In 2003, Hong Kong was hit by severe acute respiratory syndrome (SARS) from which many people died. The Hong Kong Government subsequently set up a Governmental Team Clean Committee to investigate ...possible urban design policies. Team Clean charged the task to the Planning Department, HKSAR. In 2003, it initiated a study titled: “Feasibility Study for Establishment of Air Ventilation Assessment (AVA) System.” A number of focused studies were conducted. It eventually led to the technical methodology and guidelines of the air ventilation assessment (AVA) system. Unlike many countries with guidelines for dealing with gust wind problems, AVA is a guideline for weak wind conditions specifically designed to deal with congested urban conditions. The AVA system basically establishes a method for project developers to objectively assess their designs. In 2006, the government of Hong Kong officially adopted the system and required all major publicly funded development projects to undertake the assessment. The scientific and implementation processes leading to the AVA system are reported in this paper.
► A comprehensive review of urban outdoor comfort and outdoor space use is presented. ► An introduction of the most widely used outdoor thermal comfort assessment methods is presented. ► Future ...directions of development of planning-support tools are discussed.
Outdoor spaces are important to sustainable cities because they accommodate pedestrian traffic and outdoor activities, and contribute greatly to urban livability and vitality. In the global context of climate change, outdoor spaces that provide a pleasurable thermal comfort experience for pedestrians effectively improve the quality of urban living. The influence of thermal comfort on outdoor activities is a complex issue comprising both climatic and behavioral aspects; however, current investigations lack a general framework for assessment. This paper presents a review of research over the past decade on the behavioral aspects of outdoor thermal comfort. The article focuses on perceptions of outdoor thermal comfort and the use of outdoor space in the context of urban planning. We further discuss a general framework for assessing outdoor thermal comfort based on behavioral aspects and the need for predicting tools in the design and planning of outdoor thermal comfort.
Urban climatic map studies: a review Ren, Chao; Ng, Edward Yan‐yung; Katzschner, Lutz
International journal of climatology,
December 2011, Volume:
31, Issue:
15
Journal Article
In sub-tropical city of Hong Kong where summer is hot and humid, the usage of outdoor spaces is often hindered due to thermal discomfort. In order to improve the thermal comfort of the outdoor ...environment and to make urban outdoor spaces delightful places for people to use and enjoy, better understanding of the thermal perception of people is needed. This paper presents the results of an outdoor thermal comfort study conducted in Hong Kong. The findings of the study reveal that air temperature, wind speed and solar radiation intensity are influential factors in determining the thermal sensation of people outdoor. Based on our analysis, the neutral physiological equivalent temperature (PET) in summer in Hong Kong is around 28 degree C. Under shaded condition, a wind speed of 0.9a1.3 m/s is needed for a person in light clothing to achieve neutral thermal sensation in an urban environment. This understanding can contribute towards establishing wind performance standards for thermal comfort in hot and humid tropical and sub-tropical cities.
Greening is a useful mitigation strategy for planners mainly from a visual perspective. For high-density urban living environment such as Hong Kong, urban greening helps cooling the air and providing ...shade; it also helps lowering building energy consumption by providing a better outdoor boundary condition. Many researchers have also suggested that greening may be employed as a strategy for combating the ill effects of urban Heat Island (UHI). Working towards a set of better greening guidelines for urban planners, the current paper first provides a comprehensive review of planning with urban greening. It then describes parametric studies that have been conducted to investigate the preferred location, amount, and types of vegetation for urban planning. The parametric studies employed the numerical model ENVI-met, verified using field measurements, to simulate 33 cases with different combinations of factors. For benefiting urban activities, ambient air temperatures at the pedestrian level are compared among different greening strategies and building heights. For a city such as Hong Kong, which has a high building-height-to-street-width (
H/
W) ratio, the present study reveals that roof greening is ineffective for human thermal comfort near the ground. Trees are also suggested to be more effective than grass surfaces in cooling pedestrian areas. The amount of tree planting needed to lower pedestrians level air temperature by around 1
°C is approximately 33% of the urban area. The present study allows urban planners to identify more precisely the greening principles, amount and policies necessary for better urban living environment in high-density cities.
