A methodological approach to the air-sea energy fl uxes data collection and analysis at the tropical coastal ocean

Method Article

A methodological approach to the air-sea energy fl uxes data collection and analysis at the tropical coastal ocean

Yusri Yusup

^a,b,

*, John Stephen Kayode

^a

, Abbas F.M. Alkarkhi

^c

aEnvironmentalTechnology,SchoolofIndustrialTechnology,UniversitiSainsMalaysia,USM,11800,Pulau Pinang,Malaysia

bCentreforMarine&CoastalStudies(CEMACS),UniversitiSainsMalaysia,PulauPinang,Malaysia

cMalaysianInstituteofChemical&BioengineeringTechnology,UniversitiKualaLumpur,78000,Melaka, Malaysia

ABSTRACT

ThesouthernSouthChinacoastaloceanswithintheSouthEastAsianregionaremuchlackingintheperceptionofthe surfaceenergybudgetandevaporationovertheoceanwatersinresponsetoclimaticchanges.Theeddycovariance methodwasusedtomeasuretheenergyfluxes,microclimatevariables,andsurfacewatertemperaturefrom November2015toOctober 2017at the Straits of Malacca, South China Sea; Pulau Pinang,Malaysia, situatedat latitude 528⁰06⁰⁰N,andlongitude10012⁰01⁰⁰E.Thisworkfocusedonthemethodologicalapproachtotheair-seaenergy fluxesdatacollectionandanalysis.Inthisregard,themethodappliedforthedirectmeasurementsandanalysisof energyfluxesandothermeteorologicalparametersinthesiteisconsideredandreported.

^{Thepapersummarizestheanalysisofenergyfluxes,}microclimatevariables,andsurfacewatertemperature datainatropicalcoastaloceanstationusingtheeddycovariancemethod.

^Themethodologicalapproachillustratesthemethodofanalysisappliedinthisstudywhichcanbecompared andusedforsimilarstudiesinotherplaces.

^ThereproducibledataanalysistechniquematchessimilarcomparativemethodssuchasMatlabandPython.

©2018TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://

creativecommons.org/licenses/by/4.0/).

ARTICLE INFO

Methodname:Rstatisticalanalysisofeddycovariancedata

Keywords:Rstatisticalsoftware,Microclimatevariables,Tropicalcoastalocean,Energyfluxes,Eddycovariance Articlehistory:Received12April2018;Accepted11May2018;Availableonline17May2018

*Correspondingauthorat:EnvironmentalTechnology,SchoolofIndustrialTechnology,UniversitiSainsMalaysia,USM, 11800,PulauPinang,Malaysia.

E-mailaddresses:yusriy@usm.my(Y.Yusup),jskayode@usm.my(J.S.Kayode),abbas@unikl.edu.my(A.F.M. Alkarkhi).

https://doi.org/10.1016/j.mex.2018.05.003

2215-0161/©2018TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://

creativecommons.org/licenses/by/4.0/).

ContentslistsavailableatScienceDirect

MethodsX

journalhomepage: www.elsevier.com/locate/mex

Specificationstable

Subjectarea EarthandPlanetarySciences EnvironmentalScience Morespecificsubjectarea AtmosphericScience

Methodname Rstatisticalanalysisofeddycovariancedata Nameandreferenceof

originalmethod

Rprogramming,eddycovarianceprocessingusingEddyProversion6 Resourceavailability Dataandthedataanalysisprogrammingcodes

Methoddetails

UnderstandingoftheexchangeofenergyattheEarth’ssurfaceisnecessaryfortheimprovementof regionalweatherforecastandglobalclimatemodels.ThemainsourceofenergycomefromtheSunin theformoftheglobalradiationsymbolizedbyRG.Theamountofenergyabsorbedbytheoceanis denotedasthenetradiation(RN).Partoftheenergyisstoredinsidetheoceanastheresidual,G.The oceanemittedsomeenergiesintheformofevaporationaslatentheat(LE)andsensibleheat(H)fluxes, totheatmosphere[1].Assuminghorizontalhomogeneity,thesurfaceenergybalance,(Fig.1)ofocean couldbeexpressedas,

RN=LE+H+G (1)

whereRNRnandGarepositivewhentheoceanabsorbedtheenergy.However,LEandHarepositive whentheenergyisreleasedfromtheocean.TheunitofmeasurementofthefluxesisinWm²[2].

FromEq.(1),energybalanceclosurecanbeusedtogiveanoverviewonthevalidityofsurfacelayer measurementsusingtheEddyCovariance(EC)systemofmeasurements.Thoughthereisnoperfect energybalanceequation,itisunderstandablethatenergybalanceattheEarth’ssurfacecouldnotbe closedformanyexperimentaldatasetsfromvariedsurfaces,e.g.,oceanicandterrestrialsurfaces[1].

