Study on the Micro Climate Condition Along Green Pedestrian Canyon in Singapore

Study on the microclimate condition along a green pedestrian canyon in Singapore 197 provide maximum cooling and reduce the outdoor air temperature by up to 2.7K (Parker, 1983). Wong et al. (2002) studied the impact of intensive and extensive rooftop greenery on buildings and on the environment in Singapore. Rooftop greenery can provide benefits nott on no only ly to th thee bu build ildin ing, g, bu butt al also so to th thee en envi viro ronm nmen ent’ t’ss ambient temperature condition. With an intensive system, the surface temperature may be reduced by up to 31K and the ambient temperature at 1m may be reduced by up to 1.5K. The impact of rooftop greenery is clearer for a metal roof. Without plants, the surface temperature of the metal roof can be up to 60–70 8C during daytime and lower than 20K at night-time. With plants, it ranges only from 24 8C to 328C. The benefits of reducing the surface temperature by greenery can be observed from the mean surface temperature differences between the hard metal surface and those below the plants. They are 4.7, 1.9 and 1.4K with the presence of dense plants, sparse plants and weed, respectively. However, in urban areas, having large greenery areas may be a constraint. As a result, the microclimatic conditions in these canyons become the most crucial element in influencing the city’s overall climate (Shashua-Bar and Hoffman, 2004). A city with various land uses may comprise warm and an d co cold ld ar area eass as a re resu sult lt of di dist stin inct ct ur urba ban n la land nd us usee change cha nge.. The cha change nge bet betwee ween n par park k and bui builtlt-up up are areaa can prod pr oduc ucee ur urba ban n ai airr te temp mper erat atur uree di diff ffer eren ence cess up to 7K (Spronken-Smith, 1998). Toudert (2005) studied different street designs to improve outdo ou tdoor or th ther erma mall co comf mfor ortt in th thee ho hott an and d dr dry y cl clima imate te of  Ghardia, Algeria and in the temperate climate of Freiburg, Germany. The study found that wide streets ( H  / W  W   0.5) are not favourable for either E–W or N–S street orientation. However Howe ver,, N – S ori orient entatio ation n has some adv advanta antages ges and its bene be nefi fitt in incr crea ease sess as th thee as aspe pect ct ra rati tio o ( H/W ) inc increa reases ses.. Shashua-Bar and Hoffman (2004) found similar findings in their the ir mic microc roclima limatic tic stu study dy of urb urban an str street eetss and cou courty rtyard ardss with trees. Increasing the building height from 12 to 24m reduces the air temperature by 1.5K in a street or a courtyard 24m wide. The cooling effect effect in the N – S street orientation orientation is slightly stronger, about 0.64K, than that in the E–W orientation in the H/W  1.0 cluster with high albedo. Another  study in a hot and dry climate (Johansson, 2006) also concluded that a deep canyon is favourable to providing shade in th thee su summ mmer er,, co cool oler er by as mu much ch as 10 10K K du duri ring ng th thee hottest period, as compared to a shallow canyon. In addition to canyon geometry, materials and orientation, resear res earche chers rs hav havee als also o bee been n inte interes rested ted in veg vegeta etatio tion n as a climate component in urban street design. Shashua-Bar and Hoff Ho ffman man (2 (200 000) 0) st stud udied ied th thee co cool oling ing ef effe fect ct of 11 sm smal alll urba ur ban n gr gree een n si sites tes wit with h tr tree eess in Te Tell-Av Aviv, iv, Is Isra rael. el. Tr Tree ee shading provided an average of a 3K cooling effect at noontime, while the specific cooling effect of the site due to its geometry and tree characteristics, besides the shading, was found to be relatively small at about 0.5K. Many researchers have explored the climatic impact of  vegetation and small green areas on either building energy ¼

or various canyon types. However, similar studies in tropical climates are rather limited and the cooling effects within the different green canyon forms have not been explored. This study will look into this aspect of work with the aim of providing a better understanding of the microclimatic environment in these canyons. This article involves two aspects of  study: stu dy: (i) inv invest estiga igation tion of the mic microc roclima limatic tic con conditi dition on of  two different pedestrian canyons and (ii) the cooling effect  inside ins ide the can canyon yons, s, inc includi luding ng can canyon yon abi ability lity to pro provid videe thermal comfort to pedestrians.

