of ground, winds from thunderstorms rush vertically down and spreadoutwards (Figure 2). Thus, the ground roughness has little effect on the wind. Thecharacteristics of thunderstorm winds, e.g. the mean wind profile, gust profile, gust factor andturbulence intensity, are expected to be quite different from those related to monsoons andtropical cyclones.A squall line is a cluster of severe thunderstorms or storm cells along a line. On theradar display, a squall line, which spans tens to hundreds of kilometers in length, manifests itselfas a narrow band of intense rain area with a width of about ten to a few tens kilometers.Sometimes it curves like a bow, know as “Bow Echo” [Fujita, 1978; Johns & Hirt, 1987;Staudenmaier & Cunningham, 1996]. Its passage is also marked by rapid increase in pressureand sudden drop in temperature which are usually much sharper than that of ordinary single cellthunderstorms. These surface features referred to the mesohigh and intense downdraft thattypically accompany such a system [Schmidt & Cotton, 1989; Haertel & Johnson, 2000].3. DATA AND ANALYSISTo investigate the significance of thunderstorm winds in Hong Kong, a study of the wind speeddata from five AWSs in Hong Kong, namely Waglan Island (WGL), Cheung Chau (CCH), KaiTak (KT), Star Ferry (SF) and Lau Fau Shan (LFS), is conducted.
Figure 1Mean number of thunderstormdays in Hong KongFigure 2Thunderstorm cell showing downdraftgust frontFigure 3 shows the locations of these stations in Hong Kong. WGL and CCH areislands over offshore areas, KT and SF are urban stations inside the harbour areas and LFS is arepresentative station in the northwestern part of the New Territories. The hourly maximumgusts (which provides the longest available data) recorded at these stations were used for theanalysis. Table 1 summarizes the respective periods of available data of these stations.Table 1Summary of data available periodStationData PeriodWaglan Island1968 to 2005Cheung Chau1993 to 2005Kai Tak1968 to 2005Star Ferry (Tsim Sha Tsui)1988 to 2005Lau Fau Shan1986 to 2005 114 0' E 114 10' E 114 20' E 114 30' E22 30' N 22 30' N Lau Fau ShanKai Tak 22 20' NStar Ferry (Tsim Sha Tsui)22 20' N 0246810 公里 km比例 Scale Cheung Chau 22 10' N114 0' E Figure 3 Waglan Island114 10' E 114 20' E 114 30' E 22 10' N Locations of the automatic weather stations used for this study
In this study, the highest 100 gust records in a year for each station are identifiedand ranked. Each gust event is checked against whether it was owing to thunderstorms, tropicalcyclones or monsoons. Thunderstorm events occurring during tropical cyclone or monsoonevents are not counted. Table 2 gives a sample result of the highest 10 gusts for 2005 at LFS.As can be seen from the table, 5 out of the 10 highest gust records are coming underthunderstorm events.Table 0520052005Highest 10 gust events in 2005 for 2111191116gust erstorm gustYesYesYesYesYesFigures 4a and 4b show the probability of any one year having certain number ofthunderstorm gusts using the list of top 100 for LFS and WGL respectively. At LFS theprobability of any one year having 35 to 40 thunderstorm gusts showing up on the list of top 100is about 5%, and is about 30% for having 25 to 30 thunderstorm gusts. For WGL, the picture isdifferent. There are more chances of having less number of thunderstorm gusts making to thetop 100, e.g. having less than 5 is about 26% and 10 to 15 is about 29%. On average, thethunderstorm gusts contribute about 18 % of the list of top 100 gusts every year at LFS, thehighest percentage among the five stations.0.50.4(a) Lau Fau Shan0.30.20.10Yearly probabilityYearly probability0.50.4(b) Waglan Island0.30.20.100-55-1010-15 15-20 20-25 25-30 30-35 35-40 40-45 45-50No. of thunderstorm gust records in the list of thehighest 100 gusts0-55-1010-15 15-20 20-25 25-30 30-35 35-40 40-45 45-50No. of thunderstorm gust records in the list of thehighest 100 gustsFigures 4a and 4b Yearly probability of any one year having different number of thunderstorm gust records in thelist of highest 100 gusts for LFS and WGL.We can also have an idea of how significant is the thunderstorm gust in a year bylooking at the highest rank of the thunderstorm gust in the list of the highest 100 gust records ofthat year. The highest thunderstorm gusts of 5 years out of 20 years at LFS rank the highestgust of the year whereas the corresponding number for WGL is 4 years out of 38 years.