TheratiobetweenLEandHfluxescouldbecomputedtoevaluatetheenergybalanceclosure(or EBCintheformoffractionorpercentage).Theabsoluteenergyresidual(R_es,Wm²)wascalculated usingEq.(2).Resisaddedto(1)togetacompletebalanceequationfortheenergyasshownin(3)[1,2].

Giscalculatedfromtheintegrationofthechangesintheunderwatertemperatureprofilewithtime.

Res=RNHLEG (2)

Fig.1.TheenergybudgetatthetropicalcoastaloceanstationnamedastheMukaHeadstation,PulauPinang,Malaysia,inthe southernSouthChinaSea(528⁰6⁰⁰N,10012⁰1⁰⁰E).

RN=Res+H+LE+G (3) The fluxesweremeasuredusing aneddycovariancesystem,whichwasinstalled ona stable stainless-steelplatformextendingapre-existingpiersothatthesystemwouldbedirectlyoverthe tropicalcoastal ocean. The eddy covariance system included a 3-D sonic anemometer (model 81000V,RMYoung,USA)andanopen-pathCO2/H2Ogasanalyzer(modelLI-7500A,LI-COCOR, Inc., USA) installed 4.1m above the sea water surface, (Fig. 2). The gas analyzer and sonic anemometerwerefactory-calibratedbeforedeployment.A datalogger(model LI-7550Analyzer InterfaceUnit,LI-COR,Inc.,USA)wasusedtorecordtheeddycovariancedataatafrequencyof20Hz.

Thesonicanemometerwasalsousedtomeasurewindspeed(U)andwinddirection(WD).Theflux datawasaveragedin 30-minblocks.Thefluxmovementconventionusedis,negativefluxvalue indicatingdownward-movingfluxwhilepositivevalueindicatingupward-movingflux.

To complement the eddy covariance system, the “Biomet” system of slow-response sensors measuredthemicroclimatevariables,theatmospherictemperature(TA)andrelativehumidity(RH) sensor(modelHMP155,Vaisala,Finland; theaccuracyofT_Ais0.7Cand theaccuracyofRHis 1.7%),apyranometer(RG)(modelLI-200SL,LI-COR,Inc.,USA;errorwas<5%),andanetradiometer (RN)(model NR LITE 2, Kipp& Zonen, Inc., USA; sensitivity was 13.6

m

^{V W–1 m–2). Seawater}

temperatures(TS)atdepthsof0.5m(TS1)and2.5m(TS2)weremeasuredusingtwothermistors(LI- COR,Inc.,USA)withtheaccuracyof1C. SinceTS1 waspositionednearthewatersurface, the temperaturemeasurementswereassumedtobetheseawatersurfacetemperature.Anadditional dataloggerrecordedtheBiometdata(model9210bXlite,SutronCorporation,USA)atasampling frequencyof1minandaveragedin30-minblocks.Allslow-responsesensorswerefactory-calibrated beforeinstallation.Cumulative15-minprecipitationdata(DavisVantagePro2Plus,Davis,USA)was retrievedfromanearbyweatherstationIPENANGP2(5284⁰'N,10017⁰25⁰'E).

TheEddyProVersion6software(LI-COR,Inc.,USA)wasusedtoprocesstheraw20-Hzdata[3].The softwarewasusedtofiltertherawdatasetsusingmethodsdescribedby[4]specifically,thefollowing operationswerecarried,i.e.,amplituderesolution,spikeremoval,dropouts,absolutelimitsinaddition toskewnessandkurtosis.Thesonicanemometerwassubjectedtotwo-foldrotationstechniquefor thetiltcorrectiontoguaranteethatthemeanverticalwindspeediszero.Footprintestimationas describedby[5]wasalsoemployed.

Fig.2.Locationofthestudysitenamedasthe“MukaHead”Station(528⁰6⁰⁰N,10012⁰1⁰⁰E)whiletherightpaneliszoomed-out viewofthesiteshowingtheInstrumentsPlatformtogetherwiththesensorsandtheeddycovariancesystem.

Methodvalidation

1.QualityControl:Thefluxesarequality-flaggedintheEddyProsoftwareinwhichthefinalflag outputswasbasedonacombinationoftwotests,eachprovidingapartialstandard.Thetwotestsare thesteady-statetestanddeveloped-turbulenceconditionstests.Thefinalstandardoutputswiththe value“0”indicatesthebestqualityflux data,“1”meansgoodqualityfluxeswhile“2” meansbad qualitywhichwassubsequentlyremovedfromthedata.Theclassificationofenergyfluxesflagging wasdevelopedaftertheseconddiscussionintheCarboEuropeworkshoponthequalityassuranceand qualitycontrolofECmeasurements[6,7].In addition,theratioof thestandarddeviationforthe verticalcomponentofthewindspeed,

s

w(ms¹)andthefrictionvelocity,u_*(ms¹),i.e.,

s

w/u_*was foundtoholdonlyforwinddirections0to90,whichwastypicallyfromthesurfaceoftheoceanwith ahomogeneouslyflatsurface.Thisratiodidnotholdforotherdirections,i.e.,typicallyforrough surfacesfrom90to360andthus,fluxdatafromthesequadrantswerediscarded.Thepercentageof dataretained,afterfollowingthequality-controlledprocedures,is21%forLEandH.Thedatawasnot filledusinganydata-gapmethod.