CLIMATE OF SINGAPORE 1 Located between latitudes 18090 N and 18290 N and longitudes 1038360 E and 10 104 48250 E, Sin Singa gapo pore re ca can n be cla class ssif ifie ied d as having a hot humid climate. Uniform high air temperatures, humidity and rainfall throughout the year characterize the climate. The diurnal temperature variations are small with the range for minimum and maximum air temperatures of  23–268C and 31–348C, respectively. The mean annual air  temperature was 27.4 8C between the period 1982 and 2001. Relative humidity (RH) is generally high and although it  invariably exceeds 90% in the early hours of the morning just  before sunrise, it frequently falls to 60% during the afternoons when there is no rain. During the prolonged heavy rains, RH often reaches 100%. Between the period 1982 and 2001, the mean annual RH was 83.5%. There are two main seasons in Singapore: northeast (NE) monsoon and southwest (SW) monsoon seasons. The NE monsoon occurs between November and early March, with the prevailing wind blowing from north to northeast. Meanwhile, the SW monsoon occurs between June and September, be r, wi with th th thee pr prev evai aili ling ng wi wind nd bl blow owin ing g fr from om so sout uth h to southwest. Two short inter-monsoon periods with a duration of two months separate the main seasons. There is no clear distinct wet or dry season as rainfall occurs occ urs thr throug oughou houtt the yea year. r. How Howeve ever, r, the NE mon monsoo soon n season is considered as wet weather, since the wind is generally cool and brings frequent spells of wet weather at about  48% of total annual rainfall. On the other hand, the SW monsoon wind brings about 36% of total annual rainfall.

METHODOLOGY Object of study 

The Na The Natio tiona nall Un Univ iver ersi sity ty of Sin Singa gapo pore re (N (NUS US)) ca camp mpus us complex can be considered as a ‘city’ on a smaller scale (Figure 1). From the estate-wide air temperature measurement (Wong et al., 2007a, b), daytime heat island intensity (4K) was found to be higher than night-time heat island intensity (3K). Prince George Park Residence (PGP) is one of the hottest areas in the NUS campus due to its highdensity building arrangement and less greenery.

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198 Wong and Jusuf

Figure 1 | Measuremen Measurementt points at the ENG (see Table 1 for the detailed equipment) equipment)

Two different characteristics of pedestrian canyons, in the Faculty of Engineering (ENG) and the PGP of NUS, were selected for the measurements. The first canyon was situated in ENG, between the E4 and E5 buildings. It was 14m wide,

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enclosed between two blocks of four-storey-high buildings (Figure (Figu re 1). The avera average ge  H/W  was 1.3 1.3.. The site was pre predom domininantly mature trees and they covered the canyon extensively. The second canyon was in PGP . It was on average 13m wide

Study on the microclimate condition along a green pedestrian canyon in Singapore 199 with 7 – 1313-sto storey rey-hi -high gh res reside identia ntiall bui buildi ldings ngs adj adjoini oining ng it  (Figur (Fi guree 2). The cal calcul culated ated ave averag ragee H/W  wa wass 1. 1.7. 7. Th Thee cany ca nyon on ha had d a re rela lati tive vely ly yo youn ung g an and d mo mode dera rate te or lo low w

canopy cano py he heig ight ht of tr tree eess an and d sh shru rubs bs as co comp mpar ared ed to the cany ca nyon on si side dewal walk k at EN ENG. G. Bo Both th lo loca catio tions ns ha had d th thee sa same me NW–SE orientation.

Figure 2 | Measuremen Measurementt points at the PGP (see Table 1 for the detailed equipment)

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200 Wong and Jusuf

Table 1 | Measurement equipment and parameters Symbol

Parameter

Equipment

Accuracy

Sensor height

Logging interval

 Air temperature and RH (Continuous)

HOBO H08-003-0 H08-003-02 2

+0.28C at 218C

1.8m

1min, averaged to 1h data

Ground surface temperature (Continuous)

T-type thermocoupl thermocouple e wire and HOBO U12-014

+1.58C

On surface

1min, averaged to 1h data

Wind speed/direction inside canyon and solar radiation below tree (Continuous)

S-WCA-M003 wind speed and direction sensors Silicon pyranometer # S-LIB-M003

+0.5m/s +10W/ 

1.8m 1.8m

1min, averaged to 1h data 1min, averaged to 1h data

Complete weather stations (Continuous)

HOBO weather station

1.8m

1min, averaged to 1h data

  Field measurements

Field measurements were carried out from 17 July to 20 October Octo ber 2007. 2007. Tab Table le 1 and Figure Figure 3 sho show w the complete complete measured parameters and equipment. All the meteorological equipment was calibrated for a few days in a controlled environment.

m2

speed/direction. These WSs served as reference points for the respective canyons and were configured to measure at 1-min intervals. Other HOBO weather stations (WS 2) were put in the middle of the canyon, mainly to measure wind speed/direction and solar radiation at pedestrian level inside the canyon. NUS weather station (NUS WS)

  Microclimate condition inside the canyon

Meteorological data from the NUS WS were also collected to obtai ob tain n th thee cli clima matic tic da data ta ab abov ovee th thee ca cany nyon ons. s. Th Thee WS is located on the rooftop of a building in the ENG and maintained by the Department of Geography (2008), NUS.

