Significantly higher maximum cooling was 3. In recorded 1 in sunny weather than cloudy weather summary, woodland canopy on GR furthered the weather- Table 4 c and 2 during daytime than nighttime associated moderation in thermal sensation through Table 4 a, b.
It was observed that GR significantly cooled the environment throughout sunny daytime cooling in the thermal comfort indices in cloudy weather days, with mean daytime C-TS being the most prominent at was nearly half than that in sunny weather. Table 4. In short, thermal comfort was most sig- strong heat stress, jointly composing an overwhelming majority nificantly improved on GR, especially during daytime under Overcast conditions sunny clear sky.
Still, BR experienced mainly Fig. It was noteworthy that trace periods of stress Under overcast sky, the subdued daytime insola- Under vegetation cooling in sunny weather, GR was dis- tion restrained heat storage carried over to nighttime, posed to strong heat stress Furthermore, GR showed rare instances of no stress In addition, GR featured exceptionally rare thermal stress 2.
Nighttime UTCI Discussion Despite radiative cooling of its concrete surface, BR returned Assessing microclimate of woodland green roof the most intense nighttime heat stress characterized by strong heat stress Cloudy sky reduced daytime Green roofs expand the urban green cover without sacrificing heating and retained less heat at night, thus slashing strong the scarce street-level space.
It would be useful to compare IGR heat stress occurrce by Cloudy sky even returned rare occurrence of no This discussion would consider meteorological parameters af- thermal stress 0. Comparability cooling in sunny weather Table 4 than those ushered by park was also observed in the distribution of the less frequent trees and garden vegetation by 2. Specifically compared distributions indicated the immaterial weather effect on night- against other green roofs, our IGR registered higher maximum time thermal sensation on GR.
However, the subdued insolation under cloudy previous research. At ground-level, building-induced shading sky suppressed the cumulative frequency of extreme heat stress weakened the insolation onto control sites, relative to more on BR to 8. More importantly, BR was mainly beset by exposed rooftop-level in this study. Consequently, the solar- moderate heat stress Intense insolation generated occasion- Apart from the influence of the elevation of measurement al strong and even extreme heat stress LV1 , bringing a cu- sites, vegetation characteristics contributed to superior air mulative frequency of merely 4.
With cloudy weather, the cooling. The closely by the dominance by slight heat stress In short, dense vegeta- At clear-sky night, BR was dictated by moderate heat tion communities on rooftop-level contributed to remarkable stress Int J Biometeorol — Despite the superior air cooling, trapping of the Expressing thermal comfort on intensive green roof evapotranspiration-generated atmospheric moisture inside the canopy-enclosed space could partly compromise comfort In order to assess the effectiveness of IGR in improving ther- by flattening the skin-to-air vapor pressure gradient and con- mal comfort, comparisons were conducted with parks and sequently decelerating perspiration heat loss.
The disturbance other green roofs from previous research. Our IGR provided to homeostasis would depend on the balance between the superior mean daytime C-PET to parks in Chiba; Japan; and comforting air cooling and contending sub-canopy humidifi- Utrecht, the Netherlands by nearly 9. Such consideration was not specific to subtropics, as it ; Klemm et al. However, modeling results sug- also occurred in temperate regions Saneinejad et al.
Table 4 and frequency terms Fig. Future research could But such situation was suppressed to merely 4. The results demonstrated the notable capability of this roof greening. During daytime, the woodland IGR showed mainly strong and moderate heat stress in UTCI terms but conveyed an Despite remarkable air cooling, surface cooling in this study impression of greater comfort with mostly moderate and slight was less pronounced than previous studies.
Whereas maxi- heat stress in PET terms Fig. Dissimilar impression of thermal sensation also oc- ; Fioretti et al. The more curred on bare concrete roof. UTCI showed strong to very intense insolation in the Mediterranean or tropical locations of strong daytime heat stress whereas PET demonstrated previous studies widened the difference in surface heating rate substantial extreme heat stress Fig.
On the oppo- between GR and BR. Furthermore, their monitoring sites were site, at night, bare roof showed mainly strong heat stress relatively less shielded by neighboring buildings from insola- in UTCI terms but mainly slight to moderate heat stress in tion, further magnifying the GR-versus-BR surface heating PET terms Fig.
