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UrbanClimateChange andHeatIslands Thispageintentionallyleftblank
UrbanClimateChange andHeatIslands Characterization,Impacts,and Mitigation Editedby
RICCARDOPAOLINI SchoolofBuiltEnvironment,FacultyofArts,Designand Architecture,UniversityofNewSouthWales(UNSW), Sydney,NSW,Australia
MATTHAIOSSANTAMOURIS SchoolofBuiltEnvironment,FacultyofArts,Designand Architecture,UniversityofNewSouthWales(UNSW), Sydney,NSW,Australia
Elsevier
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1.Urbanclimatechange:reasons,magnitude,impact,and mitigation1
MatthaiosSantamouris
1.1 Introduction1
1.2 Whatiscausingurbanoverheating?7
1.3 Aboutthemagnitudeoftheurbanoverheating11
1.4 Abouttheimpactofurbanoverheating13
1.5 Mitigationofurbanoverheating18
1.5.1 Decreaseoftheabsorptionofsolarradiationintheurbanfabric18
1.5.2 Increaseoftheemissionofinfraredradiationbythe urbanstructures19
1.5.3 Increaseoftheventilativecoolingincities19
1.5.4 Decreaseoftheflowofadvectiveheat19
1.5.5 Increaseoftheevapotranspirationhearflux19
1.5.6 Decreaseoftheanthropogenicheatrelease20
1.5.7 Dissipationoftheexcessheattolow-temperature environmentalsinks21
1.6 Conclusion22 References23
2.Experimentalandmonitoringtechniquestomapand documenturbanclimatechange29
RiccardoPaolini
2.1 Introduction29
2.2 Measurementapproachesinurbanclimatology32
2.2.1 Networksofweatherstations continuousmonitoring32
2.2.2 Short-termterrestrialcampaigns46
2.2.3 Remotesensing56
2.3 Climateandnonclimatedatatosupporturbanheatmitigation: challengesandprospects59
2.3.1 Measurementofadvectiveflowsandcausesofurban overheating60
2.3.2 Measurementofparametersthatinfluencetheperformanceof urbanheatmitigationtechnologies60
2.3.3 Mappingofurbanpollutionandnoiselevels61
2.4 Conclusion61 References63
3.Synergiesandexacerbations effectsofwarmerweatherand climatechange73 HassanSaeedKhan,RiccardoPaoliniandMatthaiosSantamouris
3.1 Urbanheatislandsandurbanoverheating73
3.1.1 Urbanoverheatingcauses74
3.1.2 Urbanoverheatingquantificationmethods75
3.2 Heatwaves75
3.2.1 Heatwavesidentificationmethods76
3.3 Thecombinedeffectofurbanoverheatingandheatwaveson humanhealth,economy,energy,andenvironment79
3.3.1 Mortalityandmorbidity79
3.3.2 Energy80
3.3.3 Environmentandtheeconomy80
3.4 UOinteractionwithheatwaves quantificationofenergybudget equation81
3.4.1 Alterationintheradiativeinputduringheatwaves81
3.4.2 Alterationinsensibleandlatentheatfluxesduringheatwaves82
3.4.3 Alterationinadvectiveheatfluxesduringheatwaves83
3.4.4 Alterationinanthropogenicheatfluxesduringheatwaves83
3.4.5 Alterationinheatstorageduringheatwaves84
3.4.6 UOresponsetoheatwavesinvariouscities84
3.4.7 Inconsistentresponseofurbanoverheatingto heatwaves importantfactors88
3.5 Synopticclimatology91
3.5.1 Classification92
3.5.2 Synoptic-scaleweatherconditionsandurbanoverheating94
3.6 AcasestudyfromSydney,Australia98
3.6.1 Interactionbetweenurbanoverheatingandheatwavesin Sydney104
3.6.2 Interactionbetweenurbanoverheatingandsynoptic-scale weatherconditionsinSydney105
3.7 Discussionandconclusion106 Nomenclature110 Supplementarymaterial111 References113
4.Multiscalemodelingtechniquestodocumenturban climatechange123
NeginNazarian,MathewLipsonandLeslieK.Norford
4.1 Introduction:whymodelurbanandintra-urbanclimatechange?123
4.2 Modelingtechniquestodocumenturbanandintraurbanclimate variabilityandchange126
4.2.1 Scalemodels126
4.2.2 Statisticalmethods127
4.2.3 Numericalmethods127
4.2.4 Summaryandreviewofmodelingtechniques141
4.3 Modelingurbanclimate’simpactonhumanlife141
4.3.1 Urbanclimateandclimatechangeinteraction144
4.3.2 Urbanventilation146
4.3.3 Thermalenvironmentandexposureinthebuiltenvironment148