2.WindSpeed:OneofthephysicaldriversofenergyfluxesistheWindSpeed(WS)[8].Theaverage WSinthetropicalcoastaloceanlocationwasmostlylight,withanaverageof0.520ms¹throughout thestudyperiod.AsshowninFig.3,mostofthehighWSvaluesofbetween2ms¹and4ms¹were north-easterly(0 to90), whichwas blowingfromtheseadirectionandfinally usedinfurther analysis.TheWSvalueobservedfromtheinlandalongthesouththroughtothewestdirectionstothe station,i.e.,between90and360werediscardedduringtheanalysisduetopoor-qualityflagsand deviationsfromtheratio1.3,asmentionedabove.However,thewindroseshowthatwindswere evenly-distributedamongthefourquadrants,whichsuggeststhatfluxeswerewell-representedforall seasonsthroughoutthestudyperiod.

Fig.3.WindroseobtainedfortheperiodfromNovember2015toOctober2017 withthefrequencycountinpercentage.

3.Turbulentheatfluxes:Thelatentheat(LE)andsensibleheat(H)fluxesinthisstudylocationwere relatively low, which ranged from 36W m² to 130W m² and 12W m² to 24W m², respectively.Thelow valuesrecordedcouldbeperhapsdue totheweak windsvelocitiesof this location.Theaveragehalf-hourlyLEshowedaverticalpositivefluxofwatervaporwith11.67Wm^-2 value.HwasmuchlowerthanLE,whichrecordedatthehalf-hourlymeanof1.56Wm².TheBowen ratio(theratioofH/LE=0.13)isequivalenttotheglobaloceanestimate.

4.Underwatertemperature:Inthisstudy,underwatertemperature(C)trendsattwolevelswere measured.T_S1isthetemperaturenearthesurfacewhileT_S2isthetemperaturenearertotheseabed.

Thesetwounderwaterthermistorsshowedthatthetemperaturesvariedgreatlybetween27Cand 33C.TS1wasalwayshigherthanTS2,withmeantemperaturesat30.85Cand30.27C,respectively.

ThisisbecausethethermistorthatmeasuredthetemperatureatTS1issometimesexposedtothe atmosphere,duetolowtides,andthusrecordedahighertemperatureincomparisontoTS2locatedin theoceanwater.ThereweresomedatagapsforTS1betweenthemonthofAugustandmid-September.

Thiswascausedbythebreakdownofthermistornearthewatersurface,givingsuperextremevalues andsothedatawasdiscarded.Generally,TS1andTS2didnotvarysignificantlybuttheyshowedlarge changesinthetemperaturesdiurnally.However,therewasnodifferenceinthewatertemperaturesat thetwodepths.

5.Globalandnetradiations:Thehalf-hourlyrangesofR_GandR_Nwere0Wm²to1020Wm²and -130Wm² to990Wm²,respectively.TherecordedglobalRGandnetRNradiationsduringthe reportingperiodsrecordedpeakvaluesduringthemiddleofMarch,forbothRGandRN,averagedat 217Wm²and 163Wm²,respectively.Thissituationisan indicationthat thetropicalcoastof southernSouth ChinaSeareceivedthemaximumamountof solarradiation,possiblydue tothe extremeheatwavefromthesunalongthetropicalequatorialbeltinthismonth.

6.Airtemperatureandrelativehumidity:Fluctuationsintheairtemperaturerecordedbetween22C to32Cthroughoutthestudyperiod.Thedaytimetemperature,obtainedwasconsiderablehigher thanthenighttimetemperature.Differencesinthetemperatureswasapparentparticularlyduring precipitationintheearlyhoursofthedayandatnighttime.Thelastmicroclimatevariablemeasured atthestationistherelativehumidity(RH)expressedinpercentage,%,whichrangedfrom50%to100%.

Thisrangeistypicaloftropicallocations.

Acknowledgements

WeacknowledgeUniversitiSainsMalaysia(USM)forawardingustheUSMResearchUniversity Grant(1001/PTEKIND/811316)thatmadethisresearchpossible.WealsothankLooiMeiThung,Chong YiHuan,andKulwantSinghforassistingustocollectdataandmaintaintheeddycovariancestation.

AppendixA.Supplementarydata

Supplementarymaterialrelatedtothisarticlecanbefound,intheonlineversion,atdoi:https://doi.

org/10.1016/j.mex.2018.05.003.

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