Therefore, on bare concrete roof, differentials. In summary, urban surroundings would exert PET detected daytime extreme heat stress but presented a observable influences on surface cooling performance.
Apart from landscape-related factors, the dense woodland For landscaping and urban planning practitioners, the qual- vegetation caused thermal blanket effect which limited surface itative notation of heat stress of PET and UTCI could be be- cooling.
IGR and EGR in previous research featured vegetation wildering given identical or similar nomenclature but convey- with lower density and per-area biomass than in this study, ing different biometeorological sensations.
Nonetheless, the permitting surface-to-sky radiative heat loss and convective principle of prudence should be upheld for the avoidance of surface cooling. Therefore, as a precaution, the ed heat loss via outgoing long-wave radiation. Hence, re- thermal index with qualitative notation connoting higher heat radiation from canopy bottom to the surface of IGR decelerated stress could be adopted.
For instance, on bare roof, in sunny radiative heat loss. Therefore, the biomass structure of vegeta- daytime, the various extreme heat levels shown by PET Fig. For green roof, UTCI canopy improved overall thermal comfort by filtering insola- would be preferred for whole-day period for its frequent indi- tion. Through-canopy insolation transmission decreased with cation of strong heat stress Fig. Yet, PET might serve foliage density Konarska et al. The considerably sub- as a reference during daytime because of the distinct observa- dued insolation relieved radiative heat load of green roof users, tion of extreme heat stress Fig.
Coupled with significant air cooling, the ameliora- to heat stress and encourage patronage to roof greening in tion of heat stress was reflected by PET and UTCI Fig. Although both PET and UTCI assigned Green roof can impart multifold thermal-related benefits to weights to radiative exchanges with the surroundings using city dwellers.
Compact mode of city development has stifled view angle factors, the improvement in the asymmetry in ra- the land supply for ground-level green space. IGR offers a diation caused by the canopy remained uncharted.
Upon acknowledging the heat stress mitigation func- night batches, thermal adaptation over time was largely unex- tions, building owners and users may consider establishing plored inasmuch as PET and UTCI are steady-state parameters green roof on existing bare rooftops. This can address the in which the effects of time are ignored. Future studies could problem of seldom visitation of rooftops, and make probe into the thermal comfort under continuous spatial and fruitful use of an otherwise wasted resource.
Building owners temporal variations of microclimate. Such an approach could restrict the study During outdoor passive recreation on green roofs, building to a particular locality and would limit the generalization occupants could temporarily switch off air-conditioning to in relation to green roofs situated in different urban land- reduce energy use and generate upstream benefits of reducing scapes.
Nevertheless, our results are applicable to places carbon emission at power stations. The current research was conducted by measuring the bio- The lack of other comparable woodland and other green meteorological conditions at the center of the green roof where roofs in the vicinity and elsewhere, however, frustrated the microclimatic influences by the native woodland would be implementing an expanded empirical investigation at expected to be maximal.
In future studies, a network of roofs of intensive would be expected to decline at and near the edge of the green and extensive types with multiple weather stations roof where the ambient microclimatic conditions would im- installed at each site could overcome the problem of lo- pose greater influences. For green roofs with a considerable cation-specificity. Both inter- and intra-spatial variations size, future research could measure the microclimatic condi- in thermal comfort at each roof could be explored.
This tions along a gradient from the center to the edge. Enhanced understanding of maximal thermal comfort. The retention of Alternative and additional urban green spaces could thus be cool air inside canopy-enclosed space was achieved at provided on rooftops to serve urban residents.
Such elevated the cost of lowered wind speed. This decelerated the green sites can compensate for the shortage at the street level recharging of fresh air and diminished comfort. With better thermal ambience, Psychological oppression might develop because exces- users are more inclined to stay longer on the green roof and sively dense foliage could connote gloominess and mess- enjoy more health-related benefits Thorsson et al. Increased exposure to outdoor environment in the hot season Therefore, analyses incorporating the quantifica- could equip urban dwellers with physiological resilience and tion of the contribution of thermal and psychological fac- adaptation to thermal stress to tackle the combined impacts of tors to overall comfort could be explored.
