Impacts of urbanisation on the thermal behaviour of new built up environments: A scoping study of the urban heat island in Bahrain

https://doi.org/10.1016/j.landurbplan.2013.01.013Get rights and content

Abstract

The recent process of urbanisation in the Gulf Cooperation Council Countries (GCCC), including Bahrain, can increase ambient and surface temperatures in newly developed built up areas – the urban heat island (UHI) effect. This study investigates urban expansion in Bahrain over the last few decades and assesses its impact on atmospheric urban heat islands using remote sensing, advanced statistics of weather data and geographical information system (GIS). The study examines how the thermal behaviour of new urban areas is affected by urban elements, with two district models being considered. Numerical modelling is used to assess wind flow, temperatures and heat distribution fluxes within the two models. The assessment shows that mean air temperatures increase within the range of 2 °C–5 °C. The magnitude of the UHI is mainly reinforced by urban activity such as on-going construction processes, shrinkage of green areas and sea reclamation. A comparison of results between the two studied districts reveals that the mean temperature is increased by 2 °C–3 °C in new artificial islands, and by 3 °C–5 °C in deserted urban lands with hot arid conditions. The distribution of urban radiant heat islands is varied and depends largely on the type of land cover and urban surface. Urban grids and structure have a significant impact on the thermal behaviour of newly developed built up areas. Human comfort is improved in the presence of water bodies and green areas.

Highlights

► Impacts of urbanisation on the thermal behaviour of new built up areas. ► The appearance of UHI is reinforced by massive urban construction and sea reclamation. ► Air temperatures are increased by 2–3 °C in artificial islands, and by 3–5 °C in inlands. ► Urban network grids play a significant role in the variation of UHI. ► Radiant heat islands are reduced and comfort is improved in the presence of water and vegetation.

Introduction

With the current process of urbanisation in the Gulf Cooperation Council Countries (GCCC), significant changes occur in their landscape (Lavieren et al., 2011). Such a process can lead to an increase in the temperatures of urban regions compared to their rural surroundings, forming ‘islands’ of higher temperatures. Within this context, some sources have shown that the centres of metropolitan cities in hot climates similar to those of the GCCC experience elevated temperatures within the range of 2 °C–4 °C, when compared to their rural surroundings (Wong & Jusuf, 2008). This difference is what constitutes the urban heat island (UHI) effect, which is normally evident on built environment surfaces and in the atmosphere. The increase in surface temperature impacts upon the intensity of local and downwind ambient air temperatures, especially those close to the surface, because of various convective heat fluxes from the surface (EPA, 2012a). Many factors contribute to the development of UHI. Some are related to natural factors such as weather and location. Sato, Murakami, Ooka, and Yoshida (2008), for example, showed a reduction in atmospheric UHI due to the availability of a sea breeze. Other factors are related to human activity, such as the reduction of vegetation and water bodies, urban geometry and materials, and anthropogenic heat emissions. Recent studies in the development of UHI due to urban activities have been reviewed by Wong, Jusuf, et al. (2011). Saleh (2011), for instance, evaluated the impact of urban growth in Baghdad city on the surface UHI and recommended the integration of remote sensing and geographical information system (GIS). Hung, Daisuke, Shiro, & Yoshifumi (2006) used remote sensing for the comparative assessment of UHI in 18 mega cities in both temperate and tropical climate regions and analysed the development of UHI in association with urban vegetation covers and surface energy fluxes.

Many studies have analysed urban activities that contribute to the development of UHI. Giannopoulou et al. (2011) carried out a detailed statistical analysis of UHI characteristics in Athens and concluded that the appearance of high air temperatures was reinforced by increased urbanisation and industrialisation coupled with the increased anthropogenic heat flows and the lack of vegetation. Urban elements play a major role in the development of UHI. Some studies (Jusuf, Wong, Hagen, Anggoro, & Hong, 2007) showed the various impacts of land use on urban temperature. In the daytime an industrial area was found to have the highest surface temperature, followed by a commercial location, airport, residential and park areas in descending order. At night, however, the order was commercial, residential, park, industrial and airport. Other studies, such as that of Li et al. (2011), investigated how landscape composition and configuration would impact upon the UHI in the metropolitan centres. Others have studied the impact of urban surfaces. Lopes, Saraiva, and Alcoforado (2011), for instance, examined the impact of surfaces and aerodynamic roughness and concluded that an increase in roughness would cause a 40% reduction of the wind speed. Within this context, Huang, Taniguchi, Yamano, and Wang (2009) analysed the surface air temperature records as well as bore-hole subsurface temperature records and recommended a combination of heat conduction in the subsurface and heat convection of the air in order to estimate the full extent of urban heat island impacts on the environment.

The development of UHI influences the microclimate, thermal conditions and the quality of human life as can be seen in the increased energy demand for cooling buildings, elevated greenhouse gas emissions and compromised human comfort. Lin et al. (2008) showed the influence of UHI on the climatic elements, including thunderstorms. This study found that the heat island impact could perturb thermal and dynamic processes and hence impact upon the location of thunderstorms and precipitation over tropical areas such as Taiwan's western plain.

