Jin-hai Zheng*,Sang Sang,Jin-cheng Wang,Chun-yan ZhouHong-jun Zhaoa,

aKey Laboratory of Coastal Disaster and Defence(Hohai University),Ministry of Education,Nanjing 210098,China

bCollege of Harbor,Coastal and Offshore Engineering,Hohai University,Nanjing 210098,China

Numerical simulation of typhoon-induced storm surge along Jiangsu coast, Part I:Analysis of tropical cyclone

Jin-hai Zhenga,b,*,Sang Sangb,Jin-cheng Wangb,Chun-yan Zhoua,b,Hong-jun Zhaoa,b

aKey Laboratory of Coastal Disaster and Defence(Hohai University),Ministry of Education,Nanjing 210098,China

bCollege of Harbor,Coastal and Offshore Engineering,Hohai University,Nanjing 210098,China

The Tropical Cyclone Best Track Dataset issued by the Shanghai Typhoon Institute of the China Meteorological Administration,for the period from 1949 to 2013,was analyzed,and the typhoons threatening the Jiangsu coast were categorized into three different classes in terms of track,including typhoons making straight landfall,typhoons active in offshore areas,and typhoons moving northward after landfall.On the basis of the 65-year dataset,the typhoon parameters of these three categories,including the central pressure and the maximum wind speed,were investigated.Statistical analysis suggested that the minimum central pressure increased northward and shoreward gradually.The relationship between the maximum wind speed and the minimum central pressure was established through second-order polynomial fitting.Considering typhoons No.1210,No.0012,and No.9711 as the basic typhoons,ten hypothetical cyclones with typical tracks and minimum central pressure occurring during the period from 1949 to 2013 were designed,providing the driving conditions for numerical simulation of typhoon-induced storm surges along the Jiangsu coast.

Jiangsu coast;Hypothetical tropical cyclone;Central pressure;Maximum wind speed;Typhoon track

1.Introduction

The Jiangsu coast,located on the western side of the South Yellow Sea,is affected by the unique convergence and divergence of the South Yellow Sea amphidromic wave system and the East China Sea forward tidal wave system(Chen et al., 2009;Zhang et al.,2013;Zhou et al.,2015).This stretch of coast includes the abandoned Yellow River delta in the north and the radial sand ridges in the south.Typhoon-induced storm surges are one of the severest types of disasters off the Jiangsu coast,and typhoons destroy offshore facilities and coastal structures as well as ships(Feng et al.,2012).When the wind speed is above 10 on the Beaufort scale,a storm surge that overlaps with a high astronomical tide is de fined as a superlarge storm tide,and the causes for the super-large storm tides along the Jiangsu coast have been analyzed(Chen,1991). Lu et al.(2002)analyzed the main patterns of storm surge disasters along the Jiangsu coast and their in fluence on the coastal economy.Yu et al.(2009)investigated the assessment method of direct and indirect economic losses caused by storm surges along the coast of Yancheng in Jiangsu Province and provided the storm surge bearing loss rate.According to the wind and rain intensities of typhoons affecting Jiangsu Province,the in fluence is classi fied into four levels:lightest influence,heavier in fluence,severe in fluence,and severest in fluence levels(Gan,2006).In terms of track,the typhoons are classi fied into five categories,which are typhoons moving northward after landfall,typhoons vanishing after landfall, typhoons making straight landfall,typhoons active in offshoreareas,and typhoons making landfall near the South China Sea and then exiting along the Jiangsu coast(Xiang et al.,2008), of which the first category is further sub-divided into three types.This classification system is over-detailed and unfavorable to the study of typhoon-induced storm surges.At present,the wind and rain intensities in various parts of Jiangsu Province are generally used to determine whether typhoons affect Jiangsu Province or not,leading to more accurate results.However,it is very difficult to acquire the ground wind and rainfall data covering a large area over a long period.

In this study,a method for defining the affected domain was used to conduct statistical analysis of the typhoon process affecting Jiangsu Province.According to the typhoon track, the typhoons affecting Jiangsu Province were classified into three categories.Then,statistical analysis of typhoon parameters was carried out to establish a relationship between the minimum central pressure and the maximum wind speed. Finally,ten hypothetical typhoons along the Jiangsu coast were designed on the basis of historical data during the period from 1949 to 2013.