Figures 1 and 2 show the various measured parameters and the locations of measurement points in both locations. At  ENG, there were eight measurement points stretching over  an approximate length of 135m. Points 1–5 were placed within the canyon, while Points 6–8 were placed outside Calcu Calculatio lation n of mean radian radiantt tempera temperature ture (MRT) it. At PGP Re Resi side denc nces es,, th ther eree we were re se seve ven n me meas asur urem emen ent  t   and PET  points installed over a span of 125m. To study the performance performance of two differ different ent characteristics characteristics of  Meanwhile, HOBO weather stations (WS 1) were put on a canyons in providing thermal comfort to the pedestrian, a nearby (within 100m) open place with minimal effect from the th ther erma mall co comf mfor ortt in inde dex, x, PE PET, T, wa wass ca calc lcul ulat ated ed.. PE PET T is canyon cany on and greenery. greenery. The WSs reco recorded rded the weat weather her data, defined to be equivalent to the air temperature that is required which included air temperature, RH, solar radiation and wind to reproduce, in a standardized indoor setting and for a standardiz dar dized ed per person son,, the cor coree and skin temp tempera eratur tures es tha thatt are observ obs erved ed und under er the con conditi ditions ons bei being ng ass assess essed ed (Ho (Hoppe ppe,, 1999).. Hoppe developed 1999) developed PET based on the Munich energybalance model for individuals (MEMI), which models the thermal conditions of the human body in a physiologically releva rel evant nt way way.. Thi Thiss mod model el is a mod modific ificatio ation n of Fan Fanger ger’s ’s indoor thermal comfort indices ‘predicted mean vote’ and ‘predicted percentage dissatisfied’, so that it is applicable to the out outdoo doorr con conditi ditions ons by ass assign igning ing app approp ropria riate te par par-ameter ame terss to adj adjust ust the mod model el with a muc much h mor moree com comple plex x outdoor radiation condition. The MEMI model is based on thee en th ener ergy gy ba bala lanc ncee Eq Equa uatio tion n (1 (1)) fo forr th thee hu human man bo body dy,, where the unit of all heat flows is in Watt:  M  þ W  þ R

Figure 3 | Weather station at reference reference point (left) and HOBO HOBO

on the lamppost for temperature/RH measurement measurement inside the canyon (right)

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þ C  þ E D þ E Re þ E Sw þ S  ¼ 0

ð1Þ

where M  is the metabolic rate (internal energy production); W  is the physical work output; R is the net radiation of the body; C  is the convective heat flow; E D is the latent heat  flow to evaporate water diffusing through the skin (imperceptible ceptib le persp perspiration iration); ); E Re is th thee su sum m of heat heat fl flow owss fo for  r 

Study on the microclimate condition along a green pedestrian canyon in Singapore 201 heating and humidifying the inspired air;  E Sw is the heat flow temperature data. Table 2 shows the selected dates for the due to evaporation of sweat and S  is the storage heat flow for  analysis. heating or cooling the body mass. The individual heat flows in Equation (1) are controlled by the following meteorological parameters (Hoppe, 1999):   Microclimatic condition inside the canyons † † † †

air temperature: C , E Re humidity: E D, E Re, E Sw wind velocity: C , E Sw mean radiant temperature:

  Air temperature

R.

PET has been used in many outdoor thermal comfort studies (Matzarakis et al., 1999, Spagnolo and de Dear, 2003) and is sufficient for use in a comparison study of thermal comfort in tropical climate canyons (Johansson and Emmanuel, 2006). Anoth An other er im impo port rtan antt pa para rame meter ter to de deter termin minee the therm rmal al comfor com fortt is MRT MRT,, esp especi eciall ally y in out outdoo doorr con condit dition ions, s, and solar radiation intensity is the main climatic parameter that  influences the MRT value. RayMan 1.2 software (Matzarakis et al., 2000) was used to calculate the MRT and PET values of the ENG and PGP canyons. To calculate the MRT and PET values inside the canyon, solar radiation below tree data was used rather than solar  radi ra diati ation on at the re refer feren ence ce po poin ints ts fr from om the re resp spec ectiv tivee canyons. The tree in the ‘obstacle’ menu of RayMan 1.2 software serves as a parameter to calculate the sky view factor  (SVF) value. It is important to note that using the reference point’s solar radiation data will present a misleading MRT and PET value for the ENG canyon that has mature trees fully covering its canyon. It will provide high MRT and PET values, which to the contrary is not the case. The PET threshold value of 33 8C was used as the upper  limit value to achieve outdoor thermal comfort, borrowed from fro m the Dha Dhaka ka out outdoo doorr the therma rmall com comfor fortt zon zonee (Ah (Ahmed med,, 2003). Johansson and Emmanuel (2006) used this method, since there is no established outdoor thermal comfort zone for tro tropic pical al cli climate mate.. In the Sin Singap gapore ore con contex text, t, Dha Dhaka’ ka’ss PET threshold value is probably too high, since Singapore has a greener environment and less pollution, which may give a higher tolerance for Singapore’s inhabitants towards hot and humid climate conditions. For the sub subjec jectt par parame ameter ters, s, lig light ht tro trouse users rs and sho shortrtsleeved clothing of 0.5clo were used and activity of 115W wass se wa sett as wa walk lkin ing g at a sp spee eed d of 0. 0.89 89m/ m/ss (A (ASH SHRA RAE, E, 1989). The other climatic parameters (air temperature, RH, wind speed, solar radiation) were structured as the average of 24-hour data on selected hot days.

