Hindawi Publishing CorporationAdvances in MeteorologyVolume 2013 Article ID 650971 8 pageshttpdxdoiorg1011552013650971

Research ArticleDrag Coefficient during Strong Typhoons

Binglan Wang1 Lili Song1 and Wenchao Chen2

1 Public Meteorological Service Center China Meteorological Administration Beijing 100081 China2Guangdong Climate Centre Guangzhou Guangdong 510080 China

Correspondence should be addressed to Binglan Wang wangblcmagovcn

Received 7 March 2013 Revised 13 May 2013 Accepted 21 May 2013

Academic Editor Lian Xie

Copyright copy 2013 Binglan Wang et al This is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

Using data from wind towers during typhoons Hagupit and Nuri drag coefficient was estimated The relationship between dragcoefficient and atmospheric stability was examined The results indicate that the drag coefficient decreased when atmospherestability changed from weakly stable or unstable to neutral Relationship between drag coefficient and wind speed was alsoexamined and the results indicate that the relationships between drag coefficient and wind speed were similar to other researchersrsquoresult but the wind thresholds were different due to different observation sites Quantitative expressions between drag coefficientand wind speed were derived

1 Introduction

Drag coefficient an important factor to parameterize thewind stress on the sea surface is widely used in modelingboth atmospheric and oceanic dynamics remote sensing andother applications In order to simulate the exact sea surfacecondition accurate estimation of drag coefficient is urgentlyneeded Owing to the complicated water-air interaction onthe sea surface present parameterizations of the wind stressor equivalently the drag coefficient over the sea are far fromsatisfactory [1] To help resolve this dilemma there have beenmany attempts to estimate the drag coefficient Geernaert etal [2] reported on the behavior of the drag coefficient inrelation to sea state during moderate to high wind speedsbased on data collected over a water column depth of 15mRao [3] compared drag coefficient in two different sites usingMONTBLEX-90 data Grachev et al [4] derived a a newformulation for the neutral drag coefficient in the convectiveboundary layer based on the idea that free convection canbe considered as a particular case of forced convectionMahrt et al [5] examined the dependence of the surface dragcoefficient on stability wind speed mesoscale modulation ofthe turbulent flux using data sets collected over grasslandsparse grass heather and two forest sites

Drag coefficients under tropical cyclones have beenexamined Powell et al [6] presented that the drag coefficient

levels off and starts to decrease with a further increase in thewind speed at wind speeds increasing above hurricane valuesof about 33ms which is contrary to the behavior of the dragcoefficient parameterizations that are currently used in oceanand atmosphere applications Moon et al [7] found that thedrag coefficient increasedwithwind speed at lowerwinds butthe rate of its increase was significantly reduced at high windsand therewas a leveling off or even decrease in certain sectorsMakin [8] predicted the reduction of the drag coefficient forthe wind speed exceeding hurricane values of 30ndash40ms inagreement with field data according to a resistance law of thesea surface at hurricane winds

In order to determine drag coefficient on the sea surfaceand provide a reference for modeling of typhoon dragcoefficients estimated from the turbulence measurementsduring typhoons Hagupit and Nuri were analyzed The mainaim of the present paper is to estimate the relationshipbetween drag coefficient and wind speed during typhoonusing experimental data

2 Analytical Data Description andData Quality Control

21 Data Description Two typhoon cases were selectedSong et al [9] presented two criteria according to the eddystructure of typhoon and the Beaufort scale of strong typhoon

2 Advances in Meteorology

wind (i) The wind direction with wind speed exceeding 8thgrade Beaufort scale (10min mean wind speed of 8th gradeis 172ms) successively alters over 120∘ (ii) The variationof wind speed with time shows an ldquoMrdquo shape during thepassage of typhoons and if a low wind speed less than 11msoccurs between two peak values it can be judged as thetyphoon eye area The two criteria should be fulfilled at thesame time to prove whether the typhoon center has passedover the observation site According to the two criteria twocases were selected to analyze drag coefficient during strongtyphoons typhoon Nuri (observed from Sanjiao Island) andtyphoon Hagupit (observed from Zhizai Island) Both of thetwo selected typhoon observations captured the passage oftyphoonrsquos eye wall and eye and this representative obser-vation is called typical typhoon [10] The typhoon tracksand the positions of the observation sites Sanjiao Island(where typhoonNuri was observed) and Zhizai Island (wheretyphoon Hagupit was observed) are showed in Figure 1 andTable 1

Typhoon Nuri made landfall in the coastal region of theSai Kung Sea in Hong Kong at 0455 pm on 22nd Aug 2008(Beijing time hereinafter the same) The center of typhoonNuri passed the tower located in Sanjiao Island at 1850 onAugust 22 2008 and the shortest distance fromSanjiao Islandto typhoon center is about 32 km Data from Sanjiao Islandduring typhoon Nuri were collected Sanjiao Island is locatedon the seaThe distance between northeastern coast ofMacaoand the island is about 12 km (Figure 1) The surface wascovered by grass and bushes (Figure 2(a)) The wind towerwas installed on a 93-meter high hill and was equippedby ultrasonic anemometers at the height of 10m and 60mThe sonic data from 0000 am on August 22 to 2359 pm onAugust 23 2008 were chosen to calculate drag coefficient andother parameters Drag coefficient mean wind speed winddirection and turbulence statistics have been calculated forperiod of 10min and so there are 288 samples of typhoonNuri observations

Strong typhoon Hagupit made landfall in the coastalregion of Chencun Town Dianbai County MaomingGuangdong Province at 0645 am on September 24 2008The center of typhoon Hagupit passed the tower locatedin Zhizai Island at 0510 on September 24 2008 and theshortest distance from Zhizai Island to typhoon center isabout 85 km In this paper data from Zhizai Island dur-ing typhoon Hagupit were collected The wind tower wasinstalled on a 10-meter high hill on Zhizai Island which islocated on the sea The shortest distance between the 100mhigh observation tower sites at Zhizai Island and the coast is45 km (Figure 1)The surface was covered by sand and sparseweed The terrain situation is illustrated in Figure 2(b) Anultrasonic anemometerwas placed at the height of 60mHighfrequency (10Hz) observations of turbulence and the virtualtemperature were obtained from the ultrasonic anemometerThe sonic data from 000 am to 2359 pm on September 242008 were chosen to calculate drag coefficient and otherparameters Turbulence statistics have been calculated forperiod of 10min and so there are 144 samples of typhoonNuri observation The wind data from the observation towerwere classified into ldquoonshore windrdquo and ldquooffshore windrdquo

and detailed information of the two typhoon observationsis showed in Table 1 Considering the complex surface ofalongshore direction along winds were not analyzed in thiswork

22 Data Quality Control Before analyzing all the data werepreprocessed as follows [11]

(1) Find the turbulence data spikes Referring to Hoslashjs-truprsquos [12] and Vickers andMahrtrsquos [13] methods datasatisfying the following formula can be considered asspikes

|119889119909 (119894)| ge 4120590 (1)

where 119909 is the series of 119906 V 119908 119889119909(119894) = 119909(119894 + 1) minus 119909(119894)and 120590 is the standard deviation of series 119909

(2) Remove and interpolate the spikes by Hoslashjstruprsquos [12]method

119909 (119894) = 119909 (119894 minus 1) 119877119898+ (1 minus 119877

119898)119883119898 (2)

where 119898 is a constant (119898 = 10 in this paper) 119877119898is

the correlation coefficient between the series 119909(119894minus119898 119894 minus 3) and 119909(119894 minus 119898 + 2 119894 minus 1) and 119883

119898is the mean

value of the series 119909(119894 minus 119898 119894 minus 1)

3 Analysis and Results

31 Wind Characteristics Using data from the ultrasonicanemometers on the wind tower located in Sanjiao Islandwind speed and direction during typhoon Nuri were calcu-lated and are showen in Figures 3(a) and 3(b) At 60m heightthe maximum wind speed was 327ms and the minimumwind velocity was 150ms The significant ldquoM-rdquo shapedbimodal distribution was also found The wind directionaltered more than 120∘ counterclockwise Compared with thetwo criteria typhoon Nuri is a typical typhoon

Figure 3(c) shows wind speed and direction duringtyphoon Hagupit It can be seen that the wind directionaltered 120∘ in the clockwise direction with the wind speedover 8th grade Beaufort scale The significant ldquoM-rdquo shapedbimodal distribution was found and the minimum valueof wind speed between two peaks was 119ms and themaximumvalue of thewhole typhoonwas 459ms All of thecharacteristics of the wind speed and direction well satisfiedthe two criteriamentioned above in Section 2 suggesting thattyphoon Hagupit is also a typical typhoon

32 Drag Coefficient during Strong Typhoons The drag coef-ficient is found directly by evaluating the ratio of the frictionvelocity and the mean wind speed difference squared asfollows

119862119889= (119906lowast

119880)2

(3)

Advances in Meteorology 3

Zhizai IslandSanjiao Island

Typhoon Nuri

Typhoon Hagupit

24-1200

23-0000

22-1800

22-1200

22-0600

22-0000

23-180024-0000

24-0600

23-1200

Sanjiao Island

12 km 32 km Zhizai Island

45 km

85 km

Figure 1 Typhoon tracks (yellow dotted lines) and positions of the observation sites Zhizai Island and Sanjiao Island The distance betweennortheastern coast of Macao and the Sanjiao Island is about 12 km and the shortest distance between the 100m high observation tower sitesat Zhizai Island and the coast is 45 km The center of typhoon Nuri passed the tower at 1850 on August 22 2008 and the shortest distancefrom Sanjiao Island to typhoon center is about 32 km The center of typhoon Hagupit passed the tower at 0510 on September 24 2008 andthe shortest distance from Zhizai Island to typhoon center is about 85 km

(a) (b)

Figure 2 Sites of wind tower (a) Sanjiao Island (where typhoon Nuri was observed) and (b) Zhizai Island (where typhoon Hagupit wasobserved)

4 Advances in Meteorology

0000 0600 1200 1800 0000 0600 1200 1800 00000

10

20

30

40

Time

0

90

180

270

360W

ind

spee

d (m

s)

Win

d di

rect

ion

(∘)

(a)

0000 0600 1200 1800 0000 0600 1200 1800 0000Time

0

10

20

30

40

Win

d sp

eed

(ms

)

0

90

180

270

360

Win

d di

rect

ion

(∘)

(b)

0000 0300 0600 0900 1200 1500 1800 2100 0000Time

Wind speedWind direction

8

18

28

38

48

Win

d sp

eed

(ms

)

0

90

180

270

360

Win

d di

rect

ion

(∘)

(c)

Figure 3 Wind speed and wind direction during strong typhoons (a) typhoon Nuri from 0000 am on August 22 to 2359 pm on August 232008 at 10m height (b) typhoon Nuri from 0000 am on August 22 to 2359 pm on August 23 2008 at 60m height and (c) typhoon Hagupitfrom 000 am to 2359 pm on September 24 2008 at 60m height

Table 1 Detailed information of the two typhoon observations

Observation site Type of wind Wind direction Elevation of the observation site Area of the observation site

Sanjiao Island (Nuri) Offshore wind 0ndash75∘ 270ndash360∘ 93m 062 km2

Onshore wind 75ndash225∘

Zhizai Island (Hagupit) Offshore wind 0ndash75∘ 225ndash360∘ 10m 00036 km2

Onshore wind 75ndash225∘

where 119906lowastrepresents the friction velocity calculated from the

flux data using the relation

119906lowast= (11990610158401199081015840

2

+ V101584011990810158402

)14

(4)

and 119880 is 10-minute average wind speedFigure 4 shows drag coefficient during typhoon Nuri In

general drag coefficients were different at different heightsbut the variation trends with time were similar Beforetyphoon center passed drag coefficients at 10m height weregreater than that of 60mheightWhen typhoon center passedthe wind tower increments of drag coefficient at the twoheights were observed due to the low wind speed in typhooneye region After typhoon center passed drag coefficients atthe two heights made little difference

Figure 5 shows drag coefficient during typhoon HagupitBefore the typhoon center passed there was a peak value(00050) of the drag coefficient at 410 An hour after the

typhoon center passed the greatest drag coefficient appearedwith a value of 00234 During the period from 410 to 700the drag coefficients changed dramatically which may beresulted from low wind speed in typhoon eye region

33 Variations of Drag Coefficient with Atmospheric StabilityThe use of sonic data made it possible to calculate atmo-spheric stability by the fowling formula

120589 =119911

119871= minus119896119911119892 (11990810158401205791015840V)

120579V1199063

lowast

(5)

where 120589 is atmospheric stability 119911 is height and 119871 is theMonin-Obukhov length The von Karman constant 119896 = 04and the Gravitational acceleration 119892 = 98 120579V is virtualpotential temperature 119908 is vertical wind speed and 119906

lowastis

friction velocityFigure 6 is the atmospheric stability during typhoons

Nuri and Hagupit In order to compare atmospheric stability

Advances in Meteorology 5

0000 0600 1200 1800 0000 0600 1200 1800 0000Time

Cd

10minus1

10minus2

10minus3

10minus4

10minus5

10 m60 m

Figure 4 Drag coefficients during typhoon Nuri from 0000 am onAugust 22 to 2359 pm on August 23 2008 at 10m and 60m height

Cd

10minus1

10minus2

10minus3

10minus4

10minus5

0000 0300 0600 0900 1200 1500 1800 2100 0000Time

Figure 5 Drag coefficients during typhoon Hagupit from 000 amto 2359 pm on September 24 2008 at 60m height

between different typhoons before and after typhoon centerpassed samples from 0700 am on August 22 to 0700 am onAugust 23 2008 during typhoon Nuri were selected Duringthe period the absolute values of atmospheric stability at10m height were close to zero indicating the atmospherestratification was nearly neutral At 60m height the atmo-spheric stability varied from minus2 to 2 indicating weak stableor weak unstable stratification Before typhoon eye passedthe difference of atmospheric stability between 10m and 60mheight it was small As winds were from the land beforetyphoon center passed the small difference between thetwo heights indicated that the influence of land surface onatmospheric stability was similar at different heights Aftertyphoon center passed atmospheric stability of most samplesat 10m height was greater than zero while at 60m heightwas smaller than zero After typhoon center passed windswere from the sea with the large difference of atmosphericstability indicating that the influences of the sea surface onatmospheric stability were different at different heights

Figure 7 shows variations of drag coefficient with atmo-spheric stability in onshore direction during typhoons Nuriand Hagupit During typhoon Nuri (Figure 7(a)) atmo-spheric conditions are different at different heights At 10mheight 120589 is greater than zero and drag coefficient reducesregularly as atmospheric stratification changes from neutralto weakly stable At 60mheight 120589 is lower than zero and drag

coefficient reduces regularly as atmospheric stratificationchanges from neutral to weakly unstable The curve whichcan describe the regular variation of drag coefficient wasderived from these observation data as follows

119862119889=

exp (086601205892 + 36106120589 minus 60184) exp (1305471205892 minus 158059120589 minus 59793)

(6)

During typhoon Hagupit variation of drag coefficientwith atmospheric stability is not so regular like typhoonNuriand no relationship was found between drag coefficient andatmospheric stability during typhoon Hagupit

34 Variation of Drag Coefficient withWind Speed in OnshoreDirection The data pairs (119862

119889 119880) in onshore direction are

graphically presented in Figure 8 In general variation trendsof drag coefficient withwind are similar for the two typhoonsIt can also be seen that variation trends of drag coefficientwith wind are accordant at different heights during typhoonNuri Drag coefficient increases until wind speed reaches acertain threshold and decreases when wind speed is greaterthan the threshold For different typhoons the thresholds aredifferent The threshold is about 15ms for typhoon Nuri25ms for typhoonHagupit and 40ms for other researchersrsquo[6]

