The increasing trend of the urban heat island intensity

2018·0.45 MB·English

by  Geoff Levermore

UrbanClimate24(2018)360–368
ContentslistsavailableatScienceDirect
Urban Climate
journalhomepage:http://www.elsevier.com/locate/uclim
The increasing trend of the urban heat island
intensity
GeoffLevermorea,⁎,JohnParkinsona,KwanhoLeeb,PatrickLaycocka,
SarahLindleya
aTheUniversityofManchester,UnitedKingdom
bUlsanCollege,Ulsan,SouthKorea
a r t i c l e i n f o a b s t r a c t
Articlehistory: Thispaperexaminestheurbanheatislandintensityindetailinthecity
Received16September2016 ofManchester,UK.Anincreasingintensityisfoundovertime.Theurban
Receivedinrevisedform10February2017 heatislandintensity(UHII)dataisexaminedinmoredetailgivingrela-
Accepted13February2017 tionshipsbetweenweatherparameters,cloudcover,windspeedand
theurbanmorphology.TheurbanheatislandintensityinManchester
Keywords: hasahighlysignificantrisingtrendwhichbytheendofthecentury
Urbanheatisland couldadd2.4Ktotheaverageannualurbantemperature,ontopof
Risingtrend
thepredictedclimatechangeincrease.Ananalysisoftheurbanmor-
Temperaturemeasurements
phologyshowedthattheurbansitehadindeedbecomemoreurban
over9yearsofthestudy,losinggreenspaceswhichmitigateagainst
theUHII.
©2017ElsevierB.V.Allrightsreserved.
1.Introduction
Theurbanheatislandintensity(UHII),thedifferenceintemperaturebetweenanurbansiteandarural
site,isameasureoftheurbanheatisland(UHI)effect.TheUHImeansthatcitiesandtownsarewarmerat
nightthanruralareasduetotheabsorptionofthesun'sradiationintheurbanconcreteandbuildings,the
factthattheurbanbuildingsareshadedatnightfromheatlosstothecoldclearskyandthatthereareheat
gainsintheurbanareasduetocarsandtransportandbuildings'energyuse.Thisisimportantinthedesign
andenergyassessmentsofbuildingsinurbanareas.Itwillbeespeciallyimportantinthefuturewithclimate
changeandpotentialglobalwarmingasthetemperatureislikelytoincreaseandtheUHIwilladdtoit.Recent
measurementsinManchestershowasummermaximumurbanheatislandintensityof8°C.Climatechange,
⁎ Correspondingauthor.
E-mailaddresses:[email protected](G.Levermore),[email protected](J.Parkinson),
[email protected](K.Lee),[email protected](P.Laycock),[email protected](S.Lindley)
http://dx.doi.org/10.1016/j.uclim.2017.02.004
2212-0955©2017ElsevierB.V.Allrightsreserved.
G.Levermoreetal./UrbanClimate24(2018)360–368 361
UKCP09projections(UKCIP,2016),indicatethatsummermeantemperaturesinthenorthwestoftheUK
couldriseby5°C(50%probability,7°Ctopoftherange)bythe2080s(highemissionsscenario).TheUHI
willaddtotheurbanareatemperatureontopofclimatechange.
TheUHIisimportantasbuildingsneedtocooloffatnighttogetridoftheirstoredheat.ResearchinMan-
chester(LeeandLevermore2013),modellinganinterwarhouseindicated3Kriseininternaltemperaturefor
amodestUHIIof5Kwitha60%riseindiscomforthoursin2050.Overheatinginsummerisalsoaproblemfor
modern,lowenergy,wellinsulatedhouses(Goodhomesalliance,2013)climatechangeandoverheating:op-
portunitiesandrisksfordesignersandthesupplychain.London:TheBuildingCentre,2013.http://www.
goodhomes.org.uk/events/138(accessed14November2013).Thisoverheatingresultsintheoccupantshav-
ingdifficultysleepingandpossiblyresortingtoinstallingfansorfullair-conditioningleadingtomoreelectric-
ityconsumption.Non-domesticbuildingsaresimilarlyaffectedandevenlowenergynon-domesticbuildings
utilisingnaturalventilation,havegreaterdifficultydischargingtheirstoredheatovernightinurbancentres
duetotheUHImaintainingawarmermicroclimatearoundthem.
TheUHIisprobablyacontributortotheconsiderableperformancegapofnewbuildingsbetweenthede-
clareddesignenergyconsumptionandtheactualconsumption,thelatterbeingbetween1.5and2.5greater
thantheformer(Kimpian,2013).
AlltheseproblemswillbeevenworseastheUHIincreasesasthispapersuggests,especiallyforManches-
ter,UK.
2.ManchesterUKresults
Fig.1showsthelocationofManchesterUK.
