INTERNATIONALJOURNALOFCLIMATOLOGYInt.J.Climatol.:1 - PDF document

INTERNATIONALJOURNALOFCLIMATOLOGYInt.J.Climatol.:1–26(2003)PublishedonlineinWileyInterScience(www.interscience.wiley.com).DOI:10.1002/joc.859TWODECADESOFURBANCLIMATERESEARCH:AREVIEWOFTURBULENCE,EXCHANGESOFENERGYANDWATER,ANDTHEURBANHEATISLANDA.JOHNARNFIELD*DepartmentofGeography&AtmosphericSciencesProgr *Correspondenceto:A.JohnArneld,DepartmentofGeography,TheOhioStateUniversity,154NorthOvalMall,Columbus,OH43210-1361,USA;e-mail:john.arneld@osu.edu2003RoyalMeteorologicalSociety A.J.ARNFIELDdrivenprogressinotherareasofurbanclimateresearch.Third,itwillbeconnedtoscienticliteraturepublishedintheEnglishlanguage.Thetopicalfocusofthisreviewistwofold.First,itwillassessadvancesinthestudyofselectedurbanclimaticprocessesrelatingtourbanatmosphericturbulence(includingsurfaceroughness)andexchangeprocessesforenergyandwater.Specically,thiswillbedoneforcharacteristicscalesofconsiderationfromabout10to10m,correspondingtoindividualfacetsoftheurbanenvironment,throughstreetsandcityblockstoneighbourhoods.Understandingtheseprocessesiscriticaltotheexplanationoftheclimaticcharacteristicsofurbanlandscapesatthis‘humanscale’,andourdeepeningknowledgeintheseareashasfurtheredanddrivenadvancesinotherareasofurbanclimatology.Second,thisreviewwilldescribeandevaluatewhathasbeenlearnedaboutthetemperatureeldsofcities,includingtheurbanheatislandwhichisoftenregardedasthemostwelldocumentedexampleofanthropogenicclimatemodication.Adoptionofthisparticularperspectivewillmeanthatmanyotherareasofurbanclimatologymustbeneglected.TableIprovidesreferencestosourcesofinformationonotherurbanclimatetopicsthathaveappearedshortlybeforeorduringthereviewperiod.Furthermore,thisreviewwilllargelyignoreofurbanclimatology,despitetheirsignicanceandfar-reachingscope:TableIIprovidessomerepresentativeandillustrativeexamplesfromtheseareas.2.CONCEPTUALADVANCESANDURBANCLIMATOLOGYTheperiodaddressedbythisreviewhasseentheinitiationorconsolidationofseveralconceptualadvancesrelevanttourbanclimatology.Someofthesearespecictotheurbanatmosphericenvironment,whileothersareofmoregeneralapplicabilitywithinboundary-layerclimatologybuttheyaddressissuescrucialtotheunderstandingofthecityatmosphere.Thissectionwillreviewsomeoftheseadvancesingeneralterms;manyofthesewillariseagaininsubsequenttreatmentsofparticularempiricalstudies.2.1.Scale,spatialheterogeneityandurbanclimateTheconceptofscaleisfundamentaltounderstandingthewaysinwhichelementsoftheurban‘surface’interactwithadjacentatmosphericlayers.Anindividualbuilding,forexample,consistsofwallsandrooffacets,eachwithadifferingtime-varyingexposuretosolarradiation,netlongwaveradiationexchangeandventilation(e.g.Arneld,1984,2000b;PatersonandApelt,1989;VerseghyandMunro,1989a,1989b).Horizontalground-levelsurfacesareapatchworkofelements,suchasirrigatedgardensandlawns(Oke,1979a;Suckling,1980),non-irrigatedgreenspace,andpavedareas(Dolletal.,1985;Asaedaetal.,1996;Anandakumar,1999)withcontrastingradiative,thermal,aerodynamicandmoistureproperties,frequentlyincludingtrees(Oke,1989;Grimmondetal.,1996;KjelgrenandMontague,1998).Thesedifferentsurfaceelementspossessdiverseenergybudgetsthatgeneratecontrastsinsurfacecharacteristics(e.g.‘skin’temperature),andleadtomutualinteractionsbyradiativeexchangeandsmall-scaleadvection.Thesefundamentalmorphologicalunitsmaybeaggregatedhierarchically.Buildingwallsandtheelementslyingbetweenbuildings,forexample,denetheurbancanyon(UC).UCsandtheroofsofadjacentbuildingsdenecityblocks,whichinturnscaleuptoneighbourhoods,land-usezonesand,ultimately,theentirecity.Ateachscale,unitswillpossessdistinctiveenergybalancesthat,ingeneral,representmorethanthearea-weightedaverageofthebudgetsofindividualelementsbutalsoincorporatethedistinctiveinteractionsamongtheirconstituentunits.Moreover,eachunitinteractswithadjacentonesinthesamescalecategorybyadvectionetal.,1983).Asspatialscaleincreases,spatialvariabilityislikelytobereduced;thatis,thereisprobablylessdifferenceamongtwoland-usezonesinacitythanbetweenthenorth-andsouth-facingwallsofanindividualbuildingwithinoneofthem(SchmidandOke,1992).Urbanclimatologyisrequiredtocometotermswiththisheterogeneityandcomplexity,eitherexplicitly,intermsofdetailedmappingofurbanmorphology(Ellefsen,1990–91;GrimmondandSouch,1994;CioncoandEllefsen,1998),orininterpretingobservationsataggregatescales.Linkagesamongscalesareexplicit,forexample,inparameterizationsofthestorageheatuxusingtheobjectivehysteresismodel(OHM)ofGrimmondetal.(1991),whichaggregatestheheatstorageresponsesCopyright2003RoyalMeteorologicalSocietyInt.J.Climatol.:1–26(2003) URBANCLIMATETableI.Reviews,bibliographiesandsummariesonurbanclimatologypublishedduringthereviewperiod ReferenceTopic Oke(1979b)Reviewofurbanclimatology,1973–76Changnon(1980)TheLaPorteprecipitationanomalyOke(1980)Bibliographyofurbanclimateliterature,1977–80Landsberg(1981a)MultipletopicsinurbanclimatologyChangnon(1981)AreviewandsummaryoftheMETROMEXprojectLandsburg(1981b)MultipletopicsinurbanclimatologyOke(1982)UrbanenergybalanceDouglas(1983)UrbanphysicalenvironmentingeneralwithonechapteronurbanclimateLockerby(1983)BibliographyontheclimateofcitiesLee(1984)MultipletopicsinurbanclimatologyOke(1986)UrbanclimatologywithspecialemphasisontropicalOke(1988b)UrbanenergybalanceSmithson(1989)Progressreport(includes‘urbanclimates’)Oke(1989)TreesincitiesSmithson(1990a)Progressreport(includes‘urbanclimates’)Smithson(1990b)Progressreport(includes‘urbanclimates’)Yoshino(1990–91)EmphasisonJapaneseurbanclimateworketal.(1990–91)TRUCE(TropicalUrbanClimateExperiment)etal.(1991)IncludessectiononurbanclimatologyChangnon(1992)InadvertentclimatechangeinurbanareasandlessonsforglobalclimateworkSteyn(1992)Integratedstudiesofairpollutionmeteorologyetal.(1995)WindclimatesincitiesGoldreich(1995)ReviewofIsraeliurbanclimateworkArneld(1998a)Reviewofurbanclimateworkpublishedin1996Arneld(1998b)Reviewofurbanclimateworkpublishedin1997Sturman(1998)Urbanclimateprocesses,mitigationofurbanclimateeffectsandimplicationsforurbandesignLowry(1998)UrbaneffectsonprecipitationamountincludingacritiqueofexperimentaldesignArneld(2000a)Reviewofurbanclimateworkpublishedin1998Roth(2000)AtmosphericturbulenceinurbanenvironmentsArneld(2001a)Reviewofurbanclimatework(‘urbancanyonstudies’and‘urbanclimatesoutsidethemid-latitudes’)publishedin1999Arneld(2001b)Reviewofurbanclimateworkpublishedin2000 ofdiversesurfacetypestonetradiationforcingaccordingtothespatialextentofeach.Likewise,UCenergybudgetmodels(TerjungandO’Rourke,1980a,b;Mills,1993;Arneld,2000b)generatecanyonsystemenergybudgetsbycombiningthebudgetsoftheconstituentfacets,whichincorporatemutualinteractions.Fundamentaltotheissueofscaleisthedistinctionbetweentheurbancanopylayer(UCL)andurbanboundarylayer(UBL).Thisdistinction,originallyappliedtoUHIsbyOke(1976),hasbeenaguidingprincipleinurbanclimateresearchofalltypes.IntheUCL(roughlyfromgroundtorooflevel),processesofairowandenergyexchangearecontrolledbymicroscale,site-speciccharacteristicsandprocesses.TheUBL,aboverooflevel,incontrast,isthatpartoftheplanetarybound-arylayerwhosecharacteristicsareaffectedbythepresenceoftheurbansurface(oritsland-usezones)belowandisalocal-tomeso-scalephenomenoncontrolledbyprocessesoperatingatlargerspatialandCopyright2003RoyalMeteorologicalSocietyInt.J.Climatol.:1–26(2003) A.J.ARNFIELDtemporalscales.Thedistinctiongoesbeyondmerescale,therefore:itreectsdifferentassemblagesofprocessesThepatchyandheterogeneousnatureoftheurbansurfacehassignicantimplicationsintheinterpretationofmeasuredenergybudgetsandinthedesignoftower-baseduxstudies.Fewideashavehadsuchaprofoundimpactonurbanclimatologyduringthereviewperiodthantherealizationthatturbulencecharacteristics(includinguxes)measuredatheightoveracomplexurbanlandscapemaynotbeinequilibriumwiththesurfacebelow,butmayinsteadreectthecharacteristicsofthesurfaceupwind,thesizeandlocationofwhichwilldependonwindspeedanddirection,roughnessandatmosphericstability(SchmidandOke,1990;Schmid,1994).Thesource-areamodelofSchmidandOke(1990)hasfoundapplicabilityinthesuburbanenergybalancemeasurementsofOkeetal.(1989),Schmidetal.