FINDINGS AND DISCUSSION The data analysis focuses on fairly clear and hot weather  conditions, selected by analysing the solar radiation and air 

Before dis Before discus cussin sing g the air tem temper peratu ature re pro profile file ins inside ide the canyon, canyo n, the background background air temperature of the respective cany ca nyon onss wa wass st stud udie ied d (s (see ee Fig Figur uree 4) 4).. On the ty typic pical al ho hot  t  days 5–6 August 2008, from morning until noontime, the ENG site has slightly higher air temperature than the PGP site si te an and d it re rema main inss hi high gher er un unti till ar arou ound nd 19 19.0 .00 0 ho hour urs, s, whereas the PGP site is warmer until around 08.00 hours. The heat storage flux in the ground during this period is negative and it gives heat to the surface after 16.00 hours, which is greater in the PGP site than in the ENG site since thee PG th PGP P si site te is ch char arac acte teriz rized ed by hi high gh-d -den ensi sity ty bu build ildin ing g arrangement arran gement and less greenery (Hamdi and Schay Schayes, es, 2005). The air temperature difference between NUS WS and both canyons’ reference points (WS 1) is larger during daytime than at night-time. As mentioned, NUS WS was located on the engineering building rooftop, where the wind speed is higher than at the ground level and the prevailing wind direction was relatively perpendicular to canyon orientation (see the Wind  section below). Hence, during daytime, the wind shear provides a more cooling effect at the rooftop level as compared to the ground level. Meanwhile, during night-time when there is no solar radiation, the thermal properties of the building fabrics and the pavement generate the temperature difference (Arnfield, 2003). The ave averag ragee air temp tempera eratur tures es of all the mea measur sureme ement  nt  points inside the ENG (Points 1–5) and PGP (Points 1–7) canyons are shown in Figure 5 together with their respective reference point to show the average air temperature condition alon al ong g th thee ca cany nyon on.. Th Thee ai airr te temp mpera eratur turee ins inside ide th thee EN ENG G canyon is lower around 0.7–1.1K as compared to the PGP sitee dur sit during ing daytime daytime and maintains maintains its coo coolne lness ss at abo about  ut  0.4– 0. 4– 0. 0.5K 5K du duri ring ng ni nigh ghtt-tim time. e. Th Thee ma matur turee tr tree eess pl plan anted ted along the ENG canyon provide very good shading to the pedestrian walkway (see Figure 1 as compared to Figure 2). Shashu Sha shua-Ba a-Barr and Hof Hoffma fman n (20 (2000) 00) als also o fou found nd tha thatt high higher  er  tree canopies reduce the heating effect from the surrounding environment through the shading effect. It is very clear from Figure 5 that the air temperature inside the ENG canyon is lower than its reference point. Meanwhile, the condition is reversed in the PGP site where young palm trees dominate its canyon. These palm trees are not able to provide sufficient shading and cooling to the environment due to the condition of their leaves. The LAI value is only 2.2 as compared to 5.3, the mature trees in the ENG site. The time shift of peak air temperature is

Table 2 | Selected days of fairly clear and hot weather condition (total 20 days) July

Selected dates

-

August

September

October

2, 5, 6, 7, 11, 25

5, 9, 13, 19, 20, 21, 22, 25, 28, 30

3, 4, 5, 6

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202 Wong and Jusuf

Figure 4 | Temperature condition condition at ENG, PGP reference point (WS 1) and NUS WS

Figure Fig ure 5 | Avera Average ge air air temp temperatu erature re (20 hot days days)) of of all the meas measurem urement ent poin points ts insi inside de the ENG and PGP cany canyons ons as comp compared ared

to ENG, PGP reference point (WS 1) and NUS WS during the period between 17 July and 20 October 2007

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Study on the microclimate condition along a green pedestrian canyon in Singapore 203

Figure Fig ure 6 | Average RH of all the measurement points inside the ENG and PGP canyons on clear hot days between 17 July and

20 October 2007

probably due to differentiation of solar reception inside the cany ca nyon on.. Th Thee ai airr te temp mper erat atur uree in th thee EN ENG G si site te is at th thee maxim imu um at 15.00 hours, wh whil ilee at the PGP sit itee the maximum is at 14.00 hours. It can be concluded that  the max maximu imum m air tem temper peratu ature re at dif differ ferent ent site sitess may not  occur at the same time. There is a strong influence from the site’s specific characteristics, such as canyon geometry and the existence and density of greenery. Relative humidity 

In th thee ea earl rly y mo morn rnin ing, g, th thee RH on bo both th si site tess is re relat lativ ively ely similar to the average of around 82% (see Figure 6). The RH difference is then becoming larger when the sun starts rising and reaching its maximum around 15.00 hours when the ENG site is 5% higher. This can be explained by the fact that PGP, on average, has a higher air temperature and less greenery. Solar radiation below tree and surface temperature

