Urban Heat Island and Mitigation Strategies at City and Building

Advances in the Development of Cool Materials for the Built Environment, 2013, 3-32 3

Dionysia-Denia Kolokotsa, Mattheos Santamouris and Hashem Akbari (Eds) All rights reserved- 2013 Bentham Science Publishers

CHAPTER 1

Urban Heat Island and Mitigation Strategies at City and Building Level

Nyuk Hien Wong* and Steve Kardinal Jusuf

Department of Building, National University of Singapore, 4 Architecture Drive 117566, Singapore

Abstract: Extensive urbanization has resulted to economic, social, energy & environmental challenges. The global population increase led to an increasing demand for housing. Natural land has been replaced with artificial surfaces in most cities around the world with undesirable thermal effects. This, together with industrialization growth, has caused a deterioration of the urban environment. Urban heat island (UHI) phenomenon has become a common problem in many major cities worldwide. Several factors influence the urban heat island phenomenon, such as the continuous reduction of green spaces, the changes of wind velocity due to high buildings density, the anthropogenic heat release and the alteration of surfaces albedo. The aforementioned factors lead to overheating problems in cities due to the absorption of solar radiation by the various surfaces and buildings. Hence, urban climate is one of the most important elements of urban physical environment, which is often ignored in urban planning. To design a sustainable city, it is necessary to take into account the climatic conditions holistically and strategically during the planning process. Since the 1970s, German researchers have developed the concept of urban climate map (UC-Map) that has a strong focus on applied urban climatology. UC-Map is an appropriate tool for translating climatic phenomena and problems into 2-D images including symbols for land use and spatial information suitable for the urban planner. Therefore this map is a useful tool for urban planners, architects and governors in order for them to understand more accurately and evaluate the effects of urban climatic issues on decision-making and environmental control. At the micro-climate level, several UHI mitigations can be implemented to reduce the UHI severity. First is greenery. The benefits of greenery to the built environment have been widely investigated. Greenery dissipates the incoming solar radiation on the building structures through its effective shading; it reduces long-wave radiation exchange between buildings due to the low surface temperatures created by plants shading; it reduces the ambient air temperature through evapotranspiration. The role of buildings materials, mainly determined by their optical and thermal characteristics, is crucial in reducing the thermal and solar hear gains, in the urban environment. The so-called cool materials, characterized by high reflectivity and high emissivity, can improve the thermal conditions in cities by lowering the surface temperatures that affect the thermal exchanges with the surrounding air. Urban

*Address correspondence to Nyuk Hien Wong: Department of Building, National University of Singapore, 4 Architecture Drive, Singapore 117566; Tel: +65-65163423; Fax: +65-67755502; E-mail: [email protected]

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4 Cool Materials for the Built Environment Wong and Jusuf

ventilation is another important strategy of UHI mitigation. It is important to understand the nature of air flow regimes within urban canyons in order to make further progress in describing the complex interactions between mesoscale forces and the built environment that create the urban boundary layer.

Keywords: Urban heat island, mitigation strategies, urban climatic map, urban greenery, urban ventilation, cool materials.

DEFINITION OF URBAN HEAT ISLAND

The world has experienced an unprecedented urban growth in the latest centuries. In 1800, only 3% of the worlds population lived in urban areas. This percentage reached 14% and 47% in 1900 and 2000 respectively. In 2008, it was the first time in history where more than half of the worlds population was living in the urban areas. Moreover in 2003, United Nations estimated that by the year 2030, up to 5 billion people will be living in urban areas, which corresponds to 61% of the world's population [1].

Extensive urbanization has resulted in economic, social, energy & environmental challenges. The trend in global population increase leads to an increase in housing demand. Natural land has been replaced by artificial surfaces in most cities around the world with undesirable thermal effects. This issue, together with industrialization growth, has caused a considerable deterioration of the urban environment. Buildings in cities influence the urban climate in many ways [2]:

1. Softscape (trees, grass and soil) is replaced by hard surfaces (asphalt and concrete);

2. The rounded, soft shapes of trees and bushes are replace by blocky, angular buildings;

3. Anthropogenic heat from the buildings, air conditioning systems and automobiles is released;

4. Surface infiltration is prevented due to efficient disposal of rain water in drains, sewers and gutters;

5. Contaminants that create an unpleasant urban atmosphere due to the chemical reactions are emitted.

Urban Heat Island and Mitigation Strategies at City Cool Materials for the Built Environment 5

