Sky view factor analysis of street canyons and its implications for daytime intra-urban air temperature differentials in high-rise, high-density urban areas of Hong Kong: a GIS-based simulation approach

Mitigating high air temperatures and heat waves is vital for decreasing air pollution and protecting public health. To improve understanding of microscale urban air temperature variation, this paper performed measurements of air temperature and relative humidity in a field of Wuhan City in the afternoon of hot summer days, and used path analysis and genetic support vector regression (SVR) to quantify the independent influences of land cover and humidity on air temperature variation. The path analysis shows that most effect of the land cover is mediated through relative humidity difference, more than four times as much as the direct effect, and that the direct effect of relative humidity difference is nearly six times that of land cover, even larger than the total effect of the land cover. The SVR simulation illustrates that land cover and relative humidity independently contribute 16.3% and 83.7%, on average, to the rise of the air temperature over the land without vegetation in the study site. An alternative strategy of increasing the humidity artificially is proposed to reduce high air temperatures in urban areas. The study would provide scientific support for the regulation of the microclimate and the mitigation of the high air temperature in urban areas.

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Review of methods used to estimate the sky view factor in urban street canyons

Building and Environment ◽

10.1016/j.buildenv.2019.106497 ◽

2020 ◽

Vol 168 ◽

pp. 106497 ◽

Cited By ~ 8

Author(s):

Chunping Miao ◽

Shuai Yu ◽

Yuanman Hu ◽

Huiwen Zhang ◽

Xingyuan He ◽

...

Keyword(s):

View Factor ◽

Street Canyons ◽

Urban Street ◽

Sky View Factor ◽

Urban Street Canyons

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Quantifying the Effect of Different Urban Planning Strategies on Heat Stress for Current and Future Climates in the Agglomeration of The Hague (The Netherlands)

Atmosphere ◽

10.3390/atmos9090353 ◽

2018 ◽

Vol 9 (9) ◽

pp. 353 ◽

Cited By ~ 4

Author(s):

Sytse Koopmans ◽

Reinder Ronda ◽

Gert-Jan Steeneveld ◽

Albert Holtslag ◽

Albert Klein Tank

Keyword(s):

Heat Stress ◽

Urban Planning ◽

View Factor ◽

Climate Scenarios ◽

High Rise ◽

Vegetation Fraction ◽

Sky View Factor ◽

Green Areas ◽

Planning Strategies ◽

The Hague

In the Netherlands, there will be an urgent need for additional housing by the year 2040, which mainly has to be realized within the existing built environment rather than in the spatial extension of cities. In this data-driven study, we investigated the effects of different urban planning strategies on heat stress for the current climate and future climate scenarios (year 2050) for the urban agglomeration of The Hague. Heat stress is here expressed as the number of days exceeding minimum temperatures of 20 °C in a year. Thereto, we applied a diagnostic equation to determine the daily maximum urban heat island based on routine meteorological observations and straightforward urban morphological properties including the sky-view factor and the vegetation fraction. Moreover, we utilized the Royal Netherlands Meteorological Institute’s (KNMI) climate scenarios to transform present-day meteorological hourly time series into the future time series. The urban planning strategies differ in replacing low- and mid-rise buildings with high-rise buildings (which reduces the sky-view factor), and constructing buildings on green areas (which reduces the vegetation fraction). We found that, in most cases, the vegetation fraction is a more critical parameter than the sky-view factor to minimize the extra heat stress incurred when densifying the neighbourhood. This means that an urban planning strategy consisting of high-rise buildings and preserved green areas is often the best solution. Still, climate change will have a larger impact on heat stress for the year 2050 than the imposed urban densification.

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Applications of Extensive Green-roof Systems in Contributing to Sustainable Development in Densely Populated Cities: a Hong Kong Study

Construction Economics and Building ◽

10.5130/ajceb.v11i1.1751 ◽

2011 ◽

Vol 11 (1) ◽

pp. 15-25 ◽

Cited By ~ 6

Author(s):

Vivian W. Y. Tam ◽

Xiaoling Zhang ◽

Winnie Lee ◽

LY Shen

Keyword(s):

Hong Kong ◽

Air Temperature ◽

Current Practice ◽

Green Roof ◽

Green Roofs ◽

Existing Building ◽

Extensive Green Roof ◽

High Rise ◽

Urban Greenery ◽

Extensive Green Roofs

Developed cities such as Hong Kong are usually densely populated. Since the land is limited, high-rise buildings are constructed. When the building height becomes higher, air flow is reduced and heat is trapped among high-rise buildings. Air temperature will be greatly increased and air pollution becomes a serious problem. This creates a walled building problem. To reduce air temperature caused by the wall-effects, various methods have been developed in the previous studies. One typical method is the use of green roof systems. The application of extensive green roofs on the existing building rooftops has been recommended in Hong Kong since 2001. The advantage of this practice is that no additional floor area is required and it can also improve urban greenery. Although a green roof system has been introduced and adopted in Hong Kong since 2001, the emphasis is mainly given to the application of intensive green roofs for podium garden instead of extensive green roofs. It is considered valuable and necessary of the extensive green roofs for the buildings. This paper investigates the current practice of using extensive green roofs in Hong Kong. The constraints in applying extensive green roofs are investigated, which leads to studying the solutions for mitigating these constraints and improving the future development of the implementation.

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Thermal and Geometric Controls on the Rate of Surface Air Temperature Changes in a Medium-Sized, Midlatitude City

Journal of Applied Meteorology and Climatology ◽

10.1175/jam2486.1 ◽

2007 ◽

Vol 46 (2) ◽

pp. 241-247 ◽

Cited By ~ 5

Author(s):

Tomohiko Tomita ◽

Hiroyuki Kusaka ◽

Ryo Akiyoshi ◽

Yoshiyuki Imasato

Keyword(s):

Time Dependence ◽

Air Temperature ◽

Urban Areas ◽

Mesoscale Model ◽

Thermal Environment ◽

Surface Air Temperature ◽

Anthropogenic Heat ◽

View Factor ◽

Temperature Changes ◽

The Impact

Abstract Gradual cooling in the evening forms a wintertime nocturnal urban heat island. This work, with a mesoscale model involving urban canopy physics, is an examination of how four thermal and geometric controls—anthropogenic heat QF, heat capacity C, thermal conductivity k, and sky-view factor ψs—modify the rate of surface air temperature changes ΔT/Δt. In particular, the time dependence is diagnosed through numerical experiments. The controls QF and k are major agents in the evening, when QF changes the evening ΔT/Δt linearly and k is logarithmic. The effects of C and ψs are large in the morning and in the afternoon with those of k. The impact of QF is, however, substantial only in the evening. Because the time dependence of C and k is different, the thermal inertia used as a parameter in the urban climate studies should be divided into two parameters: C and k. To improve the thermal environment in urban areas, the modification of QF and k could be effective.

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