4.4 Conclusions151 References152
5.Urbanoverheating energy,environmental,andheat-health implications165 MatthaiosSantamouris
5.1 Introduction165
5.2 Impactofurbanoverheatingonenergygenerationandenergy supplysystems166
5.2.1 Impactofurbanoverheatingontheenergyconsumptionof referencebuildings169
5.2.2 Impactofurbanoverheatingonthetemporalvariationof theenergyconsumptionofbuildings172
5.2.3 Impactofoverheatingontheenergyconsumptionofthe totalbuildingstockofacity174
5.2.4 Impactofthefutureoverheatingontheenergyconsumption ofbuildings175
5.2.5 Impactofoverheatingontheglobalelectricityconsumption ofacity,oracountry176
5.2.6 Impactofoverheatingonthepeakelectricitydemand178
5.2.7 Impactofoverheatingontheperformanceoftheelectricity productionanddistributionsystems180
5.3 Impactofurbanoverheatingofurbanvulnerability186
5.4 Impactofurbanoverheatingonairquality187
5.5 Impactofurbanoverheatingonhealth191
5.5.1 Impactofurbanoverheatingonheat-relatedmorbidity192
5.5.2 Impactofurbanoverheatingonheat-relatedmortality201
5.6 Conclusion212 References214
6.Fightingurbanclimatechange—stateoftheartof mitigationtechnologies227
JieFeng,ShamilaHaddad,KaiGao,SamiraGarshasbi,GiuliaUlpiani, MatthaiosSantamouris,GianlucaRanziandCarlosBartesaghi-Koc
6.1 Introduction227
6.2 Mitigatingtheurbanheatusingadvancedmaterialsforthe urbanfabric228
6.2.1 Introductiontomitigationmaterials228
6.2.2 Highreflectancewhitecoatings229
6.2.3 Coloredinfraredreflectivecoatings231
6.2.4 Reflectingmaterialsofhighthermalcapacity232
6.2.5 Temperature-sensitive/colorchangingmaterials233
6.2.6 Fluorescentmaterialsformitigation237
6.2.7 Photonicandmaterialsofdaytimeradiativecooling240
6.2.8 Coolingwithelastocaloricmaterials244
6.3 Usingtranspirationcoolingtomitigateurbanheat250
6.4 Mistcooling255
6.5 Urbangreenerytomitigatingtheurbanheat261
6.5.1 Progressonatmosphericheatmitigationwithgreen infrastructure263
6.6 Conclusions277 References278
7.Environmental,energy,andhealthimpactofurban mitigationtechnologies297 MatthaiosSantamouris
7.1 Introduction297
7.2 Theimpactofincreasedurbanalbedoonurbantemperature, energyconsumption,andhealth298
7.2.1 Theimpactofincreasedurbanalbedoonambienturban temperature300
7.2.2 Theimpactofincreasedurbanalbedoonheat-relatedmortality305
7.2.3 Theimpactofincreasedurbanalbedoonenergyconsumption andelectricitygeneration309
7.3 Theimpactofincreasedgreeninfrastructureonurbantemperature andhealth310
7.3.1 Introduction310
7.3.2 Dataandcharacteristics311
7.3.3 Theimpactofincreasedgreeninfrastructureonambient temperature mitigationpotential311
7.3.4 Impactofincreasedgreeninfrastructureonheat-related mortality323
7.3.5 Impactofgreeninfrastructureonheat-relatedmorbidity325
7.3.6 Impactofincreasedgreeninfrastructureonurban pollutionlevels325
7.4 Conclusion327 References328
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Listofcontributors CarlosBartesaghi-Koc
SchoolofArchitectureandBuiltEnvironment,ECMS,TheUniversityofAdelaide, Adelaide,SA,Australia
JieFeng
SchoolofBuiltEnvironment,FacultyofArts,DesignandArchitecture,Universityof NewSouthWales(UNSW),Sydney,NSW,Australia
KaiGao
SchoolofBuiltEnvironment,FacultyofArts,DesignandArchitecture,Universityof NewSouthWales(UNSW),Sydney,NSW,Australia
SamiraGarshasbi
SchoolofBuiltEnvironment,FacultyofArts,DesignandArchitecture,Universityof NewSouthWales(UNSW),Sydney,NSW,Australia
ShamilaHaddad
SchoolofBuiltEnvironment,FacultyofArts,DesignandArchitecture,Universityof NewSouthWales(UNSW),Sydney,NSW,Australia
HassanSaeedKhan
SchoolofBuiltEnvironment,FacultyofArts,DesignandArchitectureUniversityof NewSouthWales(UNSW),Sydney,NSW,A ustralia;Data-61,TheCommonwealth ScientificandIndustrialResearchOrganization(CSIRO),Kensington,Perth,WA, Australia