Limitations of this study Conclusion This pioneering study on the quantification of thermal comfort This study applied universal thermal climate index improvement by woodland IGR indicated room for improve- UTCI and physiological equivalent temperature PET ments in research design. First, PET and UTCI may be inad- to quantify the impact of intensive green roof IGR on equate to reflect spatial and temporal variations of microcli- human biometeorology, bridging the knowledge gap in matic parameters on thermal comfort.
For example, long- human thermal comfort research in the woodland IGR wave radiative exchange between the woodland floor and setting.
Under the woodland canopy, subdued insolation, the canopy bottom tends to equalize the long-wave radiation remarkable air cooling, and satisfactory surface cooling fields above and below green roof users, somehow easing significantly enhanced thermal comfort. Build Environ — Int J Climatol — Cheung PK, Jim CY c Subjective outdoor thermal comfort and Considerably greater environmental cooling and thermal urban green space usage in humid-subtropical Hong Kong.
Energy comfort improvement were registered during daytime than and Buildings — Nevertheless, caution should be Egan MD Concepts in thermal comfort. Pearson Prentice Hall, exercised in interpreting the qualitative classes of heteroge- New Jersey neous thermal indices.
This research substantiated the better- Fanger PO Thermal comfort. Analysis and applications in envi- ronmental engineering. Danish Technical Press, Copenhagen ment of human biometeorology using the innovative rooftop Fioretti R, Palla A, Lanza LG, Principi P Green roof energy and water greening approach, specifically the complex woodland green related performance in the Mediterranean climate. Build Environ roof, which has been sparingly installed. The findings inter- — The complex wood- clothing model.
Int J Biometeorol — Practitioners and policy makers could use non-technical Overview. Owners and users of build- Hong Kong. Available at: the mitigation of thermal sensation. Power Hong Kong Limited for research grant, technical support and Accessed 20 Mar permission to use its substation to conduct the field studies. Energy and Buildings — SI Ed. Arch Meteor Wind structure in and above a tropical forest. J Appl Meteorol 4: Geophy B — CO;2 mance of tropical green roof.
Landsc Ecol Eng — Geogr Pol — Landsc Urban Plan — Urban For Urban Green — Sustain thetic reactions toward green roofs in the northeastern United States. Cities Soc — Oke TR Boundary layer climates. Routledge, London landurbplan. Energies — A case study. Energy and Buildings buildenv. A transversal study in cies. Energy — Theor A Relating microclimate, human thermal comfort and health Appl Climatol — Int J Climatol various heat island mitigation measures.
Int J effect in a high latitude city. Urban Climate — Ecol Eng Environ Pollut — the thermal effectiveness of an extensive green roof. Build Environ Build building morphology on shadow patterns and mean radiant temper- Environ — Theor Appl Climatol — Build landurbplan. Matzarakis A, Mayer H Another kind of environmental stress: thermal stress.
To operationalize the main research hy- tion only in the center of the intensive green roof, these pothesis that IGR would improve thermal comfort, the exper- cooling mechanisms could be expected to decrease in magni- imental set-up, computation of thermal comfort indices, and tude towards the edges of the roof due to increased exposure data analysis procedures were described in BMaterials and to ambient conditions.
The practical implications and recommendations of the findings were elab- Instrumentation orated in Discussion. Finally, concluding remarks were pro- vided in Conclusion. To compute thermal comfort indices, meteorological variables were measured at min interval using high-precision sen- sors placed in the center of both roofs Table 1.
The 7. Progressive drop in wind speed could be Planning Department ; Hong Kong Census and Statistics anticipated with increasing distance from the edge towards the Department As a compact city with humid-subtropical center of the intensive green roof where the meteorological climate characterized by hot-humid summer and cool-dry variables were measured.