2.Statistical characteristics

The typhoon information was derived from the Tropical Cyclone Best Track Dataset issued by the Shanghai Typhoon Institute of the China Meteorological Administration(Ying et al.,2014).The diameter of a typhoon,depending on its intensity,usually ranges from 600 km to 1000 km.Therefore, centered at the Jiangsu coast,the affected distance was set to be 450 km.Typhoons entering the domain with longitudes from 116°E to 126°6'E and latitudes from 28°N to 38°N were determined to be the typhoons influencing Jiangsu Province, and the domain is shown as the rectangular area in Fig.1.The FORTRAN program was used to read the historical typhoon information from the best track dataset.A typhoon that entered the selected domain and was stronger than a tropical storm (TS)according to the Chinese national standardIntensityCategory for Tropical Cyclones(GB/T 19201-2006)was defined as a typhoon process affecting Jiangsu Province.Of the 2157 typhoons in the western North Pacific Basin that occurred during the period from 1949 to 2013,202 typhoons affected Jiangsu Province,with an annual average number of 3.2.According to the typhoon track distribution,the typhoons influencing Jiangsu Province can be classified into three categories:typhoons making straight landfall,accounting for 3.96%;typhoons active in offshore areas,which do not make landfall,accounting for 14.36%;and typhoons moving northward after landfall,which make landfall along the Zhejiang and Fujian coasts and then move northward inland and affect Jiangsu Province,accounting for 77.72%.In addition to these three categories,there are some other types of typhoons (accounting for 3.96%)with irregular tracks,which are beyond the scope of the present study.

Fig.1.Tracks of typhoons influencing Jiangsu Province from 1949 to 2013.

3.Parameters of typhoons

Parameters of typhoons include the radius to the maximum wind speed(Rmax),the forward speed(Vt),the central pressure (pc),and the ambient pressure(pn).Based on statistical information from historical typhoons,parallel movement of the real typhoon track was used to obtain the typhoon track and forward speed.The pressure value at one place was taken as the minimum value of the central pressures of typhoons that have occurred over the 65-year period at that place.Then,the relationship between pressure and wind speed was established in order to obtain the wind speed of hypothetical typhoons.

3.1.Central pressure

It was found that in the minimum central pressure distribution there existed a latitude boundary of about 27°N corresponding to 920 hPa.The minimum central pressure was less than 920 hPa,down to 885 hPa to 896 hPa,in the area south of 27°N,while it was larger than 920 hPa and increased with latitude in the area north of 27°N(Duan et al.,2004).Jiangsu Province is located in the mid-latitude area and latitude has a significant influence on the pressure distribution of typhoons (Vickery et al.,2000;Willoughby et al.,2006).Therefore,in order to obtain the information for the central pressure of typhoons in Jiangsu Province,it is necessary to understand the distribution pattern of the minimum central pressure.

The rectangulardomain with longitudes from 115°E to 133°E and latitudes from 24°Nto 38°Nwasthe study area,covering the North Yellow Sea,the East China Sea,Jiangsu Province,Zhejiang Province,and Fujian Province.The best track dataset from 1949 to 2013 was analyzed,providing a totalof431 typhoonsthat entered the domain and had an intensity equal to or larger than a grade IV typhoon(TY)according to the Chinese national standardIntensity Category for Tropical Cyclones(GB/T 19201-2006).In order to investigate the distribution pattern of the minimum central pressure,the grid cell for the selected domain was 0.5°×0.5°,and a domain with latitudes from -0.5°Nto 0.5°Nand longitudesfrom-0.5°E to 0.5°Efrom each grid pointwas the value range ofthatgrid point.The lowestvalueof central pressure occurring within the value range of each grid pointwastaken asthe historicalminimum centralpressure ofthis grid point.The typhoon track was recorded once every 6 h.For some typhoons,the distance between two consecutive recording points was larger than 2°in latitude,which was larger than the value range of the domain with latitudes from-0.5°N to 0.5°N and longitudes from-0.5°E to 0.5°E.In order to obtain the lowest value of central pressure at each grid point,the recorded typhoon tracks were interpolated to obtain the typhoon track data every 2 h,and the distance between two consecutive recording points was less than 1°both in latitude and in longitude.

