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Environ. Res. Lett. 11 (2016) 074003
doi:10.1088/1748-9326/11/7/074003
LETTER
Weekly cycles in peak time temperatures and urban heat
islandintensity
Nick Earl1, Ian Simmonds1 andNigel Tapper2
1 School of Earth Sciences, TheUniversity ofMelbourne,
Parkville,Melbourne, Victoria, 3010, Australia2 School of Earth,
Atmosphere and Environment,MonashUniversity andCooperative
ResearchCentre forWater Sensitive Cities, Clayton,
Melbourne, Victoria, 3800, Australia
E-mail: [email protected]
Keywords: anthropogenic activity, weekly cycles, diurnal cycle,
urban heat island
Supplementarymaterial for this article is available online
AbstractRegular diurnal andweekly cycles (WCs) in temperature
provide valuable insights into theconsequences of anthropogenic
activity on the urban environment. Different locations experience
arange of identifiedWCs and have very different structures. Two
important sources of urban heat arethose associatedwith the effect
of large urban structures on the radiation budget and energy
storageand those from the heat generated as a consequence of
anthropogenic activity. The former forcingwillremain relatively
constant, but aWCwill appear in the latter.WCs for specific times
of day and theurban heat island (UHI)have not been analysed
heretofore.We use three-hourly surface (2m)temperature data to
analyse theWCs of sevenmajor Australian cities at different times
of day and todetermine towhat extent one of ourmajor city’s
(Melbourne)UHI exhibits aWC.We show that theWCof temperature
inmajor cities differs according to the time of day and that theUHI
intensity ofMelbourne is affected on aWC. This provides crucial
information that can contribute toward thepush for healthier urban
environments in the face of amore extreme climate.
1. Introduction
Regular diurnal and weekly cycles (WCs) of meteor-ological
parameters provide valuable insights into theconsequences of human
activities (e.g. industrialactivity, generating electricity,
biomass burning andpowering motor vehicles, which commonly change
atweekends), particularly in urban areas. Such insightsare possible
because many human activities followfairly regular diurnal and
weekly schedules. SinceAshworth (1929) ‘accidentally’ discovered a
weeklySunday minimum when investigating rainfall in theindustrial
town of Rochdale, England, the concept ofhumans affecting their
surroundings on a daily scaleinspiredmanyWC studies as summarised
by Sanchez-Lorenzo et al (2012).
Different locations experience a wide range ofidentified WCs,
and these are often contradictory innature. Many researchers have
found significant WCsin various meteorological parameters in and
aroundurban/industrial areas (Fujibe 1987, 2010, Simmonds
and Keay 1997, Cerveny and Balling 1998, 2005, For-ster and
Solomon 2003, Shutters and Balling 2006,Gong et al 2006, 2007, Bell
et al 2008, Laux and Kunst-mann 2008, Bell et al 2009, Sitnov 2010,
Rosenfeld andBell 2011, Gruzdev 2013, Farias et al 2014,
Georgouliaset al 2015)while others found that the observed
signalsare no stronger than the background variability(DeLisi et al
2001, Barmet et al 2009, Hendricks Frans-sen et al 2009, Stjern
2011). Daniel et al (2012) suggestthat a wide variety of
statistical tests are often usedinappropriately and this is at
least partly responsiblefor the contradiction. Anthropogenic
activity linked toWCs can be thought of as due to human heat
genera-tion and the release of atmospheric pollution (Sim-monds and
Keay 1997), with the accompanyingemission of moisture also
affecting the urban energybudget (Sailor 2011). The three major
sources ofanthropogenic heat, closely related to energy
con-sumption, are transportation, buildings and industry,as
explained by Sailor (2011). However, atmosphericdynamics, aerosol
andmoisture release complicate the
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ACCEPTED FOR PUBLICATION
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PUBLISHED
1 July 2016
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observed surface air temperature relationship withanthropogenic
activity. Aerosol and moisture interac-tion with solar radiation
(direct effect) and aerosolsaltering clouds characteristics and
thermodynamics(indirect effect) vary greatly with time of year,
surfaceland use and a wide range of other influences (e.g.
citysize, location, types of local circulations; Bell andRosenfeld
2008). Therefore, the net effect of aerosol onsurface temperature
is highly variable and underdebate.