MathewLipson
ARCCentreofExcellenceforClimateExtreme,Sydney,NSW,Australia
NeginNazarian
SchoolofBuiltEnvironment,FacultyofArts,DesignandArchitecture,Universityof NewSouthWales(UNSW),Sydney,NSW,Australia;CityFuturesResearchCentre, UniversityofNewSouthWales(UNSW),Sydney,NSW,Australia;ARCCentreof ExcellenceforClimateExtreme,Sydney,NSW,Australia
LeslieK.Norford
DepartmentofArchitecture,Massach usettsInstituteofTechnology,MA, UnitedStates
RiccardoPaolini
SchoolofBuiltEnvironment,FacultyofArts,DesignandArchitecture,Universityof NewSouthWales(UNSW),Sydney,NSW,Australia
GianlucaRanzi
SchoolofCivilEngineering,TheUniversityofSydney,Sydney,NSW,Australia
MatthaiosSantamouris
SchoolofBuiltEnvironment,FacultyofArts,DesignandArchitecture,Universityof NewSouthWales(UNSW),Sydney,NSW,Australia
GiuliaUlpiani
SchoolofBuiltEnvironment,FacultyofArts,DesignandArchitecture,Universityof NewSouthWales(UNSW),Sydney,NSW,Australia
CHAPTER1 Urbanclimatechange:reasons, magnitude,impact,and mitigation MatthaiosSantamouris
SchoolofBuiltEnvironment,FacultyofArts,DesignandArchitecture,UniversityofNewSouthWales (UNSW),Sydney,NSW,Australia
1.1Introduction Citiesincreasetheirboundariesandpopulation.Whiletheurbanpopulationin1960wascloseto1billion,reaching3.33billionin2007,itgrew to4.1billionby2017,representingalmost55%oftheworldpopulation (Ritchie&Roser,2018).Futureprojectionsshowthatby2050urban populationmayreach7billionpeople.Anincreaseintheurbanpopulationisassociatedwithaspectaculargrowthofthesizeofmegacities,cities hostingmorethan10millionpopulation.Whilein1990,only10cities presentedatotalpopulationabove10million,thenumberincreasedto 28millionin2014and33millionin2017(Young,2019).
Apartfromtheurbanpopulationincrease,thedensityofcitiesinthe developedworldhassurgedtounprecedentedlevels.CitieslikeMumbai andKolkata,India,andKarachi,Pakistanhavetremendouspopulation densitiesaround77,000,62,000,and50,000peoplepersquaremile, respectively.Urbanpopulationdensitieshavealsoincreasedindeveloped countrieswithoutreachingtheaforementioneddensityfigures.Forexample,populationdensitiesinTokyomegacity,JapanandAthens,Greece, arecloseto12,300and14,000peoplepersquaremile,respectively,while thedensityinSydney,Australiaisnotexceeding1100peoplepermile. Suchatremendousincreaseoftheabsolutepopulationfiguresanddensitiesareaseriouschallengeaffectingthelocalclimate,useofresources, diseasecontrolandhealthservices,educationandemploymentopportunities,networks,infrastructures,andfacilities.Poverty,unemployment, andlackofpropersheltersobligealmost1billionpeopletoliveininformalurbansettlementsorslumsincompletelyunacceptablehygienicand
climaticconditions(UnitedNationsHumanSettlementsProgramme UN-HABITAT,2019).Unfortunately,futurepredictionsshowadramaticgrowthoftheslumspopulation,anditispredictedthatby2050it willreachabout3billion,ormorethan30%oftheworldpopulation,by themiddleofthecurrentcentury(UnitedNationsDepartmentof EconomicandSocialAffairs,2017).
Apartfromthetremendousincreaseoftheurbanpopulationand urbandensities,severalotherissuesinfluencethemagnitudeofurbantemperatures.Designandconstructionpracticesincitiesfavorhighlyabsorbingandsuperwarmmaterialsforbuildingsandopenspaceslikeconcrete andasphalt.Thegrowthinurbanizationisreflectedalsointheworldproductionofcement,whichhasincreasedfromalmost3700Mtin2010to 4100Mtin2018(InternationalEnergyAgency,2019).Inparallel,the worlddemandforasphalthasrisenfrom103.3millionmetrictonsin 2005to119.5millionmetrictonsby2015(Statista,2016).Highlyabsorbingmaterialsusedintheoutdoorskinofbuildingsandopenurbanplaces reachveryhighsurfacetemperaturesduringthesummer,transferringthe absorbedandstoredheattotheambientaircontributingenormouslyto urbanoverheating.
