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Recent rapid urbanization has rendered outdoor space a key quality-of-life factor, yet walkability studies especially in hot-humid climates have seldom considered human biometeorology. This site-level study investigated microclimatic functions of an overhead structure in improving walkability, and identified biometeorology-related factors influencing pedestrian behaviour. A walkway with overhead tinted glass cover, demarcated into sunny zone and shaded zone, was equipped with hanging aluminium fins. Human thermal sensation was approximated by physiologically equivalent temperature (PET) and universal thermal climate index (UTCI), both closely related to global solar radiation and black-globe temperature. Pedestrian flow was tallied by demographic factors to assess the choice between shaded or sunny zones. Compared with sunny zone, shaded zone slashed maximum global solar radiation by 432 W/m2 in full effect to achieve 90% solar radiation reduction. The maximum cooling in air and black-globe temperatures reached 0.8 °C and 6.1 °C respectively. The overhead structure imposed minimal effect on wind field in both zones. Contrast in maximum PET and UTCI between the two zones reached 8.2 °C and 5.3 °C respectively. In shaded zone, the dominant biometeorological condition was moderate heat stress or slightly warm sensation in contrast to strong heat stress or warm sensation in sunny zone. An overall preference for shaded zone was detected. Pedestrian gender and age, namely female and elderly, were significantly associated with shaded zone preference. The findings could inspire a biometeorological perspective in understanding walking behaviour and pedestrian-friendly facilities. Biometeorological-sensitive design of artificial shade could improve walkability in urban environment that increasingly demands climate change proofing.

Existing studies emphasizing the magnitude of green-wall thermal benefits paid little attention to temporal microclimatic behavior. To bridge the knowledge gap, we employed high-precision sensors to monitor the microclimate of two wirerope-type climber green walls and adjacent control bare walls in humid-subtropical Hong Kong. Sample days were systematically selected to represent the results on sunny, cloudy and rainy weather in summer. This study aimed at: (1) evaluating the magnitude and daily temporal occurrence of maximum temperature, cooling and insolation at different positions of the layered green walls, and (2) analyzing temporal occurrence patterns of maximum values of microclimatic variables. The results demonstrated the paramount importance of insolation on green-wall microclimate. Varied solar inputs under different weather brought a sequence in maximum insolation, temperature and cooling: sunny > cloudy > rainy. Achieving an average cooling of 8.5 °C against bare wall surface exterior on sunny days, climber foliage acted as a heat sink for the street canyon and the building shell. The posterior air gap (PAG) generated an effective insulation layer with average cooling of 2.5 °C against ambient air in sunny conditions. However, ambient air anterior to the green wall experienced minor warming up to 0.05 °C. Cooling was stronger on more solar-exposed aspect and under stronger insolation. Green walls steered humidification effect by 0.17 kPa on average on sunny days. Daily temperature oscillation was moderated most obviously at the building exterior with average 4.3 °C reduction in sunny conditions. Implications were drawn on climber-species selection and PAG provision to enhance the multi-dimensional thermal benefits of climber green wall.

Trees are invaluable to urban landscape and environmental quality. However, tree failure can jeopardize life and property. Although the sway behavior of many coniferous species in temperate forest has been comprehensively evaluated, few studies have covered broadleaf tropical trees. This research aims at demonstrating the application of a new precision triaxial accelerometer to assess branch sway of an urban tree in tropical Hong Kong, testing the use of quantitative analysis to the large volume of generated sway and wind data, and identifying the factors influencing the branch sway behavior of the sampled tree. Following the tree selection criteria, four branches that are respectively healthy, crossed, wounded and hollow on a broadleaf deciduous Delonix regia (Flame of the Forest) were monitored by high-precision Tree Motion Sensors (TMS) during a tropical cyclone event. The results indicate significant inter-branch and axial difference in time-domain and frequency domain sway behavior. The average fundamental frequency of the branches ranged from 0.471 to 0.593 Hz. Mean sway amplitude (SAMEAN) correlated the best with wind speed fluctuations (maximum ρ = 0.801). Scientific instrumentation and data processing, such as the SAMEAN approach to monitor branch sway, can inform tree-risk management actions to abate possible failure. The approach undertaken in this study is an appropriate and effective means of monitoring tree sway and interpreting the associated data.

As building-integrated vegetation, green roofs can improve thermal performance of buildings and their ambience. Research efforts tended to focus on thermal behavior of extensive green roofs outside the tropics, with inadequate attention on intensive type and tropical region. Using detailed microclimatic monitoring, this study investigated the thermal behavior of a native-woodland intensive green roof in subtropical Hong Kong. Three summer weather days denoting sunny, cloudy and rainy were chosen from long-term monitoring data for comparison with a nearby bare control roof. Shading and evapotranspiration by the woodland canopy respectively reduced roof surface temperature and air temperature by latent-heat absorption. The most notable maximum surface and air cooling, respectively at 19.80 °C and 6.21 °C, was achieved in daytime sunny condition, with cooling effect extending into nighttime. Surface cooling was mainly governed by daily pattern of solar irradiance, whereas air cooling was more pronounced nearer ground surface except in nighttime period on rainy days. Slight near-surface air warming occurred in rainy nighttime. The woodland vegetation filtered 90% of the incoming short-wave radiation during most of the daytime, serving as an insolation filter which contributed to surface cooling. The substrate experienced net heat loss in cloudy and rainy weather, exporting its antecedent stored heat. At 0.5 m substrate depth, the temperature stayed at a stable level, suggesting restricted heat exchange in subsoil. With appropriate design and regular maintenance, an intensive green roof with dense woodland canopy and 1-m substrate could provide effective above-ground and soil cooling and buffer against outdoor-to-indoor heat ingress.