The aforementioned factors influence the amount of incoming and outgoing radiation and also affect the local wind speeds [3]. Their effects on local climate, especially on environmental temperature and ventilation, are very critical. Moreover they alter the radiative, thermal, moisture and aerodynamic properties of the environment [4], causing heat concentration in urban areas compared to its neighboring rural areas. This phenomenon is called the urban heat island (UHI) effect (see Fig. 1).UHI has become a common problem in major cities worldwide while the temperature difference between urban and rural areas can be more than 10K (Table 1).

Figure 1: Typical urban heat island profile (Source: Wong and Chen, 2009).

Table 1: UHI intensity in several countries around the world

City Intensity (K) Shanghai, China [5] Up to 8.4

Tokyo, Japan [6] 3-8

Kuala Lumpur, Malaysia [7] 4-6.5

Singapore [8] 4

Newark, USA [9] 3

London, UK [10] 8 (max)

Lodz, Poland [11] 12 (max)


The severity of the UHI effect is influenced by some important factors [12-14]:

1. Canyon geometry: Urban canyons work as traps that decrease the loss of both short-wave and long-wave radiation. This is attributed to the complex exchange between buildings and the skyline screening. Urban canyons also decrease the effective albedo of the overall area due to multiple reflections of short-wave radiation by the canyon surfaces.

2. Building materials: Materials, such as concrete and brick, have large heat capacity and more sensible heat can be stored during daytime. The stored heat is then released back to the environment at night.

3. Greenhouse effect: The polluted urban atmosphere traps the long-wave radiation that is emitted by the ground to the sky.

4. Anthropogenic heat source is generated by the industrial combustion, traffic, air-conditioners and other human activities.

5. Evaporative cooling: Vegetation and water can mitigate the UHI effect since more incoming heat can be transformed into latent heat rather than sensible heat. Unfortunately, the lack of greenery in cities deteriorates the UHI effect.

6. Urban ventilation: Wind is able to remove heat by turbulent and convective transfer. However, such heat loss from urban streets is reduced due to the obstruction of wind flow by the urban settings.

The UHI phenomenon is exacerbated by human energy release overheating and by increasing demand of air conditioning, which leads to further heating and CO2 release [15]. Simulation studies have shown that the possible increase in peak cooling electricity load due to UHI could range from 0.5 to 3% for each 1K temperature increase [16]. Meanwhile, another study in Singapore shows that 1K reduction of urban air temperature reduces the energy consumption for cooling by 5% [17].

Many studies have been conducted around the world to mitigate the UHI in the cities. This chapter discusses several UHI mitigation measures at the city, estate and building level.


MITIGATION STRATEGIES Urban Climatic Map at City Level

Countries need to develop their cities to stimulate economic growth and promote their national development. Therefore the development of cities compared to the various neighborhood towns and villages attracts more inhabitants, leading to an aggravated need to further urban expansion and development. Due to climate change, there is a vision to develop cities to be more sustainable and healthy for their inhabitants. Urban climate, one of the elements of urban physical environment, has gained its momentum as an important aspect of the urban planning process together with economic and social aspects. Several studies acknowledged the influence of urban forms towards urban thermal comfort [18-20], urban temperature [21, 22] and urban heat island intensity [23].

Climate is an important factor of the built environment and all studies regarding urban climatic conditions are focusing on the improvement of the local climatic conditions with the simultaneous reduction of the negative microclimate effects. Two different difficulties appear in a climatic study at local level. Firstly, there is no suitable meteorological data that are available in local level since the usual meteorological measuring grid is too wide. Secondly, there is only a little or no time for the planners to make decisions and so is the available time for the meteorological investigation [24].

Germany is one of the leading countries in urban climate research. The first urban climatic study was conducted in Berlin as early as in the end of 19th century and was used by several methodologies in the later studies such as thermal imaging, temporary weather station, car transverses, vertical soundings that led to the urban climatic map (UC-Map) in the early 80s [25].