Theincreaseoftheurbanpopulationandimprovementofthequality oflifeofthemid-classpopulation combinedwithanincreasingeconomiccapacityofhouseholds hasresultedinadramaticgrowthofthe numberofindividualcars,increasingtheanthropogenicheatreleasedin cities.Itisexpectedthatthetotalcarsaleswillincreasefrom70million peryearin2010toabout125million/yearin2025(Boutonetal.,2015). Abouthalfofthecarsareboughtincities,whilefutureprojectionsshow thatthefleetsize,1.2billioncars,maybedoubledby2030(Bouton etal.,2015).Carsreleaseheatandpollutionincities,increasingtheurban ambienttemperatureandobviouslycausingtrafficcongestion,whichin severalcountriescostabout2% 4%ofthenationalGrossDomestic Product(GDP)(Boutonetal.,2015).
Globalclimatechangeincreasesthemagnitudeoftheambienttemperatureandthefrequencyofextremeheatevents(IPCC,2000).Becauseof theimportantsynergiesbetweentheglobalandlocalclimatechange, higherambienttemperaturesintensifythemagnitudeoftheregionaloverheating(IPCC,2000),whileincreasingthedemandforbuildingcooling andraisingthereleaseofanthropogenicheatfromtheairconditioners. Futureprojectionsoutlineaconsiderableincreaseinthecoolingdegree daysinalmosteverypartoftheplanet(Warrenetal.,2006). Fig.1.1
Figure1.1 Currentandfuturecoolingdegreedaysforthemajorareasoftheplanet andfordifferentclimaticscenarios.(1)Baseline.(2)Increasebetween0Kand1K.(3) Increasebetween1Kand2K.(4)Increasebetween2Kand3K.(5)Increasebetween 3Kand4K,and6K.(6)Increasebetween4Kand5K(Santamouris,2016a,b).
presentsthepredictedcoolingdegreedaysforthemajorpartsofthe planet,consideringanincreaseintheambienttemperaturebetween0and 5K(Santamouris,2016a,b;Warrenetal.,2006).Asshown,forspecific regionslikeSouth-EasternAsia,theexpectedincreaseofthecooling degreedaysandcoolingenergydemandisextremelyhigh.
Regionalclimatechangedependshighlyonthesocioeconomicpathwaysfollowedindevelopedanddevelopingcountries.Futureeconomic growthmaydefinethelevelsofthefuturegreenhouseemissionsandthus theneedforadaptationandmitigation.Predictionsofthefutureeconomic growthandtheworldGDPdependonthespecificassumptionsofthe modelsandpresentahighuncertainty.Existingpredictionsfor2050differ substantiallyintermsofthepredictedaverageGDPpercapita. Fig.1.2 presentstheresultsof19publishedmodels(Santamouris,2016a,b).Asshown, theaverageGDPpercapitamayvarybetween7200and26,400US$of 1990.However,acommondenominatorofallscenariosistheserious amplificationoftheeconomicdifferencesbetweenthevariousgeographic partsoftheworld(Fig.1.3).TheusedGDPpredictiondataaretakenfrom theMassachusettsInstituteofTechnologyemissionscenario(MIT,2021).
Figure1.2 PredictedglobalGDPpercapitain2050bythevariousemissionscenarios(inUS$1990). FromSantamouris,M.,2016a.Coolingthebuildings past,present andfuture.EnergyandBuildings,128,617 638. https://doi.org/10.1016/j. enbuild.2016.07.034 .Santamouris,M.,2016b.Innovatingtozerothebuildingsectorin Europe:mminimisingtheenergyconsumption,eradicationoftheenergypovertyand mitigatingthelocalclimatechange.SolarEnergy,128,61 94. https://doi.org/10.1016/j. solener.2016.01.021.
PredictionshaveshownthattheGDPinthelessdevelopingcountriesmay bealmost85%lowerthanthatofthedevelopedones.Giventhatmostof thefuturemegacitiesarelocatedinthe developingcountries,wheremostof thefutureurbanpopulationisexpected,loweconomicdevelopmentwill affectthequalityofthecities,theirinfrastructure,theenergyconsumption, andtherelatedenvironmentalpoliciesandveryprobablywillresultinaseriousincreaseoftheurbanoverheating.