The presence and/or strength of WCs in urbanareas are related to
the driving forces for the anthro-pogenic use of energy. These
include population, char-acter of industrial activity, patterns and
intensity ofvehicular traffic, typical synoptic patterns (and
impli-cations for, e.g., ventilation), physical geography of
thecity, proximity of the city to large water bodies andtheir
direction in relation to the prevailing winds, fre-quency of
inversions (and their implications for con-vection and for trapping
of heat) and typical rain-producing mechanisms of the area. The
relativeimportance of these factors differs between cities,strongly
influenced by background climate.
Diurnal rhythms also present perspectives onanthropogenic
influences. The typical diurnal warm-ing and cooling regime of
high-density urban areas isvery different from that of rural or
low-density urbanareas. After nightfall, urban sites cool
relatively slowlydue to a number of anthropogenic causal
mechan-isms, discussed in detail by Oke (1982). This steadydecline
continues throughout the night, whereas ruralareas cool fast soon
after sunset and then more slowlylater as radiation emission
decreases. After sunrise, therural sites warm faster, catching up
with the urbanlocations and generally both reach a temperature
peakin mid-afternoon. This rural warming can lead to anearly
morning urban ‘cool island’ in some cities due tothe atmospheric
boundary layer being deeper over theurban area, reducing the
apparent earlymorning heat-ing compared to the rural environment
(Oke 1982,Theeuwes et al 2015). However other factors
includingshading in urban canyons, leading to a smaller surfacearea
of accessible insolation, greater total urban sur-face area
(compared toflat surface) and enhanced earlymorning absorption of
radiation by urban materials(e.g. concrete and asphalt) may also be
involved. Thismeans that the urban heat island (UHI)
intensityundergoes a marked diurnal variation with the
largestdifference typically 3–5 h after sunset and smallest,which
may sometimes be negative, from early morn-ing tomid-afternoon. For
reasons similar to those dis-cussed above, the diurnal evolution of
the UHI candiffer significantly with location. The diurnal
cycle,combined with theWC, provides us with the opportu-nity to
examine when anthropogenic activity has thelargest influence on the
environment. This is the firstmajor focus of the paper.
There is a range of postulated causes of urban heat(Oke 1982).
Two of the more important sources of
urban heat are those associated with the effect of largeurban
structures on the radiation budget and energystorage and from the
heat generated from anthro-pogenic activity (Rizwan et al 2008).
The former for-cing will remain relatively constant, but a WC
willappear in the latter. The nature (including the
diurnalprogression) and intensity of the UHI changes withcity
characteristics (size, density, industrial activity,traffic
intensity, amount of green space etc; Stewartand Oke 2012),
latitude, time of day (e.g. Kolokotroniet al 2006), background
climate (e.g. Zhao et al 2014)and weather types (e.g. Lowry 1977,
Morris and Sim-monds 2000). This means that the UHI in cities in
tro-pical areas may contrast with those in temperateregions.With
Australia possessing cites with a range ofclimates, it is an ideal
location for this study. Lowry(1977) provides a conceptual
framework that high-lights the difficulties in defining and
measuring theUHI, with factors including background climate,
localtopography and effect of local urbanisation. Under-standing
the characteristics of the UHI is of consider-able importance due
to the rises in urban population(table S1). UHIs have a profound
effect on the lives ofurban residents and the health impact of
heatwaves isone factor that motivates the growing efforts to
miti-gateUHI (Tapper et al 2014, Zhao et al 2014).
One aspect of UHIs yet to be comprehensively stu-died is how it
varies on a WC. Furthermore, surfaceobservation based WC
temperature studies are essen-tially based on daily mean
temperature (Bäumer andVogel 2007, Laux and Kunstmann 2008, Fujibe
2010),maximum or minimum temperatures (Simmondsand Keay 1997, Laux
and Kunstmann 2008, Kimet al 2009) or daily temperature range
(Forster andSolomon 2003, Gong et al 2006, Bäumer andVogel 2007,
Laux and Kunstmann 2008, Kimet al 2009, Kim et al 2010), not at
specific times of day.Therefore, there are two main questions this
paperaims to answer, once WCs have been identified asexpected for
at least the largest cities: How does thesurface temperatureWCof
Australianmajor cities dif-fer with time of day? Is the UHI
intensity of a majorcity,Melbourne, affected on aWC?