Among the various methods, UC-Map is found very useful for urban planning purposes since it integrates the urban climatic factors and urban planning considerations. Before the development of the UC-Map methodology, the integration of urban planning and meteorology was a problem in many cities, due to the different domains and aspects of knowledge. Meteorologists do not know the planning requirements that should be considered in the urban microclimatic study, while urban planners do not understand the type of climatic data that should


be provided for their planning purposes [26]. As an information and evaluation tool, the UC-Map has two components, the Urban Climatic Analysis Map (UC-AnMap) and the Urban Climatic Recommendation Map (UC-ReMap).

The UC An-Map compiles the meteorological data, land use data, building footprint information, topography and vegetation information. The influence of these data on thermal load and thermal comfort are analyzed and classified spatially into several categories [27]. The UC-AnMap is also called climatope map and synthetic climate function map, as it represents distinct local climatic conditions and defines the climatopes. Climatopes are geographic areas in the urban space that have similar microclimatic characteristics and similar relative significance towards their surroundings. Moreover the specific tool operates on a spatial scale of several dozens to hundreds meters. The characteristics are primarily distinguished by the daily thermal variation, the vertical roughness, the topographical conditions or exposure and more importantly by the type of materials [28].

Figure 2: Workflow of UC-AnMap development for Hong Kong (Source: CUHK, 2008).

In developing a city, planners are dealing with land-use changes that usually alter the aforementioned urban characteristics and subsequently create new microclimatic conditions. The modification of roughness parameter changes the


urban ventilation. An increase of roughness usually limits the air exchange rates leading to an increase of the heat generated by the city. Moreover the pollutants emitted by the near-surface sources are insufficiently dispersed. This phenomenon causes critical environmental impacts to the inhabitants. The effects of inadequate ventilation are critical during periods of extremely low prevailing winds and high radiation [29]. The interaction between urban structures and climate becomes more prominent when the city is not located in a flat terrain. The urban ventilation or wind path within the city changes according to the topographic condition. Therefore, it is not only the land use and the urban structures considered in the UC-AnMap, but also the topography and its influence on the urban and rural ventilation. Fig. 2 depicts the workflow and data required to develop UC-AnMap for the urban climatic map of Hong Kong as an illustration.

Figure 3: UC-ReMap city of Stuttgart (Source: Ministry of Economy Baden-Wuerttemberg, 2008).


Urban Climatic Recommendation Map (UC-ReMap) targets to the planning procedure. It provides strategic and city planning guidelines to improve the microclimatic conditions based on the UC-AnMap and practical constraints. Similar climatopes obtained from UC-AnMap are grouped into classification zones, where each zone is represented in different colors and has specific planning guidelines, so that the urban climatic conditions are not worsened. Hence, the collaboration between the urban climatologist and the urban planners is very important in the development of the UC-ReMap from the UC-AnMap [30]. The recommendation map can be in a form of general guidelines for urban planners. For example the Stuttgart climatic map includes a transformation of green spaces and vegetation to preserve and reclaim natural vegetation in order to improve ventilation, reduce the release of air pollutants and support fresh air provision (Fig. 3) [31]. The recommendation map may also be in a form of more rigid guidelines. For example the Kassel climatic map includes guidelines on the building coverage, the spacing between buildings and their heights as well as the percentage of greenery [32]. The implementation of the UC-ReMap involves not only the local government but also the private sectors, such as industries and businesses. The scale of the UC-Map is 1:100000 for regional analysis and 1:5000 for district analysis. It provides an overall analysis, in which, microclimatic study can be selected and conducted.

Urban Climatic Map at Estate Level

Urban climatic map at the estate level provides an analysis for more detailed climatic conditions, i.e. urban air temperature, compared to urban climatic map at city level and it usually has the scale of 1:5000 to 1:100000 with the resolution of 100m grid.

Known as temperature map, its methodology was developed based on the fact that air temperature in urban areas is closely related to land uses [33], which physically are related to the urban morphology characteristics, such as: sky view factor [34-38], greenery condition [39, 40], thermal mass of the built environment [41, 42] and building materials [43-45].