Overpopulationandeconomicgrowthdrivethefuturedevelopment ofresidentialandcommercialbuildings.Whilein2010,thetotalfloor areaofresidentialbuildingsintheworldvariedbetween140billion squaremetersand190billionsquaremeters(GlobalEnergyAssessment WritingTeam,2012;Urge-Vorsatzetal.,2013),itisexpectedtoincrease upto180 290billionsquaremetersby2030and190 379billion squaremetersby2050.Inparallel,thetotalareaofthecommercialbuildingsisexpectedtoriseby2050between25billionsquaremetersand 30billionsquaremeterscomparedto21 24billionsquaremetersin
per Capita, (2010 $)
Figure1.3 PredictedgrowthoftheGDPpercapitabetween2010and2050forall majorareasoftheworld(inUS$2010).Eachboxplotcomprisesalldatafromall majorzonesoftheworld. FromSantamouris,M.,2016a.Coolingthebuildings past, presentandfuture.EnergyandBuildings,128,617 638. https://doi.org/10.1016/j. enbuild.2016.07.034 .Santamouris,M.,2016b.Innovatingtozerothebuildingsectorin Europe:mminimisingtheenergyconsumption,eradicationoftheenergypovertyand mitigatingthelocalclimatechange.SolarEnergy,128,61 94. https://doi.org/10.1016/j. solener.2016.01.021.
2010(John,2014;Urge-Vorsatzetal.,2013).Suchamassiveincreaseof thetotalbuildingsurfaceisexpectedtohaveasignificantimpactontheir energyconsumptionandthereleasedanthropogenicheatandalsoonthe overallconstructionactivitiesandmaterialuse.Atthesametime,itisevidentthatitwillseriouslyintensifythemagnitudeofthelocaloverheating.
Thefutureenergyconsumptionofbuildingsmaybepredictedconsideringalldriversaffectingtheirenergydemandatagloballevel.Thelevels offutureenergyconsumptiondetermineatlargethemagnitudeofthe greenhousegasemissionsandtheevolutionofglobalclimatechange, whilethereleaseoftheadditionalanthropogenicheatintheurbanenvironmentmayseriouslyintensifythelevelsoflocaloverheating.Numerous predictionmodelsconsideringmostoftheabovedriversaredeveloped aroundthefuturebuildingenergyconsumption. Fig.1.4 presentsthepredictedfutureenergydemandoftheresidentialsectorascalculatedby
Figure1.4 Predictedfutureresidentialcoolingenergyconsumptionbythevariousexistingmodels.Thebluezone(leftpartofthefigure)isfor2030,thegreen(middlepartof thefigure)for2050,andthered(rightpart),for2100. FromSantamouris,M.,2016a. Coolingthebuildings past,presentandfuture.EnergyandBuildings,128,617 638. Santamouris,M.,2016b.InnovatingtozerothebuildingsectorinEurope:mminimising theenergyconsumption,eradicationoftheenergypovertyandmitigatingthelocalclimatechange.SolarEnergy,128,61 94. https://doi.org/10.1016/j.solener.2016.01.021 .
severalmodels(McNeil&Letschert,2008;Scottetal.,2008).Asshown, averysubstantialincreaseincoolingenergyconsumptionisforeseenfor thenext20 30years.ItisimportantthatthehighestenergyconsumptionisexpectedinAsiaticcountriesandinparticularinChinaandIndia, wherethemostseriousurbanproblemsareexpected.
Globalclimatechange,anincreaseintheurbanpopulation,higher urbandensities,asignificantincreaseinthenumberofbuildings,andthe expectedtremendousenergyconsumptionofthebuildingsectorarefactorsthatmayseriouslyintensifythemagnitudeofurbanoverheating.
Higherurbantemperatureshaveaseriousimpactontheenergyconsumptionofbuildings,indoorandoutdoorthermalcomfort,theconcentrationofharmfulpollutants,heat-relatedmortalityandmorbidity,
sustainabilityandsurvivabilitylevelsoflow-incomehouseholds,while seriouslyaffectingtheglobaleconomyandwell-beingofcities (Santamouris,2020).Thepreciseimpactofurbanoverheatingiswell quantifiedandisanalyzedandpresentedintherestofthebook.
Tofacetheproblemandcounterbalancetheimpactoflocaloverheating, propermitigation,andadaptationtechnologies,measuresandpolicieshaveto bedevelopedandimplemented.Intherecentyears,seriousresearchhasbeen carriedoutaimingtocounterbalancetheimpactofregionalclimatechange whilenumerouslargescaleprojectsareundertakentoemployandimplement advancedmitigationtechnologiesandmeasures(Akbarietal.,2016). Monitoringresultsobtainedfromahighnumberofmitigationprojectshave shownthattheuseofthecurrentlyavailablemitigationtechnologiescontributesseriouslytodecreasethepeakambienturbantemperatureupto2.5° C 3°C,improvecomfortlevels,decreaseheat-relatedmorbidityandmortality, reducetheconcentrationofharmfulpollutants,andimprovethelivingconditionsindeprivedurbanareas(Santamourisetal.,2017).