2.Data andmethod
In order to investigate the temperature WCs fordifferent times
of day, three-hourly surface (2 m)observation (dating as far back
as 1943) data from theAustralian Bureau of Meteorology are
utilised. Thesegive eight separate local times of day from which
theWCs are analysed from 0000 to 2100 h. We use datafrom seven
major Australian cities (see table S2)including Melbourne, which is
also the focus of theUHI analysis, together with three additional
sites torepresent the surrounding rural area. The studyutilises
data on vehicular traffic that is a major sourceof anthropogenic
heat release and generally acts as a
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Environ. Res. Lett. 11 (2016) 074003
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good proxy for indicating when peak times occur withregard to
anthropogenic activity. To test whetherWCsare statistically
significant, we use aMonte Carlo basedmethod (Earl et al 2015).
2.1. Station temperature dataThemeteorological stations are
located at sites that arenear the centres of the urban areas, and
hencesignificantly affected by anthropogenic activities (tableS2).
Included in the analysis is every state capital, aswell as the
reasonably densely populated regionalcentre of Cairns in
Queensland, due to its long recordand tropical location. Territory
capitals will not beanalysed due to Canberra’s station being moved
manytimes and Darwin’s station being located too far fromthe urban
area.
2.2.MelbourneUHI station dataThe three non-urban sites, to be
compared with theMelbourne CBD site, include the stations located
atMelbourne Airport, Moorabbin Airport and LavertonAirport (table
S3). We use the same period of1973–2013 for the analysis, as this
is an epoch when allthree non-urban sites have good data coverage.
Thesesites were used by Morris and Simmonds (2000) intheir study of
Melbourne’s UHI, and they providefurther site description details.
These sites are locatedbetween 17 and 23 km away, in different
directionsfrom Melbourne’s centre (see figure 3) and are atsimilar
altitudes. We did not average the non-urbansites, because of the
varying microclimatic effects ateach specific location; some
important signals wouldlikely be smoothed out. Some sites are ‘more
urban’than others, depending on wind direction and are notideal
locations for rural representation, falling underthe category of
‘urban affected’ (see Lowry 1977), butare the best available.
2.3. Vehicular traffic peak timesTraffic data provided by Vic
Roads (www.vicroads.gov.au) for a typical junction located
approximately1 km from the CBD of the major Australian city
ofMelbourne are utilised.We use the traffic volume data(every 15
min total) from October 2014, which can beconsidered as a
representativemonth.
2.4. Statistical testingThe temperature data are run through the
dailydivergence from a 31 d running mean (Bäumer andVogel 2007,
Earl et al 2015) filter to remove the effectsof interannual and
intraannual cycles, with each timeof day treated as a separate time
series.We use aMonteCarlo test based on the range between the days
of theweek with the minimum and maximum temperatureanomalies. We
randomly remove 5% of the data (run10 000 times), retaining the
order of the remainingcomponents of the time series and therefore
effectivelyretaining the autocorrelation of the data, which is
important when analysing temperature data due to thememory,
meaning that independence cannot beassumed. The WC temperature
anomalies are calcu-lated and the maximum-to-minimum range
takenfrom each simulated WC (WCR), creating a MonteCarlo based PDF
(of range in temperature) (Earlet al 2015). To test whether the
weekends differ fromthe weekdays, we take the average
Monday–Fridaytemperature minus the average
Saturday–Sundaytemperature from the original data and compare
thatto each of the 10 000 Monte Carlo based WCs (WD-WE). This is
also done with mean Monday–Saturdayminus Sunday alone (WD-S).
Mondays at 0000, 0300and 0600 h are considered as representing
Sundaynights for this statistical test due to being influenced
bythe (lack of) Sunday night urban activity.