During typhoonNuri drag coefficient assumes very largevalues in the onshore direction for the wind speed reachesabout 25ms at both 10m and 60m height The relationshipbetween drag coefficient andwind speed in onshore directioncan be described by the following formulae

119862119889=10minus3 (002841198802minus39000119880+142000) 119880 le 15ms104

times 84061119880minus55597

119880 gt 15ms(7)

During typhoon Hagupit drag coefficient ranges from00007 to 0003 Drag coefficient increases slowly until windspeed reaches about 25msWhen wind speed is greater than25ms there are only several samples nevertheless it couldbe seen that drag coefficient decreases regularly with windspeed A least squares analysis on the data pairs in onshorewind direction produced the following ldquobest fitrdquo regressionequation

119862119889= 10minus3 (000631198802 minus 01499119880 + 1500) 119880 le 25ms30541119880minus23837 119880 gt 25ms

(8)

Figure 9 shows the average drag coefficient of typhoonsHagupit andNuri in onshore directionHere drag coefficientsfrom different sites at the same height are compared In theonshore direction at 60m height drag coefficient duringtyphoons Hagupit and Nuri makes some difference for differ-ent speed bins In general when wind speed is greater than10ms and lower than 25ms drag coefficient of typhoonNuri is far greater than that of typhoon Hagupit When windspeed is greater than 25ms drag coefficient of typhoon Nurireaches the same order of magnitude as typhoon Hagupit

6 Advances in Meteorology

120589

0700 1300 1900 0100 0700Time

2

1

0

minus1

minus2

10 m60 m

(a)

0000 0600 1200 1800 0000Time

1

05

0

minus05

minus1

120589

(b)

Figure 6 Atmospheric stability during strong typhoons (a) typhoon Nuri from 0000 am on August 22 to 2359 pm on August 23 2008 at10m and 60m height and (b) typhoon Hagupit from 000 am to 2359 pm on September 24 2008 at 60m height

Cd

10minus1

10minus2

10minus3

10minus4

120589

020minus1 minus02minus04minus06minus08

10 m60 m

(a)

Cd

10minus2

10minus3

10minus4

120589

02 04 060minus04 minus02

(b)

Figure 7 Variations of drag coefficient with atmospheric stability in onshore direction during strong typhoons (a) typhoon Nuri at 10m and60m height and (b) typhoon Hagupit at 60m height

5 10 15 20 25 30 35Wind speed (ms)

Nuri 10 mNuri 60 m

Cd

10minus1

10minus2

10minus3

10minus4

(a)

10 15 20 25 30 35 40Wind speed (ms)

Cd

10minus2

10minus3

10minus4

Hagupit 60 m

(b)

Figure 8 Drag coefficient as a function of wind speed during strong typhoons in onshore direction (a) typhoon Nuri and (b) typhoonHagupit

Advances in Meteorology 7

0 10 20 30 40Wind speed (ms)

Nuri 10 mNuri 60 mHagupit 60 m

Cd

10minus2

10minus3

Figure 9 Average drag coefficient of typhoons Hagupit and Nuri inonshore direction and the wind speed bin is 1ms

By comparing drag coefficients at the two heights duringtyphoon Nuri it can be found that the relationships betweendrag coefficient and wind speed are similar at differentheights

Another fact which is worth noting is that wind thresh-olds for typhoon Nuri and typhoon Hagupit are differentfrom other researchersrsquo result [6] It is maybe resulted fromdifferent observation sites Typhoon Nuri was observed froma tower located on a 93-meter high hill while typhoonHagupit was observed from a tower located on a 10-meterhigh hill Furthermore Sanjiao Island where typhoon Nuriwas observed covers an area of 062 km2 while Zhizai Islandonly covers an area of 00036 km2 The average distance fromthe tower to the edge of Sanjiao Island is about 840m and toZhizai Island is about 45m in the onshore direction Windsfrom the onshore direction are mainly affected by the seasurface but are also influenced by the land where the toweris located These observation environments make the seasurface roughness length arisen from wave or other factorsfromopen sea surface produce lesser effect on drag coefficientduring typhoon Nuri That is to say relationships betweendrag coefficient and wind speed derived from typhoon Nuriand typhoon Hagupit are not from real open sea surfacePowell et alrsquos [6] experiment was conducted in open sea andwas not affected by land These different observation sitesmay be why the wind thresholds are different for differenttyphoons Consequently whether the observation data areaffected by land may be a key factor which determines thethreshold Considering other researchersrsquo experiments fromopen sea and these experiments from islands in this workit can be concluded that the larger area of the land whereobservation data are collected is responsible for the lowerthreshold of wind speed

35 Variation of Drag Coefficient with Wind Speed in OffshoreDirection Figure 10 shows the variation of drag coefficientwith wind speed during typhoons Nuri and Hagupit in off-shore direction It can be seen that drag coefficient fluctuatesobviously from 00001 to 0001 during typhoons Nuri andHagupit and no regular relationship is found between dragcoefficient and wind speed It may be resulted from thedistance from the observation tower to shore and complexland surface in offshore direction

4 Conclusion

Using data from wind towers during typhoons Hagupitand Nuri drag coefficient was estimated The relation-ship between drag coefficient and atmospheric stability wasexamined finding that the drag coefficient decreased whenatmosphere stability changed from weakly stable or unstableto neutral Relationship between drag coefficient and windspeed was also examined and the result indicated that therelationships between drag coefficient and wind speed weresimilar to other researcherrsquos result but the wind thresholdswere different due to different observation sites Some pre-liminary conclusions are obtained as follows(1) By comparing atmospheric stability at the two heights

during typhoon Nuri in onshore direction it is found thatatmospheric conditions are different at different heights At10m height 120589 gt 0 and drag coefficient reduces regularlyas atmospheric stratification changes from neutral to weaklystable At 60m height 120589 lt 0 and drag coefficient reducesregularly as atmospheric stratification changes from neutralto unstable The curve which can describe the regular varia-tion of drag coefficient is derived from these observation dataas follows

119862119889=

exp (086601205892 + 36106120589 minus 60184) exp (1305471205892 minus 158059120589 minus 59793)

(9)

(2) In onshore direction at 60m height drag coefficientduring typhoonsHagupit andNurimakes some difference fordifferent speed In general when wind speed is greater than10ms and lower than 25ms drag coefficient of typhoonNuri is far greater than that of typhoon Hagupit When windspeed is greater than 25ms drag coefficient of typhoon Nurireaches the same order of magnitude as typhoon HagupitBy comparing drag coefficients at the two heights duringtyphoon Nuri it can be found that the relationships betweendrag coefficient and wind speed are similar at differentheights(3) In onshore direction relationships between drag

coefficient and wind speed are derived from observation dataof different typhoon cases showing considerable differencebetween different typhoons

During typhoon Nuri the relationships between dragcoefficient and wind speed are similar at 10m and 60mheight

119862119889=10minus3 (002841198802minus39000119880+142000) 119880le15ms104 times 84061119880minus55597 119880gt15ms

(10)

8 Advances in Meteorology

5 15 25 35Wind speed (ms)

Nuri 10 mNuri 60 m

Cd

10minus1

10minus2

10minus3

10minus4

(a)

10 15 20 25 30 35 40 45 50Wind speed (ms)

Hagupit 60 m

Cd

10minus2

10minus3

10minus4

(b)

Figure 10 Variation of drag coefficient with wind speed during strong typhoons in offshore direction (a) typhoon Nuri and (b) typhoonHagupit

During typhoon Hagupit at 60m height

119862119889= 10minus3 (000631198802 minus 01499119880 + 1500) 119880 le 25ms30541119880minus23837 119880 gt 25ms

(11)

Considering the same expression of typhoon Nuri at dif-ferent height the relationship at 60m height during typhoonHagupit can be thought to work at 10m height(4) Variation trends of drag coefficient with wind in this

work are similar to other researcherrsquos result but the windthresholds are different Whether the observation data areaffected by land may be a key factor which determines thethreshold Considering other researchersrsquo experiments fromopen sea and these experiments from islands in this workit can be concluded that the larger area of the land whereobservation data are collected is responsible for the lowerthreshold of wind speed Here the wind threshold is a valueDrag coefficient increases until wind speed reaches a certainthreshold and decreases when wind speed is greater than thethreshold

Acknowledgments

The comments of the two referees are gratefully acknowl-edged This material is supported by the National NaturalScience Foundation of China (Grant no 91215302) the Cleandevelopment mechanism Foundation (Grant no 1212014)and National Department Public Benefit Research Founda-tion (Grant no GYHY201006035)

References

[1] C Guan and L Xie ldquoOn the linear parameterization of dragcoefficient over sea surfacerdquo Journal of Physical Oceanographyvol 34 no 12 pp 2847ndash2851 2004

[2] G L Geernaert K B Katsaros and K Richter ldquoVariation ofthe drag coefficient and its dependence on sea staterdquo Journal ofGeophysical Research vol 91 no C6 pp 7667ndash7679 1986

[3] K G Rao ldquoRoughness length and drag coefficient at twoMONTBLEX-90 tower stationsrdquo Journal of Earth System Sci-ence vol 105 no 3 pp 273ndash287 1996

[4] A A Grachev C W Fairall and S E Larsen ldquoOn thedetermination of the neutral drag coefficient in the convectiveboundary layerrdquo Boundary-LayerMeteorology vol 86 no 2 pp257ndash278 1998

[5] L Mahrt D Vickers J Sun et al ldquoDetermination of the surfacedrag coefficientrdquoBoundary-LayerMeteorology vol 99 no 2 pp249ndash276 2001

[6] M D Powell P J Vickery and T A Reinhold ldquoReduced dragcoefficient for high wind speeds in tropical cyclonesrdquo Naturevol 422 no 6929 pp 279ndash283 2003

[7] I-J Moon I Ginis and T Hara ldquoEffect of surface waves on air-sea momentum exchange Part II behavior of drag coefficientunder tropical cyclonesrdquo Journal of the Atmospheric Sciencesvol 61 no 19 pp 2334ndash2348 2004

[8] V K Makin ldquoA note on the drag of the sea surface at hurricanewindsrdquo Boundary-LayerMeteorology vol 115 no 1 pp 169ndash1762005

[9] L L Song J B Pang C L Jiang et al ldquoField measurement andanalysis of turbulence coherence for Typhoon Nuri at MacaoFriendship Bridgerdquo Science ChinaTechnological Sciences vol 53pp 2647ndash2657 2010

[10] R S Chen Typhoon (in Chinese) Fujian Science amp TechnologyPress Fujian China 2002

[11] B Wang F Hu and X Cheng ldquoWind gust and turbulencestatistics of typhoons in South Chinardquo Acta MeteorologicaSinica vol 25 no 1 pp 113ndash127 2011

[12] J Hoslashjstrup ldquoA statistical data screening procedurerdquo Measure-ment Science and Technology vol 4 no 2 pp 153ndash157 1993

[13] D Vickers and L Mahrt ldquoQuality control and flux samplingproblems for tower and aircraft datardquo Journal of Atmosphericand Oceanic Technology vol 14 no 3 pp 512ndash526 1997

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2 Advances in Meteorology

wind (i) The wind direction with wind speed exceeding 8thgrade Beaufort scale (10min mean wind speed of 8th gradeis 172ms) successively alters over 120∘ (ii) The variationof wind speed with time shows an ldquoMrdquo shape during thepassage of typhoons and if a low wind speed less than 11msoccurs between two peak values it can be judged as thetyphoon eye area The two criteria should be fulfilled at thesame time to prove whether the typhoon center has passedover the observation site According to the two criteria twocases were selected to analyze drag coefficient during strongtyphoons typhoon Nuri (observed from Sanjiao Island) andtyphoon Hagupit (observed from Zhizai Island) Both of thetwo selected typhoon observations captured the passage oftyphoonrsquos eye wall and eye and this representative obser-vation is called typical typhoon [10] The typhoon tracksand the positions of the observation sites Sanjiao Island(where typhoonNuri was observed) and Zhizai Island (wheretyphoon Hagupit was observed) are showed in Figure 1 andTable 1

Typhoon Nuri made landfall in the coastal region of theSai Kung Sea in Hong Kong at 0455 pm on 22nd Aug 2008(Beijing time hereinafter the same) The center of typhoonNuri passed the tower located in Sanjiao Island at 1850 onAugust 22 2008 and the shortest distance fromSanjiao Islandto typhoon center is about 32 km Data from Sanjiao Islandduring typhoon Nuri were collected Sanjiao Island is locatedon the seaThe distance between northeastern coast ofMacaoand the island is about 12 km (Figure 1) The surface wascovered by grass and bushes (Figure 2(a)) The wind towerwas installed on a 93-meter high hill and was equippedby ultrasonic anemometers at the height of 10m and 60mThe sonic data from 0000 am on August 22 to 2359 pm onAugust 23 2008 were chosen to calculate drag coefficient andother parameters Drag coefficient mean wind speed winddirection and turbulence statistics have been calculated forperiod of 10min and so there are 288 samples of typhoonNuri observations

Strong typhoon Hagupit made landfall in the coastalregion of Chencun Town Dianbai County MaomingGuangdong Province at 0645 am on September 24 2008The center of typhoon Hagupit passed the tower locatedin Zhizai Island at 0510 on September 24 2008 and theshortest distance from Zhizai Island to typhoon center isabout 85 km In this paper data from Zhizai Island dur-ing typhoon Hagupit were collected The wind tower wasinstalled on a 10-meter high hill on Zhizai Island which islocated on the sea The shortest distance between the 100mhigh observation tower sites at Zhizai Island and the coast is45 km (Figure 1)The surface was covered by sand and sparseweed The terrain situation is illustrated in Figure 2(b) Anultrasonic anemometerwas placed at the height of 60mHighfrequency (10Hz) observations of turbulence and the virtualtemperature were obtained from the ultrasonic anemometerThe sonic data from 000 am to 2359 pm on September 242008 were chosen to calculate drag coefficient and otherparameters Turbulence statistics have been calculated forperiod of 10min and so there are 144 samples of typhoonNuri observation The wind data from the observation towerwere classified into ldquoonshore windrdquo and ldquooffshore windrdquo

and detailed information of the two typhoon observationsis showed in Table 1 Considering the complex surface ofalongshore direction along winds were not analyzed in thiswork

22 Data Quality Control Before analyzing all the data werepreprocessed as follows [11]

(1) Find the turbulence data spikes Referring to Hoslashjs-truprsquos [12] and Vickers andMahrtrsquos [13] methods datasatisfying the following formula can be considered asspikes

|119889119909 (119894)| ge 4120590 (1)

where 119909 is the series of 119906 V 119908 119889119909(119894) = 119909(119894 + 1) minus 119909(119894)and 120590 is the standard deviation of series 119909

(2) Remove and interpolate the spikes by Hoslashjstruprsquos [12]method

119909 (119894) = 119909 (119894 minus 1) 119877119898+ (1 minus 119877

119898)119883119898 (2)

where 119898 is a constant (119898 = 10 in this paper) 119877119898is

the correlation coefficient between the series 119909(119894minus119898 119894 minus 3) and 119909(119894 minus 119898 + 2 119894 minus 1) and 119883

119898is the mean

value of the series 119909(119894 minus 119898 119894 minus 1)

3 Analysis and Results

31 Wind Characteristics Using data from the ultrasonicanemometers on the wind tower located in Sanjiao Islandwind speed and direction during typhoon Nuri were calcu-lated and are showen in Figures 3(a) and 3(b) At 60m heightthe maximum wind speed was 327ms and the minimumwind velocity was 150ms The significant ldquoM-rdquo shapedbimodal distribution was also found The wind directionaltered more than 120∘ counterclockwise Compared with thetwo criteria typhoon Nuri is a typical typhoon