►Greening policies in the sub-tropical climate are reviewed. ►The simulation tool of ENVI-met is verified with field measurements to be applied in the study. ►Parametric studies are conducted to investigate the effect of greening on temperature reduction. ►Planning implications are derived from the simulation study.
Shape-edged buildings impose large frictional drag on the flow in the urban boundary layer. In the sub-tropics, especially during hot and humid summers, compact building blocks create stagnant air ...that worsens outdoor urban thermal comfort. The current study adapts the κ-ω SST turbulence model to simulate air flow in urban areas. The accuracy of the κ-ω SST turbulence model in detecting air flow around a rectangular block is validated by comparing it with the data from the wind tunnel experiment. In the computational parametric study, wind speed classification is derived based on Physiological Equivalent Temperature (PET) to evaluate the effect of wind speed on outdoor thermal comfort. Numerical analysis compares the effects of different building morphology modifications on pedestrian-level natural ventilation. Critical design issues are also identified. From both the accuracy and practical points of view, the current study allows city planners and architects to improve building porosity efficiently for better pedestrian-level urban ventilation, without losing land use efficacy.
•Morphology-based tree planning methods are evaluated by measurement and simulation.•Cooling on road surface is 18°C and 15°C under high and low SVF, respectively.•Comfortable Tmrt (34°C) is provided ...by trees in heavily built areas with low SVF.•Combining trees with wind path doubles cooling in temperature and sensible heat.
Hong Kong suffers from an intense urban heat island (UHI) effect of up to 4°C as a result of compact urban form and highly urbanized land cover. Enhancing the cooling efficiency of urban greenery is essential for improving the microclimate in high-density cities. This paper aims to delineate design strategies for urban greenery to maximize thermal benefits and mitigate the daytime UHI effect. Two site-specific design strategies for tree planting in the urban environment are proposed. The sky view factor (SVF)-based design approach and the wind-path design approach are evaluated in the neighbourhood scale in two climate-sensitive areas with different urban morphologies. Observed data and simulation results indicated that the cooling effect of urban trees is highly associated with SVF. Air temperature reduction (a 1.5°C reduction) is the most profound for the high SVF scenario, whereas substantial radiation shading (Tmrt reduced to 34°C) is detected in areas with medium-low SVFs. The modelling study also showed that the cooling of air temperature and sensible heat were twice as high for vegetation arranged in wind corridors than those for leeward areas. The study demonstrated that tree planting in conjunction with proper planning is an effective measure to mitigate daytime UHI.
The urban heat island (UHI) effect exacerbates the adverse impact of heat on human health. However, while the UHI effect is further intensified during extreme heat events, prior studies have rarely ...mapped the UHI effect during extreme heat events to assess its direct temperature impact on mortality. This study examined the UHI effect during extreme heat and non-extreme heat scenarios and compared their temperature-mortality associations in Hong Kong from 2010 to 2019. Four urban heat island degree hour (UHIdh) scenarios were mapped onto Hong Kong's tertiary planning units and classified into three levels (Low, Moderate, and High). We assessed the association between temperature and non-external mortality of populations living in each UHIdh level for the extreme heat/non-extreme heat scenarios during the 2010–2019 hot seasons. Our results showed substantial differences between the temperature-mortality associations in the three levels under the UHIdh extreme heat scenario (UHIdh_EH). While there was no evidence of increased mortality in Low UHIdh_EH areas, the mortality risk in Moderate and High UHIdh_EH areas were significantly increased during periods of hot temperature, with the High UHIdh_EH areas displaying almost double the risk (RR: 1.08, 95%CI: 1.03, 1.14 vs. RR: 1.05, 95 % CI: 1.01, 1.09). However, other non-extreme heat UHI scenarios did not demonstrate as prominent of a difference. When stratified by age, the heat effects were found in Moderate and High UHIdh_EH among the elderly aged 75 and above. Our study found a difference in the temperature-mortality associations based on UHI intensity and potential heat vulnerability of populations during extreme heat events. Preventive measures should be taken to mitigate heat especially in urban areas with high UHI intensity during extreme heat events, with particular attention and support for those prone to heat vulnerability, such as the elderly and poorer populations.
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•We examined temperature-mortality associations under different UHI scenarios.•Under extreme heat (EH) scenario, high UHI areas were in more suburban “new towns”.•Mortality risk was doubled in high UHI areas (vs. moderate) under the EH scenario.•Other scenarios found no contrast in mortality between high vs. moderate UHI areas.•When stratified by age, temperature-mortality risk was mainly found in elderly 75+.