(1991)andGrimmond(1992).2.2.TheroughnesssublayerSeveralexperimentalstudies(e.g.Thometal.,1975,Garratt,1978,1980)inagriculturalandforestmeteorologyhavedemonstratedthat,intheairlayersabovehorizontallyuniformsurfacetypeswithtallroughnesselements,conventionalux–prolerelationshipsandMonin–Obukhovsimilaritytheoryarelikelytobeinvalid.Inthislayer,termedtheroughnesssublayer(Raupach,1979),owconsistsoftheinteractingwakesandplumes(ofheat,humidityandpollutants)introducedbyindividualroughnesselements.Atsomeheightabovethecanopy,theblendingeffectofturbulentmixingwillerasethesignicanceofindividualroughnesselementsandcreatealayer(theinertialsublayersurfacelayerconstant-ßuxlayerTennekes,1973;Roth,2000)inwhichturbulentuxesareconstantwithheight,permittingmeasurementoflandscape-scaleenergybalanceuxesandReynoldsstress.TheroughnessandoverlyingsurfacelayerconstitutethelowestportionoftheUBLdenedintheprevioussection(Rothetal.,1989b).Thenatureoftheurbansurface,withitsrigidbuildingsofdifferentheightsandphysicalcharacteristics,separatedbytrees,canyonsandopenspaces,makesitparticularlysusceptibletothedevelopmentofaroughnesssublayerofsignicantdepth,perhapsseveraltimestheaveragebuildingheight(Raupachetal.,1980,1991;Roth,2000).Withintheroughnesssublayer,characteristicsdependonahorizontaldistancescaledeterminedbyinter-elementspacing,ratherthanheightandverticaltemperaturegradient,asinthesurfacelayer.Flowandturbulenteldsaredifferentfromthoseintheinertialsublayer(Hetal.,1982;Rotach,1993a,b;Roth,1993;RothandOke,1993;OikawaandMeng,1995).Strongverticalshear,largeturbulenceintensities,wakediffusion,formdrag,diversityandspatialseparationofsourcesandsinksofmomentumandscalars,andlocaladvectionstemmingfromextremeheterogeneityarethenormwithinthislayer(Roth,2000).Rothetal.(1989b)provideevidencethatmeasurementsofuxesandturbulentspectramadeclosetotheinterfacebetweenthesurfaceandroughnesslayersareingoodagreementwiththosefromsmoothersurfaces,andmakerecommendationsregardingimplicationsformeasurementsofeddyuxesfromtowersoversuburbanterrain.ProlesofturbulencestatisticsacrossaroughnesssublayerarepresentedbyRotach(1995),andthecaseforexplicitrecognitionoftheroughnesssublayerinoperationalmodelsforpollutiondispersionhasbeenmadebyRotach(1999)anddeHaanetal.(2001)andcanbeexpectedinurbanclimatesimulationmodelsforthesamereasons.2.3.ThediversityofUHIsTheUHIremainsacompellingfocusofclimateresearchinbuilt-upareas.However,acharacteristicfeatureofworkduringtheperiodofthisreviewistheemergenceoftheideathattherearemanyUHIs,displayingdifferentcharacteristicsandcontrolledbydifferentassemblagesofenergyexchangeprocesses.Oke’s(1976)originaldistinctionbetweentheUHIintheUBLandthatintheUCLhasbeendiscussedbrieyinSection2.1.Thoughthesepossessdifferentscalemanifestationsandarethoughttoresultfromdifferentprocesses(Table2inOke(1982)),bothhavehistoricallyfoundexpressioninantemperatureexcessoverthatintheruralenvirons.Ground-basedthermalremotesensing(andaircraft-basedthermographyatalowenoughelevationtoresolvestreets,roofsandwalls)permitsdenitionofyetanotherUHI,namelythatforthegroundsurface.Copyright2003RoyalMeteorologicalSocietyInt.J.Climatol.:1–26(2003) URBANCLIMATETableII.Examplesofresearchpublicationsinapplicationsofurbanclimatology UrbanclimateapplicationExamplesofresearch Meteorologicalandclimaticcharacteristicsattheindividualbuildingscaleetal.(1999);Hoyanoetal.(1999);MurakamiandMochida(1989):Smithetal.(2001)UrbanairpollutiondispersionGrantandWong(1999);HannaandChang(1992);KotakeandSano(1981);Taha(1997)UrbandesignandplanningdeSchillerandEvans(1996);Oke(1984,1988a);etal.(1999)Biometeorologyandhumancomfortintownsetal.(1982);deAssisandFrota(1999);Pearlmutteretal.(1999)RoadclimatologyintownsShaoandLister(1995);Shaoetal.(1994)EnergyconservationissuesBretzetal.(1998);Rosenfeldetal.(1995);Sailor(1998);SimpsonandMcPherson(1998)HistoricandculturalCamuffoetal.(1999)GlobalwarmingresearchEppersonetal.(1995);Hansenetal.(2001);Jonesetal.(1989);Kuklaetal.(1986) Thoughsurfacetemperaturesshowsomesimilarspatialandtemporalpatternstothoseforairtemperatures,thiscorrespondenceisnotexact.Inparticular,undercalm,clear,nocturnalconditions,theygenerallydisplayamuchstrongerdependenceonmicroscalesitecharacteristics,especiallyskyviewfactorreductionbroughtaboutbystreetgeometry,thandosimultaneouslyevaluatedairtemperatures(Betal.,1985;Eliasson,1990–91,1996a).Theseresultssuggestthatstreettemperaturepossessesasimplercausalitythanairtemperature,whichiscoupledtothethermalstateoftheadjacentsurfacesbutisalsosubjecttoadvectiveinuences(Betal.,1985;StollandBrazel,1992).ItisperhapsthissimplicitythathaspermittedsuccessfulhardwareandnumericalsimulationmodellingofthestreetsurfaceUHIundercalm,night-timeconditions,whensolarshadingisabsentandturbulentinteractionsbetweenstreetandairandadvectiveuxesareminimal(Oke,1981;Arneld,1990b;Johnsonetal.,1991;Okeetal.,1991;Swaid,1993a).Thermalremotesensingfromsatelliteandhigh-altitudeaircraftplatforms(Carlsonetal.,1981;Vukovich,1983;KidderandWu,1987;Rothetal.,1989a;Lee,1993;Nichol,1996)raisesadditionalissues.Whilesomestudieshavereportedsimilaritiesbetweenspatialpatternsofairtemperatureandremotelysensedsurfacetemperature(Henryetal.,1989;Nichol1994),mostsuggestsignicantdifferences,includingthetimeofdayandseasonofmaximumUHIdevelopmentandtherelationshipbetweenlanduseandUHIintensity(Rothetal.,1989a).Inparticular,remotelysensedUHIsareusuallystrongerandexhibitgreatestspatialvariabilitybyday,theoppositetoairtemperatureUHIs.Rothetal.(1989a)suggestthatthesedifferenceshavetheiroriginsinthenatureoftheurbansurface‘seen’byasatellitesensor,especiallyoneatahighelevationangle,givingabird’seyeorplanviewofthecity.Inthiscase,roofs,treetops,roadsandopenhorizontalareasareoversampledandverticalsurfacesandareasbelowtreecrownsareneglected.Thesesensorssamplepoorlythetrue‘activesurface’withinthecity,andthosetypesoffacetsthatarewellrepresentedinthethermalimagetendtohavequitedifferentphysicalpropertiesandradiativeandturbulentenvironmentsfromthosethatareundersampled.ThisissueisaddressedbyVoogtandOke(1997,1998a,b),whouseradiativethermometrytodenea‘completeurbansurfacetemperature’.Theyshowthatthistemperaturecanexhibitsignicantdeviationsfromairborneestimatesofsurfacetemperatureatnadirandoff-nadirviewinganglesandthattheurbansurfaceexhibitsastrong‘effectiveanisotropy’duetodifferentialpatternsofirradiatedandshadedsurfaceswithinasensoreldofview.Shoshanyetal.(1994)presentanimageprocessingtechniquetoeliminaterooftopsignalsfromthermalimagerytoapproximateground-levelsurfacetemperaturesmoreclosely,althoughthismethodstillprovidesnoinformationonwallfacetsfornadirviewingdirections.Copyright2003RoyalMeteorologicalSocietyInt.J.Climatol.:1–26(2003) A.J.ARNFIELDInsummary,developmentsoverthepasttwodecadeshaveunderlinedthatUHIsaremorediversethanoriginallysuspected.Wemaydeneavarietyofsuchfeaturesbasedonthemediumsensed(air,surface,evensubsurface)andthesensingsystememployed.Eachwillpossessitsownclimatologyandwillbesubjecttovaryingcausalinuences,butitisessentialthatthenatureofthemeasurementsusedtodenetheheatislandbepresentedinreportingresultsinamannerthatisinterpretabletotheurbanclimatecommunity.Likewise,greatcaremustbeexercisedincomparingUHIsifthemediumsensedandthemethodsemployedinsensingitdiffer.3.SELECTEDSMALL-SCALEURBANCLIMATEPROCESSES:ATMOSPHERICTURBULENCEANDEXCHANGESOFENERGYANDWATER3.1.UrbanroughnessparametersTheaerodynamicroughnessofcities,asexpressedintheroughnesslengthandzero-planedisplacement,exceedsthatofessentiallyallothertypesoflandscapeelement.Theseparametersareessentialinunderstandingtheprocessesoccurringwithinurbanairlayers(surfacedrag,shearingstress,windproleformsandturbulencecharacteristics),aswellassuchurbanclimaticphenomenaaswindshearovercities,inducedverticalmotions,thedepthoftheUBLandpollutantdispersionaboveurbancanopies.Rooney(2001)providesanexampleoftheexperimentaldeterminationoftheseparametersforBirmingham,UK.Grimmondetal.(1998)reviewanemometry-basedmethodsforevaluatingtheseparametersforfoursuburbanareasinNorthAmericancities.Theparticularproblemsassociatedwithmeasurementsintheurbanenvironmentcreatedifcultiesformanyconventionalmethods,andnoconclusioncouldbereachedonthebestone.Techniqueshavebeenproposedtoestimatethesecriticalparametersfrommeasuresofthegeometricformofbuildingarrays(e.g.Bottema,1997;Duijm,1999).Acomprehensivereview(GrimmondandOke,1999a)ndsthatmostmorphometricmethodsyieldplausiblevaluesofbutthatmost‘observed’valuesareofinsufcientqualitytodecidetrulybetweencompetingtechniques.Statisticalagreementbetweenthoseempiricalvaluesjudgedbestandmorphometricestimatesisnotimpressive.Theimplicationsofthesetwostudiesaresobering.Thoughmorehigh-qualityestimatesofwouldseemtobedesirable,uncertaintiesanderrorsinthecurrentmethodsofobtainingsuchdataarenotencouraging.GiventhecentralityoftheseparametersinmodellingpollutiondispersionandUBLclimates,however,GrimmondandOke(1999a)suggestpracticesforchoosingthemostplausiblevalues.3.2.TurbulenceintheurbanatmosphereJustpriortothebeginningofthisreviewperiod,Oke(1979b)remarkedthat‘evidenceofthestructureofturbulenceovercitiesislimited’.Theavailabilityofturbulencemeasurementsinurbanenvironmentshasimproveddramaticallysincethattime,however.Duringthe1980s,resultswerereportedbyMellingandList(1980),Hetal.(1982),Steyn(1982),HildebrandandAckerman(1984),Ching(1985),Fujitani(1986),YerselandGoble(1986),Unoetal.(1988)andRothetal.