Figure 3(c) shows wind speed and direction duringtyphoon Hagupit It can be seen that the wind directionaltered 120∘ in the clockwise direction with the wind speedover 8th grade Beaufort scale The significant ldquoM-rdquo shapedbimodal distribution was found and the minimum valueof wind speed between two peaks was 119ms and themaximumvalue of thewhole typhoonwas 459ms All of thecharacteristics of the wind speed and direction well satisfiedthe two criteriamentioned above in Section 2 suggesting thattyphoon Hagupit is also a typical typhoon

32 Drag Coefficient during Strong Typhoons The drag coef-ficient is found directly by evaluating the ratio of the frictionvelocity and the mean wind speed difference squared asfollows

119862119889= (119906lowast

119880)2

(3)

Advances in Meteorology 3

Zhizai IslandSanjiao Island

Typhoon Nuri

Typhoon Hagupit

24-1200

23-0000

22-1800

22-1200

22-0600

22-0000

23-180024-0000

24-0600

23-1200

Sanjiao Island

12 km 32 km Zhizai Island

45 km

85 km

Figure 1 Typhoon tracks (yellow dotted lines) and positions of the observation sites Zhizai Island and Sanjiao Island The distance betweennortheastern coast of Macao and the Sanjiao Island is about 12 km and the shortest distance between the 100m high observation tower sitesat Zhizai Island and the coast is 45 km The center of typhoon Nuri passed the tower at 1850 on August 22 2008 and the shortest distancefrom Sanjiao Island to typhoon center is about 32 km The center of typhoon Hagupit passed the tower at 0510 on September 24 2008 andthe shortest distance from Zhizai Island to typhoon center is about 85 km

(a) (b)

Figure 2 Sites of wind tower (a) Sanjiao Island (where typhoon Nuri was observed) and (b) Zhizai Island (where typhoon Hagupit wasobserved)

4 Advances in Meteorology

0000 0600 1200 1800 0000 0600 1200 1800 00000

10

20

30

40

Time

0

90

180

270

360W

ind

spee

d (m

s)

Win

d di

rect

ion

(∘)

(a)

0000 0600 1200 1800 0000 0600 1200 1800 0000Time

0

10

20

30

40

Win

d sp

eed

(ms

)

0

90

180

270

360

Win

d di

rect

ion

(∘)

(b)

0000 0300 0600 0900 1200 1500 1800 2100 0000Time

Wind speedWind direction

8

18

28

38

48

Win

d sp

eed

(ms

)

0

90

180

270

360

Win

d di

rect

ion

(∘)

(c)

Figure 3 Wind speed and wind direction during strong typhoons (a) typhoon Nuri from 0000 am on August 22 to 2359 pm on August 232008 at 10m height (b) typhoon Nuri from 0000 am on August 22 to 2359 pm on August 23 2008 at 60m height and (c) typhoon Hagupitfrom 000 am to 2359 pm on September 24 2008 at 60m height

Table 1 Detailed information of the two typhoon observations

Observation site Type of wind Wind direction Elevation of the observation site Area of the observation site

Sanjiao Island (Nuri) Offshore wind 0ndash75∘ 270ndash360∘ 93m 062 km2

Onshore wind 75ndash225∘

Zhizai Island (Hagupit) Offshore wind 0ndash75∘ 225ndash360∘ 10m 00036 km2

Onshore wind 75ndash225∘

where 119906lowastrepresents the friction velocity calculated from the

flux data using the relation

119906lowast= (11990610158401199081015840

2

+ V101584011990810158402

)14

(4)

and 119880 is 10-minute average wind speedFigure 4 shows drag coefficient during typhoon Nuri In

general drag coefficients were different at different heightsbut the variation trends with time were similar Beforetyphoon center passed drag coefficients at 10m height weregreater than that of 60mheightWhen typhoon center passedthe wind tower increments of drag coefficient at the twoheights were observed due to the low wind speed in typhooneye region After typhoon center passed drag coefficients atthe two heights made little difference

Figure 5 shows drag coefficient during typhoon HagupitBefore the typhoon center passed there was a peak value(00050) of the drag coefficient at 410 An hour after the

typhoon center passed the greatest drag coefficient appearedwith a value of 00234 During the period from 410 to 700the drag coefficients changed dramatically which may beresulted from low wind speed in typhoon eye region

33 Variations of Drag Coefficient with Atmospheric StabilityThe use of sonic data made it possible to calculate atmo-spheric stability by the fowling formula

120589 =119911

119871= minus119896119911119892 (11990810158401205791015840V)

120579V1199063

lowast

(5)

where 120589 is atmospheric stability 119911 is height and 119871 is theMonin-Obukhov length The von Karman constant 119896 = 04and the Gravitational acceleration 119892 = 98 120579V is virtualpotential temperature 119908 is vertical wind speed and 119906

lowastis

friction velocityFigure 6 is the atmospheric stability during typhoons

Nuri and Hagupit In order to compare atmospheric stability

Advances in Meteorology 5

0000 0600 1200 1800 0000 0600 1200 1800 0000Time

Cd

10minus1

10minus2

10minus3

10minus4

10minus5

10 m60 m

Figure 4 Drag coefficients during typhoon Nuri from 0000 am onAugust 22 to 2359 pm on August 23 2008 at 10m and 60m height

Cd

10minus1

10minus2

10minus3

10minus4

10minus5

0000 0300 0600 0900 1200 1500 1800 2100 0000Time

Figure 5 Drag coefficients during typhoon Hagupit from 000 amto 2359 pm on September 24 2008 at 60m height

between different typhoons before and after typhoon centerpassed samples from 0700 am on August 22 to 0700 am onAugust 23 2008 during typhoon Nuri were selected Duringthe period the absolute values of atmospheric stability at10m height were close to zero indicating the atmospherestratification was nearly neutral At 60m height the atmo-spheric stability varied from minus2 to 2 indicating weak stableor weak unstable stratification Before typhoon eye passedthe difference of atmospheric stability between 10m and 60mheight it was small As winds were from the land beforetyphoon center passed the small difference between thetwo heights indicated that the influence of land surface onatmospheric stability was similar at different heights Aftertyphoon center passed atmospheric stability of most samplesat 10m height was greater than zero while at 60m heightwas smaller than zero After typhoon center passed windswere from the sea with the large difference of atmosphericstability indicating that the influences of the sea surface onatmospheric stability were different at different heights

Figure 7 shows variations of drag coefficient with atmo-spheric stability in onshore direction during typhoons Nuriand Hagupit During typhoon Nuri (Figure 7(a)) atmo-spheric conditions are different at different heights At 10mheight 120589 is greater than zero and drag coefficient reducesregularly as atmospheric stratification changes from neutralto weakly stable At 60mheight 120589 is lower than zero and drag

coefficient reduces regularly as atmospheric stratificationchanges from neutral to weakly unstable The curve whichcan describe the regular variation of drag coefficient wasderived from these observation data as follows

119862119889=

exp (086601205892 + 36106120589 minus 60184) exp (1305471205892 minus 158059120589 minus 59793)

(6)

During typhoon Hagupit variation of drag coefficientwith atmospheric stability is not so regular like typhoonNuriand no relationship was found between drag coefficient andatmospheric stability during typhoon Hagupit

34 Variation of Drag Coefficient withWind Speed in OnshoreDirection The data pairs (119862

119889 119880) in onshore direction are

graphically presented in Figure 8 In general variation trendsof drag coefficient withwind are similar for the two typhoonsIt can also be seen that variation trends of drag coefficientwith wind are accordant at different heights during typhoonNuri Drag coefficient increases until wind speed reaches acertain threshold and decreases when wind speed is greaterthan the threshold For different typhoons the thresholds aredifferent The threshold is about 15ms for typhoon Nuri25ms for typhoonHagupit and 40ms for other researchersrsquo[6]

During typhoonNuri drag coefficient assumes very largevalues in the onshore direction for the wind speed reachesabout 25ms at both 10m and 60m height The relationshipbetween drag coefficient andwind speed in onshore directioncan be described by the following formulae

119862119889=10minus3 (002841198802minus39000119880+142000) 119880 le 15ms104

times 84061119880minus55597

119880 gt 15ms(7)

During typhoon Hagupit drag coefficient ranges from00007 to 0003 Drag coefficient increases slowly until windspeed reaches about 25msWhen wind speed is greater than25ms there are only several samples nevertheless it couldbe seen that drag coefficient decreases regularly with windspeed A least squares analysis on the data pairs in onshorewind direction produced the following ldquobest fitrdquo regressionequation

119862119889= 10minus3 (000631198802 minus 01499119880 + 1500) 119880 le 25ms30541119880minus23837 119880 gt 25ms

(8)

Figure 9 shows the average drag coefficient of typhoonsHagupit andNuri in onshore directionHere drag coefficientsfrom different sites at the same height are compared In theonshore direction at 60m height drag coefficient duringtyphoons Hagupit and Nuri makes some difference for differ-ent speed bins In general when wind speed is greater than10ms and lower than 25ms drag coefficient of typhoonNuri is far greater than that of typhoon Hagupit When windspeed is greater than 25ms drag coefficient of typhoon Nurireaches the same order of magnitude as typhoon Hagupit

6 Advances in Meteorology

120589

0700 1300 1900 0100 0700Time

2

1

0

minus1

minus2

10 m60 m

(a)

0000 0600 1200 1800 0000Time

1

05

0

minus05

minus1

120589

(b)

Figure 6 Atmospheric stability during strong typhoons (a) typhoon Nuri from 0000 am on August 22 to 2359 pm on August 23 2008 at10m and 60m height and (b) typhoon Hagupit from 000 am to 2359 pm on September 24 2008 at 60m height

Cd

10minus1

10minus2

10minus3

10minus4

120589

020minus1 minus02minus04minus06minus08

10 m60 m

(a)

Cd

10minus2

10minus3

10minus4

120589

02 04 060minus04 minus02

(b)

Figure 7 Variations of drag coefficient with atmospheric stability in onshore direction during strong typhoons (a) typhoon Nuri at 10m and60m height and (b) typhoon Hagupit at 60m height

5 10 15 20 25 30 35Wind speed (ms)

Nuri 10 mNuri 60 m

Cd

10minus1

10minus2

10minus3

10minus4

(a)

10 15 20 25 30 35 40Wind speed (ms)

Cd

10minus2

10minus3

10minus4

Hagupit 60 m

(b)

Figure 8 Drag coefficient as a function of wind speed during strong typhoons in onshore direction (a) typhoon Nuri and (b) typhoonHagupit

Advances in Meteorology 7

0 10 20 30 40Wind speed (ms)

Nuri 10 mNuri 60 mHagupit 60 m

Cd

10minus2

10minus3

Figure 9 Average drag coefficient of typhoons Hagupit and Nuri inonshore direction and the wind speed bin is 1ms

By comparing drag coefficients at the two heights duringtyphoon Nuri it can be found that the relationships betweendrag coefficient and wind speed are similar at differentheights

Another fact which is worth noting is that wind thresh-olds for typhoon Nuri and typhoon Hagupit are differentfrom other researchersrsquo result [6] It is maybe resulted fromdifferent observation sites Typhoon Nuri was observed froma tower located on a 93-meter high hill while typhoonHagupit was observed from a tower located on a 10-meterhigh hill Furthermore Sanjiao Island where typhoon Nuriwas observed covers an area of 062 km2 while Zhizai Islandonly covers an area of 00036 km2 The average distance fromthe tower to the edge of Sanjiao Island is about 840m and toZhizai Island is about 45m in the onshore direction Windsfrom the onshore direction are mainly affected by the seasurface but are also influenced by the land where the toweris located These observation environments make the seasurface roughness length arisen from wave or other factorsfromopen sea surface produce lesser effect on drag coefficientduring typhoon Nuri That is to say relationships betweendrag coefficient and wind speed derived from typhoon Nuriand typhoon Hagupit are not from real open sea surfacePowell et alrsquos [6] experiment was conducted in open sea andwas not affected by land These different observation sitesmay be why the wind thresholds are different for differenttyphoons Consequently whether the observation data areaffected by land may be a key factor which determines thethreshold Considering other researchersrsquo experiments fromopen sea and these experiments from islands in this workit can be concluded that the larger area of the land whereobservation data are collected is responsible for the lowerthreshold of wind speed

35 Variation of Drag Coefficient with Wind Speed in OffshoreDirection Figure 10 shows the variation of drag coefficientwith wind speed during typhoons Nuri and Hagupit in off-shore direction It can be seen that drag coefficient fluctuatesobviously from 00001 to 0001 during typhoons Nuri andHagupit and no regular relationship is found between dragcoefficient and wind speed It may be resulted from thedistance from the observation tower to shore and complexland surface in offshore direction

4 Conclusion

Using data from wind towers during typhoons Hagupitand Nuri drag coefficient was estimated The relation-ship between drag coefficient and atmospheric stability wasexamined finding that the drag coefficient decreased whenatmosphere stability changed from weakly stable or unstableto neutral Relationship between drag coefficient and windspeed was also examined and the result indicated that therelationships between drag coefficient and wind speed weresimilar to other researcherrsquos result but the wind thresholdswere different due to different observation sites Some pre-liminary conclusions are obtained as follows(1) By comparing atmospheric stability at the two heights

during typhoon Nuri in onshore direction it is found thatatmospheric conditions are different at different heights At10m height 120589 gt 0 and drag coefficient reduces regularlyas atmospheric stratification changes from neutral to weaklystable At 60m height 120589 lt 0 and drag coefficient reducesregularly as atmospheric stratification changes from neutralto unstable The curve which can describe the regular varia-tion of drag coefficient is derived from these observation dataas follows

119862119889=

exp (086601205892 + 36106120589 minus 60184) exp (1305471205892 minus 158059120589 minus 59793)

(9)

(2) In onshore direction at 60m height drag coefficientduring typhoonsHagupit andNurimakes some difference fordifferent speed In general when wind speed is greater than10ms and lower than 25ms drag coefficient of typhoonNuri is far greater than that of typhoon Hagupit When windspeed is greater than 25ms drag coefficient of typhoon Nurireaches the same order of magnitude as typhoon HagupitBy comparing drag coefficients at the two heights duringtyphoon Nuri it can be found that the relationships betweendrag coefficient and wind speed are similar at differentheights(3) In onshore direction relationships between drag

coefficient and wind speed are derived from observation dataof different typhoon cases showing considerable differencebetween different typhoons

During typhoon Nuri the relationships between dragcoefficient and wind speed are similar at 10m and 60mheight

119862119889=10minus3 (002841198802minus39000119880+142000) 119880le15ms104 times 84061119880minus55597 119880gt15ms

(10)

8 Advances in Meteorology

5 15 25 35Wind speed (ms)

Nuri 10 mNuri 60 m

Cd

10minus1

10minus2

10minus3

10minus4

(a)

10 15 20 25 30 35 40 45 50Wind speed (ms)

Hagupit 60 m

Cd

10minus2

10minus3

10minus4

(b)

Figure 10 Variation of drag coefficient with wind speed during strong typhoons in offshore direction (a) typhoon Nuri and (b) typhoonHagupit

During typhoon Hagupit at 60m height

119862119889= 10minus3 (000631198802 minus 01499119880 + 1500) 119880 le 25ms30541119880minus23837 119880 gt 25ms

(11)