Daily minimum (Tmin), average (Tavg) and maximum (Tmax) temperatures of a specific location are calculated as the result of temperature deviation from the temperature measured at a meteorological station nearby. The deviation is mainly


due to the urban morphological characteristics, i.e. building, pavement and greenery, within the radius of 50 meters. The independent variables for the models can be categorized into:

1. Climatic predictors. Those are the daily minimum (Ref Tmin), the average (Ref Tavg) and maximum (Ref Tmax) temperatures at a reference point. Moreover the average of daily solar radiation (SOLAR) is considered. For the SOLAR predictor, the average of the daily solar radiation is used in the Tavg model, while the daily solar radiation maximum is used in the Tmax model. SOLAR predictor is not applicable for Tmin model.

2. Urban morphology predictors. Those are the percentage of pavement area over a radius of 50m, the average height to building area ratio, the total wall surface area, the Green Plot Ratio, the sky view factor and the average surface albedo.

The planners are not able to modify the overall climatic conditions, but they can modify the urban morphological conditions. With the temperature map, planners are able to analyze the impacts of their design to the environment.

As an example, temperature maps are used to analyze and predict the impact of a new master plan as compared to the existing conditions in a Singapore estate and also to study two different greenery densities of the park, named as Green Belt. The calculated maximum temperature is shown in Fig. 4. The changes of maximum air temperature distribution pattern at different master plan models (Model 1 and Model 2) are mainly due to the change of greenery and building distributions. The removal of large greenery areas and its replacement with buildings increases the average air temperature, as seen in the Vista Xchange zone. The impact of the Green Belt in Model 2 (Fig. 4 right) that has a higher greenery density as compared to Model 1 (Fig. 4 middle) seems more noticeable, creating a larger cool island at the middle of the estate.

Researchers have conducted various investigations and measurements in relation to the built environment. As a result, they have come out with various prediction models for different purposes, including impact mitigation strategies, climatic


data predictions, improved weather and air quality forecasts [46]. Nevertheless, these prediction models are too complicated for educated non-scientists, such as urban planners. In the end, they are often kept in the shelves until the scientists are engaged to do the assessment. By the time scientists finish their assessments, the planners have no time to redesign their master plans based on the scientists findings. There is a gap between scientists and planners. Furthermore, at the building design level, CAD software has been developed and integrated with some simulation software, called the Building Information Modeling (BIM). However, at urban or estate planning level, there is still no software or tool that can equip planners to design and perform assessment at the same time. These findings emphasize the need to develop a tool for planners.

Figure 4: The calculated average air temperature of current condition (Left), master plan model 1 (Middle) and master plan model 2 (Right).

The Screening Tool for Estate Environment Evaluation (STEVE) was developed with a motivation to bridge research findings, especially air temperature prediction models, and urban planners. STEVE is a web-based application that is specific to an estate and it calculates the Tmin, Tavg and Tmax of a point of interest


for the existing conditions and future conditions (proposed master plans) of a specific area or estate. The air temperature prediction models that have been briefly mentioned above are used in this application. The map of estates existing condition or future development is displayed in STEVE interface. The viewing level of the map is set into three levels. In level 1 (Fig. 5), it displays a complete estate map including the zoning boundaries, which are darkened when the mouse is pointed to the selected zone. Users are able to zoom in the map into the second view level by clicking either the selected zone or the zoom-in button (Fig. 6).

Figure 5: First viewing level of the map.

Figure 6: Second viewing level of the map.


Figure 7: Third viewing level of the map.

The designated points appear for the users selection in this viewing level and then, users are able to predict air temperatures condition by clicking the selected point. A circle with the radius of 50 meters blinks to provide indication of the urban morphological distribution that influence the air temperatures at the selected point (Figs. 7 and 8).

Figure 8: Calculator interface.


At the left hand side of the existing or proposed master plan map, the calculator interface appears with preloaded values of different parameters for the selected point (Fig. 8). The preloaded values can be changed according to the users needs and the predicted air temperature results appear with a push on the Calculate button.

URBAN GREENERY

Planting of vegetation in urban areas is one of the main strategies employed to mitigate the UHI effect, since vegetation plays a significant role in regulating the urban climate. It is an effective measure to create oasis effect and to mitigate urban warming at both macro- and micro-levels. Trees alter the environment by moderating the climate, improving air quality, conserving water and protecting wildlife. Basically, greenery ameliorates the urban climate by moderating the effects of sun, wind and rain. Trees cool the environment through shading and evapotranspiration. Solar radiation is either absorbed or deflected by leaves during hot shinny days and is transformed into latent heat converting water from liquid to gas which in turn results in a lower leaf temperature, lower ambient air temperature and higher humidity through the process of evapotranspiration. Hence, it feels much cooler under trees shading than exposed to direct sunlight. The energy budget of plants explains the whole evapotranspiration process [47], as follows:

n C E = M + S Where:

n : Net heat gain from radiation (short-wave radiation and long-wave radiation). This is often the largest part and it drives many energy fluxes.