Thermal benefits of tropical climber green walls deserve more in-depth studies. A field experiment was designed in humid-subtropical Hong Kong to assess microclimatic effects of plant species, orientation and weather condition. In-situ measurements of air and surface temperatures of the indoor and outdoor environment were conducted respectively on the northeast and northwest oriented wirerope climber green walls with different air-gap depth. Adjacent bare walls were monitored as experimental controls. Days with sunny, cloudy and rainy weather in summer 2016 were chosen based on field-monitored incident solar radiation. The daytime and nighttime mean temperatures of the sampled days were evaluated statistically by t -test, analysis of variance (ANOVA) and Mann-Whitney U test. The best cooling benefits were realized at external building surface at 3.49, 0.52 and 1.19 °C respectively in sunny, cloudy and rainy weather during daytime. The respective values were 0.78, 0.05 and 0.03 °C during nighttime. Sunny weather featured more substantial external air and surface cooling, but indoor warming was recorded in cloudy and rainy weather. The northeast green wall achieved greater external air and surface cooling than the northwest one respectively by 0.38 and 0.77 °C in sunny weather. However, the northwest green wall registered higher external surface cooling by 0.47 °C at night. These results suggested the importance of research-informed green wall design for effective thermal regulation in urban areas. A deep air gap between the vegetation and the exterior building wall would provide more external surface cooling. Walls receiving higher solar exposure could be prioritized for greening.

Ornamental trees bearing outstanding flowers bring notable ecosystem services to cities. Existing research often treated ornamental trees as a sub-category of urban trees. To investigate the contributions of ornamental trees, this study aimed to: (1) assess their provision of ecosystem services; and (2) examine their ecological composition and diversity. Comprehensive tree survey in 93 public housing estates in Hong Kong identified 12,516 ornamental trees. To characterise the ability to provide ecosystem services, an index termed serviceable tree volume was devised. This innovative quantity aided the evaluation of ornamental trees by visual amenities, olfactory trait and ecological benefits while considering the compromise due to structural defects. The comprehensive tree survey showed a significant tendency of planting exotics for ornamental purposes. Most ornamentals could potentially deliver high-quality ecosystem services which were unfortunately restrained by prevailing very poor health conditions particularly attributed to structural defects on major branches. By relative importance value, several species dominated the ornamental tree inventory, namely Bauhinia ‘Blakeana’, Delonix regia and Bombax ceiba. By incorporating serviceable tree volume in calculating relative importance value, the importance ranking of ornamental species was re-ordered. Using tree stem as the unit of analysis, Shannon-Wiener Diversity, Reciprocal Simpson and Pielou’s Evenness of the ornamental tree stock stood at 3.08, 14.73 and 0.74 respectively, and they were reduced when the three indices incorporated serviceable tree volume. Diverse native ornamentals could be planted extensively in the urban forest in tandem with the preservation of important exotic species.

Global climate change and urban heat island effect have jointly threatened human thermal comfort in cities. Intensive green roof (IGR) could supplement urban green space to alleviate thermal comfort. Aiming at examining the impact of IGR on the thermal comfort of its users, Universal Thermal Climate Index (UTCI) and physiological equivalent temperature (PET) were applied to assess effectiveness of IGR in improving outdoor thermal comfort by IGR. Comprehensive microclimatic monitoring was conducted on urban woodland IGR and an adjacent control bare roof in humid-subtropical Hong Kong. Sunny days (n = 21) and cloudy days (n = 18) were selected from summer in 2016. Insolation, air, and surface temperatures were compared between the two roofs. UTCI and PET were computed for the daytime and nighttime period of each sampled day. On IGR, significantly higher environmental cooling was achieved during daytime in sunny weather, with mean surface and air cooling reaching 4.9 °C and 1.6 °C respectively. Woodland IGR successfully improved thermal sensation by cooling mean UTCI and PET by 5.5 °C and 10.9 °C respectively during daytime in sunny scenario. Woodland IGR reduced the occurrence of extreme and very strong heat stress which struck the control bare roof. Recommendations on about thermal sensation were provided for building owners, town planners, and landscape architects.

Socio-economically underprivileged urban communities might suffer from restricted access to urban forests. Environmental injustice research on urban greenery in Asian cities is lacking. Public housing estates in Hong Kong, accommodating low-income households and over half of the 7.45 million population, were investigated for injustice in tree provision. Two clustering schemes used socio-economic and ecological characteristics to classify 93 estates. Factor analysis of 14 socio-economic variables identified four factors related to deprivation, namely ageing population, overcrowding, working poor with high academic qualifications, and marginalised language minorities. Principal component analysis of six ecological indices returned two components related to tree density and diversity. Estates with ageing population and more language minorities had the least urban tree provision indicated by low tree density. However, estates beset by overcrowding and working poor with high academic qualifications had the highest tree diversity indicated by the highest species evenness. Estates with the lowest language minorities had the densest but the least diverse tree stands. An alternative clustering scheme delineating ecological clusters substantiated the observed urban-tree patterns with respect to socio-economic characteristics. Estates with higher tree density and diversity had higher population density indicating overcrowding. Estates with lower tree density and diversity had more ageing population language minorities. The results highlighted the need to modify tree provision in response to changing estate demographics. From the research findings, five strategies were developed to adjust the amount and composition of the estate tree stocks to achieve greater social inclusion.