(1989b).Inthefollowingdecade,attentionwasdirectedmoretoturbulenceprocessesinandclosetotheUCL,althoughthisfocuswasnotuniversal(Rotach,1993a,b,1995,1999;Roth,1993;RothandOke,1993,1995;KalogirosandHelmis,1995;OikawaandMeng,1995;Yadavetal.,1996;Xuetal.,1997;Feigenwinteretal.,1999).Zhangetal.(2001)compareturbulencespectraforsuburbanandurbanareaswiththoseforruralsurfaces.AcomprehensiveandperceptivereviewofturbulenceintheurbanatmosphereisprovidedbyRoth(2000).Someimplicationsofthiswork,generallynotconsideredtobeconventionally‘climate’,dohaveprofoundimplicationsforclimate-orientedwork.Forexample,RothandOke(1995)suggestthatinequalitiesintransferefcienciesforheat,massandmomentummayinvalidateturbulentuxevaluationsoverurbansurfacesusingconventionalprolemethods.3.3.StorageandanthropogenicheatßuxesintheurbanenergybalanceNounderstandingofurbanenergybalancecanavoidwrestlingwiththeimplicationsofscale(Section2.1)andtheissueoftheprecisedenitionofthe‘surface’towhichthebalancerefers.TheseareinterrelatedCopyright2003RoyalMeteorologicalSocietyInt.J.Climatol.:1–26(2003) URBANCLIMATEmatters:whileitisgenerallyuncontestedwhatconstitutesthe‘surface’towhichtheenergybalanceofabuildingwall,suburbanlawnorwarehouseroofapplies,thisbecomesincreasinglyuncertainaswescaleupthroughindividuallandscapeunits,likeUCs,toland-usezonesandevenwholecities.Theenergybalanceforasimpleplanefacetmaybewrittenasisnetradiation,aretheturbulentuxesofsensibleandlatentheatrespectively,andisthe(primarily)conductiveheatuxintooroutofthematerialthatconstitutesthesurface.BehaviourforsimplefacetshasbeendescribedbyDolletal.(1985)andAnandakumar(1999)andcannormallybeevaluatedusingheatuxplatesorbymeasuringtimeratesoftemperaturechangeiftheheatcapacityofthesubstrateisknown.Oke(1988b)suggeststhat,atlargerscales,forurbanlandscapes,ausefulapproachistoevaluatetheequivalentenergyuxesthroughthetopofanimaginaryvolume,extendingfromadepthinthesubstratebelowwhichenergyexchangesarenegligibleatthetimescaleofconsiderationtoalevelroughlyatrooflevel,attheuppermarginsoftheUCL.Theenergybudgetforthisvolumecanbewrittenasrepresentsanthropogenicenergyreleaseswithinthevolume,isnetadvectionthroughthesidesofthevolume,andisthestorageheatuxandrepresentsallenergystoragemechanismswithinthevolume,inair,trees,buildingfabric,soil,etc.Inpractice,byvirtueofthesizesofheatcapacitiesforairandsolidfabric,cannormallybeequatedtotheaggregateforallair–solidinterfaceswithinthevolume.Itisgenerallybelievedthatislarge(asafractionof)forurbanlandscapes,butdirectmeasurementofthistermbyaggregatingheatuxplatedeterminationsforarepresentativesampleofurbanfacetsisnotfeasible.Accordingly,methodshavebeensoughttoparameterizethestorageheatuxforuseintotalenergybalancestudiesofurbanlandscapes.NunezandOke(1980)andOkeetal.(1981)proposesimpleregressionrelationshipsbetweenfordifferentsurfacetypes,weightedbytheareaoccupiedbyeach.ThisschemewasusedinevaluatingsuburbanenergybalancesbyOkeandMcCaughey(1983)andCleughandOke(1986).OkeandCleugh(1987)notethatheatstoragedisplaysahysteresisloopwhenplottedagainstnetradiationandthatabetterparameterizationwouldinvolvebothandthetimederivativeof.TheOHMofGrimmondetal.(1991)makesuseofthisconceptandemploysurbanlandcoverdatatoscaleupindividualhysteresisrelationshipsfordifferentsurfacetypestothetotalsuburbanlandscape,anapproachthatisvalidatedbyRothandOke(1994)andGrimmondandOke(1999b).ArneldandGrimmond(1998)useanumericalsimulationapproachtoattempttoidentifythemainsourcesofhysteresisforcanyons,oneofthesurfacetypesdenedintheOHM,andconcludethatsubstratethermaladmittanceandcanyongeometryareofmostimportance.TheOHMwasusedbyTaha(1999)inamesoscalemeteorologicalmodelofurbanenergybalance.Itisalsoincorporatedinneighbourhood-scaleenergybudgetevaluationsbySchmidetal.(1991),Okeetal(1992)andGrimmond(1992),usingahighlydetaileddatabaseofthree-dimensionalsurfacetypesforthelocale,whichissampleddynamicallytoprovideappropriatecoefcientweightsfortheturbulentuxsourcearea(Section2.1).ThestateofknowledgeonanthropogenicheatuxiswellsummarizedbyOke(1988b).Table3inthatsourcelistswhatwasknownofthisuxdensityatthattime,primarilyshownforwholecities.Incorporationofanthropogenicheatuxinsimulationmodelsofurbanclimateisrelativelystraightforward,involvingtheadditionofa(usuallyconstant)terminthesurfaceenergybudgetequation.SwaidandHoffman(1990–91)andSwaid(1993b)assesstheroleof(55and11Wminthesetwostudies)onairtemperatureinasimulationofUCforms,whileKimuraandTakahashi(1991)incorporatealarge-scaleanthropogenicheatuxinasimulationoftemperatureeldsinTokyo.Steinecke(1999)foundacity-averagedabout35WmforReykjavik,andIchinoseetal.(1999)providehighlydetailed,spatiallyandtemporallyCopyright2003RoyalMeteorologicalSocietyInt.J.Climatol.:1–26(2003) A.J.ARNFIELDdifferentiatedestimatesofthistermforTokyo,suggestinganaverageofabout30Wmforresidentialareasinthesummer,whiletypicalvaluesforthecentralcityreach400Wmbydayandanastounding1590Wminwinter.Evaluationsofatlocalscales,forincorporationintoenergybalancesforsuburbanterrain,aregivenbyGrimmondandOke(1991)andSchmidetal.(1991).Theycomputethistermaswheretheindividualcomponentsaretheheatreleasedbyvehicles,stationarysourcessuchashousefurnacesandmetabolism,requiringinputssuchasvehiclenumbersbyroadtype,lengthofroadinthecontributingarea,numberofelectricityandgasconsumersbyclass(house,school,etc.)andthenumberofpeopleandanimals.UseofthistechniquebyGrimmond(1992)gaveatotalof7–14Wm,withstationarysourcesdominatingandthemetaboliccontributionbeingnegligible.Itshouldbenotedthat,insomecases,isincluded.Forexample,theTerjungandLouie(1974),Mills(1993)andArneld(2000b)energybudgetsimulationmodelsincorporateconstantinternalbuildingtemperatures,whichreecttheinvestmentofanthropogenicenergyinheatingandcoolinglivingspaces.Thoughnotappearingasanexplicititemintheurbansurfaceenergybudget,suchschemesinuenceexternalclimateviaconductiveheatuxesthroughbuildingwallsandroofs.Theydonot,ofcourse,incorporateenergyreleasestotheexternalenvironment,likemotorvehiclewasteheat,uegasesorair-conditionerheatoutput.3.4.UrbanradiationbalanceAtthebeginningofthereviewperiod(Oke,1979b,1982),additionalattenuationbyurbanboundary-layerpollutantswasbelievedtoreducesolarirradianceatthetopoftheUCLby0–10%ingeneral(butupto20%insomecases:seeTable1inOke(1988b)).Morerecentexperimentshave,forthemostpart,substantiatedthesegeneralconclusionsforfull-spectrumirradiances.PetersonandStoffel(1980)found3%urbandepletionforcentralStLouis,withlargeramountsinthewinterthaninthesummer,andsmallerreductionsatsuburbansites.Asimilar-sizedurbaneffectwasreportedforToulousebyEstourneletal.(1983)andforVancouverbyHay(1984).SomewhatlargerattenuationsaregivenforStLouis(7%:MethodandCarlson,1982)andHangzhou(9%:WangandLiu,1982).However,somemorerecentinvestigationshavefoundextremelylargeexcessattenuationsincitieswithsignicantindustrialaerosolorphotochemicalsmog.StanhillandKalma(1995)detecteda33%decreaseininHongKongovera35yearperiodthatcannotbeascribedtoachangeincloud-coverfrequencyandtowhichtheyattributethevirtuallyabsenturbanwarmingofthatcity.CentralMexicoCity,notoriousforitspoorairquality,showsanaverage22%smaller,withlargerdepletionduringperiodsofweakwindsandhighrelativehumidity(JaureguiandLuyando,1999).ThetemporalvariabilityofabsorptionbyurbanaerosolsisemphasizedbyHetal.(1990)forFrankfurt.Cerveny(1989)usesaradiativetransfermodeltoshowthat‘shadowing’ofneighbourhoodsdistantfromapollutantcloudmayproducelargepercentageattenuationsatlowsolarelevations,butthatthiseffecttendstobelostindailytotalsbecauseofthesmallirradiancesatthosetimes.Recentyearshavebroughtanincreaseininterestinurbaneffectsonthespectrumofsolarradiation,frequentlybecauseofthepotentialbiologicalimplicationsofsuchchanges.Lorenteetal.(1994)measuredspectraldistributionsfordifferentturbidityconditionsinBarcelona.Hugechangesinthebeamultraviolet(UV)portionofthespectrumarenoted(25%),withcompensatingincreasesindiffuseUV.Jacovidesetal(1998)andRepapisetal.(1998)foundsimilarresultsforAthens,Greece,andnotethatUV-BismorestronglyattenuatedthanthelongerwavelengthUV-A.Photosyntheticallyactiveradiationisalsosignicantlydepletedunderverypollutedatmosphericconditions:Jacovidesetal.(1997)foundreductionsintheglobalirradianceofthisquantityofmorethan18%insomecircumstances.Incominglongwaveradiationisgenerallybelievedtobeincreasedbyurbanareas,eitherduetotheincreasedwarmthoftheurbanatmosphere(UHI)orenhancedatmosphericemissivitybroughtaboutbythepresenceofparticulateandgaseouspollutants(Oke,1979b,1982).Suckling(1981)foundconsistentlylargerinBrandon,Manitoba,comparedwiththeruralenvirons(average10%increase,maximum20%).SlightlysmallerpercentagechangesfoundbyEstourneletal.