C : Net sensible heat loss, which is the sum of all heat losses to the surroundings by conduction and convection.

E : Net latent heat loss, which is required to convert all water evaporated from the liquid to the vapor state and is given by the product of the evaporation rate E and the latent heat of vaporization of water ( = 2.454 MJ kg-1 at 20oC).


M : Net heat stored in biochemical reactions, which represents the storage of heat energy as chemical bond energy and is dominated by photosynthesis of respiration.

S : Net physical storage of thermal energy, which includes the energy used for heating the plant as well as heat used to raise the temperature of the air.

Some researchers reported that the energy transferred to latent heat through plants can be very high. Almost 1460kg of water is evaporated from an average tree during a summer sunny day, a cooling effect that is equal to five average air conditioners [48].

A single tree has the capability to moderate the surrounding microclimate, but its impacts are limited. Large urban parks, on the other hand, extend the positive effects to the surrounding built environment. Chen and Wong studied two urban parks in Singapore and concluded that a maximum 1.3K average temperature difference was observed around the parks. The temperatures measured within parks also have strong relationship with the density of plants, i.e. Leaf Area Index (LAI). Plants with higher LAIs may cause lower ambient temperatures. Results derived from the simulation study showed that a significant amount (almost 10% reduction of the cooling load) of energy consumption for cooling may be saved when buildings are built near parks [49]. A study in Japan [50] showed that even a small area of 60x40m can create a notable cooling effect. The maximum difference between inside and outside of the small greenery area was 3K. The study also showed that the air temperature distribution was closely related to the distribution of greenery in the city. Jauregui found that in a large urban park in Mexico City, the ambient temperature was 2-3K lower than its surrounding built-up area and its influence reached a distance of 2km, about the same as its width [51]. Therefore it can be stated that groups of trees may effectively improve the thermal environment of the urban area.

Trees have impressive shading effects towards the built environment. Dense foliage trees are able to intercept the incoming solar radiation by 70% 90% [52-54]. The shading effect provided by the plants on the surface of the buildings


lowers the surface temperatures and subsequently lowers the cooling energy consumption. Strategically located plants may reduce the cooling energy consumption between 25% 80% [55-61]. They can be in the form of trees located at the eastern or western side of the buildings wall, as well as rooftop or vertical greenery. Extra savings can be observed when air conditioning units are well shaded by plants [62].

The development of roof and wall planting has also been increased in the recent years, with a number of installations for roof gardens and vertical greenery in various building types, such as airports, hotels, residential and educational buildings, shopping malls and other facilities. Wong studied the impact of intensive and extensive rooftop greenery to the buildings energy behavior and environment [63]. Rooftop greenery can provide benefits not only to the building but also to the ambient temperature conditions. With the intensive rooftop garden system, the surface temperature may reduce up to 3.1K and the ambient temperature at 1 m may reduce up to 1.5K as shown in Fig. 9. The impact of rooftop greenery is even more pronounced for metal roofs. Without plants, the metal surface can be up to 60-70C during daytime and lower than 20C at night. With plants, it ranges only from 24C to 32C.

Figure 9: Comparison of surface and ambient temperatures measured with different plants (Source: Wong et al. 2007).

Rooftop greenery research in Japan concluded that the temperature above the rooftop greenery can be reduced of around 2-5K compared to a hard surface [64,


65]. Meanwhile, the surface temperature under the plants is reduced of almost 25-30K compared to the metal surface during the peak hour of clear days. Fioretti investigated the rooftop greenery performance in Mediterranean climate of Italy. The study also found a 5-6K temperature reduction above the green roof while the foliage reduced the solar radiation incidence on the roof surface between 80% -90% [66].