Considering the same expression of typhoon Nuri at dif-ferent height the relationship at 60m height during typhoonHagupit can be thought to work at 10m height(4) Variation trends of drag coefficient with wind in this

work are similar to other researcherrsquos result but the windthresholds are different Whether the observation data areaffected by land may be a key factor which determines thethreshold Considering other researchersrsquo experiments fromopen sea and these experiments from islands in this workit can be concluded that the larger area of the land whereobservation data are collected is responsible for the lowerthreshold of wind speed Here the wind threshold is a valueDrag coefficient increases until wind speed reaches a certainthreshold and decreases when wind speed is greater than thethreshold

Acknowledgments

The comments of the two referees are gratefully acknowl-edged This material is supported by the National NaturalScience Foundation of China (Grant no 91215302) the Cleandevelopment mechanism Foundation (Grant no 1212014)and National Department Public Benefit Research Founda-tion (Grant no GYHY201006035)

References

[1] C Guan and L Xie ldquoOn the linear parameterization of dragcoefficient over sea surfacerdquo Journal of Physical Oceanographyvol 34 no 12 pp 2847ndash2851 2004

[2] G L Geernaert K B Katsaros and K Richter ldquoVariation ofthe drag coefficient and its dependence on sea staterdquo Journal ofGeophysical Research vol 91 no C6 pp 7667ndash7679 1986

[3] K G Rao ldquoRoughness length and drag coefficient at twoMONTBLEX-90 tower stationsrdquo Journal of Earth System Sci-ence vol 105 no 3 pp 273ndash287 1996

[4] A A Grachev C W Fairall and S E Larsen ldquoOn thedetermination of the neutral drag coefficient in the convectiveboundary layerrdquo Boundary-LayerMeteorology vol 86 no 2 pp257ndash278 1998

[5] L Mahrt D Vickers J Sun et al ldquoDetermination of the surfacedrag coefficientrdquoBoundary-LayerMeteorology vol 99 no 2 pp249ndash276 2001

[6] M D Powell P J Vickery and T A Reinhold ldquoReduced dragcoefficient for high wind speeds in tropical cyclonesrdquo Naturevol 422 no 6929 pp 279ndash283 2003

[7] I-J Moon I Ginis and T Hara ldquoEffect of surface waves on air-sea momentum exchange Part II behavior of drag coefficientunder tropical cyclonesrdquo Journal of the Atmospheric Sciencesvol 61 no 19 pp 2334ndash2348 2004

[8] V K Makin ldquoA note on the drag of the sea surface at hurricanewindsrdquo Boundary-LayerMeteorology vol 115 no 1 pp 169ndash1762005

[9] L L Song J B Pang C L Jiang et al ldquoField measurement andanalysis of turbulence coherence for Typhoon Nuri at MacaoFriendship Bridgerdquo Science ChinaTechnological Sciences vol 53pp 2647ndash2657 2010

[10] R S Chen Typhoon (in Chinese) Fujian Science amp TechnologyPress Fujian China 2002

[11] B Wang F Hu and X Cheng ldquoWind gust and turbulencestatistics of typhoons in South Chinardquo Acta MeteorologicaSinica vol 25 no 1 pp 113ndash127 2011

[12] J Hoslashjstrup ldquoA statistical data screening procedurerdquo Measure-ment Science and Technology vol 4 no 2 pp 153ndash157 1993

[13] D Vickers and L Mahrt ldquoQuality control and flux samplingproblems for tower and aircraft datardquo Journal of Atmosphericand Oceanic Technology vol 14 no 3 pp 512ndash526 1997

Submit your manuscripts athttpwwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

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Geology Advances in

Advances in Meteorology 3

Zhizai IslandSanjiao Island

Typhoon Nuri

Typhoon Hagupit

24-1200

23-0000

22-1800

22-1200

22-0600

22-0000

23-180024-0000

24-0600

23-1200

Sanjiao Island

12 km 32 km Zhizai Island

45 km

85 km

Figure 1 Typhoon tracks (yellow dotted lines) and positions of the observation sites Zhizai Island and Sanjiao Island The distance betweennortheastern coast of Macao and the Sanjiao Island is about 12 km and the shortest distance between the 100m high observation tower sitesat Zhizai Island and the coast is 45 km The center of typhoon Nuri passed the tower at 1850 on August 22 2008 and the shortest distancefrom Sanjiao Island to typhoon center is about 32 km The center of typhoon Hagupit passed the tower at 0510 on September 24 2008 andthe shortest distance from Zhizai Island to typhoon center is about 85 km

(a) (b)

Figure 2 Sites of wind tower (a) Sanjiao Island (where typhoon Nuri was observed) and (b) Zhizai Island (where typhoon Hagupit wasobserved)

4 Advances in Meteorology

0000 0600 1200 1800 0000 0600 1200 1800 00000

10

20

30

40

Time

0

90

180

270

360W

ind

spee

d (m

s)

Win

d di

rect

ion

(∘)

(a)

0000 0600 1200 1800 0000 0600 1200 1800 0000Time

0

10

20

30

40

Win

d sp

eed

(ms

)

0

90

180

270

360

Win

d di

rect

ion

(∘)

(b)

0000 0300 0600 0900 1200 1500 1800 2100 0000Time

Wind speedWind direction

8

18

28

38

48

Win

d sp

eed

(ms

)

0

90

180

270

360

Win

d di

rect

ion

(∘)

(c)

Figure 3 Wind speed and wind direction during strong typhoons (a) typhoon Nuri from 0000 am on August 22 to 2359 pm on August 232008 at 10m height (b) typhoon Nuri from 0000 am on August 22 to 2359 pm on August 23 2008 at 60m height and (c) typhoon Hagupitfrom 000 am to 2359 pm on September 24 2008 at 60m height

Table 1 Detailed information of the two typhoon observations

Observation site Type of wind Wind direction Elevation of the observation site Area of the observation site

Sanjiao Island (Nuri) Offshore wind 0ndash75∘ 270ndash360∘ 93m 062 km2

Onshore wind 75ndash225∘

Zhizai Island (Hagupit) Offshore wind 0ndash75∘ 225ndash360∘ 10m 00036 km2

Onshore wind 75ndash225∘

where 119906lowastrepresents the friction velocity calculated from the

flux data using the relation

119906lowast= (11990610158401199081015840

2

+ V101584011990810158402

)14

(4)

and 119880 is 10-minute average wind speedFigure 4 shows drag coefficient during typhoon Nuri In

general drag coefficients were different at different heightsbut the variation trends with time were similar Beforetyphoon center passed drag coefficients at 10m height weregreater than that of 60mheightWhen typhoon center passedthe wind tower increments of drag coefficient at the twoheights were observed due to the low wind speed in typhooneye region After typhoon center passed drag coefficients atthe two heights made little difference

Figure 5 shows drag coefficient during typhoon HagupitBefore the typhoon center passed there was a peak value(00050) of the drag coefficient at 410 An hour after the

typhoon center passed the greatest drag coefficient appearedwith a value of 00234 During the period from 410 to 700the drag coefficients changed dramatically which may beresulted from low wind speed in typhoon eye region

33 Variations of Drag Coefficient with Atmospheric StabilityThe use of sonic data made it possible to calculate atmo-spheric stability by the fowling formula

120589 =119911

119871= minus119896119911119892 (11990810158401205791015840V)

120579V1199063

lowast

(5)

where 120589 is atmospheric stability 119911 is height and 119871 is theMonin-Obukhov length The von Karman constant 119896 = 04and the Gravitational acceleration 119892 = 98 120579V is virtualpotential temperature 119908 is vertical wind speed and 119906

lowastis

friction velocityFigure 6 is the atmospheric stability during typhoons

Nuri and Hagupit In order to compare atmospheric stability

Advances in Meteorology 5

0000 0600 1200 1800 0000 0600 1200 1800 0000Time

Cd

10minus1

10minus2

10minus3

10minus4

10minus5

10 m60 m

Figure 4 Drag coefficients during typhoon Nuri from 0000 am onAugust 22 to 2359 pm on August 23 2008 at 10m and 60m height

Cd

10minus1

10minus2

10minus3

10minus4

10minus5

0000 0300 0600 0900 1200 1500 1800 2100 0000Time

Figure 5 Drag coefficients during typhoon Hagupit from 000 amto 2359 pm on September 24 2008 at 60m height

between different typhoons before and after typhoon centerpassed samples from 0700 am on August 22 to 0700 am onAugust 23 2008 during typhoon Nuri were selected Duringthe period the absolute values of atmospheric stability at10m height were close to zero indicating the atmospherestratification was nearly neutral At 60m height the atmo-spheric stability varied from minus2 to 2 indicating weak stableor weak unstable stratification Before typhoon eye passedthe difference of atmospheric stability between 10m and 60mheight it was small As winds were from the land beforetyphoon center passed the small difference between thetwo heights indicated that the influence of land surface onatmospheric stability was similar at different heights Aftertyphoon center passed atmospheric stability of most samplesat 10m height was greater than zero while at 60m heightwas smaller than zero After typhoon center passed windswere from the sea with the large difference of atmosphericstability indicating that the influences of the sea surface onatmospheric stability were different at different heights

Figure 7 shows variations of drag coefficient with atmo-spheric stability in onshore direction during typhoons Nuriand Hagupit During typhoon Nuri (Figure 7(a)) atmo-spheric conditions are different at different heights At 10mheight 120589 is greater than zero and drag coefficient reducesregularly as atmospheric stratification changes from neutralto weakly stable At 60mheight 120589 is lower than zero and drag

coefficient reduces regularly as atmospheric stratificationchanges from neutral to weakly unstable The curve whichcan describe the regular variation of drag coefficient wasderived from these observation data as follows

119862119889=

exp (086601205892 + 36106120589 minus 60184) exp (1305471205892 minus 158059120589 minus 59793)

(6)

During typhoon Hagupit variation of drag coefficientwith atmospheric stability is not so regular like typhoonNuriand no relationship was found between drag coefficient andatmospheric stability during typhoon Hagupit

34 Variation of Drag Coefficient withWind Speed in OnshoreDirection The data pairs (119862

119889 119880) in onshore direction are

graphically presented in Figure 8 In general variation trendsof drag coefficient withwind are similar for the two typhoonsIt can also be seen that variation trends of drag coefficientwith wind are accordant at different heights during typhoonNuri Drag coefficient increases until wind speed reaches acertain threshold and decreases when wind speed is greaterthan the threshold For different typhoons the thresholds aredifferent The threshold is about 15ms for typhoon Nuri25ms for typhoonHagupit and 40ms for other researchersrsquo[6]

During typhoonNuri drag coefficient assumes very largevalues in the onshore direction for the wind speed reachesabout 25ms at both 10m and 60m height The relationshipbetween drag coefficient andwind speed in onshore directioncan be described by the following formulae

119862119889=10minus3 (002841198802minus39000119880+142000) 119880 le 15ms104

times 84061119880minus55597

119880 gt 15ms(7)

During typhoon Hagupit drag coefficient ranges from00007 to 0003 Drag coefficient increases slowly until windspeed reaches about 25msWhen wind speed is greater than25ms there are only several samples nevertheless it couldbe seen that drag coefficient decreases regularly with windspeed A least squares analysis on the data pairs in onshorewind direction produced the following ldquobest fitrdquo regressionequation

119862119889= 10minus3 (000631198802 minus 01499119880 + 1500) 119880 le 25ms30541119880minus23837 119880 gt 25ms

(8)

Figure 9 shows the average drag coefficient of typhoonsHagupit andNuri in onshore directionHere drag coefficientsfrom different sites at the same height are compared In theonshore direction at 60m height drag coefficient duringtyphoons Hagupit and Nuri makes some difference for differ-ent speed bins In general when wind speed is greater than10ms and lower than 25ms drag coefficient of typhoonNuri is far greater than that of typhoon Hagupit When windspeed is greater than 25ms drag coefficient of typhoon Nurireaches the same order of magnitude as typhoon Hagupit

6 Advances in Meteorology

120589

0700 1300 1900 0100 0700Time

2

1

0

minus1

minus2

10 m60 m

(a)

0000 0600 1200 1800 0000Time

1

05

0

minus05

minus1

120589

(b)

Figure 6 Atmospheric stability during strong typhoons (a) typhoon Nuri from 0000 am on August 22 to 2359 pm on August 23 2008 at10m and 60m height and (b) typhoon Hagupit from 000 am to 2359 pm on September 24 2008 at 60m height

Cd

10minus1

10minus2

10minus3

10minus4

120589

020minus1 minus02minus04minus06minus08

10 m60 m

(a)

Cd

10minus2

10minus3

10minus4

120589

02 04 060minus04 minus02

(b)

Figure 7 Variations of drag coefficient with atmospheric stability in onshore direction during strong typhoons (a) typhoon Nuri at 10m and60m height and (b) typhoon Hagupit at 60m height

5 10 15 20 25 30 35Wind speed (ms)

Nuri 10 mNuri 60 m

Cd

10minus1

10minus2

10minus3

10minus4

(a)

10 15 20 25 30 35 40Wind speed (ms)

Cd

10minus2

10minus3

10minus4

Hagupit 60 m

(b)

Figure 8 Drag coefficient as a function of wind speed during strong typhoons in onshore direction (a) typhoon Nuri and (b) typhoonHagupit

Advances in Meteorology 7

0 10 20 30 40Wind speed (ms)

Nuri 10 mNuri 60 mHagupit 60 m

Cd

10minus2

10minus3

Figure 9 Average drag coefficient of typhoons Hagupit and Nuri inonshore direction and the wind speed bin is 1ms

By comparing drag coefficients at the two heights duringtyphoon Nuri it can be found that the relationships betweendrag coefficient and wind speed are similar at differentheights

Another fact which is worth noting is that wind thresh-olds for typhoon Nuri and typhoon Hagupit are differentfrom other researchersrsquo result [6] It is maybe resulted fromdifferent observation sites Typhoon Nuri was observed froma tower located on a 93-meter high hill while typhoonHagupit was observed from a tower located on a 10-meterhigh hill Furthermore Sanjiao Island where typhoon Nuriwas observed covers an area of 062 km2 while Zhizai Islandonly covers an area of 00036 km2 The average distance fromthe tower to the edge of Sanjiao Island is about 840m and toZhizai Island is about 45m in the onshore direction Windsfrom the onshore direction are mainly affected by the seasurface but are also influenced by the land where the toweris located These observation environments make the seasurface roughness length arisen from wave or other factorsfromopen sea surface produce lesser effect on drag coefficientduring typhoon Nuri That is to say relationships betweendrag coefficient and wind speed derived from typhoon Nuriand typhoon Hagupit are not from real open sea surfacePowell et alrsquos [6] experiment was conducted in open sea andwas not affected by land These different observation sitesmay be why the wind thresholds are different for differenttyphoons Consequently whether the observation data areaffected by land may be a key factor which determines thethreshold Considering other researchersrsquo experiments fromopen sea and these experiments from islands in this workit can be concluded that the larger area of the land whereobservation data are collected is responsible for the lowerthreshold of wind speed

35 Variation of Drag Coefficient with Wind Speed in OffshoreDirection Figure 10 shows the variation of drag coefficientwith wind speed during typhoons Nuri and Hagupit in off-shore direction It can be seen that drag coefficient fluctuatesobviously from 00001 to 0001 during typhoons Nuri andHagupit and no regular relationship is found between dragcoefficient and wind speed It may be resulted from thedistance from the observation tower to shore and complexland surface in offshore direction