(1983)forToulousewereshowntobelargelyaresultofincreasedurbanatmospherictemperaturesratherthanpollution.Incontrast,Tapper(1984)attributestheCopyright2003RoyalMeteorologicalSocietyInt.J.Climatol.:1–26(2003) URBANCLIMATEinabilityofestimationequationstoreproducemeasuredtoaneglectoftheemissivityexcessinducedbypollutants.RadiativeprocessestheUCLhavebeenamajorfocusofactivityduringthepasttwodecades.Buildingsandotherroughnesselementsintownsinterceptthedirectsolarandreducediffusesolarandsky-derivedlongwaveuxdensities,yetmayalsoenhanceirradiancebyreectionofsolaroremissionoflongwaveradiation.MeasurementsoftheseeffectsortheirconsequencesarepresentedbyAdebayo(1990),forIbadan,andMills(1997),forascalemodelofanidealizedurbancanopy.ModelsforcomputingradiationloadsonthefacetsofarbitrarybuildinggroupsincludethoseofArneld(1984)andVerseghyandMunro(1989a,b).ComputationoftheradiationexchangesamongtheinclinedandobstructedfacetsofUCs,andbetweenthemandexternalradiationsources,isarelativelystraightforwardgeometricalexerciserequiringcomputationofskylineobstructions(e.g.Franketal.,1981a,b;Arneld,1982a,1984)andviewfactorrelationshipsamongthefacetsofthecanyonform(Steyn,1980;JohnsonandWatson,1984,1985;SteynandLyons,1985;Steynetal.,1986;WatsonandJohnson,1987;Chapmanetal.,2001;Grimmondetal.,2001).Perhapsasaresult,anumberofcanyonradiationsimulationmodelsexist.TheUCradiationmodelofArneld(1976)seekstoprovideageneralmethodofcomputingsolarandlongwaveirradiancesoncanyonfacets,usingviewfactorgeometryandaschemeforrepresentingmultiplereections,basedbothoncanyongeometryanddistributionsofsurfacematerials.Thismodelwasemployedtoevaluatecanyonalbedo,emissivityandradiationbudgetsfordifferentland-usezoneswithinColumbus,Ohio,byArneld(1982b)andtoassesstheroleofcanyongeometryonsolarradiationaccesstoUCsbyArneld(1990a).ThismodelsuccessfullysimulatedtheurbansurfacetopalbedosobtainedinscalemodelexperimentsbyAida(1982)(Arneld,1988)andwasvalidatedfornocturnallongwaveuxesbyVoogtandOke(1991).AidaandGotoh(1982)andKondoetal.(2001)offermethodsforsimulatingcanyontopalbedos,asafunctionofaspectsofUCgeometry.TerjungandLouie(1973,1974),BruhlandZdunkowski(1983)andZdunkowskiandBruhl(1983)employtheprinciplesofradiationgeometrytoevaluatethesolarand/orlongwaveradiationbalancesofcanyonfacets.SieversandZdunkowski(1985)employtheBruhlandZdunkowski(1983)schemetosimulatethealbedoofstreetcanyons,againusingtheAida(1982)scalemodelmeasurementsasvalidation.UrbanalbedomeasurementsfromsatelliteplatformsarepresentedbyBrest(1987),KidderandWu(1987),SolerandRuiz(1994)andVukovich(1983).ThelargespatialvariabilityinalbedofoundbyKidderandWu(1987)forurban–ruralareaswithasnowcoverisconsistentwiththesimulationresultsofArneld(1982b).Althougheachoftheradiationstreams(incomingandoutgoingsolarandlongwaveradiation)ismodiedbyurbanization,thereappearstobelittlereasoninmostcircumstancestochallengethenotion,rstofferedbyOke(1974),thatisunlikelytovarybyalargeamountbetweenurbanandruralareas.Decreasedwithinacityispartiallycompensatedforbythegenerallyloweralbedoscharacteristicofbuiltlandscapes(Arneld,1982b).IncreasedduetoenhancedurbanatmosphericemissivityandtheUBLUHIwilltendtobeoffsetbytheeffectofthesurfacetemperatureUHIonlongwaveemittance.Arneld(1982b)simulatedrural–centralcitydifferencesandconcludedthattypicalpollutionandheatislandeffectsontheincominglongwaveandsolaruxesaresuchthatthedifferencecouldbepositiveornegativebut,intheabsenceofasnowcover,willtendtobesmall.ThisgeneralizationhasbeenshowntobevalidtoafewpercentbythemeasurementsofOkeandMcCaughey(1983),KerschgensandHacker(1985)andCleughandOke(1986),hasalsobeenshowntobeconservativewithinparticularland-useclasses(e.g.Schmidetal.,1991).Therearesomeinterestingexceptionstothisgeneralization,however:Grimmondetal.(1996)measured19%foraLosAngelessuburbwith30%treeandshrubcoverthanforanearbyneighbourhoodwith10%cover.3.5.UrbanfacetenergybalanceVirtuallyallobservationalandmodelledestimatesofthecomponentsoftheenergybalanceofurbansurfaceshavepostdatedtheyear1970andsothepreponderanceofourknowledgeonthismostfundamentalareaofurbanclimatologyhasbeenaccumulatedduringthereviewperiod.Thisdiscussionwillstartwiththeenergybalancesoffairlydistinct,planefacets,basedonEquation(1),andwillmovetolargerspatialscalesinsubsequentsections,includingbothobservationalandmodellingapproaches.Copyright2003RoyalMeteorologicalSocietyInt.J.Climatol.:1–26(2003) A.J.ARNFIELDAsaedaandCa(1993)developandvalidateanumericalmodeltoinvestigatetheroleofheatandmoisturetransportwithinbothexposedsoilandsoilcoveredbyasphaltandconcrete.‘Waterproong’ofthesoilbytheimperviouslayerreducestheevaporativesink,leadingtohighsurfacetemperaturesandanupward-,evenatnight.Increasingthedepthofthelayerincreasesduringthedayandproducesmorewithdrawalofheatatnight.Anandakumar(1999)measuredonanannualbasisforadryasphaltsurface,withbothuxesexhibitinganout-of-phaserelationshipwith.IrrigatedlawnvegetationwasfoundbyOke(1979a)toexhibitveryhigh,inexcessof,supportedbymicroscaleadvectionfromcontiguousdryimpermeablesurfaces,suchasdriveways,roadsandbuildingfacets.Incontrast,Suckling(1980)foundforasuburbanlawn,similartomoistruralsurfaces,foranenvironmentinwhichadvectionwaslesslikely.Contrastsintheenergybalancesofrooftoplawnsandaconventionaltar–gravelroofwerefoundbyJonesandSuckling(1983)tosuppresssurfaceheatinggreatly.KjelgrenandMontague(1998)usedatwo-layercanopymodeltoevaluatetheroleofthegroundsurfacetype(asphaltorturfgrass)incontrollingfortrees.Nosimpleconclusionwasreached,asthehighersurfacetemperaturesoftheasphaltbothincreasethelongwaveuxtothecanopy,henceincreasingtranspiration,andleadtohigherleaftemperatures,whichcausedstomatalclosure.Atslightlylargerscales,urbanparkenergybudgetshaveattractedtheattentionofSpronken-SmithandOke(1999).Nocturnalcoolingofascalemodelparkwasshowntodependontheinteractionofskyviewfactor,controllingnetlongwaveradiation,andthermaladmittance,whichdetermines.Latentheatlossesplaylittleroleatnight,buttheymayhavebeeninstrumentalinestablishingtherelativecoolnessoftheparkatsunset.Theroleofadvectioninmaintaininghighfromanirrigatedpark,withadownward-directedsensibleheatux,wasdemonstratedbySpronken-Smithetal.(2000).3.6.UCenergybalanceWithintheUCL,energybudgetsaregovernedbymicroscaleprocessesmediatedbythesiteconditionsoftheimmediatesurroundings.Theseconditions,consistingofthespecicsofsurfacethree-dimensionalgeometry,substratematerialsandwetness,windexposure,shadingandthelike,aresubjecttomyriadvariationswithinrealcities.Asameansofdistillingwhatiscommontomanyurbanlandscapesfromwhatisuniquetotheparticulararchitectural,cultural,andgeographicalmilieu,muchurbanclimateworkhasadoptedtheconstructoftheUC.TheUCconsistsofthespacebetweenadjacentbuildings,comprisingthesolidsurfacesonthefacesofthosebuildingsandthestreet,theenclosedairvolume,theopen‘top’atrooflevelandthe‘ends’ofthecanyonatstreetintersections,throughwhichmassandenergyuxesmayoccurhorizontally.Thecanyonaspectratio(AR),theratioofwallheighttobuildingseparation,hasbeensuggestedbymanyasamajorcontrolonowwithintheUC,onturbulentintensities,onradiativeenvironmentsand,hence,onthetotalenergybudget.Theliteratureofurbanclimatologyyieldsveryfewempiricalenergybudgetstudiesinanexplicitframework.NunezandOke’s(1977)pioneeringinvestigationofanorth–southUCinVancouver,Canada,showedthatthetimingandmagnitudeoftheenergyexchangesonthedifferentfacetsdifferedconsiderably,inresponsetopatternsofradiationreceiptandloss,butthattheenergybalanceforthesystemasawholewasrelativelysmoothandsymmetric.Byday,canyontopnetradiationismainlydissipatedinthesensibleheatuxtotheUBLabove,withatnoonandthemajorityoftheremaindergoingtothesubstrateheatux.Alatentheatuxfromthecanyonooranddown-canyonadvectionwereothersecondarycomponentsofthebudget.Bynight,andthenetlongwaveirradiancebalancedeachother.Incontrast,theYoshidaetal.(1990–91)experimentalcanyon(Kyoto,Japan)wasdry,orientedeast–west,withanAR94.Byday,attheUCtop,withtheremainderbeingstoredwithintheurbanfabric.Nomeasurementsorestimatesofhorizontaluxeswithinthecanyonweremade.Theseauthorspointoutthatthecanyontopactedasaweakerheatsourcethantheadjacentroofareas,whichtheyattributetotheextensiveshadingofcanyonsurfacesandthelowwindspeedsthere.Bynight,theenergybudgetofthecanyonasawholewassimilartothatofNunezandOke(1977).ArneldandMills(1994b)studiedamarkedlyasymmetricdry,east–westUC.Diurnalpatternsresembledthoseintheprevioustwostudies,butmeasurementsshowednotablecontrasts.Byday,Copyright2003RoyalMeteorologicalSocietyInt.J.Climatol.:1–26(2003) URBANCLIMATEboththeturbulentandcirculation-drivencomponentsofthisuxweresmallinabsolutemagnitude,subjecttoirregulaructuationsanduncorrelatedwith.Bynight,theturbulentuxwasweaklyupward-directed.Severalhypotheseswereofferedtoaccountforthesecharacteristics,includingthelackofaclearlydenedvortexcirculation,theadvectionofwarmairoverthecanyontopfromadjacentroofsandtheshading/shelterargumentadvancedbyYoshidaetal.