Thisbookaimstopresentandanalyzethecausesofurbanoverheating, thefuturechallengesconcerningthelocalclimatechange,demonstrate thelatestdevelopmentsoftheexperimentalandmonitoringtechnologies toquantifythecharacteristicsandthemagnitudeofthelocaloverheating, identifyandquantifytheimpactofoverheatingonenergy,health,environmentalqualityandeconomy,andfinallypresenttherecentachievementsinthefieldofurbanmitigationandadaptationtechnologies.
1.2Whatiscausingurbanoverheating? Thethermalbalanceinthebuiltenvironmentisdefinedasthesumofthe heatgains,heatstorage,andheatlosses.Asurbanareascomparedtothe surroundingruralorsuburbanenvironmentpresenthigherthermalgains andlessthermallosses,theirthermalbalanceismorepositive,andcities presentahigherambienttemperaturecomparedtotheirsurroundings. Thisdynamicsiswellknownastheurbanheatislandphenomenon (Fig.1.5),whichprobablyisthemostdocumentedphenomenonofclimatechange(Akbarietal.,1992).
Citiesreceiveandabsorbsolarradiation.Theexactamountofabsorbed radiationdependsonthesolarabsorbanceofthematerialsandotherurban structures,whichisasurfaceproperty.Mostmaterialsused,likeconcreteand asphalt,presentahighsolarabsorbance(i.e.,theratioofabsorbedtoincident solarradiation).Theabsorbedenergyisstoredintothemassofthematerials,
Sketch of an Urban Heat-Island Profile Figure1.5 Sketchofatypicalheatislandurbanprofile. FromTaha,H.,Akbari,H., Sailor,D.,Ritschard,R.,1992.Causesandeffectsofheatislands:sensitivitytosurface parametersandanthropogenicheating.
increasingtheirtemperature,anditis releasedintotheatmosphereintheform ofconvectiveheatandinfraredradiation. Inparallel,materialsabsorbinfrared radiationemittedbytheatmosphereandtheothersurfacesinthebuiltenvironment.Convectivelossesorgainsbetweentheurbansurfacesandtheambientairdependmainlyonthecorrespondingtemperaturedifferenceandthe windspeedandturbulence.Anthropogenicheataddedtotheatmosphereas releasedbycars,industry,powerplants,andtheenergysystemsofbuildings increasestheenergyfluxesinthebuilt environment.Heattransferbyadvection incitiesaffectsinapositiveornegativewaytheenergybudgetasafunctionof thetemperaturedifferencebetweentheambientandtheadvectedair.Finally, latentheatreleasedthroughtheevaporationofwaterbyurbanvegetationand watersurfaceshelpstodecreasetheambienttemperature.
Thustheenergybalanceofthesurface ambientairsystemcanbe writtenas:
whereQr isthesumofthenetradiativeflux,QT isthereleasedanthropogenicheat,QE isthesumofthesensibleheat,QL isthelatentheat,Qs isthestoredenergy,whileQA isthenetenergytransferredtoorfromthe urbansystemthroughadvectionundertheformofsensibleorlatentheat. Theadvectivetermcanbeignoredincentralurbanareassurroundedby analmostuniformbuildingdensity.Still,itmaybeimportedintothe boundariesbetweentheurbanandtheruralenvironment.
Usually,theabsorbedsolarradiationisthetermpresentingthehighest magnitude,andthatcontributesmoretoincreasetheurbantemperature. Thusadecreaseinthesolarabsorbanceorincreaseofthesolarreflectance oftheurbansurfacesiscrucialtominimizethereleaseofthesensibleheat totheatmosphereanddecreasetheambienttemperature.Theinfrared radiationemittedbytheurbanstructurescontributeshighlytolowerthe ambienttemperature,especiallyduringthenighttime.Theemissivecapacity,thatis,theemissivityoftheurbanmaterialsandstructures,highly determinesthemagnitudeoftheemittedradiation.However,asthespectralemissivityisequaltothespectralabsorptivityofthematerials,high emissivityvaluesmayresultinincreasedabsorptionoftheemittedatmosphericradiation.Especiallyinurbanzoneswithahighcontentofwater vapororatmosphericpollution,themagnitudeoftheincomingatmosphericradiationmaybequitehigh.Materialspresentingahighspectral emissivityintheso-calledatmosphericwindow,thatis,between8and 13micrometers,presentanadditionaladvantageastheatmosphericradiationatthesewavelengthsisminimum.Latentheatreleasedbyurbanvegetationandwatersurfacesisconsiderablyreducedincitiescomparedto theruralareasasaresultofthelimitedgreenandwaterzones.Anincrease oftheevapotranspirationalfluxcontributesconsiderablytodecreasethe ambienttemperatureandrisethewatercontentintheatmosphere.