The solar radiation inside the PGP canyon is higher as compared to the ENG canyon (see Figure 7). Although both the solar sensors were placed below the tree, it shows that palm tree tr ees, s, wh which ich do domin minat atee th thee PGP ca cany nyon on,, ar aree no nott ab able le to provide sufficient shading. The tree’s leaves density has a strong correlation with the tree’s ability to intercept solar 

radiation that is described in Beer’s law (Jones, 1992). The sudden drop in the solar radiation intensity at the PGP site and the increase at the ENG site at 14.00 hours are mainly due to the positionin positioning g of the solar radiation radiation sensors. sensors. At  14.0 14 .00 0 ho hour urs, s, at th thee PGP si site te,, mu multi ltipl plee la laye yers rs of pa palm lm leaves coincidentally blocked the solar radiation. Similarly, at the ENG site, the solar radiation was able to penetrate through the gap between mature trees. The ground surface temperature at the PGP site is higher, on average, by 2K as compared to the ground surface temperature at the ENG site (Figure 8). This is because the solar  radiation received by the PGP canyon is higher. However, this surface temperature is not as high as compared to the bare asphalt surface, which can reach a maximum surface temperature up to 61 8C (Santamouris, 2001). The pavement  of the PGP site is made from yellow sandstone, which is considered as a light colour. A light colour pavement has been proved to have a lower surface temperature, since it helps to re refl flec ectt the he heat at (D (Dou oulo loss et al al., ., 20 2004 04). ). Th Thee me meas asur ured ed average solar reflectivity from the ENG pavement is 0.25 as compared to 0.3 in the PGP site. However, the high reflectivity of a sandstone pavement reduces the visual comfort of  pedestrians. The ground surface temperature in the ENG site is similar  to the ambient air temperature, although the pavement is

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204 Wong and Jusuf

Figure 7 | Average solar radiation below tree (20 hot days) measured at point WS 2 in the ENG and PGP canyons during the

period between 17 July and 20 October 2007

Figure 8 | Average ground surface temperature inside inside the ENG and PGP canyons (20 hot days) during the period between 17

July and 20 October 2007

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Study on the microclimate condition along a green pedestrian canyon in Singapore 205 made of black pebble stone. This is because the mature trees can provide very good shading. Meanwhile, in the PGP site, the ground surface temperature can be up to 2K higher as comp co mpare ared d to th thee am ambie bient nt te temp mper erat atur ure. e. Wi With th a hig highe her  r  surface temperature, the thermal comfort of pedestrians can be af affe fecte cted d du duee to a hi high gher er me mean an ra radia diant nt te temp mper erat atur uree (Johansson and Emmanuel, 2006).

Wind 

In Figure 9, the average wind speed over the 20 hot days is between 1.0 and 4.0m/s measured at the NUS WS (rooftop). Meanwhile, the wind speed at the pedestrian level is low, less than tha n 1.5 1.5m/s m/s dur during ing day daytime time and les lesss tha than n 0.5 0.5m/s m/s dur during ing night-time. The graph also shows a pattern in which the wind speed starts to increase at 09.00 hours and reaches its maximum speed at 15.00 hours. The ENG reference point  (WS 1) has the lowest average wind speed due to its location, surrounded by some mature trees and building blocks, which prevents it from prevailing wind blow most of the time, although the location was relatively open to the sky. On average, the wind speed inside the ENG canyon is lower  than inside the PGP canyon. The dense greenery inside the ENG EN G ca cany nyon on is be belie lieve ved d to re reduc ducee th thee win wind d sp spee eed d tha that  t  passed through inside the canyon. Trees are able not only

to provide shading, but also to block and redirect wind direction (Olgyay, 1992). Figure 10 shows that the prevailing wind direction during the per period iod of mea measur sureme ement nt cam camee fro from m the southern southern and southe sou theaste astern rn dir direct ection ions, s, rel relativ atively ely per perpen pendicu dicular lar to the canyon can yon’s ’s ori orient entatio ation, n, whe where re aro around und 22% was cal calm m win wind d (0.5–2.1m/s). The wind inside the ENG canyon followed the canyon orientation. The wind blew in a direction from Point 1 to Point 5. This is understandable because the area outside the canyon edge in Point 1 is an open space, which has a higher altitude than the other side of the canyon Meanwhile, the observation found inside the ENG canyon is different from that inside the PGP canyon. In the PGP canyon, the wind direction follows the southern prevailing wind direction. The position of WS 2 for wind measurement  was located in a small open space in the middle of the canyon (see Figure 2). Hence, the wind direction at this location was not influenc influenced ed by the canyon canyon ori orient entatio ation n as in the ENG canyon. Calculated MRT and PET values

Figures 11 and 12 show the calculated MRT and PET values inside the ENG and PGP canyo canyons ns and their respective respective reference points. The representative point for the ENG canyon is Point 3 (ENG 3) located in the middle of the canyon and