(1990–91).Indirectlinesofevidenceareofferedthatmayaccountforthebulkoftheenergybudgetresidualbynight,suggestingasimilarbalancetothatfortheVancouverandKyotocanyonsByday,however,analysissuggestsaremainingresidualofupto200Wm,which,itisargued,isdissipatedbydown-canyonadvectionand,toalesserextent,bybulkowthroughthecanyontop,evenwithperpendicularwinds.TheseresultssuggesttheintriguingpossibilitythatthedaytimepartitioningofnetradiantenergywithindrycanyonsmayperhapsbeafunctionoftheARofthecanyon,withsensibleheatproductionthroughthecanyontopdecreasingasaspectratioincreases.Unfortunately,thesethreestudiesdonotrepresentawell-structuredexperimentalinvestigationofthishypothesisbecausetheARwasnottheonlydifferencebetweenthesestudies—weatherconditions,canyonorientation,canyonasymmetryandotherfactorswerenotheldconstant,andsomayhaveinterferedinanypossiblerelationshipbetweenARandcanyonNumericalsimulationusinganappropriatelyvalidatedcanyonenergybudgetmodelisafarmoreeffectivemethodofassessingthedependenceofUCenergybudgetquantitiesonsynopticconditionsandcanyoncharacteristics.That,andperhapstheassumedgeometricalsimplicityoftheUCform,havemadesimulationthemethodologyofchoiceinmostUCenergybudgetinvestigations.AmajorlimitationinmodellingtheenergybalancesofthefacetswithinUCsisthelackofwell-foundedtheoryfortheturbulentuxeswithinthecanyonairspace,asisavailableforthehorizontal,uniformsurfacesmorecharacteristicofruralsites.NunezandOke(1980)provideamodelofthedaytimeUCenergybudget,buttheprocedureistheoreticallyrigorousonlyinitscalculationsoftheabsorbedsolarradiation.Computationand,hence,(asaresidual)ishighlyparameterized,basedontheempiricaldataofNunezandOke(1977).Suchaprocedureiscontextspecicandcannotbeexpectedtoyieldgoodestimatesofenergybudgetsforcanyonsandconditionsmuchdifferentfromtheoneusedtocreatetheparameterizations.TheURBAN3modelofTerjungandLouie(1974)hasbeenusedextensivelyininvestigatingtheclimaticenvironmentsofcities,asthesedependoncitysurfacestructureandweatherconditions,andhascontributedsignicantlytothecausalexplanationofnear-surfacecityclimates.URBAN3isnotstrictlyaasitsimulatesenergybudgetsforthetotalurbansurfacesystem,includingroofs.ItsvalidationconsistedofacomparisonwithsurfacetemperaturedataforLosAngelescitysurfacesduringclear,relativelycalmconditions,anevaluationthatisprobablynotasrigorousasvalidatingwithuxdata.Theradiationuxesarehandledtheoretically,usingradiationgeometryandviewfactorconcepts,andtheturbulentuxesemployan‘exchangecoefcient’approach(e.g.ColeandSturrock,1977),basedonwindspeed.TerjungandO’Rourke(1980a)concludethattwoprimarycausalfactorsintheUHI,sensibleheatuxandoutgoinglongwaveradiation,showconsiderablespatialvariabilitywithinamodelcityoftypicalNorthAmericanstructureandcastdoubtonthevalidityoftheenergeticlowerboundaryconditionsinmanymesoscalemodels.TerjungandO’Rourke(1980b)showthatmanyaspectsofurbanfacetenergybudgetsincityzonesofdifferingphysicalstructureexhibitcounterintuitivecharacteristics.ThisversatilemodelwasusedbyTerjungandO’Rourke(1981a)toanalyseenergybudgetsastheyvarywithcitystructureandwascombinedwithaplantcanopymodel(TerjungandO’Rourke,1980c)toelucidatethephotosyntheticandevaporativeroleofstreet-levelvegetationbyTerjungandO’Rourke(1981b)andO’RourkeandTerjung(1981a,b).ThemodelwasdrivenwithcharacteristicweatherconditionsforseveralsynopticweathertypesbyTodhunter(1989)andTodhunterandTerjung(1990)torevealthedependenceofthemagnitudeanddirectionofenergybudgetcomponentsonsynoptic-scalecontrols.Todhunter(1990)investigatedtheroleofcanyonorientationandasymmetryoncanyonenergybudget,emphasizingtheroleofthediurnalpatternofsolarirradiationonthetotalenergybudget.AnotherUCenergybudgetmodelthatwasdescribedearlyinthereviewperiodisthatofSieversandZdunkowski(1986).Thiscombinestheirgeneralizedstream-function–vorticitymethodwindmodelwiththeradiationmodelsofBruhlandZdunkowski(1983)andZdunkowskiandBruhl(1983)tosimulatetheCopyright2003RoyalMeteorologicalSocietyInt.J.Climatol.:1–26(2003) A.J.ARNFIELDtemperaturedistributionoverthecross-sectionofacanyonundermid-latitudesummerconditions,withandwithoutacentralvegetatedstriponthecanyonoor.Theresultingdistributionsoftemperatureshowsomepromisingcorrespondenceswithobservedtemperaturepatterns(e.g.NakamuraandOke,1988),althoughtheauthorsadmittothesomewhatarbitrarynatureofmanyoftheinputparameters,whichwerechosentodemonstratethecapabilitiesofthemodel.Theschemeisatwo-dimensionaloneanddoesnotincorporateadvectioneffects.GiventhecomplexityoftheUCsystem,severalworkershavesoughttorestrictconsiderationtoconditionsunderwhichsimplicationsintheenergybalancecanbeintroducedwithoutunduelossofrealism.Inparticular,undernocturnalconditions,withcalmwinds,solarradiationandtheturbulentuxesofheatandwatervapourmaybeneglected,leadingtoasimpliedenergybudgetinwhichsubstrateheatlossisdissipatedinthenetlongwaveirradiance.Suchabudgethassomeempiricaljustication(e.g.NunezandOke,1977).Simulationsofnight-timesurfacecoolingunderthesesimpliedconditionshavebeentheobjectiveofmodellingexercisesbyArneld(1990b),Okeetal.(1991),Johnsonetal.(1991)andSwaid(1993a),allofwhichhavedemonstratedtheimportanceofbothcanyongeometryandmaterialsingivingrisetotheurbansurfacetemperatureexcessovertheruralenvirons.TheSTTCmodelofSwaidandHoffman(1989)employstheconceptoftheUCtoadjustbackgroundmeteorologicalstationtemperaturedataforuseinbuildingenergy-usemodels.ItwasextendedbySwaidandHoffman(1990–91)andSwaid(1993b)toincorporatetheeffectsofanthropogenicheatsourcesandarticialshadingofcitystreetsfortemperaturecontrolpurposes.Themodelissubjecttoanumberofphysicalassumptionsandconstraints(ElnahasandWilliamson,1997),however,whichlimititsgeneralapplicabilitytoaparticularcanyonandmeteorologicalsituation.ElnahasandWilliamson(1997)haveproposedmeansofavoidingtheselimitations,butthishasbeenachievedbyemployinganumberofparameterizations(e.g.exchangecoefcients,anevapotranspirationmodel,windvelocitymultipliers)thatmaylimitthemodel’sgenerality.TheschemeofMills(1993)combinesthewindmodelofNicholson(1975),theArneld(1976,1982b)canyonradiationbudgetmodelandanovelprocedureforevaluatingtemperaturechangeintheairasitcirculateswithintheUC.Themodelwasfoundtopredictsatisfactorilythecanyonfacetsurfacetemperaturesandnetradiation(MillsandArneld,1993),andtocharacterizetheofthecanyontopsensibleheatuxtotheUBL.However,themechanismresponsibleforthisheatexchange(thevortexcirculation)wasnotapparentinthedatasetusedtovalidatethescheme(ArneldandMills,1994a,b);theobservationsalsosuggestedtheimportanceofdown-canyonadvectiveuxesandbulkowthroughthecanyontop,neitherofwhichispresentintheMillsmodel.ArneldandGrimmond(1998)employthesameradiationschemeasMills(1993),butasimpliedwithin-canyonwindmodel(applicationofaconstant‘shelterfactor’toabove-canyonwinds)toinvestigatethephaserelationshipsbetweencanyonsystemsubstrateheatuxandnetradiation.ThemodelreproducedthecanyonsystemenergybudgetofNunezandOke(1977)reasonablywell,butitshowedsomeerrorfortheindividualfacetbalances.Virtuallyidenticalschemesforhandlingtheradiationbudgetandsubstrateheatow,butwithdifferentapproachestoturbulentexchangewithinthecanyonairspace,havebeenpresentedbyHerbertetal.(1997,1998),Arneldetal.(1998)andArneld(2000b).Theworkreportedin1997and1998usesacomputationaluiddynamicalapproachtoairowaroundandwithinthecanyonwithaturbulencesub-model,basedonPatersonandApelt(1989),coupledwiththescalardispersionmodelofJohnsonandHunter(1995).ThismodelisabletoreproducemanyobservedaspectsofUCairow,thermalbehaviourandenergyexchange,butithasnotyetbeenformallyvalidated.Arneld(2000b)replacesthewindanddispersionmodelwithaparameterizedschemeinwhichthewindatrooflevelisresolvedintoacross-anddown-canyoncomponents.TheformerdrivesthecanyonvortexmodelofHotchkissandHarlow(1973)andthelatterdenesalogarithmicproleofwindvelocityforthedown-canyonow.Thesecomponentsofthewindarecombinedwithinthecanyonandareusedinanexchangecoefcientparameterizationtoevaluatesensibleheatux.ThemodeliscomputationallyeconomicalandreplicatestheNunezandOke(1977)canyonfacetandsystemenergyuxeswithexcellentveracity.Sakakibara(1996)alsopresentsacanyonmodelandusesittocontrastthecanyonenergybudgetwiththatofanopenparkinglotsite.Copyright2003RoyalMeteorologicalSocietyInt.J.Climatol.:1–26(2003) URBANCLIMATE3.7.UrbanneighbourhoodenergybalanceFewareasinurbanclimatologyhaveundergonesucharadicaltransformationasourunderstandingofenergybudgetsatthelocalscale,correspondingtodistinctiveneighbourhoodswithinacity.Inparticular,verylittlewasknownabouttheenergybalanceoflandusespriortothereviewperiod,despitethefactthatthistypeoflandscapeoccupiesthemajorityoftheareaofmostcitiesinNorthAmerica.Thisisparticularlycritical,sincethesesuburbanareasoftenpossesslargegreenspacefractions,areoftenirrigated,andpotentiallydeviatemarkedlyfromtheviewofthecityasa‘desert’thatfrequentlyprevailedpriorto1980.Theseareasofferparticulardifcultiesintermsoftemporalandspatialsamplingandtheapplicabilityofconventionalboundary-layertheory.Inparticular,theenergybalanceframeworkadoptedatthisscaleisthatofEquation(2),withtheuppersurfaceofthevolumewellabovethebuildingroofstoavoidthecomplexityoftheroughnesssublayer.Thisenhancesthedangerofradiativeandturbulentuxdivergenceovertheatmosphericlayerbetweenmeasurementheightandtheroughnesselements(Funk,1960;FuggleandOke,1976)andnecessitatesconsiderationofthegeometryoftheuxsourceareaupwindofthemeasurementpoint(Section2.1).Okeetal.