Figures 5a and 5b show the probability of any one year having the highest thunderstorm gustwith a particular ranking in the list of top 100 for LFS and WGL respectively. From the figureswe can see that at LFS there is about 70% chance for any one year to have the highestthunderstorm gust to rank 1 to 10 while the corresponding chance for WGL is only 29%. Otherthree stations also have percentages less than that of LFS (figures not shown here).0.80.80.7(a) Lau Fau Shan0.6Yearly probabilityYearly probability0.70.50.40.30.20.1(b) Waglan 4150th5160th6170th7180th8191- 100th90th 100th1-10thRanking of the highest thunderstorm gust recordin the list of highest 100 gusts in a yearFigures 5a and - 100th90th 100thRanking of the highest thunderstorm gust recordin the list of highest 100 gusts in a yearYearly probability for the highest rank of thunderstorm gust with a particular ranking in thelist of top 100 gusts in a year for LFS and WGL.THUNDERSTORM GUST AT LAU FAU SHANOwing to its geographical position, LFS is prone to the attack of squall lines moving across thePearl River Estuary which are described by commoners’ as "Shi Hu Feng", or "Wind of theStony Lake". Accompanying a southward moving cold front or trough of low pressure, thistype of squall line usually affects southern China and its coastal area in late spring and earlysummer.Annual Maximum Gust (m/s) underThunderstorm Situations359 May 20019 May 2005YearFigure 6Annual maximum gust recorded under thunderstorm situations at LFS from 1996 to 2005Figure 6 shows the annual maximum gust associated with thunderstorms in LFS from1996 to 2005. The highest and the second highest thunderstorm gusts during this periodoccurred on 9 May 2001 and on 9 May 2005 respectively. Both of these events are due to thepassage of a severe squall line moving from the northwest to southeast across the Pearl RiverEstuary and the maximum gusts recorded were much larger than 100km/hr. In these two cases,
the damaging winds of the squall lines resulted in collapsed containers and scaffoldings as wellas numerous fallen trees.4.1 Squall line event on 9 May 2005On the morning of 9 May 2005, a squall line spawned from a trough of low pressure oversouthern China and edged towards the south China coast[9]. The Doppler Weather Radar at TaiMo Shan of the Hong Kong Observatory detected intense echoes of the impeding rainbandevolving to a bow shape squall line when it reached the northwest corner of Hong Kong nearLFS at around 12:12 HKT. While keeping its strength, the squall line raced east-southeast at aspeed of about 80 km/hr (22.2m/s) towards the urban area and reached the Kowloon peninsulaabout 25 minutes later (Figure 7).Figure 7Radar reflectivity image of TMS radar at 1230 HKT on 9 May 20053526100929.7 m/sPressureTemperature251 -m in P re s s u re (h P a )1 m inu te m axim um gu st (m 3013:00100411:3013:30Time (HKT)Figure 812:0012:3013:002013:30Time (HKT)1-minute maximum gustFigure 91-minute pressure and temperaturerecorded at Lau Fu Shan from 11:30 to 13:30 HKT on 9 May 2005.1 -m in T e m p e ra tu re (o C )30
The time series of 1-minute maximum gust, temperature and pressure at LFS duringthe passage of squall line are shown in Figures 8 and 9. The squalls of the system triggered animpulse of maximum gust which increased from about 10 m/s to a peak value of 29.7 m/s withina few minutes. At the same time, sharp and substantial pressure rise of up to a rate of 3 hPa per15-minute and temperature drop of over 3 oC within 15 minutes were observed. Analysis of thesurface pressure recorded at 12:30HKT by the automatic weather station (AWS) network of theHong Kong Observatory also depicted a well defined mesohigh system right behind the gustfront, a typical feature of a squall line (Figure 10).Figure 105.Surface pressure field analysis over Hong Kong at 12:30 HKT on 9 May 2005 (isobars are in 0.5 hPa interval).EXTREME THUNDERSTORM GUSTWe have seen from the previous section that the occurrence as well as the ranking of the gustsdue to thunderstorm are significant. To further look at the problem, the magnitudes of thethunderstorm gusts are investigated. Extreme wind study is carried out for gusts at LFS. Asmentioned earlier Hong Kong is under a mixed weather system (Choi 2002), individual weathersystem is investigated separately. Gust from thunderstorm and peak gust of the