4 Conclusion

Using data from wind towers during typhoons Hagupitand Nuri drag coefficient was estimated The relation-ship between drag coefficient and atmospheric stability wasexamined finding that the drag coefficient decreased whenatmosphere stability changed from weakly stable or unstableto neutral Relationship between drag coefficient and windspeed was also examined and the result indicated that therelationships between drag coefficient and wind speed weresimilar to other researcherrsquos result but the wind thresholdswere different due to different observation sites Some pre-liminary conclusions are obtained as follows(1) By comparing atmospheric stability at the two heights

during typhoon Nuri in onshore direction it is found thatatmospheric conditions are different at different heights At10m height 120589 gt 0 and drag coefficient reduces regularlyas atmospheric stratification changes from neutral to weaklystable At 60m height 120589 lt 0 and drag coefficient reducesregularly as atmospheric stratification changes from neutralto unstable The curve which can describe the regular varia-tion of drag coefficient is derived from these observation dataas follows

119862119889=

exp (086601205892 + 36106120589 minus 60184) exp (1305471205892 minus 158059120589 minus 59793)

(9)

(2) In onshore direction at 60m height drag coefficientduring typhoonsHagupit andNurimakes some difference fordifferent speed In general when wind speed is greater than10ms and lower than 25ms drag coefficient of typhoonNuri is far greater than that of typhoon Hagupit When windspeed is greater than 25ms drag coefficient of typhoon Nurireaches the same order of magnitude as typhoon HagupitBy comparing drag coefficients at the two heights duringtyphoon Nuri it can be found that the relationships betweendrag coefficient and wind speed are similar at differentheights(3) In onshore direction relationships between drag

coefficient and wind speed are derived from observation dataof different typhoon cases showing considerable differencebetween different typhoons

During typhoon Nuri the relationships between dragcoefficient and wind speed are similar at 10m and 60mheight

119862119889=10minus3 (002841198802minus39000119880+142000) 119880le15ms104 times 84061119880minus55597 119880gt15ms

(10)

8 Advances in Meteorology

5 15 25 35Wind speed (ms)

Nuri 10 mNuri 60 m

Cd

10minus1

10minus2

10minus3

10minus4

(a)

10 15 20 25 30 35 40 45 50Wind speed (ms)

Hagupit 60 m

Cd

10minus2

10minus3

10minus4

(b)

Figure 10 Variation of drag coefficient with wind speed during strong typhoons in offshore direction (a) typhoon Nuri and (b) typhoonHagupit

During typhoon Hagupit at 60m height

119862119889= 10minus3 (000631198802 minus 01499119880 + 1500) 119880 le 25ms30541119880minus23837 119880 gt 25ms

(11)

Considering the same expression of typhoon Nuri at dif-ferent height the relationship at 60m height during typhoonHagupit can be thought to work at 10m height(4) Variation trends of drag coefficient with wind in this

work are similar to other researcherrsquos result but the windthresholds are different Whether the observation data areaffected by land may be a key factor which determines thethreshold Considering other researchersrsquo experiments fromopen sea and these experiments from islands in this workit can be concluded that the larger area of the land whereobservation data are collected is responsible for the lowerthreshold of wind speed Here the wind threshold is a valueDrag coefficient increases until wind speed reaches a certainthreshold and decreases when wind speed is greater than thethreshold

Acknowledgments

The comments of the two referees are gratefully acknowl-edged This material is supported by the National NaturalScience Foundation of China (Grant no 91215302) the Cleandevelopment mechanism Foundation (Grant no 1212014)and National Department Public Benefit Research Founda-tion (Grant no GYHY201006035)

References

[1] C Guan and L Xie ldquoOn the linear parameterization of dragcoefficient over sea surfacerdquo Journal of Physical Oceanographyvol 34 no 12 pp 2847ndash2851 2004

[2] G L Geernaert K B Katsaros and K Richter ldquoVariation ofthe drag coefficient and its dependence on sea staterdquo Journal ofGeophysical Research vol 91 no C6 pp 7667ndash7679 1986

[3] K G Rao ldquoRoughness length and drag coefficient at twoMONTBLEX-90 tower stationsrdquo Journal of Earth System Sci-ence vol 105 no 3 pp 273ndash287 1996

[4] A A Grachev C W Fairall and S E Larsen ldquoOn thedetermination of the neutral drag coefficient in the convectiveboundary layerrdquo Boundary-LayerMeteorology vol 86 no 2 pp257ndash278 1998

[5] L Mahrt D Vickers J Sun et al ldquoDetermination of the surfacedrag coefficientrdquoBoundary-LayerMeteorology vol 99 no 2 pp249ndash276 2001

[6] M D Powell P J Vickery and T A Reinhold ldquoReduced dragcoefficient for high wind speeds in tropical cyclonesrdquo Naturevol 422 no 6929 pp 279ndash283 2003

[7] I-J Moon I Ginis and T Hara ldquoEffect of surface waves on air-sea momentum exchange Part II behavior of drag coefficientunder tropical cyclonesrdquo Journal of the Atmospheric Sciencesvol 61 no 19 pp 2334ndash2348 2004

[8] V K Makin ldquoA note on the drag of the sea surface at hurricanewindsrdquo Boundary-LayerMeteorology vol 115 no 1 pp 169ndash1762005

[9] L L Song J B Pang C L Jiang et al ldquoField measurement andanalysis of turbulence coherence for Typhoon Nuri at MacaoFriendship Bridgerdquo Science ChinaTechnological Sciences vol 53pp 2647ndash2657 2010

[10] R S Chen Typhoon (in Chinese) Fujian Science amp TechnologyPress Fujian China 2002

[11] B Wang F Hu and X Cheng ldquoWind gust and turbulencestatistics of typhoons in South Chinardquo Acta MeteorologicaSinica vol 25 no 1 pp 113ndash127 2011

[12] J Hoslashjstrup ldquoA statistical data screening procedurerdquo Measure-ment Science and Technology vol 4 no 2 pp 153ndash157 1993

[13] D Vickers and L Mahrt ldquoQuality control and flux samplingproblems for tower and aircraft datardquo Journal of Atmosphericand Oceanic Technology vol 14 no 3 pp 512ndash526 1997

Submit your manuscripts athttpwwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

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EarthquakesJournal of

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Applied ampEnvironmentalSoil Science

Volume 2014

Mining

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Journal of

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International Journal of

Geophysics

OceanographyInternational Journal of

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Journal ofPetroleum Engineering

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

GeochemistryHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Atmospheric SciencesInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

OceanographyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Advances in

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MineralogyInternational Journal of

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MeteorologyAdvances in

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ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

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Geological ResearchJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Geology Advances in

4 Advances in Meteorology

0000 0600 1200 1800 0000 0600 1200 1800 00000

10

20

30

40

Time

0

90

180

270

360W

ind

spee

d (m

s)

Win

d di

rect

ion

(∘)

(a)

0000 0600 1200 1800 0000 0600 1200 1800 0000Time

0

10

20

30

40

Win

d sp

eed

(ms

)

0

90

180

270

360

Win

d di

rect

ion

(∘)

(b)

0000 0300 0600 0900 1200 1500 1800 2100 0000Time

Wind speedWind direction

8

18

28

38

48

Win

d sp

eed

(ms

)

0

90

180

270

360

Win

d di

rect

ion

(∘)

(c)

Figure 3 Wind speed and wind direction during strong typhoons (a) typhoon Nuri from 0000 am on August 22 to 2359 pm on August 232008 at 10m height (b) typhoon Nuri from 0000 am on August 22 to 2359 pm on August 23 2008 at 60m height and (c) typhoon Hagupitfrom 000 am to 2359 pm on September 24 2008 at 60m height

Table 1 Detailed information of the two typhoon observations

Observation site Type of wind Wind direction Elevation of the observation site Area of the observation site

Sanjiao Island (Nuri) Offshore wind 0ndash75∘ 270ndash360∘ 93m 062 km2

Onshore wind 75ndash225∘

Zhizai Island (Hagupit) Offshore wind 0ndash75∘ 225ndash360∘ 10m 00036 km2

Onshore wind 75ndash225∘

where 119906lowastrepresents the friction velocity calculated from the

flux data using the relation

119906lowast= (11990610158401199081015840

2

+ V101584011990810158402

)14

(4)

and 119880 is 10-minute average wind speedFigure 4 shows drag coefficient during typhoon Nuri In

general drag coefficients were different at different heightsbut the variation trends with time were similar Beforetyphoon center passed drag coefficients at 10m height weregreater than that of 60mheightWhen typhoon center passedthe wind tower increments of drag coefficient at the twoheights were observed due to the low wind speed in typhooneye region After typhoon center passed drag coefficients atthe two heights made little difference

Figure 5 shows drag coefficient during typhoon HagupitBefore the typhoon center passed there was a peak value(00050) of the drag coefficient at 410 An hour after the

typhoon center passed the greatest drag coefficient appearedwith a value of 00234 During the period from 410 to 700the drag coefficients changed dramatically which may beresulted from low wind speed in typhoon eye region

33 Variations of Drag Coefficient with Atmospheric StabilityThe use of sonic data made it possible to calculate atmo-spheric stability by the fowling formula

120589 =119911

119871= minus119896119911119892 (11990810158401205791015840V)

120579V1199063

lowast

(5)

where 120589 is atmospheric stability 119911 is height and 119871 is theMonin-Obukhov length The von Karman constant 119896 = 04and the Gravitational acceleration 119892 = 98 120579V is virtualpotential temperature 119908 is vertical wind speed and 119906

lowastis

friction velocityFigure 6 is the atmospheric stability during typhoons

Nuri and Hagupit In order to compare atmospheric stability

Advances in Meteorology 5

0000 0600 1200 1800 0000 0600 1200 1800 0000Time

Cd

10minus1

10minus2

10minus3

10minus4

10minus5

10 m60 m

Figure 4 Drag coefficients during typhoon Nuri from 0000 am onAugust 22 to 2359 pm on August 23 2008 at 10m and 60m height

Cd

10minus1

10minus2

10minus3

10minus4

10minus5

0000 0300 0600 0900 1200 1500 1800 2100 0000Time

Figure 5 Drag coefficients during typhoon Hagupit from 000 amto 2359 pm on September 24 2008 at 60m height

between different typhoons before and after typhoon centerpassed samples from 0700 am on August 22 to 0700 am onAugust 23 2008 during typhoon Nuri were selected Duringthe period the absolute values of atmospheric stability at10m height were close to zero indicating the atmospherestratification was nearly neutral At 60m height the atmo-spheric stability varied from minus2 to 2 indicating weak stableor weak unstable stratification Before typhoon eye passedthe difference of atmospheric stability between 10m and 60mheight it was small As winds were from the land beforetyphoon center passed the small difference between thetwo heights indicated that the influence of land surface onatmospheric stability was similar at different heights Aftertyphoon center passed atmospheric stability of most samplesat 10m height was greater than zero while at 60m heightwas smaller than zero After typhoon center passed windswere from the sea with the large difference of atmosphericstability indicating that the influences of the sea surface onatmospheric stability were different at different heights

Figure 7 shows variations of drag coefficient with atmo-spheric stability in onshore direction during typhoons Nuriand Hagupit During typhoon Nuri (Figure 7(a)) atmo-spheric conditions are different at different heights At 10mheight 120589 is greater than zero and drag coefficient reducesregularly as atmospheric stratification changes from neutralto weakly stable At 60mheight 120589 is lower than zero and drag

coefficient reduces regularly as atmospheric stratificationchanges from neutral to weakly unstable The curve whichcan describe the regular variation of drag coefficient wasderived from these observation data as follows

119862119889=

exp (086601205892 + 36106120589 minus 60184) exp (1305471205892 minus 158059120589 minus 59793)

(6)

During typhoon Hagupit variation of drag coefficientwith atmospheric stability is not so regular like typhoonNuriand no relationship was found between drag coefficient andatmospheric stability during typhoon Hagupit

34 Variation of Drag Coefficient withWind Speed in OnshoreDirection The data pairs (119862

119889 119880) in onshore direction are

graphically presented in Figure 8 In general variation trendsof drag coefficient withwind are similar for the two typhoonsIt can also be seen that variation trends of drag coefficientwith wind are accordant at different heights during typhoonNuri Drag coefficient increases until wind speed reaches acertain threshold and decreases when wind speed is greaterthan the threshold For different typhoons the thresholds aredifferent The threshold is about 15ms for typhoon Nuri25ms for typhoonHagupit and 40ms for other researchersrsquo[6]

During typhoonNuri drag coefficient assumes very largevalues in the onshore direction for the wind speed reachesabout 25ms at both 10m and 60m height The relationshipbetween drag coefficient andwind speed in onshore directioncan be described by the following formulae

119862119889=10minus3 (002841198802minus39000119880+142000) 119880 le 15ms104

times 84061119880minus55597

119880 gt 15ms(7)

During typhoon Hagupit drag coefficient ranges from00007 to 0003 Drag coefficient increases slowly until windspeed reaches about 25msWhen wind speed is greater than25ms there are only several samples nevertheless it couldbe seen that drag coefficient decreases regularly with windspeed A least squares analysis on the data pairs in onshorewind direction produced the following ldquobest fitrdquo regressionequation

119862119889= 10minus3 (000631198802 minus 01499119880 + 1500) 119880 le 25ms30541119880minus23837 119880 gt 25ms

(8)

Figure 9 shows the average drag coefficient of typhoonsHagupit andNuri in onshore directionHere drag coefficientsfrom different sites at the same height are compared In theonshore direction at 60m height drag coefficient duringtyphoons Hagupit and Nuri makes some difference for differ-ent speed bins In general when wind speed is greater than10ms and lower than 25ms drag coefficient of typhoonNuri is far greater than that of typhoon Hagupit When windspeed is greater than 25ms drag coefficient of typhoon Nurireaches the same order of magnitude as typhoon Hagupit

6 Advances in Meteorology

120589

0700 1300 1900 0100 0700Time

2

1

0

minus1

minus2

10 m60 m

(a)

0000 0600 1200 1800 0000Time

1

05

0

minus05

minus1

120589

(b)

Figure 6 Atmospheric stability during strong typhoons (a) typhoon Nuri from 0000 am on August 22 to 2359 pm on August 23 2008 at10m and 60m height and (b) typhoon Hagupit from 000 am to 2359 pm on September 24 2008 at 60m height

Cd

10minus1

10minus2

10minus3

10minus4

120589

020minus1 minus02minus04minus06minus08

10 m60 m

(a)

Cd

10minus2

10minus3

10minus4

120589

02 04 060minus04 minus02

(b)

Figure 7 Variations of drag coefficient with atmospheric stability in onshore direction during strong typhoons (a) typhoon Nuri at 10m and60m height and (b) typhoon Hagupit at 60m height

5 10 15 20 25 30 35Wind speed (ms)

Nuri 10 mNuri 60 m

Cd

10minus1

10minus2

10minus3

10minus4

(a)

10 15 20 25 30 35 40Wind speed (ms)

Cd

10minus2

10minus3

10minus4

Hagupit 60 m

(b)

Figure 8 Drag coefficient as a function of wind speed during strong typhoons in onshore direction (a) typhoon Nuri and (b) typhoonHagupit

Advances in Meteorology 7

0 10 20 30 40Wind speed (ms)

Nuri 10 mNuri 60 mHagupit 60 m

Cd

10minus2

10minus3

Figure 9 Average drag coefficient of typhoons Hagupit and Nuri inonshore direction and the wind speed bin is 1ms

By comparing drag coefficients at the two heights duringtyphoon Nuri it can be found that the relationships betweendrag coefficient and wind speed are similar at differentheights