In the city, the amount of buildings faade surfaces is quite large compared to the roof surfaces. Hence, greening of building faade surface, known as vertical greenery system, has a great potential in mitigating the UHI effect through evapotranspiration and shading [67]. Vegetation can dramatically reduce the maximum temperature of a building by shading its walls from the sun, with daily temperature fluctuation being reduced by as much as 50% [68]. In addition, a faade fully covered with greenery is protected from the solar radiation intensity in the summer and it can reflect or absorb by its leaf cover between 40% - 80% of the received radiation, depending on the amount and the type of greenery [31].

A research in the humid climate of Hong Kong showed a maximum temperature decrease of 8.4K by vertical greenery systems in an urban canyon [69]. The reduction is significant as the distribution of ambient air in a canyon influences the energy consumption of buildings. Higher temperatures in a canyon increase the heat convection into the building and correspondingly increase the cooling load [70]. Surface temperatures of vertical greenery systems have been observed in different settings at the University of Toronto since 1996 [71]. These results have consistently demonstrated that areas with vertical greenery are cooler than light-colored brick walls and black surfaces that are typically found in urban areas. In Japan, experiments show that vines can reduce the surface temperature of a veranda with south-western exposure by 13-15K and the air temperature by 1-3K [52]. In Africa, a temperature reduction of 2.6K was observed behind vegetated panels of vines [72]. Therefore, together with the insulation effect of vegetation, the temperature fluctuations at the wall surface can be reduced between 10-60K to 5K-30K [73].

In Singapore, eight different vertical greenery systems were studied vs. their thermal performance [74]. The reason of the differences in the thermal


performance of these vertical greenery systems can be a combination of various factors, including the substrate type, the insulation of the system structure, the substrate moisture content as well as the shading and insulation of greenery coverage. At the same time, the interactions between leaf area, geometry, color and other microclimatic parameters such as solar radiation are complex and may result in different cooling efficiencies during day and night. Maximum reduction of 11.58K in the wall surface temperature on clear days is observed by the vertical greenery systems (Fig. 10) and a reduction of up to 3.33K in ambient air temperature from a distance of 0.15m away (Fig. 11) compared to a regular concrete wall.

Figure 10: Wall and substrate surface temperature of Vertical Greenery System no.5 (Source: Wong et al. 2010).

Figure 11: Ambient temperatures at a distance of 0.15m away from Vertical Greenery System no 1, 2 and 4 (Source: Wong et al. 2010).


URBAN VENTILATION

Wind speed in the urban areas is seriously decreased compared to the undisturbed wind speed. Moreover wind direction may be altered. The roughness of buildings and urban structures (geometry) affects wind within the cities and slows down wind speeds, thus decreases the natural ventilation potential and increases the pollutants concentration. One of the causes of UHI is the reduced turbulent transfer of heat within streets, due to the decrease of wind speed. The serious reduction of wind speed in urban canyons hampers the application of airflow through natural ventilation for dense urban environments [70]. The decrease in outdoor ventilation increases the outdoor pollutant concentrations and creates poor thermal comfort conditions. This has a trickling effect to the indoor environments as well. Experimental evaluation of airflow reduction in urban canyons has shown a reduction of 90% [75].

Natural ventilation is a good strategy for achieving acceptable thermal comfort, dilution of pollutant concentrations and dispersion of heat flux. Air movements determine the convective heat and mass exchange of the human body with the surrounding air. Higher air velocities increase the evaporative rate of skin surface and consequently enhance the cooling sensation [76].

Oke classified the wind variation with height over the cities with a two-layer classification of urban modification, the urban canopy layer (UCL) and urban boundary layer (UBL), see Fig. 12. As the air flows from rural to urban areas, the boundary layer must adjust to the new boundary conditions defined by the skyline of the city [77].

The urban canopy layer (UCL) or obstructed sub-layer extends from the ground surface up to the height of the buildings. The climatic conditions inside the UCL are determined by various urban configurations and material properties. In general, the wind speed inside this layer significantly decreases relative to the undisturbed wind speed. The turbulence decreases with increasing height due to ground obstacles and thermal airflow instabilities. The urban boundary layer (UBL), or free surface layer, lies above the buildings roof tops. Its thickness (from hundreds to thousands of meters) is determined by the gradient height at


which surface friction of the ground no longer affects the general wind flow. It varies from one point to another because of the variable heights of the buildings below and wind speed. It is also more homogeneous in its properties over the urban area than the UCL. In the UBL, the complex terrain increases the roughness of the surface and therefore increases the turbulence.