(1989)suggestthatthedifcultiesofmeasurementinsuchlandscapescanbeovercomewithcarefulsiteselection,considerationgiventotheheightofmeasurementandappropriatetemporalsamplingprocedures.etal.(1980)madepioneeringBowenratio–energybudgetdeterminationsofenergyexchangeforasuburbanareainVancouver.wasalwayssignicantandBowenratiosweremostlyintherange0.5–1.0.Abruptday-to-daychangesinenergypartitioningbetweenthetwoturbulentuxeswerefoundthatwereunrelatedtoprecipitationevents.Itisspeculatedthatthisfeaturereectsirrigationpracticesbyhomeownersinresponsetodryingcycles.Ratesoflatentheatlossoftenequalledorexceededthe‘equilibrium’rate,whichetal.(1980)attributetoenhancedratesofevapotranspirationfromirrigatedlawns,fedbythecity’spipedwatersupplyanddrivenbymicroscaleadvection(Oke,1979a).OkeandMcCaughey(1983)presentsimultaneouslymeasuredenergybudgetsforasuburbanandruralsurfaceformostlyclearconditionsafteraperiodofhighprecipitation,withmoistsoil.Unexpectedly,suburbanevaporationratesweregreaterthantheruralrates,averagingabout80%of.Theauthorssuggestthatmicroscaleadvectionfrommoistgardensandlawns,especiallyunderhighradiationconditionswhenimpermeablesurfacesaredryandhot,isresponsibleforthisphenomenon,sincesuburbanwaterlosswassmallerthantheruralwaterlossundercloudierconditionswhensuchadvectionislikelytobeweaker.ThesamesiteswereusedbyCleughandOke(1986),but,inthiscase,thedispositionofenergybetweentheturbulentuxeswasasexpected.was4%largerandwasabout50%largerinthesuburbanarea,leadingtoBowenratiosofabout0.5fortheruralsiteand1.3forthecity.Largeday-to-dayvariationsinuxpartitioningatthesuburbansitewereobservedandareinterpretedintermsoftheMcNaughtonandJarvisparameter,whichindexestherelativedependenceofevaporationonradiativeandadvectiveforcing.Theaerodynamicroughnessinthesuburbanenvironmentcouplesthesurfacecloselytothemixedlayer,thecharacteristicsofwhichvarywithsynopticconditions.CleughandOke(1986)alsosuggestthatthismaybetheexplanationforthedifferencebetweentheirresultsandthoseofOkeandMcCaughey(1983).Spatialvariabilityinenergybalancesoveranapparently‘homogeneous’suburbanareainVancouverwasfoundbySchmidetal.(1991)tobefargreaterthanexpected.Thoughwasrelativelyconservativespatially(attributedtothesurfacealbedo–surfacetemperaturefeedback),turbulentenergyuxdensitieswerefoundtovaryby25–40%athorizontalscalesof10–10m,asaresultoftheshiftingovertimeofthesourceareawithwindspeedandstability(Section2.1).Storageandanthropogenicheatuxesalsoexhibitedsignicantspatialvariabilitybasedonthemorphologyoftheurbansurface.Hence,variabilityinmeasuredturbulentuxesisamanifestationofvariabilityinsurfacecovertypesatthemicroscaleastheyaresensedbytheturbulentuxinstrumentationonthemeasurementtower.Grimmond(1992)describestherstspringandwinterenergybalancemeasurementsforsuburbanterrain.Forspringtime,therelativesizesofthedifferentuxesweresimilartothesummercasesdescribedabove,butwintertimedatashowedanenhancedrelativemagnitudeforlatentheatux,whichwasthelargestuseofthenetradiationunderthemoistwintermaritimeWestCoastclimate.Notunexpectedly,anthropogenicheatuxplayedamoresignicantroleinwinter.Copyright2003RoyalMeteorologicalSocietyInt.J.Climatol.:1–26(2003) A.J.ARNFIELDSummertimeenergybudgetsforfourNorthAmericancities(Tucson,Sacramento,Chicago,LosAngeles)withdifferentmorphologiesandprevailingclimateswerecomparedbyGrimmondandOke(1995).Thoughtheabsolutemagnitudesoftheensembleaverageenergyuxesvaried,asexpected,thediurnaltrendsinuxpartitioningwerequitesimilar.Theresultssuggestthat,forcitieswhereirrigationispractised,atypicalvaluefortheBowenratiois1.5butthatthisissomewhatlower(0.8–1.0)forlocationswithfrequentnaturalwaterinputsviaprecipitation.Typicalpartitioningofwas40%,30%and30%,respectively.ThedaytimeaverageBowenratiowasalsofoundtobeinverselyrelatedtotheareairrigated.etal.(1996)exploredtheroleoftreesinthesuburbanenergybudgetandfoundthat,inabsolutevalue,alluxes,including,wereincreasedatasitewithgreatertreecover,asaresultofincreasedbroughtaboutbyreducedalbedoandlowerradiativetemperature.However,asafractionofwereincreasedandwasreduced.MexicoCityhasbeenthesiteofthreeenergybudgetstudiesatasimilarspatialscaletothosedescribedaboveforNorthAmericancities.Okeetal.(1992)measuredatasitewithinamixedresidential,commercialandindustrialarea,usedtheOHMofGrimmondetal.(1991)toparameterizeandobtained(summedwith)astheresidual.DerivedBowenratiosfortheensembleaverageenergybudgetwerenotdissimilartothosefortemperatecities(around1.1onadailybasis),aremarkablysmallvalueforacityinwhichonly21%oftheplanareaand8%ofthethree-dimensionalsurfaceisvegetated,afactattributedtotrees(excludedfromthesurfaceanalysis),anduseandleakageofpipedwater.Thetermswereofsimilarmagnitude,whichimpliesamuchlargerroleforthelatterthanistypicaloftemperateurbanareas.Therstenergybalanceforadry,denselybuilt,city-centresite(forMexicoCity)isdescribedbyOkeetal.(1999).Asexpected,evaporationwassmallandtheBowenratiowaslarge(abouteight).Surprisingly,duringtheday,thefractionofthatwaschannelledintostoragewithintheurbanfabricwasremarkablylarge(58%of,evenlargerthaninOkeetal.(1992)),sothatwassmallerthanexpectedforsuchasite.Atnight,withdrawalofthisheatbalancedthenetlongwavelossor,insomecases,exceededit,leadingtoan(implyinganegativepotentialtemperaturegradient).Incontrasttothisstudy,Barradasetal.(1999)measuredtheenergybalanceatavegetatedsuburbansiteinMexicoCityduringthedryandwetseasons.Budgettermswereverysimilartothoseexpectedforruralsitesunderconditionsofsoilwateravailabilityandmoisturestress,withaverageBowenratiosof1.9andnearzeroforthedryandwetseasons3.8.TheurbanneighbourhoodwaterbalanceTheurbanwaterbudgetforavolumeextendingfromaboutroofleveltothedepthinthesubstratewherenonetexchangeofwatertakesplaceduringthetimeperiodrelevanttotheprocessunderinvestigationcanbewrittenas
S((GrimmondandOke,1991)whereisprecipitation,isthepipedwatersupplyofthecity,isthewatervapourreleasedduetoanthropogenicactivities,suchascombustion,isevapotranspiration,isrunoff,isthenetadvectionofmoistureforthevolumeandisthechangeinwaterstorageduringthetimeperiod.Thewaterbalanceislinkedtotheenergybalancethrough,whichisvirtuallyproportionaltoThetermsinthisequationmaybeexpressedasmassesordepthsperunittime.Whentheformerunitsare,whereisthelatentheatofvaporizationofwater.etal.(1986)employthisbalanceequation,withsettozero,toconstructamodelforthecomponentsoftheurbanwaterbalance,intendedtobedrivenbystandardclimatedataandemployingsimplesite-specicparameters.Theschemeincludesanevaporationmodelofthecombinationtype(PriestleyandTaylor,1972)andperformscalculationsforsuburbanareasbasedonthreedistinctsurfacetypes:impervious,perviousirrigatedandperviousunirrigated.ThismodelisappliedtoaVancouversuburbancatchmentbyGrimmondandOke(1986)forperiodsrangingfromadaytoayear.Aparticularfocusistheroleofirrigation(mainlygardensprinklingbyhomeowners)inthewaterbalance.PipedwateruseisCopyright2003RoyalMeteorologicalSocietyInt.J.Climatol.:1–26(2003) URBANCLIMATErelatedtoprevailingweatherconditions,butinamannerthatiscomplexandinvolvesafeedbacksystemthatinvolveshumanperceptionanddecisionmakingaswellasbiophysicalcontrols.Inparticular,followingdeclinesaswaterstoresaredepletedandpipedwateruseremainslow(below)forseveraldays,afterwhichincreasestoroughlyequal,whichreboundsinresponsetotheavailabilityofirrigationwater.Theauthorsspeculatethathighratesofevaporationaremaintainedbymicroscaleadvectionofwarm,dryairfromimpervioustovegetatedsurfacesandbytheentrainmentofdryairintotheUBLbypenetrativeconvectionenhancedbytheroughnessofthecitysurfaceandanunstableatmosphere.etal.(1980),OkeandMcCaughey(1983),Okeetal.(1992)andGrimmond(1992)allreportunexpectedlyhighevaporationratesfromcitysites;theGrimmondetal.