Another fact which is worth noting is that wind thresh-olds for typhoon Nuri and typhoon Hagupit are differentfrom other researchersrsquo result [6] It is maybe resulted fromdifferent observation sites Typhoon Nuri was observed froma tower located on a 93-meter high hill while typhoonHagupit was observed from a tower located on a 10-meterhigh hill Furthermore Sanjiao Island where typhoon Nuriwas observed covers an area of 062 km2 while Zhizai Islandonly covers an area of 00036 km2 The average distance fromthe tower to the edge of Sanjiao Island is about 840m and toZhizai Island is about 45m in the onshore direction Windsfrom the onshore direction are mainly affected by the seasurface but are also influenced by the land where the toweris located These observation environments make the seasurface roughness length arisen from wave or other factorsfromopen sea surface produce lesser effect on drag coefficientduring typhoon Nuri That is to say relationships betweendrag coefficient and wind speed derived from typhoon Nuriand typhoon Hagupit are not from real open sea surfacePowell et alrsquos [6] experiment was conducted in open sea andwas not affected by land These different observation sitesmay be why the wind thresholds are different for differenttyphoons Consequently whether the observation data areaffected by land may be a key factor which determines thethreshold Considering other researchersrsquo experiments fromopen sea and these experiments from islands in this workit can be concluded that the larger area of the land whereobservation data are collected is responsible for the lowerthreshold of wind speed

35 Variation of Drag Coefficient with Wind Speed in OffshoreDirection Figure 10 shows the variation of drag coefficientwith wind speed during typhoons Nuri and Hagupit in off-shore direction It can be seen that drag coefficient fluctuatesobviously from 00001 to 0001 during typhoons Nuri andHagupit and no regular relationship is found between dragcoefficient and wind speed It may be resulted from thedistance from the observation tower to shore and complexland surface in offshore direction

4 Conclusion

Using data from wind towers during typhoons Hagupitand Nuri drag coefficient was estimated The relation-ship between drag coefficient and atmospheric stability wasexamined finding that the drag coefficient decreased whenatmosphere stability changed from weakly stable or unstableto neutral Relationship between drag coefficient and windspeed was also examined and the result indicated that therelationships between drag coefficient and wind speed weresimilar to other researcherrsquos result but the wind thresholdswere different due to different observation sites Some pre-liminary conclusions are obtained as follows(1) By comparing atmospheric stability at the two heights

during typhoon Nuri in onshore direction it is found thatatmospheric conditions are different at different heights At10m height 120589 gt 0 and drag coefficient reduces regularlyas atmospheric stratification changes from neutral to weaklystable At 60m height 120589 lt 0 and drag coefficient reducesregularly as atmospheric stratification changes from neutralto unstable The curve which can describe the regular varia-tion of drag coefficient is derived from these observation dataas follows

119862119889=

exp (086601205892 + 36106120589 minus 60184) exp (1305471205892 minus 158059120589 minus 59793)

(9)

(2) In onshore direction at 60m height drag coefficientduring typhoonsHagupit andNurimakes some difference fordifferent speed In general when wind speed is greater than10ms and lower than 25ms drag coefficient of typhoonNuri is far greater than that of typhoon Hagupit When windspeed is greater than 25ms drag coefficient of typhoon Nurireaches the same order of magnitude as typhoon HagupitBy comparing drag coefficients at the two heights duringtyphoon Nuri it can be found that the relationships betweendrag coefficient and wind speed are similar at differentheights(3) In onshore direction relationships between drag

coefficient and wind speed are derived from observation dataof different typhoon cases showing considerable differencebetween different typhoons

During typhoon Nuri the relationships between dragcoefficient and wind speed are similar at 10m and 60mheight

119862119889=10minus3 (002841198802minus39000119880+142000) 119880le15ms104 times 84061119880minus55597 119880gt15ms

(10)

8 Advances in Meteorology

5 15 25 35Wind speed (ms)

Nuri 10 mNuri 60 m

Cd

10minus1

10minus2

10minus3

10minus4

(a)

10 15 20 25 30 35 40 45 50Wind speed (ms)

Hagupit 60 m

Cd

10minus2

10minus3

10minus4

(b)

Figure 10 Variation of drag coefficient with wind speed during strong typhoons in offshore direction (a) typhoon Nuri and (b) typhoonHagupit

During typhoon Hagupit at 60m height

119862119889= 10minus3 (000631198802 minus 01499119880 + 1500) 119880 le 25ms30541119880minus23837 119880 gt 25ms

(11)

Considering the same expression of typhoon Nuri at dif-ferent height the relationship at 60m height during typhoonHagupit can be thought to work at 10m height(4) Variation trends of drag coefficient with wind in this

work are similar to other researcherrsquos result but the windthresholds are different Whether the observation data areaffected by land may be a key factor which determines thethreshold Considering other researchersrsquo experiments fromopen sea and these experiments from islands in this workit can be concluded that the larger area of the land whereobservation data are collected is responsible for the lowerthreshold of wind speed Here the wind threshold is a valueDrag coefficient increases until wind speed reaches a certainthreshold and decreases when wind speed is greater than thethreshold

Acknowledgments

The comments of the two referees are gratefully acknowl-edged This material is supported by the National NaturalScience Foundation of China (Grant no 91215302) the Cleandevelopment mechanism Foundation (Grant no 1212014)and National Department Public Benefit Research Founda-tion (Grant no GYHY201006035)

References

[1] C Guan and L Xie ldquoOn the linear parameterization of dragcoefficient over sea surfacerdquo Journal of Physical Oceanographyvol 34 no 12 pp 2847ndash2851 2004

[2] G L Geernaert K B Katsaros and K Richter ldquoVariation ofthe drag coefficient and its dependence on sea staterdquo Journal ofGeophysical Research vol 91 no C6 pp 7667ndash7679 1986

[3] K G Rao ldquoRoughness length and drag coefficient at twoMONTBLEX-90 tower stationsrdquo Journal of Earth System Sci-ence vol 105 no 3 pp 273ndash287 1996

[4] A A Grachev C W Fairall and S E Larsen ldquoOn thedetermination of the neutral drag coefficient in the convectiveboundary layerrdquo Boundary-LayerMeteorology vol 86 no 2 pp257ndash278 1998

[5] L Mahrt D Vickers J Sun et al ldquoDetermination of the surfacedrag coefficientrdquoBoundary-LayerMeteorology vol 99 no 2 pp249ndash276 2001

[6] M D Powell P J Vickery and T A Reinhold ldquoReduced dragcoefficient for high wind speeds in tropical cyclonesrdquo Naturevol 422 no 6929 pp 279ndash283 2003

[7] I-J Moon I Ginis and T Hara ldquoEffect of surface waves on air-sea momentum exchange Part II behavior of drag coefficientunder tropical cyclonesrdquo Journal of the Atmospheric Sciencesvol 61 no 19 pp 2334ndash2348 2004

[8] V K Makin ldquoA note on the drag of the sea surface at hurricanewindsrdquo Boundary-LayerMeteorology vol 115 no 1 pp 169ndash1762005

[9] L L Song J B Pang C L Jiang et al ldquoField measurement andanalysis of turbulence coherence for Typhoon Nuri at MacaoFriendship Bridgerdquo Science ChinaTechnological Sciences vol 53pp 2647ndash2657 2010

[10] R S Chen Typhoon (in Chinese) Fujian Science amp TechnologyPress Fujian China 2002

[11] B Wang F Hu and X Cheng ldquoWind gust and turbulencestatistics of typhoons in South Chinardquo Acta MeteorologicaSinica vol 25 no 1 pp 113ndash127 2011

[12] J Hoslashjstrup ldquoA statistical data screening procedurerdquo Measure-ment Science and Technology vol 4 no 2 pp 153ndash157 1993

[13] D Vickers and L Mahrt ldquoQuality control and flux samplingproblems for tower and aircraft datardquo Journal of Atmosphericand Oceanic Technology vol 14 no 3 pp 512ndash526 1997

Submit your manuscripts athttpwwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

ClimatologyJournal of

EcologyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EarthquakesJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom

Applied ampEnvironmentalSoil Science

Volume 2014

Mining

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Hindawi Publishing Corporation httpwwwhindawicom Volume 2014

International Journal of

Geophysics

OceanographyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of Computational Environmental SciencesHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofPetroleum Engineering

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

GeochemistryHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Atmospheric SciencesInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

OceanographyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Advances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

MineralogyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

MeteorologyAdvances in

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Paleontology JournalHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Geological ResearchJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Geology Advances in

Advances in Meteorology 5

0000 0600 1200 1800 0000 0600 1200 1800 0000Time

Cd

10minus1

10minus2

10minus3

10minus4

10minus5

10 m60 m

Figure 4 Drag coefficients during typhoon Nuri from 0000 am onAugust 22 to 2359 pm on August 23 2008 at 10m and 60m height

Cd

10minus1

10minus2

10minus3

10minus4

10minus5

0000 0300 0600 0900 1200 1500 1800 2100 0000Time

Figure 5 Drag coefficients during typhoon Hagupit from 000 amto 2359 pm on September 24 2008 at 60m height

between different typhoons before and after typhoon centerpassed samples from 0700 am on August 22 to 0700 am onAugust 23 2008 during typhoon Nuri were selected Duringthe period the absolute values of atmospheric stability at10m height were close to zero indicating the atmospherestratification was nearly neutral At 60m height the atmo-spheric stability varied from minus2 to 2 indicating weak stableor weak unstable stratification Before typhoon eye passedthe difference of atmospheric stability between 10m and 60mheight it was small As winds were from the land beforetyphoon center passed the small difference between thetwo heights indicated that the influence of land surface onatmospheric stability was similar at different heights Aftertyphoon center passed atmospheric stability of most samplesat 10m height was greater than zero while at 60m heightwas smaller than zero After typhoon center passed windswere from the sea with the large difference of atmosphericstability indicating that the influences of the sea surface onatmospheric stability were different at different heights

Figure 7 shows variations of drag coefficient with atmo-spheric stability in onshore direction during typhoons Nuriand Hagupit During typhoon Nuri (Figure 7(a)) atmo-spheric conditions are different at different heights At 10mheight 120589 is greater than zero and drag coefficient reducesregularly as atmospheric stratification changes from neutralto weakly stable At 60mheight 120589 is lower than zero and drag

coefficient reduces regularly as atmospheric stratificationchanges from neutral to weakly unstable The curve whichcan describe the regular variation of drag coefficient wasderived from these observation data as follows

119862119889=

exp (086601205892 + 36106120589 minus 60184) exp (1305471205892 minus 158059120589 minus 59793)

(6)

During typhoon Hagupit variation of drag coefficientwith atmospheric stability is not so regular like typhoonNuriand no relationship was found between drag coefficient andatmospheric stability during typhoon Hagupit

34 Variation of Drag Coefficient withWind Speed in OnshoreDirection The data pairs (119862

119889 119880) in onshore direction are

graphically presented in Figure 8 In general variation trendsof drag coefficient withwind are similar for the two typhoonsIt can also be seen that variation trends of drag coefficientwith wind are accordant at different heights during typhoonNuri Drag coefficient increases until wind speed reaches acertain threshold and decreases when wind speed is greaterthan the threshold For different typhoons the thresholds aredifferent The threshold is about 15ms for typhoon Nuri25ms for typhoonHagupit and 40ms for other researchersrsquo[6]

During typhoonNuri drag coefficient assumes very largevalues in the onshore direction for the wind speed reachesabout 25ms at both 10m and 60m height The relationshipbetween drag coefficient andwind speed in onshore directioncan be described by the following formulae

119862119889=10minus3 (002841198802minus39000119880+142000) 119880 le 15ms104

times 84061119880minus55597

119880 gt 15ms(7)

During typhoon Hagupit drag coefficient ranges from00007 to 0003 Drag coefficient increases slowly until windspeed reaches about 25msWhen wind speed is greater than25ms there are only several samples nevertheless it couldbe seen that drag coefficient decreases regularly with windspeed A least squares analysis on the data pairs in onshorewind direction produced the following ldquobest fitrdquo regressionequation

119862119889= 10minus3 (000631198802 minus 01499119880 + 1500) 119880 le 25ms30541119880minus23837 119880 gt 25ms

(8)

Figure 9 shows the average drag coefficient of typhoonsHagupit andNuri in onshore directionHere drag coefficientsfrom different sites at the same height are compared In theonshore direction at 60m height drag coefficient duringtyphoons Hagupit and Nuri makes some difference for differ-ent speed bins In general when wind speed is greater than10ms and lower than 25ms drag coefficient of typhoonNuri is far greater than that of typhoon Hagupit When windspeed is greater than 25ms drag coefficient of typhoon Nurireaches the same order of magnitude as typhoon Hagupit

6 Advances in Meteorology

120589

0700 1300 1900 0100 0700Time

2

1

0

minus1

minus2

10 m60 m

(a)

0000 0600 1200 1800 0000Time

1

05

0

minus05

minus1

120589

(b)

Figure 6 Atmospheric stability during strong typhoons (a) typhoon Nuri from 0000 am on August 22 to 2359 pm on August 23 2008 at10m and 60m height and (b) typhoon Hagupit from 000 am to 2359 pm on September 24 2008 at 60m height

Cd

10minus1

10minus2

10minus3

10minus4

120589

020minus1 minus02minus04minus06minus08

10 m60 m

(a)

Cd

10minus2

10minus3

10minus4

120589

02 04 060minus04 minus02

(b)

Figure 7 Variations of drag coefficient with atmospheric stability in onshore direction during strong typhoons (a) typhoon Nuri at 10m and60m height and (b) typhoon Hagupit at 60m height

5 10 15 20 25 30 35Wind speed (ms)

Nuri 10 mNuri 60 m

Cd

10minus1

10minus2

10minus3

10minus4

(a)

10 15 20 25 30 35 40Wind speed (ms)

Cd

10minus2

10minus3

10minus4

Hagupit 60 m

(b)

Figure 8 Drag coefficient as a function of wind speed during strong typhoons in onshore direction (a) typhoon Nuri and (b) typhoonHagupit

Advances in Meteorology 7

0 10 20 30 40Wind speed (ms)

Nuri 10 mNuri 60 mHagupit 60 m

Cd

10minus2

10minus3

Figure 9 Average drag coefficient of typhoons Hagupit and Nuri inonshore direction and the wind speed bin is 1ms

By comparing drag coefficients at the two heights duringtyphoon Nuri it can be found that the relationships betweendrag coefficient and wind speed are similar at differentheights

Another fact which is worth noting is that wind thresh-olds for typhoon Nuri and typhoon Hagupit are differentfrom other researchersrsquo result [6] It is maybe resulted fromdifferent observation sites Typhoon Nuri was observed froma tower located on a 93-meter high hill while typhoonHagupit was observed from a tower located on a 10-meterhigh hill Furthermore Sanjiao Island where typhoon Nuriwas observed covers an area of 062 km2 while Zhizai Islandonly covers an area of 00036 km2 The average distance fromthe tower to the edge of Sanjiao Island is about 840m and toZhizai Island is about 45m in the onshore direction Windsfrom the onshore direction are mainly affected by the seasurface but are also influenced by the land where the toweris located These observation environments make the seasurface roughness length arisen from wave or other factorsfromopen sea surface produce lesser effect on drag coefficientduring typhoon Nuri That is to say relationships betweendrag coefficient and wind speed derived from typhoon Nuriand typhoon Hagupit are not from real open sea surfacePowell et alrsquos [6] experiment was conducted in open sea andwas not affected by land These different observation sitesmay be why the wind thresholds are different for differenttyphoons Consequently whether the observation data areaffected by land may be a key factor which determines thethreshold Considering other researchersrsquo experiments fromopen sea and these experiments from islands in this workit can be concluded that the larger area of the land whereobservation data are collected is responsible for the lowerthreshold of wind speed