The distribution pattern of the minimum central pressure is shown in Fig.2.It can be seen that the minimum central pressure is large in the northwest and small in the southeast in general.Two obvious characteristics can be found.The first is a significant difference between the minimum central pressures of typhoons over ocean and land.The minimum central pressure of typhoons over oceans is relatively lower,down to 900 hPa,and it increases gradually landward.The lowest value of the minimum central pressure of typhoons over oceans within the entire selected domain is 899.98 hPa,occurring over the western part of the North Pacific Ocean,and the highest value is about 986 hPa,occurring over the northern end of the South Yellow Sea.The highest value of the minimum central pressure over land is about 1003.64 hPa,occurring over the northern end of Shandong Province with higher latitudes,and the lowest value is about 926 hPa,occurring over the Zhejiang coast.The minimum central pressure increases significantly along with the landfall.The other characteristic is that the minimum central pressure increases with the latitude.The minimum central pressure increases gradually from the East China Sea to the Yellow Sea.The minimum central pressure is different on either side of the red line(as shown in Fig.2),where the Yangtze Estuary is located.The minimum central pressure in the domain area south of the Yangtze Estuary can be as low as 900 hPa.The isobars show a relatively regular distribution pattern that varies with the latitude and in the direction from sea to land.The minimum central pressure is higher than 940 hPa in the domain area north of the Yangtze Estuary,and there is no obvious pattern on the isobars,whose trend is irregular.

Fig.2.Distribution of minimum central pressure of typhoon(units: hPa).

3.2.Wind speed

Using the same method of conducting statistical analysis of the central pressure,the maximum wind speed over the 65 years in each grid cell in the study area was computed and the results are shown in Fig.3.The distribution of the maximum wind speed is contrary to the minimum central pressure.The maximum wind speed decreases as the latitude increases,and decreases gradually shoreward.The contrary distributions for the minimum central pressure and the maximum wind speed indicate a close relationship between the two parameters.

The relationship between the minimum central pressure and the maximum wind speed at each record point is shown in Fig.4,which suggests that there is a significant correlation between the two parameters,basically within an arc belt.

A second-order polynomial is used for fitting these two parameters,and the fitting equation is

whereWis the maximum wind speed,andpis the minimum central pressure.The determination coef ficient(R2)of the fitting polynomial above is 0.87,indicating that the polynomial fits the data well.There are two envelopes for the arc belt.The upper one is as follows:

When choosing the final relationship between speed and pressure,according to the principle that it should be larger than the standard value,as proposed by Gao et al.(2009),the value at the midpoint between the upper envelope and the fitting polynomial,which is demonstrated by the larger fitting curve shown in Fig.4,is used for the computation of the hypothetical typhoon in section 4.The formula for this larger fitting curve is written as follows:

3.3.Typical tracks

Fig.3.Distribution of maximum wind speed of typhoon(units:m/s).

According to the historical surge at the coastal tide gauges and direct economic loss of Jiangsu Province induced bytyphoons,the typhoons causing significant direct economic loss were No.1210(a typhoon making straight landfall); No.0012 and No.1109(typhoons active in offshore areas); and No.9711,No.9216,and No.0509(typhoons moving northward after landfall).Ranked by the storm surge caused by typhoonsofeach category,one typhoon was chosen to represent each category above,corresponding to No.1210,No.0012, and No.9711,respectively(Fig.5).

Fig.4.Relationship between minimum central pressure and maximum wind speed.

4.Hypothetical typhoons

The track ofthe representative typhoon foreach category was chosen and then moved parallelto coverallthe typhoon tracksof each type.Combined with the distribution of the minimum central pressure during the 65 years outlined in section 3.1,the minimum centralpressure ateach place wasdetermined foreach typhoon type.Finally,the maximum wind speed of a hypothetical typhoon was obtained according to Eq.(3).