Figure 12: Schematic representation of the urban atmosphere illustrating a 2-layer classification of urban modification (Source: Oke 1987).

In highly urbanized cities like Hong Kong or Singapore, most residential estates have high canyon geometry ratios e.g. 2-3. According to Oke, 70-80% of daytime radiant energy surplus within urban canyons is dissipated to the air through turbulent transfer. The balance 20-30% is stored and released at night [78].

Some key parameters that affect the air flow in urban landscape [14, 79] are the following:

1. The prevailing breezeway or air path. The overall permeability of the district has to be increased at ground level. This is to ensure that the prevailing wind travels along breezeways and major roads can penetrate deeply into the district. This can be achieved by proper linking of open spaces, creation of open plazas at roads junctions, maintaining low-rise structures along prevailing wind direction.

2. Variation of building height. Varying the height of buildings significantly improves the penetration of the airflow in the urban landscape. Stepping building height in rows would also create better


wind at higher levels if the differences in building heights between rows are significant.

3. Orientation and layout of the buildings/streets with adequate gaps between buildings are essential for good airflow. The main streets and breezeways should be aligned in parallel or up to 30 to the prevailing wind direction, in order to maximize the penetration of prevailing winds through the estate. Building axis should be parallel to the prevailing wind to avoid obstruction (Fig. 13).

4. Increasing the permeability of building blocks by the provision of void decks at ground level or at mid-span. This helps improve the ventilation for pedestrians, and to remove the pollutants and heat generated at ground level.

Figure 13: Orientation of street grids (Source: Ng E. 2009).

When the prevailing wind blows perpendicular to the street canyon, there are three regimes of wind patterns, which are a function of building (L/H) and canyon geometries (H/W) [70, 80, 81]. These are the isolated roughness flow (IRF), the wake interference flow (WIF) and the skimming flow (SF), see Fig. 14. Wind flows are considered perpendicular when the predominant airflow direction is approximately (30) to the long axis of the street canyon.


Figure 14: (a), (b), (c) Perpendicular flow regimes in urban canyons for different aspect ratios (Source: Oke 1988).

The airflow pattern of the widely spaced buildings, i.e. H/W < 0.4 for cubic and < 0.3 for row buildings, is similar to the airflow pattern of the isolated ones (Fig. 14a). For a closer spacing, such as H/W up to 0.7 for cubes and 0.65 for row buildings, the airflow pattern changes to wake interference flow (Fig 14b). It is characterized by a reverse (with respect to upwind flow direction) horizontal flow in the lower canyon and forward flow along the canyon top. A small vortex may appear behind the upwind building but it is not dominant. An area of low wind speed appears in the canyon center. Maximum wind occurs at the top of the canyon and relatively high wind speed occurs down the face of the downwind building. At the higher building geometry (H/W) and density, the main airflow skims over the building rooftops and the bulk of the airflow does not enter the canyon. This flow is named as skimming flow (Fig. 14c).

The relationship between the three principle airflow regimes and their respective canyon H/W and L/H ratios has been summarized by Oke [80], as shown in Fig. 15.


Figure 15: Threshold for flow regimes in urban canyons as functions of urban canyon H/W & L/W ratios (Source: Oke 1988).

Cool Materials

The optical and thermal characteristics of building materials that determine the energy consumption in the built environment, is the solar radiation albedo and the emissivity of long wave radiation. Those parameters have a very important impact on the urban energy balance as seen in Fig. 16. The albedo of a surface is defined as its reflectivity, integrated hemi-spherically over the wavelength. The usage of high albedo materials keeps the surface temperature lower by reducing the amount of solar radiation absorbed into the buildings and the ambient air temperature at urban level [82]. Table 2 shows the albedo of various typical urban materials and areas [70, 83-85].

Figure 16: Diagram of cool roof system (Source: U.S. Cool Roof Rating Council).