35 Variation of Drag Coefficient with Wind Speed in OffshoreDirection Figure 10 shows the variation of drag coefficientwith wind speed during typhoons Nuri and Hagupit in off-shore direction It can be seen that drag coefficient fluctuatesobviously from 00001 to 0001 during typhoons Nuri andHagupit and no regular relationship is found between dragcoefficient and wind speed It may be resulted from thedistance from the observation tower to shore and complexland surface in offshore direction

4 Conclusion

Using data from wind towers during typhoons Hagupitand Nuri drag coefficient was estimated The relation-ship between drag coefficient and atmospheric stability wasexamined finding that the drag coefficient decreased whenatmosphere stability changed from weakly stable or unstableto neutral Relationship between drag coefficient and windspeed was also examined and the result indicated that therelationships between drag coefficient and wind speed weresimilar to other researcherrsquos result but the wind thresholdswere different due to different observation sites Some pre-liminary conclusions are obtained as follows(1) By comparing atmospheric stability at the two heights

during typhoon Nuri in onshore direction it is found thatatmospheric conditions are different at different heights At10m height 120589 gt 0 and drag coefficient reduces regularlyas atmospheric stratification changes from neutral to weaklystable At 60m height 120589 lt 0 and drag coefficient reducesregularly as atmospheric stratification changes from neutralto unstable The curve which can describe the regular varia-tion of drag coefficient is derived from these observation dataas follows

119862119889=

exp (086601205892 + 36106120589 minus 60184) exp (1305471205892 minus 158059120589 minus 59793)

(9)

(2) In onshore direction at 60m height drag coefficientduring typhoonsHagupit andNurimakes some difference fordifferent speed In general when wind speed is greater than10ms and lower than 25ms drag coefficient of typhoonNuri is far greater than that of typhoon Hagupit When windspeed is greater than 25ms drag coefficient of typhoon Nurireaches the same order of magnitude as typhoon HagupitBy comparing drag coefficients at the two heights duringtyphoon Nuri it can be found that the relationships betweendrag coefficient and wind speed are similar at differentheights(3) In onshore direction relationships between drag

coefficient and wind speed are derived from observation dataof different typhoon cases showing considerable differencebetween different typhoons

During typhoon Nuri the relationships between dragcoefficient and wind speed are similar at 10m and 60mheight

119862119889=10minus3 (002841198802minus39000119880+142000) 119880le15ms104 times 84061119880minus55597 119880gt15ms

(10)

8 Advances in Meteorology

5 15 25 35Wind speed (ms)

Nuri 10 mNuri 60 m

Cd

10minus1

10minus2

10minus3

10minus4

(a)

10 15 20 25 30 35 40 45 50Wind speed (ms)

Hagupit 60 m

Cd

10minus2

10minus3

10minus4

(b)

Figure 10 Variation of drag coefficient with wind speed during strong typhoons in offshore direction (a) typhoon Nuri and (b) typhoonHagupit

During typhoon Hagupit at 60m height

119862119889= 10minus3 (000631198802 minus 01499119880 + 1500) 119880 le 25ms30541119880minus23837 119880 gt 25ms

(11)

Considering the same expression of typhoon Nuri at dif-ferent height the relationship at 60m height during typhoonHagupit can be thought to work at 10m height(4) Variation trends of drag coefficient with wind in this

work are similar to other researcherrsquos result but the windthresholds are different Whether the observation data areaffected by land may be a key factor which determines thethreshold Considering other researchersrsquo experiments fromopen sea and these experiments from islands in this workit can be concluded that the larger area of the land whereobservation data are collected is responsible for the lowerthreshold of wind speed Here the wind threshold is a valueDrag coefficient increases until wind speed reaches a certainthreshold and decreases when wind speed is greater than thethreshold

Acknowledgments

The comments of the two referees are gratefully acknowl-edged This material is supported by the National NaturalScience Foundation of China (Grant no 91215302) the Cleandevelopment mechanism Foundation (Grant no 1212014)and National Department Public Benefit Research Founda-tion (Grant no GYHY201006035)

References

[1] C Guan and L Xie ldquoOn the linear parameterization of dragcoefficient over sea surfacerdquo Journal of Physical Oceanographyvol 34 no 12 pp 2847ndash2851 2004

[2] G L Geernaert K B Katsaros and K Richter ldquoVariation ofthe drag coefficient and its dependence on sea staterdquo Journal ofGeophysical Research vol 91 no C6 pp 7667ndash7679 1986

[3] K G Rao ldquoRoughness length and drag coefficient at twoMONTBLEX-90 tower stationsrdquo Journal of Earth System Sci-ence vol 105 no 3 pp 273ndash287 1996

[4] A A Grachev C W Fairall and S E Larsen ldquoOn thedetermination of the neutral drag coefficient in the convectiveboundary layerrdquo Boundary-LayerMeteorology vol 86 no 2 pp257ndash278 1998

[5] L Mahrt D Vickers J Sun et al ldquoDetermination of the surfacedrag coefficientrdquoBoundary-LayerMeteorology vol 99 no 2 pp249ndash276 2001

[6] M D Powell P J Vickery and T A Reinhold ldquoReduced dragcoefficient for high wind speeds in tropical cyclonesrdquo Naturevol 422 no 6929 pp 279ndash283 2003

[7] I-J Moon I Ginis and T Hara ldquoEffect of surface waves on air-sea momentum exchange Part II behavior of drag coefficientunder tropical cyclonesrdquo Journal of the Atmospheric Sciencesvol 61 no 19 pp 2334ndash2348 2004

[8] V K Makin ldquoA note on the drag of the sea surface at hurricanewindsrdquo Boundary-LayerMeteorology vol 115 no 1 pp 169ndash1762005

[9] L L Song J B Pang C L Jiang et al ldquoField measurement andanalysis of turbulence coherence for Typhoon Nuri at MacaoFriendship Bridgerdquo Science ChinaTechnological Sciences vol 53pp 2647ndash2657 2010

[10] R S Chen Typhoon (in Chinese) Fujian Science amp TechnologyPress Fujian China 2002

[11] B Wang F Hu and X Cheng ldquoWind gust and turbulencestatistics of typhoons in South Chinardquo Acta MeteorologicaSinica vol 25 no 1 pp 113ndash127 2011

[12] J Hoslashjstrup ldquoA statistical data screening procedurerdquo Measure-ment Science and Technology vol 4 no 2 pp 153ndash157 1993

[13] D Vickers and L Mahrt ldquoQuality control and flux samplingproblems for tower and aircraft datardquo Journal of Atmosphericand Oceanic Technology vol 14 no 3 pp 512ndash526 1997

Submit your manuscripts athttpwwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

ClimatologyJournal of

EcologyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EarthquakesJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom

Applied ampEnvironmentalSoil Science

Volume 2014

Mining

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Hindawi Publishing Corporation httpwwwhindawicom Volume 2014

International Journal of

Geophysics

OceanographyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of Computational Environmental SciencesHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofPetroleum Engineering

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

GeochemistryHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Atmospheric SciencesInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

OceanographyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Advances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

MineralogyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

MeteorologyAdvances in

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Paleontology JournalHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Geological ResearchJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Geology Advances in

6 Advances in Meteorology

120589

0700 1300 1900 0100 0700Time

2

1

0

minus1

minus2

10 m60 m

(a)

0000 0600 1200 1800 0000Time

1

05

0

minus05

minus1

120589

(b)

Figure 6 Atmospheric stability during strong typhoons (a) typhoon Nuri from 0000 am on August 22 to 2359 pm on August 23 2008 at10m and 60m height and (b) typhoon Hagupit from 000 am to 2359 pm on September 24 2008 at 60m height

Cd

10minus1

10minus2

10minus3

10minus4

120589

020minus1 minus02minus04minus06minus08

10 m60 m

(a)

Cd

10minus2

10minus3

10minus4

120589

02 04 060minus04 minus02

(b)

Figure 7 Variations of drag coefficient with atmospheric stability in onshore direction during strong typhoons (a) typhoon Nuri at 10m and60m height and (b) typhoon Hagupit at 60m height

5 10 15 20 25 30 35Wind speed (ms)

Nuri 10 mNuri 60 m

Cd

10minus1

10minus2

10minus3

10minus4

(a)

10 15 20 25 30 35 40Wind speed (ms)

Cd

10minus2

10minus3

10minus4

Hagupit 60 m

(b)

Figure 8 Drag coefficient as a function of wind speed during strong typhoons in onshore direction (a) typhoon Nuri and (b) typhoonHagupit

Advances in Meteorology 7

0 10 20 30 40Wind speed (ms)

Nuri 10 mNuri 60 mHagupit 60 m

Cd

10minus2

10minus3

Figure 9 Average drag coefficient of typhoons Hagupit and Nuri inonshore direction and the wind speed bin is 1ms

By comparing drag coefficients at the two heights duringtyphoon Nuri it can be found that the relationships betweendrag coefficient and wind speed are similar at differentheights

Another fact which is worth noting is that wind thresh-olds for typhoon Nuri and typhoon Hagupit are differentfrom other researchersrsquo result [6] It is maybe resulted fromdifferent observation sites Typhoon Nuri was observed froma tower located on a 93-meter high hill while typhoonHagupit was observed from a tower located on a 10-meterhigh hill Furthermore Sanjiao Island where typhoon Nuriwas observed covers an area of 062 km2 while Zhizai Islandonly covers an area of 00036 km2 The average distance fromthe tower to the edge of Sanjiao Island is about 840m and toZhizai Island is about 45m in the onshore direction Windsfrom the onshore direction are mainly affected by the seasurface but are also influenced by the land where the toweris located These observation environments make the seasurface roughness length arisen from wave or other factorsfromopen sea surface produce lesser effect on drag coefficientduring typhoon Nuri That is to say relationships betweendrag coefficient and wind speed derived from typhoon Nuriand typhoon Hagupit are not from real open sea surfacePowell et alrsquos [6] experiment was conducted in open sea andwas not affected by land These different observation sitesmay be why the wind thresholds are different for differenttyphoons Consequently whether the observation data areaffected by land may be a key factor which determines thethreshold Considering other researchersrsquo experiments fromopen sea and these experiments from islands in this workit can be concluded that the larger area of the land whereobservation data are collected is responsible for the lowerthreshold of wind speed

35 Variation of Drag Coefficient with Wind Speed in OffshoreDirection Figure 10 shows the variation of drag coefficientwith wind speed during typhoons Nuri and Hagupit in off-shore direction It can be seen that drag coefficient fluctuatesobviously from 00001 to 0001 during typhoons Nuri andHagupit and no regular relationship is found between dragcoefficient and wind speed It may be resulted from thedistance from the observation tower to shore and complexland surface in offshore direction

4 Conclusion

Using data from wind towers during typhoons Hagupitand Nuri drag coefficient was estimated The relation-ship between drag coefficient and atmospheric stability wasexamined finding that the drag coefficient decreased whenatmosphere stability changed from weakly stable or unstableto neutral Relationship between drag coefficient and windspeed was also examined and the result indicated that therelationships between drag coefficient and wind speed weresimilar to other researcherrsquos result but the wind thresholdswere different due to different observation sites Some pre-liminary conclusions are obtained as follows(1) By comparing atmospheric stability at the two heights

during typhoon Nuri in onshore direction it is found thatatmospheric conditions are different at different heights At10m height 120589 gt 0 and drag coefficient reduces regularlyas atmospheric stratification changes from neutral to weaklystable At 60m height 120589 lt 0 and drag coefficient reducesregularly as atmospheric stratification changes from neutralto unstable The curve which can describe the regular varia-tion of drag coefficient is derived from these observation dataas follows

119862119889=

exp (086601205892 + 36106120589 minus 60184) exp (1305471205892 minus 158059120589 minus 59793)

(9)

(2) In onshore direction at 60m height drag coefficientduring typhoonsHagupit andNurimakes some difference fordifferent speed In general when wind speed is greater than10ms and lower than 25ms drag coefficient of typhoonNuri is far greater than that of typhoon Hagupit When windspeed is greater than 25ms drag coefficient of typhoon Nurireaches the same order of magnitude as typhoon HagupitBy comparing drag coefficients at the two heights duringtyphoon Nuri it can be found that the relationships betweendrag coefficient and wind speed are similar at differentheights(3) In onshore direction relationships between drag

coefficient and wind speed are derived from observation dataof different typhoon cases showing considerable differencebetween different typhoons

During typhoon Nuri the relationships between dragcoefficient and wind speed are similar at 10m and 60mheight

119862119889=10minus3 (002841198802minus39000119880+142000) 119880le15ms104 times 84061119880minus55597 119880gt15ms

(10)

8 Advances in Meteorology

5 15 25 35Wind speed (ms)

Nuri 10 mNuri 60 m

Cd

10minus1

10minus2

10minus3

10minus4

(a)

10 15 20 25 30 35 40 45 50Wind speed (ms)

Hagupit 60 m

Cd

10minus2

10minus3

10minus4

(b)

Figure 10 Variation of drag coefficient with wind speed during strong typhoons in offshore direction (a) typhoon Nuri and (b) typhoonHagupit

During typhoon Hagupit at 60m height

119862119889= 10minus3 (000631198802 minus 01499119880 + 1500) 119880 le 25ms30541119880minus23837 119880 gt 25ms

(11)

Considering the same expression of typhoon Nuri at dif-ferent height the relationship at 60m height during typhoonHagupit can be thought to work at 10m height(4) Variation trends of drag coefficient with wind in this

work are similar to other researcherrsquos result but the windthresholds are different Whether the observation data areaffected by land may be a key factor which determines thethreshold Considering other researchersrsquo experiments fromopen sea and these experiments from islands in this workit can be concluded that the larger area of the land whereobservation data are collected is responsible for the lowerthreshold of wind speed Here the wind threshold is a valueDrag coefficient increases until wind speed reaches a certainthreshold and decreases when wind speed is greater than thethreshold

Acknowledgments

The comments of the two referees are gratefully acknowl-edged This material is supported by the National NaturalScience Foundation of China (Grant no 91215302) the Cleandevelopment mechanism Foundation (Grant no 1212014)and National Department Public Benefit Research Founda-tion (Grant no GYHY201006035)

References

[1] C Guan and L Xie ldquoOn the linear parameterization of dragcoefficient over sea surfacerdquo Journal of Physical Oceanographyvol 34 no 12 pp 2847ndash2851 2004

[2] G L Geernaert K B Katsaros and K Richter ldquoVariation ofthe drag coefficient and its dependence on sea staterdquo Journal ofGeophysical Research vol 91 no C6 pp 7667ndash7679 1986

[3] K G Rao ldquoRoughness length and drag coefficient at twoMONTBLEX-90 tower stationsrdquo Journal of Earth System Sci-ence vol 105 no 3 pp 273ndash287 1996

[4] A A Grachev C W Fairall and S E Larsen ldquoOn thedetermination of the neutral drag coefficient in the convectiveboundary layerrdquo Boundary-LayerMeteorology vol 86 no 2 pp257ndash278 1998

[5] L Mahrt D Vickers J Sun et al ldquoDetermination of the surfacedrag coefficientrdquoBoundary-LayerMeteorology vol 99 no 2 pp249ndash276 2001

[6] M D Powell P J Vickery and T A Reinhold ldquoReduced dragcoefficient for high wind speeds in tropical cyclonesrdquo Naturevol 422 no 6929 pp 279ndash283 2003

[7] I-J Moon I Ginis and T Hara ldquoEffect of surface waves on air-sea momentum exchange Part II behavior of drag coefficientunder tropical cyclonesrdquo Journal of the Atmospheric Sciencesvol 61 no 19 pp 2334ndash2348 2004

[8] V K Makin ldquoA note on the drag of the sea surface at hurricanewindsrdquo Boundary-LayerMeteorology vol 115 no 1 pp 169ndash1762005