3. Results and discussion
InAustralian cities (and in developed nations through-out the
world), there tends to be two peaks whenmostpeople travel to and
fromwork, take children to schooletc. These occur during working
weekdays (Mondayto Friday) in the morning (typically running
from7–9 am) and there is a less pronounced afternoon/evening peak
(4–7 pm). At the weekend these peaks donot occur, so there is a
large difference in anthropo-genic activity between weekday and
weekend activityat these times, especially in the morning.
Traditionallythere is less total traffic seen in cities
likeMelbourne onSaturdays and especially Sundays (Simmonds andKeay
1997). During these times on weekdays otherhuman activities also
occur in city centres, such as theuse of air conditioning units or
heating systems, andfurther afield with electricity generation and
biomassburning (Earl et al 2015), producing additional wasteheat
and aerosols. In western cultures, with Saturdaysand Sundays being
days off work for a significantproportion of the population, Friday
and Saturdayevenings have become popular with people going
torestaurants, pubs, theatres etc. This means that thereare
anthropogenic activities in the city centres on theseevenings
through to the early hours of the followingday. This activity does
not include typical industrialactivities seen during weekday
daytimes, e.g. biomassburning, but increased usage of patio heaters
or airconditioning units and perhaps even metabolic heatfrom the
increased number of people in the area (QuahandRoth 2012).
3.1. Typical traffic volumesFigure 1 displays the traffic volume
of a typical roadjunction near the Melbourne CBD. A similar
patternwas found on a suburban arterial road 10 km southeastof the
CBD (www.vicroads.gov.au), indicating thatthis activity is not
confined to inner Melbourne. Ithighlights the difference between
the weekdays andweekend (figure 1(a)) showing the general
nocturnal
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Environ. Res. Lett. 11 (2016) 074003
http://www.vicroads.gov.auhttp://www.vicroads.gov.auhttp://www.vicroads.gov.au
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increase and morning reduction at weekends compared with the
weekdays. There is a distinct morningand afternoon peak with the
weekend showing a lessextreme single peak during the middle of the
day.When comparing the averaged Monday–Thursdayand Friday, Saturday
and Sunday volumes (1b), theFriday and Saturday night activity is
highlighted. Theweekend nocturnal increase is comparable to
theweekday evening peak hour volume. Friday followsthe weekday
(Monday–Thursday) pattern until 2100 hwhere activity is increased
into Saturday night, similarto Saturday evening and Sunday night.
Figure 1(c)displays the weekend dip in traffic volume,
especiallySunday, with Friday being a weekday and weekendnight,
possessing the most traffic. Monday has slightlyless total traffic
than Saturday, due to the high volumesin the early hours of
Saturday and the fact thatMondayhas slightly lower traffic volumes
throughout the daythan the other weekdays (not shown).
3.2.Morning peak timeIn the diurnal cycle of an average day, the
morningpeak occurs the time at which the atmosphere is in
thetransition between nocturnal and diurnal conditions,which means
that the incoming solar radiation is onlybeginning to warm the
surface and start the processesof mixing in the lower troposphere
(Oke 1982). At thistime there will often be a more stable surface
layer oreven a temperature inversion relatively close to
thesurface, whichmay act to trap the anthropogenic waste
heat, which will build up more than later in the day,especially
during winter in certain stable weather types(Morris and Simmonds
2000). Also, this is the timewhen the weekday to weekend
anthropogenic activitydifferential is at its maximum. This means
that wewould expect a strong WC of surface temperatureduring
themorning peak, especially in the larger cities.This time provides
the clearest insight into the short-term effects of anthropogenic
activity.
Figure 2 shows that Australia’s largest cities havelower 0900 h
temperatures on the weekend, with a sig-nal well above the
statistical noise level. The strength ofthe Melbourne and Sydney
signals is surprisingbecause they include all times of the year and
allsynoptic weather types, not just those which arefavourable. Both
cities have signals that are stronglystatistically significant both
in the WCR and WD-WE(figure 2(a)). The Melbourne signal displays a
WCsinusoidal pattern, peaking on Thursday with a Sun-day minimum.
