Editorial: Urban Heat Island (UHI) and its Mitigation through Urban Planning, Design, and Landscaping

The most significant thermal phenomenon of a city in relation to the climate is urban heat island (UHI). The UHI intensity is the temperature difference between an urban and a rural site under the same climate conditions. Air temperatures in the densely built area are higher compared to the ones in the surrounding sub-urban areas due to positive urban thermal balance.

To date, UHI is one of the most studied phenomenon relative to increased urbanization. Results have shown that cities and towns are warmer at night than rural areas due to the absorption of solar radiation by the urban pavements and buildings. According to United Nations report, almost 54 per cent of the world’s population lives in urban areas, a proportion that is expected to increase to 66 per cent by 2050. Projections show that urbanization could add another 2.5 billion people to urban populations by 2050, with close to 90 percent of the increase concentrated in Asia and Africa. This trend will most likely further deteriorate the built environment if city planning goes on without environmental consideration.

There are several factors influencing the UHI intensity: urban characteristics (size and population of the city, the local meteorological features, the topography, the type of urban materials and the presence (or lack) of green areas. From the planning aspect, the significance of urban morphology factor is paramount toward the UHI impact. Parameters such as urban plan area density, geometry of the buildings and topographical features influence airflows in and around buildings and energy consumption on a regional scale. While the role of planners, architects, and policy makers have become more important, the UHI mitigation strategies will be especially imperative in the future scenario of climate change andadaptation.

Back in 2016, the Fourth International Conference on Countermeasures to Urban Heat Island (IC2UHI), held in the National University of Singapore on 30-31 May and 1 June 2016, was held to highlight the growing attention of UHI and its impact on urban dwellers. Furthermore, the conference also brought mitigation strategies into the discussion, which came through various field: urban planning, design, and landscaping.

To celebrate this achievement, Architectural Science Review publish a Special Edition with selected papers from the conference which address several subject areas including urban microclimate and thermal comfort, UHI mitigation and urban landscaping.

The first paper by Tan et. al presents an overview of the inter relationship between thermal sensation and its immediate environment in high-density cities. Although the thermal comfort issue has been raised by many other researchers, Tan explores the thermal comfort matter from design perspectives, particularly in outdoor urban space, using Hong Kong as its case study for subtropical high-density area. As subjective perceptions are paramount to determine the thermal perception; apart from familiar weather parameters, Tan also stated that these perceptions are moderated by seemingly unrelated parameters, such as the acoustic environment, air quality and appreciation of urban landscape.

To complement this, Wang and Png also venture into thermal comfort discussion by looking into solar irradiance criterion. They develop a method where solar irradiance is simulated to assess thermal comfort in residential areas in Singapore., Wang and Png investigated the relationship between solar irradiance and surface parameter; using the latter as the proxy to thermal comfort in urban areas. Simulation was conducted with parametric approach of four neighbourhood with various designs, where the iteration impacts on thermal comfort were investigated.

The next two papers observe the urban geometry aspect and its relationship with heat island, energy, and thermal comfort. Lee and Levermore look into sky view (SVF) and sunshine factor (SF) of Manchester and Ulsan, while Takkanon and Chantarangul investigate height-to-width (H/W) ratios of street canyon and greenery presence in Bangkok. SVF and SF approach, with fish-eye image method, are used to calculate hourly diffuse irradiation and hourly beam irradiation for annual surface irradiation according to the geometry and buildings. On the other hand, H/W ratio has an impact on elevating ambient temperatures, especially in medium ratios where insufficient shading and ventilation occur.

From urban landscape point of view, study by Kumakura et. al look into detached house arrangement in Solar Town Fuchu, an area of 16 high performance solar houses with shared common garden path. This study investigated the potential for creating cool spots through resident behaviours, such as plant watering, and their associated outdoor environment. Findings suggest that the positional relation of houses and common garden paths can affect the lifestyles of house residents, as reflected by their behaviours, in which could improve and cultivate the living environment.

Issues on contextual aspect regarding local urban geometry characteristics affecting its urban ventilation issue is represented by the following two papers by Zhen. First, he conducted a review of urban environmental problems, research methods of wind environment in residential areas of Xi’an. The objective is to provide ideas to improve local urban ventilation, by looking at spatial shape and existing wind environment condition through field tests, wind tunnel experiments and numerical simulations. The study constructs a multiple regression model and proposes an optimal design method for the spatial shape of the ‘breathable residential area’.

Next, he conducted research review on wind environment of urban residential blocks, by exploring local policies, existing studies, and potential of implementation to improve the wind environment-related policies. The research review concluded that the cited studies indicate that the spatial configuration of urban residential blocks is closely related to their wind environments. Other aspects such as wind breezes from land and sea, mountain and valley winds, were also observed. As high-density, high-level blocks continue to be constructed in cities, the optimization and regulation of the wind environment in these blocks is a crucial factor in ensuring environmental sustainability.

Finally, to close this special issue, Zhao presents a summary of green building research through bibliometric approach, reviewing 2,980 related articles between 2000-2016. This bibliometric review aims to detect the status quo and trends of global green building research. Co-word and co-citation analyses of the 2980 bibliographic records, which were gathered from the Web of Science (WOS) database, were undertaken to examine and visualize the current state and trends of green building research. This study presents an in-depth understanding of the status quo, gaps and future agenda of green building research for both researchers and practitioners.

Special Issue Associate Editors