ForManchester,UK,theweatherdataatHulme,just2kmsouthofthecitycentrewasavailableforanum-
berofyears.AsitisnotinthetruecentreofthecitytheUHIeffectmeasuredherewouldbeexpectedtobe
slightlydiminishedbycomparisonwiththetruecentre.TheruralsiteisactuallyManchesterAirport,referred
toasRingway,whichisabout12kmfromthecitycentreHulmeandRingwayareUKMetOfficesites.Butsince
Fig.1.ThemainmapshowsManchesterCity(red)withinGreaterManchester(lightgrey).InsetshowsManchesterUKintheNorthWest
ofEngland,UK.
362 G.Levermoreetal./UrbanClimate24(2018)360–368
2005theMetofficesiteRingwaywasreplacedbyasiteatWoodford.Woodfordisasiteabout11kmsouth
eastofRingway.Itisamoreruralsitenexttoasmallairfieldnexttoanaircraftfactory.Thereare132,992
hourlymeasurementsofUHIIasmeasuredbythedifferenceintemperaturebetweenHulmeandRingway/
Woodfordduringtheyears1996to2011.Thefirstmeasurementwasat1amon1stJanuary1996andthe
lastmeasurementat11pmon31stDecember2011,implyingapotentialsetof140,256UHIhourlyvalues,
butthereareasmallproportion(5.2%)missing.Thesemissingvaluesshouldhavenosignificantimpacton
theconclusionsreachedinthispaper.
Fig.2showstheyearlyaveragesforUHI.Thereisaclearupwardtrendintime.
Thefittedtrendline:
UHI¼0:021(cid:2)YEAR−40:6 ð1Þ
hasastatisticallysignificant(Pb0.1%)slopeof0.021°Cperannum.Ifthistrendweretocontinuethenover
thecenturytheUHIwouldbe2.42°C.Thisisapproximatelyequaltothelowerpredictionsofclimatechange
andisinadditiontoclimatechange.
Thissetofyearlymeans(eachpointrepresenting8760hourlyvalues)hasaminimumof0.479°C(in
1997),amaximumof1.004°C(in2010)withanoverallaveragevalueof0.762°C.Itisalsoimportanttore-
alisethattheindividualhourlymeasurementsshowmuchvariationaboutthistrendline(residualvarianceof
0.776°C)andhenceitisnotalwaysapositivenumber.Inthedataitsvaluesrangefrom−11.00°Cto12.90°C.
Theseextremevaluesmayhavearisenasaresultofrecordingerrorsatoneorothersiteatthattimeonthat
particularday.Theassumptioninthisworkisthatanysucherrorsarerandomlydistributedandrelatively
rare,soinducingnosystematicbiasinthedataorweatherfrontscrossingtheurbanandruralsitesatdifferent
times.
3.Manchesterurbanmorphology
Fig.3showsthelocationoftheurbanmeasuringstationinHulme,partofManchesterCity.Itisquitebuilt
upbutmainlywithlowrisebuildings.
Fig.4showsthedigitisedareasofgreenspaceneartothemeasurementstation.Itwaspossibletoassess
thegreenspaceanditschangesbetween2000and2009.Itcanbeseenthatthegreenareahasreducedbyup
to11%overthewholeareashownalthoughitisonlya1.5%reductionwithin200mofthestation(the
smallestcircle)(Levermoreetal.,2015.
4.Manchesterclimatology
AlthoughthechangeinUHIIisconsideredtobemainlyduetothereductioningreenareasaroundthe
urbansiteofHulmetheweathercouldalsohavebeenchangingaswell.TheUHIisinfluencedbythecloud
coverandwindspeed.TheUHIreducesasbothincreaseandviceversa.Fig.5showsthatthewindspeed
hasindeedreducedalittlebutthecorrelationwithtimeispoorindicatingthatitisalmostinsignificant.
Fig.2.UHIYearlyaveragesplustrendlineforManchesterUK.
G
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/
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n
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lim
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te
2
4
(2
0
1
8
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3
6
0
–
3
6
8
Fig.3.TheareaaroundHulme,theurbansite(weatherstationsiteshownbystar)(Googlemap).
3
6
3
364 G.Levermoreetal./UrbanClimate24(2018)360–368
Fig.4.Digitisedareasofgreenspacerelativetothezonesofassessment.
Fig.6showsamoresignificantbutslightlylowerreducingtrendwithtime.
5.Manchestermulti-parametermodelledurbanheatislandintensity
Althoughthemorphologyoftheareasaroundtheprimarymeasurementsite(Hulme)ischangingwith
time,losinggreenareas,thelocalclimatehaschangedslightlyasshownabove.Toexaminetheinfluenceof
thelocalclimateontheUHIIastatisticalmodelwasmadefromthehourlydatafortheurbanandrural
sites(Levermoreetal.2015).Thiswasachievedbyusingthemultipleregressionfacilitiesofthestatistical
G.Levermoreetal./UrbanClimate24(2018)360–368 365
Fig.5.Thechangeofwindspeedattheruralsiteovertime.
packageSPSS.Afterfitting,thematchedpairsofthesineandcosinelinearmodeltermsusedforthediurnal
andseasonalvariationwereeachcombinedintosinglenonlinearsinetermsincorporatingaphaseshift.