Sydney has a less coherent pattern,peaking on Friday, with a Sunday
minimum. Bris-bane’s old monitoring site (1955–1972) also displays
astatistically significantWD-WE, but not forWCR, andnot as strongly
asMelbourne’s probably because of sitecharacteristics (it is
located in the City Botanic Gar-dens, close to the river and
further from the source ofheat, though still clearly affected by
the morninganthropogenic activity). Adelaide does not display
astrongly statistically significant signal, except for whenSunday
alone is compared to weekdays (WD-S) as
Figure 1.Mean four-week, 1st October–28thOctober 2014, traffic
volume at a typical junction in centralMelbourne (300 m
frommeteorological station). (a)Average weekday, weekend and
overall (per 15 min). (b)AverageMonday–Thursday volumeswith
Friday,Saturday and Sunday (per 15 min). (c)Total vehicles (per
day).
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Environ. Res. Lett. 11 (2016) 074003
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Figure 2. (a)Major Australian city 0900 h temperature weekly
cycles.WCR test p-value (left) andWD-WE (right).
(b)Melbournemaximumandminimum temperatureweekly cyclesWCR test
p-value (left) andWD-WE (right),WE considered as Sunday andMonday
forminimum temperature. (c)Aswith (a) but for 1800 h.
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Environ. Res. Lett. 11 (2016) 074003
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discussed below. The signals in the other cities did nothave
statistically significant WCs at 0900 h, probablybecause these are
not large or busy enough to producea signal above the statistical
noise level. Fujibe (2010)also found this in daily mean temperature
minimumfor major Japanese cities. However, these Australianpatterns
are different from German cities, studied byBäumer and Vogel (2007)
and Laux and Kunstmann(2008), most of which had aWednesday (or
Thursday)peak and Saturday minimum. These studies are
notsufficiently similar to ours for any robust comparisondue to
them using mean temperature rather than0900 h temperature.
The Melbourne 0900 h signal is stronger statisti-cally than any
of the Melbourne maximum and mini-mum seasonal temperature analyses
carried out bySimmonds and Keay (1997). The 0900 h signal is
alsostronger than the updated Melbourne maximum andminimum
temperature WCs (figure 2(b)). This dis-plays the Monday–Friday
working week, which has asimilar pattern to the 0900 h, though not
as amplifiedor as strong statistically and with a Friday
maximum.The minimum Melbourne temperature WC(figure 2(b)) is very
different from the 0900 h, with aclear Sunday and especially Monday
minimum, withagain smaller amplitude and is less strong
statistically.The Sunday night (Monday morning) minimum isapparent
(discussed below).
3.3. Evening peak timeIn summer, especially in temperate cites,
the weekdayevening peak time occurs before or soon after sunset,so
urban areas have a positive convective sensible heatflux with
significant mixing occurring in the unstablelower atmosphere (Oke
1982). During winter how-ever, the Sun has already set at this time
and nocturnalprocesses have begun. Like the morning, the
increasedvolume of traffic (figure 1) and relatively stable
atmo-spheric conditions, waste heat and aerosols can buildup. This
means that we would expect to see a WCduring the evening peak,
though not as pronounced asfor themorning.
Australia’s two largest cities display lower tem-peratures on
the weekend than during the week, at1800 h (figure 2(c)),
statistically significant for WD-WE, but not as strong as for 0900
h and the WCR wasnot significant. TheMelbourne signal is not
sinusoidalas for the morning, though does display a Thursdaypeak
and Sunday minimum. The 1800 h temperaturedoes not correspond
particularly closely with max-imum temperature as seen forMelbourne
(figures 2(b)and (c)), though is likely to be more similar
duringsummer, with the maximum temperature oftenoccurring around
this time. The Sydney 1800 h alsoshow a Fridaymaximum and
Sundayminimum, simi-lar to the 0900 h results. The other cities did
not dis-play statistically significantWCs at this time due to
the
daytime processes causing a great deal ofmixing henceinhibiting
the build-up of heat in these smaller cities.
3.4. Night life effectAnthropogenic activity during the night
that createsany waste heat is more likely to produce a WC signaldue
to the more stable lower atmosphere during thenight, especially on
calm nights. This means that wewould expect a Saturday and Sunday
0000 h and0300 h maxima rather than the minima seen at 0900and 1800
h.