Figure 9 | Average wind speed (20 hot days) in ENG, PGP and NUS WS (20 hot days) during the period between 17 July and

20 October 2007

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206 Wong and Jusuf

Figure 10 | Wind speed and prevailing wind (‘blowing (‘blowing from’) direction in ENG, PGP and NUS WS

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Study on the microclimate condition along a green pedestrian canyon in Singapore 207

Figure 11 | Calculated MRT inside the ENG and PGP canyons and its respective reference point on selected clear hot days

Figure 12 | Calculated PET value inside the ENG and PGP canyons and its respective reference point on selected clear hot

days

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208 Wong and Jusuf fully covered below by mature trees and that for the PGP cayn ca ynon on is Po Point int 3 lo locat cated ed in the middle middle of th thee ca cany nyon on under a young palm tree. As mentioned in the methodology, the calculation made use of the solar radiation data below the trees. Consequently, for the PGP canyon, the calculated MRT and PET values for a subject who walks along the PGP canyon were expected to be higher, close to the value of the PGP reference point value, since young palm trees are not able to fully cover the canyons and high solar radiation intensity inside the canyon can be expected. Thee ca Th calc lcula ulated ted MR MRTT-EN ENG G 3 va valu luee is lo lowe werr tha than n th thee MRT-PGP 3 one (see Figure 11). The young palm tree is not able to provide shading to the subject as compared to thee ma th matu ture re tr tree ee.. On th thee ot othe herr ha hand nd,, th thee MR MRT T of th thee ENG reference point is slightly higher than the PGP reference point due to a higher SVF value (0.91 compared to 0.86). Thee ca Th calc lcul ulate ated d PE PET T va value lue ins inside ide the ENG an and d PG PGP P canyons is shown in Figure 12. At the ENG canyon, the mature trees are able to provide thermal comfort at almost  all daytime hours, except at 14.00 hours, when the solar radiation penetrated in between the trees. On the other hand, youn yo ung g pa palm lm tr tree eess in th thee PG PGP P ca cany nyon on ar aree no nott ab able le to provide comfort to pedestrians due to their less dense leave characteristics.

Cooling effect of greenery along the canyon

In th this is se sect ction ion,, th thee ai airr te temp mper eratu ature re be beha havio viour ur in ea each ch measurement point along the canyon and the cooling effect  of gr gree eene nery ry ar aree di disc scus usse sed. d. Th Thee co cool oling ing ef effe fect ct of ea each ch measurement point was calculated by the subtraction of the air temperature temperature at resp respective ective points with the reference reference point.  At the ENG site

To determine the time of maximum cooling effect occurrence ren ce ins inside ide the can canyon, yon, the coo cooling ling effect effect pro profile filess of  each measurement point were plotted against the 24h timeline li ne (s (see ee Fi Figu gure re 13 13). ). Th This is sh show owss th that at at mo most st of th thee points, especially points inside the canyon (Points 1–5), the maximum cooling effect occurs twice a day, in the morn mo rning ing at 11 11.0 .00 0 ho hour urss an and d in the af afte tern rnoo oon n at 15 15.0 .00 0 hours. This phenomenon is different from the findings of  Shashu Sha shua-B a-Bar ar and Hof Hoffma fman n (20 (2000) 00),, who des descri cribe be in the methodology that the maximum cooling effect occurred only at 15.00 hours, which coincided with the maximum daily dail y air temp tempera eratur turee in Tel Tel-Avi -Aviv, v, Isr Israel. ael. For Poin Points ts 5 and 6, the maximum cooling effect occurred only once in the morning at around 10.00 hours, while in the afternoon the cooling effect was negligible. These Points 5 and 6 were located at the edge of the canyon and outside the canyon can yon,, res respec pective tively, ly, wher wheree ther theree is less sha shadin ding g fro from m

Figure 13 | Averaged cooling effect (20 hot days) measured measured at points inside the canyon in the ENG site

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Study on the microclimate condition along a green pedestrian canyon in Singapore 209

Figure 14 | Averaged cooling effect (20 hot days) measured measured at different points in the ENG site