Advectiongainsorlossescanbeadeterminantofheatfluxincities.In coastalzones,theimpactofseabreezehelpstoreducethelevelsofthe ambienttemperatureconsiderablyandfightoverheating,especiallyduring theafternoonhours.Inparallel,urbanzoneslocatedclosetohotandarid zoneslikethedesertorotherheatsources,likepowerplantsorlargephotovoltaicplants,mayhaveaverynegativeimpactashotorwarmairmay betransferredtothecity.
Anthropogenicheatreleasedintheurbanambientairvariesasafunctionofthespecificcharacteristicsofthecityandtherelativeanthropogenicactivitiesliketransport,industry,energysystemsofthebuildings, etc.Althoughtheaverageanthropogenicheatfluxissmallcomparedto
thesummertimemid-daysolarradiation,wasteheatfromurbananthropogenicactivitiesmayplayanimportantroleintheformationandmagnitudeoftheheatislandphenomenon.Manyexperimentalandmodeling studieshavedocumentedthatwasteheat,mainlyfromurbanenergy, transportationsystems,andpowergeneration,contributestoincreased heatislandintensities(Khan&Simpson,2001;Sailor&Lu,2004).A methodologytoestimatethemagnitudeoftheanthropogenicheatgeneratedincitiesisproposedin SailorandLu(2004).
Manystudieshavebeenperformedtocalculatetheanthropogenic heatfluxinurbanareas,andavaluecloseto100W/m2 issuggestedasan average(Grimmond,1992;Kłysik,1996).However,muchhighervalues havebeenreportedforvariouscities.Inthepast,itwasestimatedthatthe anthropogenicheatindowntownManhattanwascloseto198W/m2 (Couttsetal.,2007),whilethemaximumfluxincentralLondonwas closeto234W/m2,withanaveragevaluecloseto100W/m2 (Harrison etal.,1984).AnanalysisoftheanthropogenicheatreleasedinUScities reportsanaveragefluxbetween20and40W/m2 forthesummerand between70and210W/m2 forthewinterperiod,consideringtheupper valueasanextreme(Hosler&Landsberg,1997).InMoscow,Budapest, Reykjavik,andBerlin,theaverageanthropogenicheatfluxisestimated closeto127,43,35,and21W/m2,respectively,whileforMontrealand Vancouver,itis99and26W/m2 (Steinecke,1999;Tahaetal.,1992).
Morerecentstudiesshowthattheanthropogenicheatfluxintheurban areainTokyoexceeds400W/m2 inthedaytime,whilethemaximum valueiscloseto1590W/m2 inwinter(Ichinoseetal.,1999).Another analysisoftheanthropogenicheatdistributionforcentralBeijingshows thatat0800a.m.localtime,itrangesbetween40and220W/m2 insummerand60toW/m2 inwinter(Chenetal.,2007).Finally,estimations forToulouse,France,showedthatanthropogenicheatflowsarearound 15W/m2 duringsummerand70W/m2 duringthewinterwithpeaksof 120W/m2 (Pigeonetal.,2007).
Anthropogenicheatcanbeanimportantcontributortothethermal environmentsofcities.Numericalsimulationsoftheurbantemperature regimehaveshownthatanthropogenicheatmayincreaseurbantemperaturesbyupto3°C(Narumietal.,2003).UsingamesoscalemodelcalculatedthattheadditionofanthropogenicheatinOsakaincreasestheurban temperaturetoabout1°C.DetailedsimulationsfortheTokyoarea, reportedin KondoandKikegawa(2003),showthatanthropogenicheatingintheOtemachiarearesultedinatemperatureincreaseofabout1°C.
In Ichinoseetal.(1999),itisestimatedthatthetemperatureincreasein thesameareaofTokyoduringthesummerperiodwasaround1.5°Cat 10p.m.,whilemuchhigherdifferenceshavebeencalculatedforsurface energybalancecomponents.
1.3Aboutthemagnitudeoftheurbanoverheating Urbanoverheatingisexperimentallydocumentedinmorethan450large citiesintheworld.Whiletheexistingknowledgeonthemagnitudeof urbanoverheatingisquiterich,itisovershadowedbytheproblem’ saccuracyandrepresentativenessoftheresultsandinconsistenciesoftheexperimentaldataandtheoreticalconclusions(Stewart&Oke,2012).Thereare threemainmonitoringtechniquesemployedtomeasurethemagnitudeof urbanoverheating:(1)Thosebasedontheuseofmobiletraverses.(2) Thoseusingstandardfixedobservationstations.(3)Thoseusingnonstandardobservationstations(Santamouris,2015).Inparallel,measurements varyasafunctionofthenumberofmeasuringstationsused,theduration ofdatacollection,thereportingformat,andthecriteriatoselectthereferencestation(Stewart,2011).