The surface temperature TS of BR winter, the year normal monthly mean temperature of concrete ground and GR floor were measured. June, July, and August reached respectively The summer rainfall Computation of thermal indices reached mm, which was about half of the annual total of mm. BioKlima Blazejczyk et al. Tmrt was computed using the following equation ISO Havenith et al. To exclude the carry-over effect of previous daytime ter 0. The 0. Rainy scenario was excluded because of The required measurement height of wind speed for PET the confounding influence of rainwater evaporation.
The wind speed at The selection of sample days relied on the weather sum- height x vx was extrapolated by the logarithmic wind profile mary from Hong Kong Observatory a, b, c. Oke : The summer daily mean values of insolation, bright Table 1. Thermal indices at each time- MJ, 6. Sunny days step were graded. Then, relative frequency distributions were were defined as days featuring above-mean insolation and computed from all time-steps for a comprehensive graphical bright sunshine hours, below-mean cloud amount, and zero evaluation.
Cloudy days were defined as days showing above- mean cloud amount, below-mean insolation and bright sun- shine hours, and below mm rainfall. The systematic se- Results lection yielded 21 sunny days and 18 cloudy days.
Their corresponding daily insolation ranges were First, the mean and maximum values of three mi- 5. But the difference was enlarged to 2. This implied that roof greening exerted After the description, pairwise tests were employed to test greater impact on daily mean SI than on daily maximum SI. After checking data nor- TS and TA mality using Shapiro-Wilk test, t tests were conducted for normally distributed data whereas Mann-Whitney U test for Mean temperatures non-normally distributed data.
Cooling effectiveness was test- ed by comparing differences in microclimatic and biometeo- The highest mean temperatures were registered during day- rological parameters between the roofs GR-versus-BR.
Statistics of a daily mean and b daily maximum values of was greater than that in BR-TA. But the decrease in Standard error 7 17 4 5 mean temperature values was relatively weaker on GR. Maximum 71 Weather-related temperature changes somehow altered night- Minimum 98 52 10 time heat transfer processes. At cloudy night, BR-TS was b Daily maximum Average lower than BR-TA, triggering air-to-surface heat transfer, ren- Standard error 14 63 39 55 dering BR concrete surface a nighttime heat sink as opposite Maximum to a heat source under clear night sky.
Mean differences were shown to display the results of pairwise comparison between daytime and nighttime values and between sunny and cloudy weather, with positive values indicating respectively larger daytime than nighttime differences day-night or larger sunny than cloudy differences sunny-cloudy.
Therefore, the effects of Comparing against mean temperatures, greater differences cloudy weather were manifested by respectively lower tem- were observed in maximum temperatures between daytime peratures and altered nighttime heat transfer processes. Absence of insola- TS to But the canopy foliage hindered air flow tion and lower temperatures jointly relieved thermal sen- and the ensuing heat dissipation, resulting in slightly sation at night Fig. The cooling effects. Comparable to BR, thermal severity on most observable daytime-nighttime difference occurred under GR lessened at night Fig.
Comparable to the case of mean temperature Thermal comfort significantly improved on cloudy days values, the daytime-versus-nighttime difference was greater in for both roofs Fig.
Yet, GR benefitted relatively less by temperatures throughout the day Fig. Nevertheless, the most desir- ments than mean temperature values Table 3 b. Daytime mean C-TA in sunny conditions was weaker but still significant, reaching 1. Under subdued insolation in cloudy scenario, both C- meteorological environment.
GR counterparts on BR on cloudy night. Therefore, strong insolation during daytime would lower nighttime figures. Cloudy weather improved thermal comfort Fig. For Maximum C-TS and C-TA connoted the mollification in instance, compared against sunny conditions, daytime maxi- the environmental thermal extremities. Comparable decrements were found sunny daytime conditions, reaching 9. Significantly higher maximum cooling was 3.
In recorded 1 in sunny weather than cloudy weather summary, woodland canopy on GR furthered the weather- Table 4 c and 2 during daytime than nighttime associated moderation in thermal sensation through Table 4 a, b. It was observed that GR significantly cooled the environment throughout sunny daytime cooling in the thermal comfort indices in cloudy weather days, with mean daytime C-TS being the most prominent at was nearly half than that in sunny weather.