(1986)modelprovidesatheoreticalframeworkwithinwhichtoanalysesuchoccurrences,astheserelatetotheparticularcharacteristicsofurbanareasandtheprovisionofpipedwatertotheexternalenvironmentthroughirrigation,streetwashingandthelike.AmodeltopermitcalculationofevapotranspirationfromandinterceptionbyurbanandsuburbanareasisgivenbyGrimmondandOke(1991).Itadaptsthewell-establishedPenman–Monteith–Rutter–Shuttleworthevapotranspiration–interceptionmodel(Penman,1948;Monteith,1965;Rutteretal.,1971:Shuttleworth,1978),originallydevelopedforforests,tosuburbanuse,andemploystheSchmidandOke(1990)sourceareamodelandtheOHMforetal.,1991).Anthropogenicheatuxisincludedandsurfacewateravailabilityisdescribedusingasurfaceresistanceconcept.Themodelprovidesrealistichourlyanddailyestimatesofurbanevapotranspirationandsurfacewaterstate.4.THEURBANHEATISLAND4.1.ObservationalstudiesofheatislandsThestateofknowledgeontheintensity,spatialandverticalstructure,dynamicsanddeterminantsoftheUHIatthebeginningofthereviewperiodissummarizedbyOke(1982).Hisreviewrelatestotemperaturepatternsderivedfromnear-surface(approximatelystandardscreen-level)airtemperatureobservations.ThemethodsusedtodescribeUHIsduringthepasttwodecadesdonotdiffersignicantlyfromthoseemployedpreviously,andallpossessthelimitationsoutlinedbyLowry(1977)fortheunequivocalidenticationof‘urbaneffects’.Examplesofthesemethodsaretimetrendsatasingleurbanstation(e.g.TarletonandKatz,1995;Montetal.,2000;TereshchenkoandFilonov,2001),comparativetimetrendsatoneormoreurbanstationsandoneormoreruralones(e.g.Schmidlin,1989;Mageeetal.,1999;Philandrasetal.,1999),statisticsonurban–ruraldifferencesbasedonpairsofstationsorgroupsofstations(e.g.Ackerman,1985;etal.,1990;Moreno-Garcia,1994),networksofxedstationswithinandaroundacity(e.g.Hsu,1984;Kuttleretal.,1996;Morrisetal.,2001),transectsacrossanurbanarea(e.g.Yamashitaetal.,1986;Westendorfetal.,1989;GohandChang,1999;Kumaretal.,2001;Ungeretal.,2001)andweekday–weekenddifferences(FiguerolaandMazzeo,1998).Winkleretal.(1981)showedtheimportanceofcorrectingforobservationtimeandsitelocationincorrectlydeningthemagnitudeoftheUHIandTarletonandKatz(1995)stresstheimportanceofincorporatingtrendsintemperature,aswellasin,inexplainingchangesinextremeUHItemperatureevents.Oke’s(1982)observationthat‘[w]hilstthereisarelativeabundanceofresearchonthenatureofheatislandsintemperateclimates,thereisadearthregardingthoseofequatorial,tropical,sub-polarandpolarsettlements’isprobablylessvalidtoday.Heatislandstudiesreportedinthepasttwodecadesincludethosefrom:(a)equatorialwetclimates—Singapore(Tso,1996;GohandChang,1999),KualaLumpur(Tso,1996)andIbadan(Adebayo,1987);(b)tropicalwet–dryandmonsoonalclimates—Guadalajara(TereshchenkoandFilonov,2001)andMumbaietal.,2001);(c)tropicalhighlandclimates—MexicoCity(Jauregui,1997);(d)tropicaldeserts—KuwaitCity(Nasrallahetal.,1990),Phoenix(Hsu,1984;TarletonandKatz,1995);(e)subtropicalclimates—Johannesburg(Goldreich,1992);Copyright2003RoyalMeteorologicalSocietyInt.J.Climatol.:1–26(2003) A.J.ARNFIELD(f)high-latitudelocations—Goteborg(EliassonandHolmer,1990;Eliasson,1996b;HolmerandEliasson,1999),Fairbanks(Mageeetal.,1999)andReykjavik(Steinecke,1999).Mediterraneanclimatesseemtohavebeenparticularlywellservedbyheatislandstudiesduringtheperiod;resultshavebeenreportedforAthens,Greece(Philandrasetal.,1999),thethreeSpanishcitiesofBarcelona(Moreno-Garcia,1994),Madrid(Yagetal.,1991)andGranada(Montetal.,2000)andvariouslocationsinIsrael(Goldreich,1995).TheverticalstructureoftheUHIisexploredbyTapper(1990)andShahgedanovaetal.(1997).Thefor-merfoundevidenceof‘crossover’(i.e.urbantemperaturesthanruralonesaloft)inChristchurch,NewZealand,aboveabout290m(DuckworthandSandberg,1954;Bornstein,1968).ThelatterfoundanunexpectedlyhighfrequencyofsurfaceinversionsforasiteinMoscowduringthesummer,whichwasattributedtotheopenstructureoftheareaandthelackofanthropogenicenergyreleaseinthatseason.4.2.DeterminantsofheatislandmagnitudeandstructureThoughtheUHIstudiespublishedinthelast20yearshavebroadenedthegeographicscopeofthephenomenon,thegeneralizationsofferedbyOke(1982)largelyremainunchanged(TableIII).MuchofthisTableIII.ConrmationofUHIgeneralizationsfromOke(1982)inempiricalworkfromthereviewperiod EmpiricalgeneralizationReference UHIintensitydecreaseswithincreasingwindspeedAckerman(1985);Park(1986);Travisetal.(1987);KidderandEssenwanger(1995);Eliasson(1996b);etal.(1996);FiguerolaandMazzeo(1998);etal.(1999);Morrisetal.(2001);UngeretalUHIintensitydecreaseswithincreasingcloudcoverAckerman(1985);Travisetal.(1987);KidderandEssenwanger(1995);Eliasson(1996b);Ripleyetal(1996);FiguerolaandMazzeo(1998);Mageeetal(1999);Morrisetal.(2001);Ungeretal.(2001)UHIintensityisgreatestduringanticyclonicconditionsUnwin(1980);Unger(1996);Shahgedanovaetal(1997);Tumanovetal.(1999);MorrisandSimmondsUHIintensityisbestdevelopedinthesummerorwarmhalfoftheyearSchmidlin(1989);KysikandFortuniak(1999);etal.(1999);Morrisetal.(2001)UHIintensitytendstoincreasewithincreasingcitysizeand/orPark(1986);Yamashitaetal.(1986);HoganandFerrickUHIintensityisgreatestatUnwin(1980);Adebayo(1987);Schmidlin(1989);Djen(1992);Ripleyetal.(1996);Jauregui(1997);Mageeetal.(1999);Montetal.(2000);TereshchenkoandFilonov(2001)UHImaydisappearbydayorthecitymaybecoolerthantheruralenvironsUnwin(1980);Tapper(1990);Steinecke(1999)RatesofheatingandcoolingaregreaterinthecountrysidethanthecityJohnson(1985) Copyright2003RoyalMeteorologicalSocietyInt.J.Climatol.:1–26(2003) URBANCLIMATEworkis,hence,conrmatoryinnature.Thisisnottosuggest,however,thatnewinsightshavenotbeenforthcoming.Nasrallahetal.(1990)foundthattheUHIforKuwaitCitywaspoorlydeveloped,unlikethatforPhoenix,Arizona,inasimilarclimate,andexplainthisdifferenceintermsofcityformandlocationontheArabianGulf.Ripleyetal.(1996)foundmaximumUHIintensitiesforsunnyinSaskatoonunderclear,calmconditions.UnusualseasonalpatternsofUHIdevelopmentwerereportedbyUnwin(1980),whodetectedthegreatesturban–ruraldifferenceinspringandautumn(Birmingham,UK),andbyMageeetal.(1999)andKumaretal.(2001),whofoundthisinwinter(forFairbanks,Alaska,andMumbai,India,respectively).Reykjavik,Iceland,showsatendencyfornegativeheatislandintensities(ruralareawarmerthanurbanarea)insummerandonlyweakdevelopmentatothertimesoftheyear(Steinecke,1999).BrandBudikova(1999)detectedalargerrateofgrowthofPrague’sUHIsincethe1920sinwinterandspringthaninsummer.Intropicalregions,winter–summerdifferenceshaveproventobelesssignicantthanwet–dryseasoncontrasts.BothAdebayo(1987),forIbadan,andJauregui(1997),forMexicoCity,foundalargerheatislandeffectinthedryseasonthanthewetseason,aconclusionconsistentwithlargerthermaladmittanceintheruralenvironsduringtimesofmoistsoils.Indeed,TereshchenkoandFilonov(2001),forGuadalajara,Mexico,foundnegativeheatislandintensitiesintherainymonths.ThehardwarescalemodelsimulationofUCcoolingofOke(1981)presentsthehypothesisthat,forclearskies,calmairandanabsenceofsignicantanthropogenicheating,maximumUHIintensitycanberelatedtothe‘openness’oftheurbanstructure,representedbycanyonARorskyviewfactor,sincethiscontrolstherateofurbancoolingbylimitingnetlongwavelossatstreetlevel.Okewasabletoshowthatthisconceptwascapableofdescribingtheheatislandintensitiesforsettlementsfrommanygeographicalregionswithgreatprecision.Hecautioned,however,thatsucharelationshipmaynotbeexpectedtodescribespatialpatternsoftemperatureaparticularcity,evenforclear,calmconditions,becauseoftheinterferingeffectofthermaladmittance,anthropogenicheatreleaseandotherdifferences.Nevertheless,severalempiricalstudieshaveshownthatairtemperaturesandratesoftemperaturechangewithtimewithincitiesdoshowarelationshipwithmeasuresofurbangeometry;examplesincludeBetal.(1985),Johnson(1985),Yamashitaetal.(1986),Westendorfetal.(1989),Eliasson(1990–91,1992,1994),GohandChang(1999),andMontetal.(2000).TheapplicationofthisconceptinsuccessfullysimulatingthedependenceofUHIintensityonurbangeometryandthermaladmittance(aswellasanthropogenicheatinputsandcloudcover)hasalreadybeendiscussed(Arneld,1990b;Okeetal.,1991;Johnsonetal.,1991;Swaid,1993a).TheroleofallofthehypothesizedcausesoftheUCLheatislandwasexploredintensivelybyOkeetal.(1991),whoconcludethatgeometryandthermaladmittancedifferencesmaybeofapproximatelyequalsignicance.Thereleaseofanthropogenicheatfromwithinbuildingsisalsopotentiallyimportant,butitdependsontheroleofbuildinginsulation.Averyimportantconclusionofthisstudywastheroleofthermaladmittanceingeneratingaheatisland.Moistsoilspossessthermaladmittancevaluesthatarenotdissimilarfromthosefortypicaldenseurbanbuildingmaterials,whichmayexplainthesmall(andnegative)UHIintensitiesfoundforthemoisttropicallocationsabove.Otherfactorsthathavebeendemonstratedtocontroltheform,magnitudeanddynamicsoftheUHIincludesitefactors(e.g.Ackerman,1985;Goldreich,1992;Kuttleretal.,1996),interactionwithurban–ruralhumiditydifferences(HolmerandEliasson,1999)andadvectioncausedbythecirculationgeneratedbytherural–urbantemperaturegradient(Haeger-EugenssonandHolmer,1999).Recently,RunnallsandOke(2000)describedanempiricalmodelintendedtodisentanglethemultiplecontrolsonVancouverUHIintensity,includingtimeoftheday,windspeed,cloudcoverandruralthermaladmittance.4.3.Large-scaleheatislandobservationsandsimulationsItisbeyondthescopeofthisreviewtosummarizemodellingeffortsatscalesinwhichcitiesarerepresentedbysurfaceparametersthatareuniformorapplytolarge-scaleland-usezonesandtheplentifulremotelysensedheatislandstudiesatsimilarscales.Thefollowingisasampleofsuchwork.