the greenery, and it was observed that anthropogenic heat  the tre trees es sta stabili bilize ze the air temp tempera eratur turee ins inside ide the can canyon yon,, from fro m the opp opposi osite te bui buildin lding’s g’s air air-co -condi ndition tioning ing coo cooling ling with the result of a further cooling effect at Point 4. tower further dismisses the cooling effect from the surAt 15.00 hours, Points 2–4 are found to display a higher  rounding trees. Meanwhile, although located outside the cooling effect than the other points at the edge of the canyon canyon can yon,, the maxi maximum mum coo cooling ling eff effect ect occu occurre rred d twic twicee at  and outside the canyon. This is because the points are situated Points 7 and 8. This is because they were located near  at the mid-section of the canyon, well shaded by dense canomature trees. pies and buildings. The cooling effect at Point 1 is slightly This twice twice-max -maximum imum cool cooling ing effe effect ct phen phenomeno omenon n can be lower as it is located at the edge of the canyon, with increased explai exp laine ned d as the ef effec fectiv tivene eness ss of mat mature ure tre trees es to pr provi ovide de exposure to direct sunshine. It can be realized that leaving the shading to the canyon. In the morning (09.00–11.00 hours), canyon can result in an immediate air temperature increase of  the sun radiation has not radiated the canyon directly due to ab abou outt 2. 2.1K 1K.. Th Ther eree is no co cool olin ing g ef effe fect ct fr from om th thee tr tree eess at  the shadowing effect of the building and upper tree canopies. Po Poin ints ts 5 an and d 6. Th Thee pr prob obab able le re reas ason onss ar aree th that at th they ey ar aree The air tem temper peratu ature re ins inside ide the can canyo yon n inc incre rease asess gra gradua dually lly exposed to the open space, a reduction of tree shade at their  alon al ong g wi with th th thee in incr crea ease se of ba back ckgr grou ound nd te temp mper erat atur ure. e. At  imm immedi ediate ate en envir vironm onment ent and and,, as men mentio tioned ned,, the there re may be around 11.00 hours, the shading effect of trees gives its first  anthropogenic heat from the opposite building’s cooling tower. maximum cooling effect. When the sun moves gradually to At Point 8, the cooling effect is found to be around 1.2K. exactly above the head at around 13.00 hours in Singapore, This is higher than the cooling effect at Points 5–7, even the canyon receives the maximum solar heat gain. Then, as though they are all located outside the canyon. The dense the sun sets (lower solar elevation), the trees provide shading tre treee clu cluste sterr pla plante nted d at the mea measur suremen ementt poi point nt cau causes ses the to the canyon and the second maximum cooling effect occurs. lower ambient temperature. It has provided shade to its surThe max maximum imum coo coolin ling g eff effect ectss in the mor mornin ning g (11 (11.00 .00 round roundings ings and has lowered the air temperature beneath. beneath. hours) hou rs) and in the aft aftern ernoon oon (15.00 (15.00 hou hours) rs) are sho shown wn in Figur Fig uree 14 14.. In ge gene nera ral, l, th thee co cool olin ing g ef effe fect ct wa wass ob obse serv rved ed inside ins ide the canyon canyon sid sidewa ewalk. lk. It ran ranges ges from 0.9 to 1.5 1.5K. K.  At the PGP site The cooling effect pattern of both times is similar to the Figure 15 shows the 24h cooling effect pattern of each point at  cool co olin ing g ef effe fect ct in insi side de th thee ca cany nyon on (P (Poi oint ntss 2– 4) 4).. Wh When en the PGP site. On average, no significant cooling was observed the air temperature reaches the maximum at 15.00 hours, along the canyon sidewalk at the PGP site as compared to the

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210 Wong and Jusuf

Figure 15 | Averaged cooling effect (20 hot days) measured measured at points inside the canyon in the PGP site

Figure 16 | Averaged cooling effect (20 hot days) measured measured at different points in the PGP site

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Study on the microclimate condition along a green pedestrian canyon in Singapore 211 ENG site, with only at around 0.5K. The cooling effects are noticeable at some points with the maximum of 0.8K, that is, Poin Po ints ts 2, 3 an and d 5. At th thes esee po poin ints ts,, th thee ma maxi ximu mum m co cool olin ing g effect pattern follows the phenomenon as in the ENG site. It  occurs twice a day, but at a different time, at 10.00 hours in the mor mornin ning g and at 17. 17.00 00 hou hours rs in the aft after ernoo noon. n. Thi Thiss dif differ ferenc encee is be belie lieved ved to be du duee to the thedif differ ferenc encee of sit sitee ch chara aracte cteris ristic tics, s, suc such h as the building density, H/W ratio and greenery condition. The cooling effects at Points 2 and 3 were at the same intensity for both times (see Figure 16). Point 3 is situated at the centre of a semi-open space, planted with groups of shrubs, small sma ll pla plants nts and pal palms, ms, and re recei ceived ved no sha shadin ding g fr from om adj adjoin oining ing buildi bui lding ngs. s. The pla plants nts bro brough ughtt abo about ut the max maximu imum m coo coolin ling g effect for the location. At Point 2, young palm trees were at  thee le th left ft an and d ri righ ghtt si side dess of th thee wa walk lkwa ways ys.. Th Thee co colle llect ctiv ivee impacts from the cooling effect of young palm trees near the open op en sp spac acee an and d th thee sh shad adin ing g fr from om th thee bu buil ildi ding ngss pr prov ovid idee the sam samee coo coolin ling g ef effec fectt as the air tem temper peratu ature re mea measu sured red at  the centre of the semi-open space. The ‘heating’ effect was identified at Points 6 and 7 and a very minimal cooling effect at Point 5 in the morning. At  these points, the cooling effect intensity increases at almost  the same rate as in the afternoon at 17.00 hours. The effect of  sola so larr ra radi diat atio ion n is th thee ma main in re reas ason on fo forr th this is ai airr te temp mper erat atur uree be beha havviour. The curved form of the PGP canyon makes these points moree exp mor expos osed ed to the ea easte stern rn so solar lar rad radiat iation ion tha than n the oth other  er  points. Thus, although not intensively planted with greenery, these the se po point intss rec receiv eivee som somee sh shade adefr from om the sur surrou roundi nding ng bui buildi ldings ngs.. It is believed that the shade provided by these buildings is the main factor contributing to the cooling effect in the vicinity. Both Points 1 and 7 are situated at the ends of the canyon, where they are constantly exposed to direct solar radiation. In addition, there are only small trees and shrubs planted around the poin points, ts, whic which h are inadequate inadequate to cool the env environ ironment  ment  through their shading. The high building densities and concrete pavement at the site have also resulted in more heat  being bein g radiated radiated back to the built enviro environmen nment, t, thus cont contrib ributin uting g to the heating effect. Comparable to Points 1 and 7, Point 4 is situated at the edge of a semi-open space, exposed to similar  cond co nditi ition onss as Po Point intss 1 an and d 7: he henc nce, e, a ‘h ‘heat eating ing effect’ effect’ is found. The ‘heating effect’ identified at this point is 0.6K.