Table 4. In short, thermal comfort was most sig- strong heat stress, jointly composing an overwhelming majority nificantly improved on GR, especially during daytime under Overcast conditions sunny clear sky. Still, BR experienced mainly Fig. It was noteworthy that trace periods of stress Under overcast sky, the subdued daytime insola- Under vegetation cooling in sunny weather, GR was dis- tion restrained heat storage carried over to nighttime, posed to strong heat stress Furthermore, GR showed rare instances of no stress In addition, GR featured exceptionally rare thermal stress 2.
Nighttime UTCI Discussion Despite radiative cooling of its concrete surface, BR returned Assessing microclimate of woodland green roof the most intense nighttime heat stress characterized by strong heat stress Cloudy sky reduced daytime Green roofs expand the urban green cover without sacrificing heating and retained less heat at night, thus slashing strong the scarce street-level space.
It would be useful to compare IGR heat stress occurrce by Cloudy sky even returned rare occurrence of no This discussion would consider meteorological parameters af- thermal stress 0. Comparability cooling in sunny weather Table 4 than those ushered by park was also observed in the distribution of the less frequent trees and garden vegetation by 2.
Specifically compared distributions indicated the immaterial weather effect on night- against other green roofs, our IGR registered higher maximum time thermal sensation on GR. However, the subdued insolation under cloudy previous research. At ground-level, building-induced shading sky suppressed the cumulative frequency of extreme heat stress weakened the insolation onto control sites, relative to more on BR to 8.
More importantly, BR was mainly beset by exposed rooftop-level in this study. Consequently, the solar- moderate heat stress Intense insolation generated occasion- Apart from the influence of the elevation of measurement al strong and even extreme heat stress LV1 , bringing a cu- sites, vegetation characteristics contributed to superior air mulative frequency of merely 4.
With cloudy weather, the cooling. The closely by the dominance by slight heat stress In short, dense vegeta- At clear-sky night, BR was dictated by moderate heat tion communities on rooftop-level contributed to remarkable stress Int J Biometeorol — Despite the superior air cooling, trapping of the Expressing thermal comfort on intensive green roof evapotranspiration-generated atmospheric moisture inside the canopy-enclosed space could partly compromise comfort In order to assess the effectiveness of IGR in improving ther- by flattening the skin-to-air vapor pressure gradient and con- mal comfort, comparisons were conducted with parks and sequently decelerating perspiration heat loss.
The disturbance other green roofs from previous research. Our IGR provided to homeostasis would depend on the balance between the superior mean daytime C-PET to parks in Chiba; Japan; and comforting air cooling and contending sub-canopy humidifi- Utrecht, the Netherlands by nearly 9.
Such consideration was not specific to subtropics, as it ; Klemm et al. However, modeling results sug- also occurred in temperate regions Saneinejad et al.
Table 4 and frequency terms Fig. Future research could But such situation was suppressed to merely 4. The results demonstrated the notable capability of this roof greening. During daytime, the woodland IGR showed mainly strong and moderate heat stress in UTCI terms but conveyed an Despite remarkable air cooling, surface cooling in this study impression of greater comfort with mostly moderate and slight was less pronounced than previous studies.
Whereas maxi- heat stress in PET terms Fig. Dissimilar impression of thermal sensation also oc- ; Fioretti et al. The more curred on bare concrete roof. UTCI showed strong to very intense insolation in the Mediterranean or tropical locations of strong daytime heat stress whereas PET demonstrated previous studies widened the difference in surface heating rate substantial extreme heat stress Fig.
On the oppo- between GR and BR. Furthermore, their monitoring sites were site, at night, bare roof showed mainly strong heat stress relatively less shielded by neighboring buildings from insola- in UTCI terms but mainly slight to moderate heat stress in tion, further magnifying the GR-versus-BR surface heating PET terms Fig. Therefore, on bare concrete roof, differentials. In summary, urban surroundings would exert PET detected daytime extreme heat stress but presented a observable influences on surface cooling performance.
Apart from landscape-related factors, the dense woodland For landscaping and urban planning practitioners, the qual- vegetation caused thermal blanket effect which limited surface itative notation of heat stress of PET and UTCI could be be- cooling.