[9] L L Song J B Pang C L Jiang et al ldquoField measurement andanalysis of turbulence coherence for Typhoon Nuri at MacaoFriendship Bridgerdquo Science ChinaTechnological Sciences vol 53pp 2647ndash2657 2010

[10] R S Chen Typhoon (in Chinese) Fujian Science amp TechnologyPress Fujian China 2002

[11] B Wang F Hu and X Cheng ldquoWind gust and turbulencestatistics of typhoons in South Chinardquo Acta MeteorologicaSinica vol 25 no 1 pp 113ndash127 2011

[12] J Hoslashjstrup ldquoA statistical data screening procedurerdquo Measure-ment Science and Technology vol 4 no 2 pp 153ndash157 1993

[13] D Vickers and L Mahrt ldquoQuality control and flux samplingproblems for tower and aircraft datardquo Journal of Atmosphericand Oceanic Technology vol 14 no 3 pp 512ndash526 1997

Submit your manuscripts athttpwwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

ClimatologyJournal of

EcologyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EarthquakesJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom

Applied ampEnvironmentalSoil Science

Volume 2014

Mining

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Hindawi Publishing Corporation httpwwwhindawicom Volume 2014

International Journal of

Geophysics

OceanographyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of Computational Environmental SciencesHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofPetroleum Engineering

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

GeochemistryHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Atmospheric SciencesInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

OceanographyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Advances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

MineralogyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

MeteorologyAdvances in

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Paleontology JournalHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Geological ResearchJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Geology Advances in

Advances in Meteorology 7

0 10 20 30 40Wind speed (ms)

Nuri 10 mNuri 60 mHagupit 60 m

Cd

10minus2

10minus3

Figure 9 Average drag coefficient of typhoons Hagupit and Nuri inonshore direction and the wind speed bin is 1ms

By comparing drag coefficients at the two heights duringtyphoon Nuri it can be found that the relationships betweendrag coefficient and wind speed are similar at differentheights

Another fact which is worth noting is that wind thresh-olds for typhoon Nuri and typhoon Hagupit are differentfrom other researchersrsquo result [6] It is maybe resulted fromdifferent observation sites Typhoon Nuri was observed froma tower located on a 93-meter high hill while typhoonHagupit was observed from a tower located on a 10-meterhigh hill Furthermore Sanjiao Island where typhoon Nuriwas observed covers an area of 062 km2 while Zhizai Islandonly covers an area of 00036 km2 The average distance fromthe tower to the edge of Sanjiao Island is about 840m and toZhizai Island is about 45m in the onshore direction Windsfrom the onshore direction are mainly affected by the seasurface but are also influenced by the land where the toweris located These observation environments make the seasurface roughness length arisen from wave or other factorsfromopen sea surface produce lesser effect on drag coefficientduring typhoon Nuri That is to say relationships betweendrag coefficient and wind speed derived from typhoon Nuriand typhoon Hagupit are not from real open sea surfacePowell et alrsquos [6] experiment was conducted in open sea andwas not affected by land These different observation sitesmay be why the wind thresholds are different for differenttyphoons Consequently whether the observation data areaffected by land may be a key factor which determines thethreshold Considering other researchersrsquo experiments fromopen sea and these experiments from islands in this workit can be concluded that the larger area of the land whereobservation data are collected is responsible for the lowerthreshold of wind speed

35 Variation of Drag Coefficient with Wind Speed in OffshoreDirection Figure 10 shows the variation of drag coefficientwith wind speed during typhoons Nuri and Hagupit in off-shore direction It can be seen that drag coefficient fluctuatesobviously from 00001 to 0001 during typhoons Nuri andHagupit and no regular relationship is found between dragcoefficient and wind speed It may be resulted from thedistance from the observation tower to shore and complexland surface in offshore direction

4 Conclusion

Using data from wind towers during typhoons Hagupitand Nuri drag coefficient was estimated The relation-ship between drag coefficient and atmospheric stability wasexamined finding that the drag coefficient decreased whenatmosphere stability changed from weakly stable or unstableto neutral Relationship between drag coefficient and windspeed was also examined and the result indicated that therelationships between drag coefficient and wind speed weresimilar to other researcherrsquos result but the wind thresholdswere different due to different observation sites Some pre-liminary conclusions are obtained as follows(1) By comparing atmospheric stability at the two heights

during typhoon Nuri in onshore direction it is found thatatmospheric conditions are different at different heights At10m height 120589 gt 0 and drag coefficient reduces regularlyas atmospheric stratification changes from neutral to weaklystable At 60m height 120589 lt 0 and drag coefficient reducesregularly as atmospheric stratification changes from neutralto unstable The curve which can describe the regular varia-tion of drag coefficient is derived from these observation dataas follows

119862119889=

exp (086601205892 + 36106120589 minus 60184) exp (1305471205892 minus 158059120589 minus 59793)

(9)

(2) In onshore direction at 60m height drag coefficientduring typhoonsHagupit andNurimakes some difference fordifferent speed In general when wind speed is greater than10ms and lower than 25ms drag coefficient of typhoonNuri is far greater than that of typhoon Hagupit When windspeed is greater than 25ms drag coefficient of typhoon Nurireaches the same order of magnitude as typhoon HagupitBy comparing drag coefficients at the two heights duringtyphoon Nuri it can be found that the relationships betweendrag coefficient and wind speed are similar at differentheights(3) In onshore direction relationships between drag

coefficient and wind speed are derived from observation dataof different typhoon cases showing considerable differencebetween different typhoons

During typhoon Nuri the relationships between dragcoefficient and wind speed are similar at 10m and 60mheight

119862119889=10minus3 (002841198802minus39000119880+142000) 119880le15ms104 times 84061119880minus55597 119880gt15ms

(10)

8 Advances in Meteorology

5 15 25 35Wind speed (ms)

Nuri 10 mNuri 60 m

Cd

10minus1

10minus2

10minus3

10minus4

(a)

10 15 20 25 30 35 40 45 50Wind speed (ms)

Hagupit 60 m

Cd

10minus2

10minus3

10minus4

(b)

Figure 10 Variation of drag coefficient with wind speed during strong typhoons in offshore direction (a) typhoon Nuri and (b) typhoonHagupit

During typhoon Hagupit at 60m height

119862119889= 10minus3 (000631198802 minus 01499119880 + 1500) 119880 le 25ms30541119880minus23837 119880 gt 25ms

(11)

Considering the same expression of typhoon Nuri at dif-ferent height the relationship at 60m height during typhoonHagupit can be thought to work at 10m height(4) Variation trends of drag coefficient with wind in this

work are similar to other researcherrsquos result but the windthresholds are different Whether the observation data areaffected by land may be a key factor which determines thethreshold Considering other researchersrsquo experiments fromopen sea and these experiments from islands in this workit can be concluded that the larger area of the land whereobservation data are collected is responsible for the lowerthreshold of wind speed Here the wind threshold is a valueDrag coefficient increases until wind speed reaches a certainthreshold and decreases when wind speed is greater than thethreshold

Acknowledgments

The comments of the two referees are gratefully acknowl-edged This material is supported by the National NaturalScience Foundation of China (Grant no 91215302) the Cleandevelopment mechanism Foundation (Grant no 1212014)and National Department Public Benefit Research Founda-tion (Grant no GYHY201006035)

References

[1] C Guan and L Xie ldquoOn the linear parameterization of dragcoefficient over sea surfacerdquo Journal of Physical Oceanographyvol 34 no 12 pp 2847ndash2851 2004

[2] G L Geernaert K B Katsaros and K Richter ldquoVariation ofthe drag coefficient and its dependence on sea staterdquo Journal ofGeophysical Research vol 91 no C6 pp 7667ndash7679 1986

[3] K G Rao ldquoRoughness length and drag coefficient at twoMONTBLEX-90 tower stationsrdquo Journal of Earth System Sci-ence vol 105 no 3 pp 273ndash287 1996

[4] A A Grachev C W Fairall and S E Larsen ldquoOn thedetermination of the neutral drag coefficient in the convectiveboundary layerrdquo Boundary-LayerMeteorology vol 86 no 2 pp257ndash278 1998

[5] L Mahrt D Vickers J Sun et al ldquoDetermination of the surfacedrag coefficientrdquoBoundary-LayerMeteorology vol 99 no 2 pp249ndash276 2001

[6] M D Powell P J Vickery and T A Reinhold ldquoReduced dragcoefficient for high wind speeds in tropical cyclonesrdquo Naturevol 422 no 6929 pp 279ndash283 2003

[7] I-J Moon I Ginis and T Hara ldquoEffect of surface waves on air-sea momentum exchange Part II behavior of drag coefficientunder tropical cyclonesrdquo Journal of the Atmospheric Sciencesvol 61 no 19 pp 2334ndash2348 2004

[8] V K Makin ldquoA note on the drag of the sea surface at hurricanewindsrdquo Boundary-LayerMeteorology vol 115 no 1 pp 169ndash1762005

[9] L L Song J B Pang C L Jiang et al ldquoField measurement andanalysis of turbulence coherence for Typhoon Nuri at MacaoFriendship Bridgerdquo Science ChinaTechnological Sciences vol 53pp 2647ndash2657 2010

[10] R S Chen Typhoon (in Chinese) Fujian Science amp TechnologyPress Fujian China 2002

[11] B Wang F Hu and X Cheng ldquoWind gust and turbulencestatistics of typhoons in South Chinardquo Acta MeteorologicaSinica vol 25 no 1 pp 113ndash127 2011

[12] J Hoslashjstrup ldquoA statistical data screening procedurerdquo Measure-ment Science and Technology vol 4 no 2 pp 153ndash157 1993

[13] D Vickers and L Mahrt ldquoQuality control and flux samplingproblems for tower and aircraft datardquo Journal of Atmosphericand Oceanic Technology vol 14 no 3 pp 512ndash526 1997

Submit your manuscripts athttpwwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

ClimatologyJournal of

EcologyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EarthquakesJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom

Applied ampEnvironmentalSoil Science

Volume 2014

Mining

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Hindawi Publishing Corporation httpwwwhindawicom Volume 2014

International Journal of

Geophysics

OceanographyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of Computational Environmental SciencesHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofPetroleum Engineering

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

GeochemistryHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Atmospheric SciencesInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

OceanographyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Advances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

MineralogyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

MeteorologyAdvances in

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Paleontology JournalHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Geological ResearchJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Geology Advances in

8 Advances in Meteorology

5 15 25 35Wind speed (ms)

Nuri 10 mNuri 60 m

Cd

10minus1

10minus2

10minus3

10minus4

(a)

10 15 20 25 30 35 40 45 50Wind speed (ms)

Hagupit 60 m

Cd

10minus2

10minus3

10minus4

(b)

Figure 10 Variation of drag coefficient with wind speed during strong typhoons in offshore direction (a) typhoon Nuri and (b) typhoonHagupit

During typhoon Hagupit at 60m height

119862119889= 10minus3 (000631198802 minus 01499119880 + 1500) 119880 le 25ms30541119880minus23837 119880 gt 25ms

(11)

Considering the same expression of typhoon Nuri at dif-ferent height the relationship at 60m height during typhoonHagupit can be thought to work at 10m height(4) Variation trends of drag coefficient with wind in this

work are similar to other researcherrsquos result but the windthresholds are different Whether the observation data areaffected by land may be a key factor which determines thethreshold Considering other researchersrsquo experiments fromopen sea and these experiments from islands in this workit can be concluded that the larger area of the land whereobservation data are collected is responsible for the lowerthreshold of wind speed Here the wind threshold is a valueDrag coefficient increases until wind speed reaches a certainthreshold and decreases when wind speed is greater than thethreshold

Acknowledgments

The comments of the two referees are gratefully acknowl-edged This material is supported by the National NaturalScience Foundation of China (Grant no 91215302) the Cleandevelopment mechanism Foundation (Grant no 1212014)and National Department Public Benefit Research Founda-tion (Grant no GYHY201006035)

References

[1] C Guan and L Xie ldquoOn the linear parameterization of dragcoefficient over sea surfacerdquo Journal of Physical Oceanographyvol 34 no 12 pp 2847ndash2851 2004

[2] G L Geernaert K B Katsaros and K Richter ldquoVariation ofthe drag coefficient and its dependence on sea staterdquo Journal ofGeophysical Research vol 91 no C6 pp 7667ndash7679 1986

[3] K G Rao ldquoRoughness length and drag coefficient at twoMONTBLEX-90 tower stationsrdquo Journal of Earth System Sci-ence vol 105 no 3 pp 273ndash287 1996

[4] A A Grachev C W Fairall and S E Larsen ldquoOn thedetermination of the neutral drag coefficient in the convectiveboundary layerrdquo Boundary-LayerMeteorology vol 86 no 2 pp257ndash278 1998

[5] L Mahrt D Vickers J Sun et al ldquoDetermination of the surfacedrag coefficientrdquoBoundary-LayerMeteorology vol 99 no 2 pp249ndash276 2001

[6] M D Powell P J Vickery and T A Reinhold ldquoReduced dragcoefficient for high wind speeds in tropical cyclonesrdquo Naturevol 422 no 6929 pp 279ndash283 2003

[7] I-J Moon I Ginis and T Hara ldquoEffect of surface waves on air-sea momentum exchange Part II behavior of drag coefficientunder tropical cyclonesrdquo Journal of the Atmospheric Sciencesvol 61 no 19 pp 2334ndash2348 2004

[8] V K Makin ldquoA note on the drag of the sea surface at hurricanewindsrdquo Boundary-LayerMeteorology vol 115 no 1 pp 169ndash1762005

[9] L L Song J B Pang C L Jiang et al ldquoField measurement andanalysis of turbulence coherence for Typhoon Nuri at MacaoFriendship Bridgerdquo Science ChinaTechnological Sciences vol 53pp 2647ndash2657 2010

[10] R S Chen Typhoon (in Chinese) Fujian Science amp TechnologyPress Fujian China 2002

[11] B Wang F Hu and X Cheng ldquoWind gust and turbulencestatistics of typhoons in South Chinardquo Acta MeteorologicaSinica vol 25 no 1 pp 113ndash127 2011

[12] J Hoslashjstrup ldquoA statistical data screening procedurerdquo Measure-ment Science and Technology vol 4 no 2 pp 153ndash157 1993

[13] D Vickers and L Mahrt ldquoQuality control and flux samplingproblems for tower and aircraft datardquo Journal of Atmosphericand Oceanic Technology vol 14 no 3 pp 512ndash526 1997

Submit your manuscripts athttpwwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

ClimatologyJournal of

EcologyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EarthquakesJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom

Applied ampEnvironmentalSoil Science

Volume 2014

Mining

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Hindawi Publishing Corporation httpwwwhindawicom Volume 2014

International Journal of

Geophysics

OceanographyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of Computational Environmental SciencesHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofPetroleum Engineering

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

GeochemistryHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Atmospheric SciencesInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

OceanographyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Advances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

MineralogyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

MeteorologyAdvances in

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Paleontology JournalHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Geological ResearchJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Geology Advances in

Submit your manuscripts athttpwwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

ClimatologyJournal of

EcologyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

EarthquakesJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom

Applied ampEnvironmentalSoil Science

Volume 2014

Mining

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Hindawi Publishing Corporation httpwwwhindawicom Volume 2014

International Journal of

Geophysics

OceanographyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of Computational Environmental SciencesHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal ofPetroleum Engineering

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

GeochemistryHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

Atmospheric SciencesInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

OceanographyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Advances in

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

MineralogyInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

MeteorologyAdvances in

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Paleontology JournalHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Geological ResearchJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Geology Advances in