Studiesbasedonmobiletraversesandnonstandardmeasuringstations areusuallybasedondatacollectedforarelativelyshortperiod,andthe reportedmagnitudeofurbanoverheatingisusuallythemaximumorthe averagemaximumvaluemeasuredduringtheexperimentalperiod.When standardmeasuringstationsareused,datamaybeavailableforseveral years,whileeithertheannualaverage,theannualaveragemaximum,or theannualabsolutemaximumarereportedasthemagnitudeofurban overheating.
Ananalysisofthespecificlevelsofurbanoverheatingin100Asianand Australiancitiesisgivenin Santamouris(2015).Inparallel,asimilaranalysisfor110Europeancitiesisreportedin Santamouris(2015).AsitconcernstheAustralianandAsiancities,whenmobiletraversesareused,the magnitudeoftheurbanoverheatingvariesbetween0.4°Cand11.0°C (Santamouris,2015),whiletheaverageintensityiscloseto4.1°C.For about23%and58%oftheexaminedcities,themagnitudewasbelow 2°Cand4°C,respectively,while27%ofthecitiespresentedanoverheatingintensityhigherthan5°C.Whennonstandardmeteorologicalstations areused,theoverheatingmagnitudeisfoundtovarybetween1.5°Cand 10.7°C,withanaveragevaluecloseto5°C.Almost42%ofthecitiespresentedanoverheatingintensityhigherthan5°C(Santamouris,2015).
Whenmeasurementsarebasedonmultiyeardatacollectedbystandard meteorologicalstations,theannualaverage,annualmaximumaverage, andannualabsolutemaximumintensityoftheoverheatingisreportedas themagnitudeofurbanoverheating. Figs.1.6and1.7 reportthemagnitudeoftheannualaverageandabsolutemaximumannualoverheating magnitude(Santamouris,2015).Thereportedaverageintensityofthe annualmean,meanmaximum,andabsolutemaximumoverheatingintensityis1.0°C,3.1°C,and6.2°C,respectively.Itischaracteristicthatfor about20%ofthecities,theannualabsolutemaximumintensityofoverheatingexceeded8°C(Santamouris,2015).
InEurope,whenstandardmeteorologicalstationsareused,thecorrespondingaverageannual,averagemaximum,andannualabsolutemaximumintensityoftheurbanoverheatingis1.1°C,2.6°C,and6.2°C, respectively.Inparallel,theaveragemaximummagnitudeoftheoverheatingwhenmobiletraversesareusedisfoundcloseto6°C (Santamouris,2016a,b).
Figure1.6 Reportedintensityoftheannualaverageurbanheatislandforstudies basedonstandardmeasuringequipment. FromSantamouris,M.,2015.Analyzingthe heatislandmagnitudeandcharacteristicsinonehundredAsianandAustraliancities andregions.ScienceoftheTotalEnvironment,512 513,582 598. https://doi.org/ 10.1016/j.scitotenv.2015.01.060 .
Figure1.7 Reportedintensityofthemax maxurbanheatislandforstudiesbased onstandardmeasuringequipment. FromSantamouris,M.,2015.Analyzingtheheat islandmagnitudeandcharacteristicsinonehundredAsianandAustraliancitiesand regions.ScienceoftheTotalEnvironment,512 513,582 598. https://doi.org/10.1016/j. scitotenv.2015.01.060 .
Itisevidentthattheintensityofurbanoverheatingisquitehigh,especiallyduringthesummerperiod,andonaverage,mayexceed5°C.Such atemperatureincreasehasasevereimpactontheenergydemandfor coolingpurposes,whileitincreasestheconcentrationofharmfulpollutantsandraisesthelevelsofheat-relatedmortalityandmorbidity.
1.4Abouttheimpactofurbanoverheating Importantresearchworkhasbeencarriedoutintherecentyearsaiming toquantifytheimpactofurbanoverheatingonenergy,peakelectricity demand,pollutionlevels,heat-relatedmortalityandmorbidity,aswellas onurbansustainabilityandeconomy.
Inparticular,theimpactofhigherambienttemperatureontheenergy systemsiswellquantifiedanddocumented.Asdescribedin Santamouris (2020),urbanoverheatingaffectsboththeenergydemandandsupplysectorsadversely.Asitconcernstheenergyconsumptionofbuildings,overheatingraisesthecoolingenergyconsumptionincities.Itisreportedthat