(a)Scalemodels:Noto(1996),Poreh(1996)andLuetal.(1997).Copyright2003RoyalMeteorologicalSocietyInt.J.Climatol.:1–26(2003) A.J.ARNFIELD(b)Numericalmodels:Baik(1992),BaikandChun(1997),Baiketal.(2001),KimuraandTakahashi(1991),RichiardoneandBrusasca(1989)andTapperetal.(1981).(c)Remotelysensedheatislands:BallingandBrazel(1988),Galloetal.(1993a,b),HafnerandKidder(1999)(alsoincludesmodelling),Kim(1992),Rothetal.(1989a)andVukovich(1983).5.CONCLUDINGREMARKSThisreviewhasattemptedtocharacterizesignicantadvancesin,andthecurrentstatusof,twoareaswithinthebroadereldofurbanclimatologyfortheperiodoftwodecadessincetheestablishmentoftheJournalofClimatology.Therstoftheseisaselectedsetofsmall-scaleclimaticprocessesencompassingatmosphericturbulenceandexchangesofenergyandwater.ThesecondistheUHI.Themajorndingsandrecommendationsofthisexercisemaybesummarizedasfollows.1.Urbanclimatologyhasbenetedprofoundlyfromconceptualdevelopmentsinboundary-layerclimatology,broadlydened.Inparticular,recentenhancementsinourunderstandingoftheroughnesssublayerandoftheimplicationsofheterogeneityanduxsourceareashasfoundextensiveapplicationinboththeinterpretationofurbanenergybalancesandtheirmeasurement.2.Simulationexperimentsinparticular,andothermethodologicalapproachesingeneral,wouldbenetfromstudiesthatcontributetoourdatabaseofthefundamentalphysicalparameterswhichdeterminethedistinctiveclimatesofurbanareas,orthatprovideestimationalgorithmsbywhichtheseparametersmightbeestimatedwithadequateprecisionfrommorereadilyobservableentities.Exampleswouldbemethodsofcharacterizingtheradiativeeffectsofurbanpollutantson,methodsoffromurbanform,thermalandradiativepropertiesofbuildingmaterialsandothercommonsurfacetypesincities,waysofassigningsurfaceresistancevaluesandwaterstoragecapacitiesforpermeableurbanelementsandsimilarparametersusefulinthenumericalmodellingandinterpretationofurbanclimaticsystems.3.ThoughinformationontheformanddynamicsoftheUHIisnowavailableforclimaticzonesoutsidethetemperateregions,itwouldnowbeadvantageoustoextendenergybalanceevaluationsbeyondthoseareas,asexempliedbythepioneeringeldprogramsofOkeetal.(1992,1999)forMexicoCity.ThisisconsistentwiththeobjectivesoftheTRUCEinitiative(Okeetal.,1990–91)andcancontributetothegeneralityofurbanclimatology’sknowledgebase.4.Inaddition,energybalancemeasurementsofthetypenowemployedroutinelyforsuburbanlocationsarerequiredforcentralcitysites.Thesampling,measurementheightandotherobservationalrequirementsforsuchprogramswill,however,provedifculttomeetforcitycentreswithhigh-risebuildings.5.Completewaterbudgetsforurbanareasofdifferenttypesshouldbeconstructed,includingallcomponents(naturalandanthropogenic).Suchstudiesareneededtoelucidatetheroleofevapotranspi-rationinurbanareas,especiallyitsinteractionwithboundary-layergrowth,surfacewateravailability,thepipedwatersupplyandmicroscaleadvection.6.Additionalempiricalobservationsandmethodsofestimationarerequiredforthetwoanthropogenic.Theseareessentiallyuniquetourbanenvironmentsandareparticularlyfascinatinginthattheyintroducephasecharacteristicsintourbanenergyandwaterbalancesthatareindependentofthesolarcycle,beingdeterminedbyhumanperceptionanddecision-makingimperatives.Explorationsofwaysinwhichtrafccounts,populationdata,homeownerquestionnaires,utilityrecords,beincorporatedintoclimaticresearchaturbanscalesshouldcontinuetobepursued.7.Oke(1982)statedthaturbanheatislandswere‘welldescribedbutratherpoorlyunderstood’.Twodecadeslater,thisstatementcouldnotbemadewithasmuchcondence.Nevertheless,simplemethodsarestillneededtoestimateUHIintensityurbanareas,asafunctionoftime,weatherconditionsandstructuralattributes,forpracticalapplicationssuchasroadclimatology,phenology,energyconservation,andweatherforecasting.Copyright2003RoyalMeteorologicalSocietyInt.J.Climatol.:1–26(2003) URBANCLIMATE8.Methodsarerequiredtolinksmall-scaleandmesoscaleurbanclimatework.Suchlinkagesaretroublesomeandchallengingbutnotimpossible.Forexample,VoogtandGrimmond(2000)explorethelinksbetweenremotelysensedsurfacetemperatures,completesurfacetemperaturesandcalculationsofsensibleheatuxusingabulktransferapproachandMasson(2000)employsconceptsofcanyongeometrytocalculateurbansurfaceenergybudgetsinmesoscalemodels.9.Numericalsimulation,amethodologyperfectlysuitedtodealingwiththecomplexitiesandnon-linearitiesofurbanclimatesystems,continuestogrowinpopularity.Validationofmodels,unfortunately,lagsbehindtheircreationand,whenperformed,isoftenweak,relyingmoreonplau-sibilityofoutputsthandirectcomparisonwithprocessvariables.Thisisnotsurprising,becausethedifcultyofmeasuringsuchvariablesisaprimereasonthatnumericalsimulationissopopular.Closercollaborationbetweenmodellersandeldclimatologistsisencouragedtoclosethemethodologicalgap.10.Urbanclimatologycontinuestomigratemethodologicallyfromdescriptiveandinductive‘blackbox’approachestoprocessstudiesandprocess-response(simulation)modelling.Thismigrationisapositiveaspectoftheeld’srecenthistory(Terjung,1976;Oke,1982),becauseitenhancestheexplanatorypowerurbanclimatologistshaveattheirdisposal.Thistrendshouldcontinue.AckermanB.1985.TemporalmarchoftheChicagoheatisland.JournalofClimateandAppliedMeteorology:547–554.AdebayoYR.1987.AnoteontheeffectofurbanizationontemperatureinIbadan.JournalofClimatology:185–192.AdebayoYR.1990.AspectsofthevariationinsomecharacteristicsofradiationbudgetwithintheurbancanopyofIbadan.AtmosphericEnvironmentB:9–17.AidaM.1982.Urbanalbedoasafunctionoftheurbanstructure—amodelexperiment(partI).Boundary-LayerMeteorology405–413.AidaM,GotohK.1982.Urbanalbedoasafunctionoftheurbanstructure—atwo-dimensionalnumericalsimulation(partII).Boundary-LayerMeteorology:415–424.AnandakumarK.1999.Astudyonthepartitionofnetradiationintoheatuxesonadryasphaltsurface.AtmosphericEnvironment3911–3918.ArneldAJ.1976.Numericalmodellingofurbansurfaceradiativeparameters.InPapersinClimatology:TheCamAllenMemorialVolume,DaviesJA(ed.).DiscussionPaperNo.7.DepartmentofGeography,McMasterUniversity.ArneldAJ.1982a.Estimationofdiffuseirradianceonsloping,obstructedsurfaces:anerroranalysis.ArchivesforMeteorology,Geophysics,andBioclimatologyB:303–320.ArneldAJ.1982b.Anapproachtotheestimationofthesurfaceradiativepropertiesandradiationbudgetsofcities.PhysicalGeography:97–122.ArneldAJ.1984.Simulatingradiativeenergybudgetswithintheurbancanopylayer.ModelingandSimulation:227–233.ArneldAJ.1988.Validationofanestimationmodelforurbansurfacealbedo.PhysicalGeography:361–372.ArneldAJ.1990a.Streetdesignandurbancanyonsolaraccess.EnergyandBuildings:117–131.ArneldAJ.1990b.Canyongeometry,theurbanfabricandnocturnalcooling:asimulationapproach.PhysicalGeography:220–239.ArneldAJ.1998a.Micro-andmesoclimatology.ProgressinPhysicalGeography:103–113.ArneldAJ.1998b.Micro-andmesoclimatology.ProgressinPhysicalGeography:533–544.ArneldAJ.2000a.Micro-andmesoclimatology.ProgressinPhysicalGeography:261–271.ArneldAJ.2000b.Asimplemodelofurbancanyonenergybudgetanditsvalidation.PhysicalGeography:305–326.ArneldAJ.2001a.Micro-andmesoclimatology.ProgressinPhysicalGeography:123–133.ArneldAJ.2001b.Micro-andmesoclimatology.ProgressinPhysicalGeography:560–569.ArneldAJ,GrimmondCSB.1998.Anurbancanyonenergybudgetmodelanditsapplicationtourbanstorageheatuxmodeling.EnergyandBuildings:61–68.ArneldAJ,MillsGM.1994a.Ananalysisofthecirculationcharacteristicsandenergybudgetofadry,asymmetric,east–westurbancanyon.I.Circulationcharacteristics.InternationalJournalofClimatology:119–134.ArneldAJ,MillsGM.1994b.Ananalysisofthecirculationcharacteristicsandenergybudgetofadry,asymmetric,east–westurbancanyon.II.Energybudget.InternationalJournalofClimatology:239–261.ArneldAJ,HerbertJM,JohnsonGT.1998.Anumericalsimulationinvestigationofurbancanyonenergybudgetvariations.InPreprintVolume,AmericanMeteorologicalSocietySecondUrbanEnvironmentSymposiumand13thConferenceonBiometeorologyandAerobiology,2–6November,Albuquerque,NM;2–5.AsaedaT,CaVT.1993.Thesubsurfacetransportofheatandmoistureanditseffectontheenvironment:anumericalmodel.Boundary-LayerMeteorology:159–179.AsaedaT,CaVT,WakeA.1996.Heatstorageofpavementanditseffectontheloweratmosphere.AtmosphericEnvironment413–427.BaikJ-J.1992.Responseofastablystratiedatmospheretolow-levelheating—anapplicationtotheheatislandproblem.JournalofAppliedMeteorology:291–303.BaikJ-J,ChunH-Y.1997.Adynamicalmodelforurbanheatislands.Boundary-LayerMeteorology:463–477.BaikJ-J,KimY-H,ChunH-Y.2001.Dryandmoistconvectionforcedbyanurbanheatisland.JournalofAppliedMeteorology1462–1475.Copyright2003RoyalMeteorologicalSocietyInt.J.Climatol.:1–26(2003) 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