This statistical approach has been applied in typhoon analysis including random generation of typhoons or storms (Emanuel et al.,2006;Lin et al.,2010;Kim and Suh,2016). The parallel moving method used in this study was simple and explicit for the three types of tropical cyclones.The track of typhoon No.1210 was moved parallel southward by 1°and 2°. Adding the original track of No.1210,there were three hypothetical paths(Fig.6).The cross in Fig.6 represents the information recording point of the typhoon.The pressure intensity for the recording point was designed as the minimum central pressure for the 65 years and the corresponding maximum wind speed was calculated according to Eq.(3).We thus obtained three hypothetical typhoons making straight landfall,whose minimum central pressure and maximum wind speed variations with time are shown in Fig.7.

Fig.5.Typhoon tracks along Jiangsu coast.

Fig.6.Paths of hypothetical typhoons making straight landfall.

Fig.7.Variations of minimum central pressure and maximum wind speed for typhoons making straight landfall.

Fig.8.Paths of hypothetical typhoons active in offshore area.

Asimilarmethod to thatused fortyphoon No.1210 was used for typhoons No.0012 and No.9711,and the corresponding hypothetical tracks for typhoons active in offshore areas(B1, B2,and B3)and typhoons moving northward after landfall(C1, C2,C3,and C4)are shown in Fig.8 and Fig.9,respectively.Path B1 in Fig.8 is the path of typhoon No.0012,butwith the lowest central pressure that has occurred in the 65 years and the corresponding maximum wind speed calculated by Eq.(3).Paths B2 and B3 are the paths shifted from path B1,and the westernmost path B3 is parallel to the Jiangsu coast.Path C1 has the same track as typhoon No.9711,but with the lowest central pressure that has occurred over the 65 years examined and the corresponding maximum wind speed calculated by Eq.(3).Path C1 wasmoved paralleleastward by 1.4°(path C2)and 2.8°(path C3),respectively.The north part of path C3 was then moved paralleleastward by 1.4°(C4).Path C2 passed through Haizhou Bay,path C3 entered the sea along the straight Jiangsu coast between Haizhou Bay and the radialsand ridge area,and path C4 entered the sea near the Yangtze Estuary.

Fig.9.Paths of hypothetical typhoons moving northward after landfall.

There were three hypothetical paths corresponding to typhoons making straight landfall(A1,A2,and A3),three corresponding to typhoons active in offshore areas(B1,B2,and B3),and four corresponding to typhoons moving northward after landfall(C1,C2,C3,and C4).That is ten hypothetical paths in total.Each hypothetical path with the minimum central pressure for the 65 years and the corresponding maximum wind speed drove the numerical model in Part II in order to study the typhoon-induced storm surge along the Jiangsu coast.

5.Conclusions

On the basis ofthe Tropical Cyclone Best Track Dataset from the Shanghai Typhoon Institute of the China Meteorological Administration for the period from 1949 to 2013,the typhoons affecting Jiangsu Province were classified into three categories according to the track:typhoons making straight landfall,typhoons active in offshore areas,and typhoons moving northward after landfall.Statistical analysis indicates that the minimum central pressure increased gradually shoreward and northward. The relationship between the minimum central pressure and the maximum wind speed was established by second-order polynomial fitting.The patterns of typhoon parameters were investigated based on statistical analysis of the minimum central pressure,the maximum wind speed,and the typhoon track.Ten hypothetical tropical cyclones along the Jiangsu coast were designed based on the historical information from the 65 years examined,providing the driving conditions for numerical simulation of typhoon-induced storm surges along the Jiangsu coast.

Acknowledgements

The authors are grateful to the Shanghai Typhoon Institute of the China Meteorological Administration for providing the typhoon data and information.

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Received 22 June 2016;accepted 15 December 2016

Available online 9 March 2017

This work was supported by the National Science Fund for Distinguished Young Scholars(Grant No.51425901)and the National Natural Science Foundation of China(Grant No.41606042).

*Corresponding author.

E-mail address:jhzheng@hhu.edu.cn(Jin-hai Zheng).

Peer review under responsibility of Hohai University.

http://dx.doi.org/10.1016/j.wse.2017.03.004

1674-2370/?2017 Hohai University.Production and hosting by Elsevier B.V.This is an open access article under the CC BY-NC-ND license(http:// creativecommons.org/licenses/by-nc-nd/4.0/).

?2017 Hohai University.Production and hosting by Elsevier B.V.This is an open access article under the CC BY-NC-ND license(http:// creativecommons.org/licenses/by-nc-nd/4.0/).