Changes in the climatic elements, particularly air temperatures, can influence building energy consumption. Flor and Dominguez (2004) showed that energy consumption was related to solar loads, wind flow patterns and the external air temperature. So, improvements in urban microclimate have direct and indirect consequences for energy savings. Strømann-Andersen and Sattrup (2011) examined how the energy performance of low energy buildings was affected by the local context and found that the geometry of urban canyons influenced the total energy consumption by up to 19% for housing and 30% for offices. In Tokyo metropolis, Huang, Ooka, and Kato (2005) investigated the actual status of an urban thermal environment in a complex urban area covering a large district heating and cooling system, and Kikegawa, Genchi, Kondo, and Hanaki (2006) quantified the possible impacts of UHI counter measures upon buildings energy consumption during the summer and suggested two measures to reduce the cooling energy, namely a reduction in the air-conditioning anthropogenic heat generation and an increase in the green cover fraction. Simultaneously, changes in urban temperatures impact upon the thermal behaviour of new built up areas. Robitu, Musy, Inard, and Groleau (2006) assessed this impact by examining the role of vegetation and water bodies. Fahmy and Sharples (2009) examined how urban form can be designed to act as a passive thermal comfort system. Wong, Nicol, and Ng (2011) assessed the blockage impact of buildings on the urban temperature within dense districts and stated that removing some coastal buildings was essential to increase air ventilation in urban areas inland. In their studies to investigate the cooling performance in traditional and modern urban contexts in Dubai. Al-Sallal and Al-Rais, 2011, Al-Sallal and Al-Rais, 2012 found that narrow street canyons in modern urban patterns could accelerate the wind speed passing through them, resulting in a better passive cooling performance.

Technically, the UHI effect is caused by urbanisation when buildings, roads, and other infrastructure elements replace open land, water areas or the sea. This situation is very obvious in the GCCC where the current demand for housing and infrastructure is increasing. This demand is leading to the changing of the landscape, from vegetation, sand and water to hard surface building blocks. Exposing these blocks to the direct sun increases the temperature of their surfaces and the atmosphere, and consequently impacts upon the regional weather, energy consumption and thermal comfort through the modification of climatic variables. However, there are limited studies assessing this impact on the new urban regions of the GCCC. Bahrain is an interesting place to study the development of UHI and its implications since it is a small island state in the Gulf region where there is only a minor distinction between urban and rural lands. Analysis of the urban expansion in Bahrain is suitable as an indicative study of the trend of increased urban temperatures in the GCCC.

This work, therefore, aims at assessing the impact of urbanisation on the thermal behaviour of new built up areas in Bahrain. The main focus is placed on the summer period with the following objectives:

  • To assess the role of urban expansion in the development of the atmospheric UHI within the canopy layer.

  • To examine how the summer thermal behaviour in new built up areas is affected by different urban elements.

The outcome of this work presents recommendations for urban designers and city planners concerning the improvement of thermal behaviour with minimum energy consumption. The first and most logical step is a brief to Bahrain.

As depicted in Fig. 1, Bahrain is a small archipelago, comprising of a group of 40 islands. Recent statistics indicate that the land area consists of 735 km2 or about 3.5 times the size of Washington, DC (Fowler, Stephens, Santiago, & Bruin, 2006). It is located in the Persian Gulf (also known as the Arabian Gulf) and hereafter is referred to as the ‘Gulf region’, with a geographic location extending from 50° 20′E to 50° 20′E in longitude, and from 25°32′N to 26°26′N in latitude. Bahrain's climate can be described as moderate in winter and extremely hot in summer. The general characteristics of this climate resemble those of arid and semi-arid zones (Elagib & Abdu, 1997). Temperatures are variable but high, and relative humidity is also high, particularly in the rainy seasons. Rainfall is low, irregular, seasonal and variable. Fig. 2 shows a brief overview the climatic elements in Bahrain. The analysis shows an overall yearly average ambient temperature of 26.5 °C with a monthly average maximum temperature of 38.0 °C in August and a monthly average minimum temperature of 14.5 °C in January. The monthly average of humidity is 60%, with a maximum average of 70% and a minimum average of 50%. Wind from the North-East direction over the year is a characteristic of Bahrain. Wind speeds are generally low from April to December, with an average of 4.2 m/s, while they are well above 5.1 m/s from January to March, reaching a monthly average of 5.2 m/s in February. Bahrain is blessed with a high level of solar radiation. The highest averages of total and direct radiation are 585 W/m2 and 383 W/m2 in June and August.

The aforementioned analysis gives a general picture of the climatic elements measured at the Bahrain international airport, which is quite different from local conditions in many sites. This work assesses variations in the microclimate due to the urbanisation process, and examines how such a process impacts upon the summer thermal behaviour of urban regions.

Section snippets

Assessment methodology

As introduced in Section 1, there are two types of UHI, namely, atmospheric and surface UHI. Different methods have been reported (EPA, 2012b) to identify the two types, including direct and indirect methods, numerical modelling, and estimates based on empirical models. This work uses direct methods and numerical modelling to assess the Bahraini UHI. The assessment passed through two main steps: first, assessing the impact of urbanisation on the atmospheric UHI within the canopy layer, and

Result and discussion

This work first assesses the impact of urban expansion on the UHI. It then examines the variations in the thermal behaviour of new built up areas due to various urban elements and, finally, numerical simulations for actual complex urban areas, covering two mixed-use districts, are undertaken.

Conclusion and future work

This work performed a two step assessment: first, it assessed the role of urban expansion in the development of atmospheric UHI, and second, it examined the impact of various urban elements on the thermal behaviour of new built up areas. For the purpose of the first step, a in depth statistical analysis of atmospheric UHI characteristics was carried out using detailed temperature data from 14 sites (Table 2) distributed all over Bahrain. The results indicated that the recent process of

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