There is a statistically significant 0000 h WC inMelbourne and
the new Brisbane site in the WCR test(table 1). TheWE-WD test is
also significant but at theend of the other tail (compared to 0900
and 1800 hanalysis), weekends are warmer than weekdays asexpected.
Sydney does not display a Saturday and Sun-day maxima, the reason
for which is unclear, howeverwe can speculate that the location of
the Sydney site ison relatively high ground, especially compared to
theadjacent highway, so perhaps the site is relatively unaf-fected
by the nocturnal anthropogenic heat island andis also well
ventilated. Table 1 indicates that the 0300 htemperatures do not
show strong statistically sig-nificant patterns seen at 0000 h for
any location, otherthan Melbourne when Sunday is compared to the
restof theweek.
All cities have a noticeableMondaymorning (Sun-day night)
minimum, statistically significant in theWD-S test for Sydney,
Melbourne and Brisbane’s newsite, and close to significance in
Perth (old site),Hobart and Cairns as shown in table 1. This may
bedue to aerosol loading during the week, with Sundaynight being
the longest time (over two days) since thelast aerosol-producing
weekday. Thismeans that thesecity sites may be relatively clean,
cloud free and betterventilated on Monday morning, allowing for
moreefficient cooling (e.g. Cerveny and Balling 2005).Simmonds and
Keay (1997) found a sharp Sundaydrop in aerosols in Melbourne and
other studies havefound a similar pattern (Cerveney and Balling
1998,Kim et al 2009), so this could be the causalmechanism.Kim et
al (2009) found for Korean cities that there wasoften a
Tuesday/Wednesday minimum in minimumtemperature and a Saturday
peak, whichmay occur fora similar reason, with urban activities
occurring onSaturday night, though the authors link the peak
toincreased cloud cover from aerosol interaction. Itcould also be
due to less Sunday urban activity asfigure 1 shows with fewer
vehicles on the roadsthroughout the day, meaning less build-up of
wasteheat. This pattern is not seen in Europe for
minimumtemperature (Laux andKunstmann 2008), though thisdoes not
always correspond to 0000 h temperature.
3.5.Other times of the dayThe table 1 column for Melbourne
indicates that thesite displays a WC in temperature at all times of
day
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Environ. Res. Lett. 11 (2016) 074003
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Table 1. Summary of statisticalWC testsa.
Time Sydney (1955–2013) Melbourne (1955–2013) Brisbane
(1955–1972) Brisbane (2000–2013) Perth (1964–1991) Perth
(1995–2014) Adelaide (1979–2013) Hobart (1995–2014) Cairns
(1979–2013)
Weekly cycle range
0000 ** SaM ** SaM
0300
0600 *WSa
0900 *** F Su ***Th Su *Th Su *Th F
1200 **MoTh
1500 ***MTh **ThF
1800 *Th Su **MTh *Th F
2100 *Th Su
Weekdayminusweekend
0000 −−− −−− +0300 − −0600 ++ +0900 +++ +++ +++ + +1200 − +1500
+ − −1800 +++ +++ +2100 ++ ++ +Monday–Saturdayminus Sunday
0000 ++ +++ + ++ + + +0300 + ++ +0600 ++ ++0900 +++ +++ ++
++1200 + −− ++1500 −− ++ ++1800 ++ +++ + −− ++2100 + +++ ++ −− +
+
a Tests include WCR, WD-WE and WD-S (see section 2.4) for each
city (in order of population) station. * Indicates significance in
the WCR test and the day of maximum (left) and minimum (right), +
indicates a significant positivedifference and—a significant
negative difference. The number of *,+ or− shows significance
level, one is to the 10% level, two the 5% level and three to the
1% level.