Table 2: Albedo of typical urban materials and areas [70, 83-85]

Surface Albedo Streets Asphalt (fresh 0.05, aged 0.2)

0.05 - 0.20

Walls Concrete Brick/Stone Whitewashed stone White marble chips Light colored brick Red brick Dark brick and slate Limestone

0.10 - 0.35 0.20 - 0.40 0.80 0.55 0.30 - 0.50 0.20 - 0.30 0.20 0.30 - 0.45

Roofs Smooth-surface asphalt (weathered) Asphalt Tar and gravel Tile Slate Thatch Corrugated iron Highly reflective roof (weathered)

0.07 0.10 - 0.15 0.08 - 0.18 0.10 - 0.35 0.10 0.15 - 0.20 0.10 - 0.16 0.60 - 0.70

Paints White, whitewash Red, brown, green Black

0.50 - 0.90 0.20 - 0.35 0.02 - 0.15

Urban Areas Range Average

0.10 - 0.27 0.15

Other Light-colored sand Dry grass Average soil Dry sand Deciduous plants Deciduous forests Cultivated soil Wet sand Coniferous forests Dark cultivated soil Grass and leaf mulch

0.40 - 0.60 0.30 0.30 0.20 - 0.30 0.20 - 0.30 0.15 - 0.20 0.20 0.10 - 0.20 0.10 - 0.15 0.07 - 0.10 0.05

Extensive studies on cool coating materials for roofs or other buildings surfaces as one of solutions to mitigate UHI have been conducted during the last decade.


Cool roof is identified as a roofing system that is able to deliver high solar reflectance and high thermal emittance [86]. Cool roof system is purposed to reduce heat load for air conditioning system, energy usage and CO2 released to atmosphere. Bretz and Akbari [87] studied the relation between albedo of three different coatings, which were applied on three buildings roofs, and building energy consumption. From the two months up to six years measurements, it is shown that the higher the roof albedo value, the higher the percentage of energy savings. However, over the time, albedo values of the coated roofs dropped due to the accumulation of dirt and microbial growth causing at the same time, a similar reduction of the energy saving percentage. Experiments of washing the roof showed that albedo value drops was only temporary and it would recover 90% of its original value upon the washing although it may not be cost effective.

ENERGY STAR labeled roof products are able to reduce surface temperature up to 100F (equal to 37.79oC) and peak cooling demand by 10-15%. White or light colors coated roofs have been promoted widely in the U.S. to achieve cooler roof surface temperatures by increasing solar reflectance as a complementary alternative to metal roofing system which has high thermal emittance but low solar reflectance [88].

Synnefa et al. [89] conducted a study on the thermal performance of non-white cool colored coatings. Ten prototypes of cool colored coating tiles were compared with standard coating tiles for a period of three months during daytime and nighttime. The results show that cool colored coating tiles are more selective in absorbing infrared band, resulting in higher solar reflectance and lower surface temperature. From their experiment, there is a correlation between solar reflectance and surface temperature during daytime. These cool coatings can be applied to other building materials besides roofs.

Another experimental study done by Simpson and McPherson [90] on three identical scaled model houses in Tucson, Arizona for period of three months showed that white roof, which has a higher albedo compared to silver or gray and brown roofs, reduced the surface temperature to almost 20oC compared to a gray or silver roof and to almost 30oC compared to a brown roof. However, detailed observation also showed that increasing building surface's albedo may not be effective in reducing its temperature if the emissivity is also reduced.


The issues of aesthetic and maintenance require darker roof colors more desirable than white or light coated color roofs. Karlessi et al. [91] conducted a comparative study between thermochromic coatings, cool and common coatings. The research showed that thermochromic coatings are able to respond thermally to the environment. Thermochromic coating colors faded or became colourless when the ambient temperature was higher than the transition temperature. Under these conditions the surface reflects more solar radiation, hence, reduces the surface temperature.

With similar principles with cool roofs, cool pavements have been promoted aggressively the last years. Akbari et al. [92] believes that by implementing cool roofs and cool pavements, the urban areas overall albedo can be increased by about 0.1. The study predicted that by increasing albedo of urban roofs and paved surfaces worldwide offsets 44Gt of emitted CO2. Kinouchi [93] studied the structure of pigment and coating with low reflectivity in the visible part of sunlight spectrum and high reflectivity of near infrared. The field measurement on paint coated asphalt pavement indicated 15C lower than conventional asphalt pavement.

ACKNOWLEDGEMENTS

Part of information included in this chapter has been previously published in author's own publication.

CONFLICT OF INTEREST

The author(s) confirm that this article content has no conflict of interest.

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