year regardlessof its origin (coming from ALL weather systems: thunderstorm, tropical cyclone and monsoon)are studied. Figure 11 shows the two lines, one for ALL and the other for thunderstorms. It isevident that gusts for ALL are of higher magnitude than those for TS which is to be expected.However the important thing is the magnitude of the thunderstorm gusts. They are not small.The extreme gust magnitude for return periods of 10, 20 and 50 years obtained from the fittedcurve in Figure 11 are given in Table 3. The gusts for TS are all higher than 30m/s which aresignificant for wind loading especially for temporary structures.6.CONCLUSIONThe frequency of occurrence and intensity of thunderstorm gusts in Hong Kong are significant ascompared with those associated with tropical cyclones and monsoons which also often affect theterritory. The thunderstorm gusts often ranked high in the list of the top hundred annual highestgusts and out of the hundred peak gusts, the number of gusts due to thunderstorm event was quitesubstantial. Preliminary result also shows that extreme thunderstorm gust speeds are relativelyhigh. Furthermore, owing to higher exposure to squall lines moving from northwest tosoutheast across the Pearl River Estuary, the thunderstorm gusts are more significant at the
northwestern part of the territory than those at the southeastern part. Besides resulting inrapid increase in wind speed, these squall lines are also characterized by sharp and substantialpressure rise and temperature fall during their passage.Table 3Returnperiod (yr)102050Extreme gusts at LFSALL(m/s)41.245.751.6TS(m/s)31.534.337.960Gust speed (m/s)5040A ll Data30A ll FittedTS Data20TS Fitted100-4-20246Reduced variateFigure 115.Extreme gust distribution at LFSREFERENCES1. Koo E., “Some characteristics of winds related to the building code in Hong Kong” Seminar on wind engineeringorganized by the Hong Kong Institution of Engineers, Hong Kong, 28 January 19882. Tam C.M., “Wind regime in Hong Kong” The CICIND (Comite International des Cheminees Industrielles)Meeting, Hong Kong, 8 October 19873. Glickman T.S., “Glossary of meteorology” American Meteorological Society 20004. Theodore Fujita, “Manual of downburst identification for project NIMROD” Satellite and Mesometeorology Res.Pap. No. 156, University of Chicago, Dept. of Geophysical Sciences, pp. 104, 1978.5. Johns, R.H. and Hirt, W.D., “Derechos : Widespread Convectively Induced Windstorms” Weather andForecasting, Vol.2, p.32-49, 1987.6. Staudenmaier, M.J. and S. Cunningham, “An examination of a dynamic cold season bow echo in California, NWSWestern Region Technical Attachment” 96-10, 1996.7. Schmidt J.M. and Cotton W.R., “A high plains squall line associated with severe surface winds” Journal of theAtmospheric Sciences, Vol. 46, p281-302, 19898. Haertel P.T. and Johnson R.H., “The linear dynamics of squall line mesohighs and wake lows” Journal of theAtmospheric Sciences, Vol. 57, p93-107, 20009. Hong Kong Observatory, “Monthly Weather Summary of May 2005” 2005.10 CHOI E.C.C., Tanurdjaja A., (2002) "Extreme wind studies in Singapore. An area with mixed weather system"JWEIA, Netherlands, 90 (2002) pp1611-1630.
Occurrence and significance of thunderstorms in Hong Kong Choi E.C.C.a, Mok H. Y. b. & Lee T. C. b a Department of Building and Construction, City University of Hong Kong, Hong Kong bHong Kong Observatory, Hong Kong ABSTRACT: Hong Kong is situ
[The building block of thunderstorms is the thunderstorm cell. A thunderstorm can be made of one cell or multiple cells. A single-cell thunderstorm can be an ordinary cell or a supercell thunderstorm. Thunderstorms with more than one cell can be multi-cell clusters or multi-cell lines, which are also called squall lines.] Ordinary Cell As the .
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Turbulence: Pilot reports from aircraft encountering thunderstorms have noted up and down drafts exceeding 6000 feet per minute. Turbulence exceeding the performance capability of most aircraft can be found in and around thunderstorms. Wind
An ordinary cell thunderstorm consists of one cell with an updraft and downdraft and produce strong winds, rain, lightning, and even hail. Multicell cluster thunderstorms consist of several ordinary cell thunderstorms in the vi
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