Urban greenery, especially trees, has been proven to be one of the effective measures for urban heat island mitigation. However, no consistent findings have been found for the relationship between ...greenery abundance and cooling magnitude; while some previous studies discovered a linear relationship, others opined it could a non-linear one. In addition, there are rare studies exploring whether or not the strength of the relationship is dependent on urban density. Therefore, in this study, we aim to discuss cooling efficiency by measuring the relationship between greenery coverage ratio and the cooling effects of greenery. Parametric studies were conducted in a validated ENVI-met model, with different combinations of urban densities (Low, Mid, and High) and tree coverage ratios (2–30% at 2% interval, and 56% for full area coverage other than building). Then the pattern of the cooling efficiency was explored under three urban densities scenarios and two selected temporal periods on a typical summer daytime. For a subtropical climate background, results showed a non-linear (logarithmic) pattern for tree coverage ratio (TCR) and cooling effects, irrespective of the urban density, temporal periods, and heat indicators. When TCR reached 20–30%, the optimal cooling efficiency of trees were achieved, irrespective of building densities and temporal periods. The optimal threshold for greenery coverage in this study can provide science-based suggestions to urban planners and designers for better microclimate and thermal comfort environments at the neighborhood scale.
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•Relationship between tree coverage and its cooling efficiency was established.•The cooling efficiency of tree coverage under different urban densities were studied.•Non-linear pattern between tree coverage ratios and cooling effects was revealed.•The optimal cooling efficiency could be achieved by 20–30% tree coverage ratio.•The implication of cooling efficiency pattern for urban planning was discussed.
The estimation of the vertical components of built-up areas from free Digital Elevation Model (DEM) global data filtered by multi-scale convolutional, morphological and textural transforms are ...generalized at the spatial resolution of 250 meters using linear least-squares regression techniques. Six test cases were selected: Hong Kong, London, New York, San Francisco, Sao Paulo, and Toronto. Five global DEM and two DEM composites are evaluated in terms of 60 combinations of linear, morphological and textural filtering and different generalization techniques. Four generalized vertical components estimates of built-up areas are introduced: the Average Gross Building Height (AGBH), the Average Net Building Height (ANBH), the Standard Deviation of Gross Building Height (SGBH), and the Standard Deviation of Net Building Height (SNBH). The study shows that the best estimation of the net GVC of built-up areas given by the ANBH and SNBH, always contains a greater error than their corresponding gross GVC estimation given by the AGBH and SGBH, both in terms of mean and standard deviation. Among the sources evaluated in this study, the best DEM source for estimating the GVC of built-up areas with univariate linear regression techniques is a composite of the 1-arcsec Shuttle Radar Topography Mission (SRTM30) and the Advanced Land Observing Satellite (ALOS) World 3D-30 m (AW3D30) using the union operator (CMP_SRTM30-AW3D30_U). A multivariate linear model was developed using 16 satellite features extracted from the CMP_SRTM30-AW3D30_U enriched by other land cover sources, to estimate the gross GVC. A RMSE of 2.40 m and 3.25 m was obtained for the AGBH and the SGBH, respectively. A similar multivariate linear model was developed to estimate the net GVC. A RMSE of 6.63 m and 4.38 m was obtained for the ANBH and the SNBH, respectively. The main limiting factors on the use of the available global DEMs for estimating the GVC of built-up areas are two. First, the horizontal resolution of these sources (circa 30 and 90 meters) corresponds to a sampling size that is larger than the expected average horizontal size of built-up structures as detected from nadir-angle Earth Observation (EO) data, producing more reliable estimates for gross vertical components than for net vertical component of built-up areas. Second, post-production processing targeting Digital Terrain Model specifications may purposely filter out the information on the vertical component of built-up areas that are contained in the global DEMs. Under the limitations of the study presented here, these results show a potential for using global DEM sources in order to derive statistically generalized parameters describing the vertical characteristics of built-up areas, at the scale of 250x250 meters. However, estimates need to be evaluated in terms of the specific requirements of target applications such as spatial population modelling, urban morphology, climate studies and so on.
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