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Environ.R
es.Lett.11(2016)074003
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except at mid-afternoon 1200 and 1500 h. This is to beexpected
as this is the time when the atmosphere is atits most energetic and
maximum mixing occurs(Oke 1982), not allowing near surface heat
build-upfrom anthropogenic activities. The afternoon is thetime
when the anthropogenic activity weekday toweekend difference is at
a minimum (figure 1). Also,weekday build-up of aerosols during
themorningmaybe reducing the amount of shortwave irradiancereaching
the surface (e.g. Georgoulias et al 2015)reducing the effect of any
increased waste heat. Thetimes of day that Sydney exhibits a WC are
0900, 1800and 0000 h as discussed above. With the site
relativelyexposed, as mentioned, any signal from other times ofday
may be overcome by advection of heat out of thearea, alongwith the
anthropogenic activity beingmoresimilar at these times on weekends
and weekdays. TheBrisbane sites have differing WCs, with the new
sitehaving a strong WD-WE negative anomaly at 0000 hand the old
site a strong positive anomaly at 0900 h asmentioned above. This is
likely to be due to a change inlifestyle and Brisbane’s weekend
nocturnal anthropo-genic activity, with Brisbane’s population more
thandoubling during the two time periods (table S1) andmany parts
of the inner city becoming gentrified,leading to more nocturnal
weekend activity and lessweekday industrial activity. It could also
be due to thechange in location to a site closer to
anthropogenicactivity and further from the moderating effects of
theBrisbane River. The original Perth station does notshow any
significant WC signal though it has a weakpositive WD-WE and WD-S
at 0900 h and 0000 hrespectively. The new site has a very different
WCpattern (also from all other Australian urban stations)with
Thursday minima and Monday maxima in theafternoon and evening,
along with negative Sundayanomalies at these times. This is due to
the sites havingvery disparate source areas, with the old site
located tothe west of the city centre, upstream of the urbancentre
from the prevailing south-west winds and thenew site located
north-east of the urban centre, next toa golf course, which is
often irrigated at mandatedtimes and days
(www.watercorporation.com.au). Thishighlights the dangers of
interpreting long-termmicro-scale data when site changes are
involved(Stewart and Oke 2012), especially with variable
sitecharacteristics. Adelaide and Hobart only have a WCsignal
forWD-S, indicating that Sunday is the only daywhere anthropogenic
activity in these urban centres islow enough to produce a signal.
The only significantWC signal in Cairns is a Thursday maximum
andFriday minimum for the WCR test. It is unclear whythis
occurs.
These results show, for the first time, how there aretemperature
WCs that occur at different times of dayand that vary in character.
This is highly significantbecause understanding the temperature
character-istics of the urban environment is becoming more
important due to the rise in global urban populationand
heat-health implications.
3.6.MelbourneUHI WCMelbourne is a growing metropolis and has
increasedin population by 65% from 1973 to 2014 (table S1). Ithas
aWC for almost all times of the day (table 1), so wecan expect the
UHI to also display a WC, assumingthat the surrounding rural areas
do not display a WCas strongly.
The Melbourne UHI peaks between 2100 and0000 h and is at its
minimum between 0900 and1500 h, depending on the day of the week
and whichlocation is used to represent the rural environment(figure
3). This is generally similar to the typical UHIdiurnal cycle
described by Oke (1982) with the Mel-bourne CBD—Laverton anomaly on
weekdays themost similar to Oke’s cycle. The weekend 0900 h dip
isthe main contrast with the classic diurnal UHI cycle,but is in
agreement with the result of Theeuwes et al(2015), and is likely to
be masked by averaging in non-WC UHI studies. The weekend CBD 0900
h anoma-lies compared to all rural sites contrast strongly withthe
other days of the week, the extent of which is high-lighted by the
divergence from the mean graphs infigure 3. The 0900 h WCs are all
highly significant forboth the WCR and WD-WE tests. This is of no
sur-prise when we consider the weekday to weekend dif-ference in
the morning peak in vehicular activity(figure 1), which is likely
to be less extreme in themorerural areas. UHI are often associated
with the changesin energy budgets of the surface materials,
howeverthis confirms that the anthropogenic activity withinthese
areas also makes an important contribution,with the reduction of
the UHI especially at 0900 h onSunday. The Sunday 0900 h minimum in
the CBD isso extreme that there is a slight urban ‘cool island’when
compared to Moorabbin airport. This may bepartly due to the fact
that theMoorabbin site has someurban characteristics compared with
Laverton andespecially Tullamarine. The 1800 h UHI is not asextreme
as the 0900 h one but is also statistically sig-nificant at each
site for theWD-WE test but not for theWCR. This is to be expected
since the anthropogenicactivity weekday toweekend evening peak
difference isnot as extreme as during the morning. The Saturdayand
Sunday 0000 h anomalies have the opposite signfor the WD-WE test,
with the weekend morning (Fri-day and Saturday nights) UHI anomaly
significantlyhigher than during the rest of the week as
expected.The line graphs display how the Saturday and SundayUHI
anomalies transition from being the highest at0000 h, to the lowest
by 0900 h when compared toeach location. This is consistent with
the change intraffic volumes (figure 1) representing
anthropogenicactivity, also displaying the extreme weekday
versusweekend anomalies being at 0900 h.