Thefollowingformulawasobtained:
y¼b þb x þb x þb x þb x þb x þb x þe ð2Þ
0 1 1 2 2 3 3 4 4 5 5 6 6
wherethedependentvariable
y¼UHI ðKÞ
ð3Þ
¼ðHulmetemp−RingwaytempÞ
andthesixpredictorvariablesare:
x =Year-2000
1
x =airtempin°CatRingwaytruncatedabovezero
2
x =windspeedatRingwaytruncatedat10knots
3
x =windspeed(knots)pluscloudcover(oktas)truncatedat14
4
x =SIN(2π((MONTH=1.2,…,12)+1.248)/12)
5
x =SIN(2π((HOUR=0.1,…,23)+5.646)/24)
6
Theresidualerrortermwasmodelledashavinganormalprobabilitydistribution:
e~N(0,0.553)
Theindividualregressioncoefficientswerefoundtobe:
b =1.834954b =0.018173b =−0.235607b =−0.016058
0 1 2 3
b =−0.091833b =0.104268549b =0.21678237
4 5 6
Althoughthismodelisforhourlyvaluesitwasusedforyearlyaveragevalues.
Fig.6.Thechangeofcloudcoverwithtimeattheruralsite.
366 G.Levermoreetal./UrbanClimate24(2018)360–368
Fig.7.TheaccuracyoftheSPSS-basedmodeloftheUHIIfortheannualdataforall15years.
Thesinetermsaveragedoverayeardisappearasdothemonthlysineterms.Alsotheairtemperatures
truncatedabovezerodisappearsastheaverageairtemperatureisabovezero.
However,themodelusingyearlyaveragedparametersproducedareasonablecorrelationwiththeaver-
agedvaluesfromthemeasureddata(seeFig.7).Althoughthereissomebiasinthemodel,evidencedby
thelineartrendlinenotgoingthroughzero,themodelisusefulforcomparingtheinfluenceofthecloud
coverandwindspeedontheUHII.
Fig.8showstheindividualcontributionsfromthemodeltotheUHII.Thewind(x withb coefficient)has
3 3
littleinfluencetotheincreasingUHIIwithtime.Theincreaseisveryshallowwithaslopeof0.0006Kyear−1
andapoorcorrelation(R2of0.1429).Thecloudismoreinfluential(x withb coefficient)buttheYearofthe
4 4
averageUHIIishighlyinfluential.Thisindicatesthatnon-weatherfactorssuchastheurbanitymostlikelycon-
tributetotheincreasingUHII.
6.Manchesterruralsitemicroclimatology
FordeterminingtheUHIIthemeteorologicalsiteoutsidetheurbanareawasRingway,ManchesterAirport.
Thisisnotafullyruralsitehowever,andtheairporthasexpandedfrom1996to2011withasecondrunway
anditsinfrastructurebuilt.Theofficialnon-ruralsitewaschangedtoWoodford,nearRingwaybutamore
ruralsite.Thesiteisarunwaynearanaircraftmanufacturingsitewithasignificantagriculturalareaoffields
Fig.8.ThecontributionofwindandcloudcovertotheincreaseofUHIIovertime.
G.Levermoreetal./UrbanClimate24(2018)360–368 367
Fig.9.Thecomparisonoftheairtemperatureatthe“rural”sites,RingwayandWoodford.
Fig.10.Thecomparisonofthewindspeedatthe“rural”sites,RingwayandWoodford.
surroundingit.TherewerethreeyearsofoverlappingweatherdataforbothRingwayandWoodford.Fig.9
showsthehigherairtemperaturesatRingway,asmallUHIexistingthere.ThiswouldlessentheactualUHII
measuredintheurbanareaofHulme.However,thewindspeedatWoodfordissignificantlylowerthan
Ringway(seeFig.10).AsthewinddispersestheUHIthiscounterstheairtemperatureincreaseatRingway.
7.Conclusions
InManchester,UK,theUHIIisincreasingwithtime.Ifitcontinuestoriseitwillbesimilarattheendofthe
centurytothepredictedriseduetoglobalwarming(UKCIP,2016,MetOffice,2016,LeeandLevermore2013).
Thecloudcoverhasreducedwithtime,contributingtotheUHIIrisealittle.Butthisdoesnotaccountforthe
significantrise.Itisconsideredthattheincreasedurbanitymaywellbetheprimecause.Thegreenarea
aroundtheurbansitehasreducedby11%overpartoftheperiodofthisstudy(2000to2009).TheUHII
risemayactuallybehigherasthesemi-ruralsitewasRingway(ManchesterAirport)whichhasbeenexpand-
edandhasaslightUHIIitself.
ThisincreaseinUHIIhasseriousimplicationsforbuildingsandoccupantsinManchester.Thebuildings
willbecomemoreoverheatedinthesummerwiththeconsequentdiscomfortforoccupants.Thereisalso
theimplicationfortheinstallationofmoreairconditioningandtheincreaseinenergyuseandcarbon
emissions.
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