SUMMARY AND CONCLUSIONS A comprehensive field measurement has been conducted to study the microclimatic condition in two different canyons, ENG and PGP. ENG has the characteristic of mature trees covering its canyon, while PGP has less greenery along the canyon, dominated by young palm trees. The objective of  this study is to investigate the microclimatic condition of  thes th esee tw two o di diff ffer eren entt ca cany nyon ons. s. Th Thee su summa mmari ries es fr from om th this is study are as follows: †

The mature trees inside the ENG canyon have the ability to lower the air temperature rather than the young palm

†

†

†

†

trees in the PGP. The average air temperature inside the ENG canyon is lower by around 0.7–1.1 8C as compared to the PGP canyon during daytime and maintains its coolness ne ss at ab abou outt 0. 0.4– 4– 0. 0.5 58C dur during ing nig nightht-time time.. Howe However ver,, higher RH can be expected by having dense greenery, measured up to 5% on average as compared to the PGP canyon. The lower air temperature temperature in the ENG canyon is the result  of good shading provided by the dense foliage, which is ablee to int abl interc ercept ept muc much h more incoming incoming sol solar ar rad radiati iation. on. The mature tree is able to reduce the solar radiation radiation intensity to less than 150W/m2 as compared to the young palm treee to les tre lesss th than an 30 300W 0W/m /m2. The lower gro ground und surface surface temperatures at the ENG canyon will in turn result in less heating of the air. The wind speeds inside both canyons are calm ( , 0.5m/s) with more than 80% of the total occurrence. The wind speed inside the canyon is reduced quite substantially at  the ma maxim ximum um of ar aroun ound d 0. 0.5– 5– 1. 1.5m/ 5m/ss as co compa mpare red d to above the canyon, which can be up to 4m/s. The calculated MRT and PET show that the mature tree is able to provide thermal comfort to the pedestrian walking along the ENG canyon. On the other hand, assuming that  the young palm trees cover the whole stretch of the PGP cany ca nyon on,, th they ey ar aree st stil illl no nott ab able le to pr prov ovid idee th ther erma mall comf co mfor ortt to its pe pede destr stria ian. n. Th This is is ma mainl inly y du duee to th thee inability of young palm trees to provide sufficient shading. Mature trees and young palm trees are able to generate a cool co oling ing ef effe fect ct up to 1. 1.5 5 an and d 0. 0.5K, 5K, re resp spec ectiv tively ely.. Th Thee maximum max imum coo cooling ling eff effect ect ins inside ide the can canyon yon,, esp especi eciall ally y the location below the trees, occurs twice a day, in the morning (around 10.00–11.00 hours) and in the afternoon. The afternoon maximum cooling effect may not  happen at the same time for every canyon. It depends on the chara characterist cteristics ics of the canyons.

This stu This study dy con conclu cludes des tha thatt two mai main n con contrib tributor utory y fac factor torss inf influluence the thermal effect within the canyons. They are vegetation tati on cov covers ers and sha shadin ding g fro from m bui buildi ldings ngs and tree trees. s. Thi Thiss study has provided an indication that planting of vegetation can be an effective passive measure to improve the microclimate inside a canyon. However, in general, it can be noted that th at in incr creas easing ing th thee gr gree eene nery ry de dens nsity ity wil willl re resu sult lt in a hi high gher er RH RH..

ACKNOWLEDGEMENTS This research was supported by the Department Building and Office of Estate and Development (OED), NUS. The authors express their sincere thanks to Ms Lina Goh for providing the necessary information.

NOTE 1 Based on on meteorologic meteorological al data on 1982– 1982– 2001, National National Environmental Agency, Singapore.

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