IGR and EGR in previous research featured vegetation wildering given identical or similar nomenclature but convey- with lower density and per-area biomass than in this study, ing different biometeorological sensations. Nonetheless, the permitting surface-to-sky radiative heat loss and convective principle of prudence should be upheld for the avoidance of surface cooling. Therefore, as a precaution, the ed heat loss via outgoing long-wave radiation.
Hence, re- thermal index with qualitative notation connoting higher heat radiation from canopy bottom to the surface of IGR decelerated stress could be adopted. For instance, on bare roof, in sunny radiative heat loss. Therefore, the biomass structure of vegeta- daytime, the various extreme heat levels shown by PET Fig.
For green roof, UTCI canopy improved overall thermal comfort by filtering insola- would be preferred for whole-day period for its frequent indi- tion. Through-canopy insolation transmission decreased with cation of strong heat stress Fig. Yet, PET might serve foliage density Konarska et al. The considerably sub- as a reference during daytime because of the distinct observa- dued insolation relieved radiative heat load of green roof users, tion of extreme heat stress Fig.
Coupled with significant air cooling, the ameliora- to heat stress and encourage patronage to roof greening in tion of heat stress was reflected by PET and UTCI Fig. Although both PET and UTCI assigned Green roof can impart multifold thermal-related benefits to weights to radiative exchanges with the surroundings using city dwellers. Compact mode of city development has stifled view angle factors, the improvement in the asymmetry in ra- the land supply for ground-level green space.
IGR offers a diation caused by the canopy remained uncharted. Upon acknowledging the heat stress mitigation func- night batches, thermal adaptation over time was largely unex- tions, building owners and users may consider establishing plored inasmuch as PET and UTCI are steady-state parameters green roof on existing bare rooftops.
This can address the in which the effects of time are ignored. Future studies could problem of seldom visitation of rooftops, and make probe into the thermal comfort under continuous spatial and fruitful use of an otherwise wasted resource.
Building owners temporal variations of microclimate. Such an approach could restrict the study During outdoor passive recreation on green roofs, building to a particular locality and would limit the generalization occupants could temporarily switch off air-conditioning to in relation to green roofs situated in different urban land- reduce energy use and generate upstream benefits of reducing scapes. Nevertheless, our results are applicable to places carbon emission at power stations. The current research was conducted by measuring the bio- The lack of other comparable woodland and other green meteorological conditions at the center of the green roof where roofs in the vicinity and elsewhere, however, frustrated the microclimatic influences by the native woodland would be implementing an expanded empirical investigation at expected to be maximal.
In future studies, a network of roofs of intensive would be expected to decline at and near the edge of the green and extensive types with multiple weather stations roof where the ambient microclimatic conditions would im- installed at each site could overcome the problem of lo- pose greater influences. For green roofs with a considerable cation-specificity. Both inter- and intra-spatial variations size, future research could measure the microclimatic condi- in thermal comfort at each roof could be explored.
This tions along a gradient from the center to the edge. Enhanced understanding of maximal thermal comfort. The retention of Alternative and additional urban green spaces could thus be cool air inside canopy-enclosed space was achieved at provided on rooftops to serve urban residents.
Such elevated the cost of lowered wind speed. This decelerated the green sites can compensate for the shortage at the street level recharging of fresh air and diminished comfort. With better thermal ambience, Psychological oppression might develop because exces- users are more inclined to stay longer on the green roof and sively dense foliage could connote gloominess and mess- enjoy more health-related benefits Thorsson et al.
Increased exposure to outdoor environment in the hot season Therefore, analyses incorporating the quantifica- could equip urban dwellers with physiological resilience and tion of the contribution of thermal and psychological fac- adaptation to thermal stress to tackle the combined impacts of tors to overall comfort could be explored.
Limitations of this study Conclusion This pioneering study on the quantification of thermal comfort This study applied universal thermal climate index improvement by woodland IGR indicated room for improve- UTCI and physiological equivalent temperature PET ments in research design.
First, PET and UTCI may be inad- to quantify the impact of intensive green roof IGR on equate to reflect spatial and temporal variations of microcli- human biometeorology, bridging the knowledge gap in matic parameters on thermal comfort.
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