8
Environ. Res. Lett. 11 (2016) 074003
http://www.watercorporation.com.au
-
These results show that theUHI of amajor city canhave a WC,
especially at 0900 h, but also at 1800 and0000 h, while it is not
significant at other times of day(not shown). This result gives us
an indication of theinfluence of anthropogenic activity in
enhancing theUHI significantly, confounding the effect of the
urbansurface characteristics that produce the UHI by chan-ging the
local energy budgets.
4. Conclusions
These results show, for the first time, that the WC
oftemperature in major cities differs according to thetime of day
and that theUHI intensity of amajor city isaffected on a WC. This
has profound implicationsduring extreme heatwaves, showing that
humans havethe capability to manage anthropogenic heat flux
bychanging the activities to the characteristics of aweekend day,
and vice versa for the night. With thenumber and severity of
heatwaves set to increase(Cowan et al 2014), this could have
significant implica-tions for the health of urban populations.
Recent workfor Melbourne (Nicholls et al 2008, Tapper et al
2014)has shown distinct threshold temperatures abovewhich human
mortality greatly increases. Advancewarning of extreme heat events
provides opportunities
to better manage (e.g., incentives for drivers to avoidusing
their cars at the warmest part of the day) humanactivity that will
add to the heat. Australian govern-ment concern with the need to
create cooler, greenerandmore liveable cities in the face of
climate change isevidenced in the recent (January 2016)
ministerialstatement on cities. Clearly the results reported
hereprovide further important information that can con-tribute to
this push for healthier urban environmentsin the face of amore
extreme climate.
Introducing the concepts ofWCs at different timesof day and
identifying a WC in the UHI were the keyaims of the study. Planned
future workwill address theimportant questions of seasonality,
weather typing(e.g. Morris and Simmonds 2000), normalising
theSunrise and sunset times (e.g. Fortuniak et al 2006)and the
impact of windspeed (e.g. Morris et al 2001) totheWC signals.
Acknowledgments
Parts of this research were made possible by fundingfrom the
Australian Research Council (Project Num-ber DP130103562).
Temperature data can be down-loaded from the Bureau ofMeteorology
(http://www.bom.gov.au/climate/data-services/station-data.
Figure 3.Temperature anomalies from theMelbourne site (blue dot
in the centre of themap)minus the three airport site (red
dots)temperature weekly cycles with the test p-value for (left)
theWCR and (right)WD-WE. The top line graphs represent the
diurnaltemperature anomalies for each day. The lower graphs plot
the divergence from themean for each observation time.
9
Environ. Res. Lett. 11 (2016) 074003
http://bom.gov.au/climate/data-services/station-data.shtmlhttp://bom.gov.au/climate/data-services/station-data.shtml
-
shtml) and traffic data fromVic Roads (www.vicroads.gov.au).
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1. Introduction2. Data and method2.1. Station temperature
data2.2. Melbourne UHI station data2.3. Vehicular traffic peak
times2.4. Statistical testing
3. Results and discussion3.1. Typical traffic volumes3.2.
Morning peak time3.3. Evening peak time3.4. Night life effect3.5.
Other times of the day3.6. Melbourne UHI WC
4. ConclusionsAcknowledgmentsReferences