Importance of Greenery in Hot Areas

Importance of Greenery in Hot Areas: Comparison

Please note this is a comparison between Version 2 by Amina Yu and Version 1 by Sundus Shareef.

Global warming and climate change are critical issues that raise attention on a global level. Worldwide, some cities are warming twice as fast as the surrounding rural areas. Greenery is one of the most influential factors in reducing the outdoor air temperature and enhancing the microclimate in hot areas.

Urban Canopy Layer (UCL)
Urban Boundary Layer (UBL)
ENVI-met
tree simulation
Dubai
UAE

1. Introduction

Global warming and climate change are critical issues that raise attention on a global level. Worldwide, some cities are warming twice as fast as the surrounding rural areas [1,2]. Apart from that, a rising temperature has been recorded in Dubai/UAE, with an increase of 2.7 °C between 1975 and 2013 [3]. Generally, the increase in air temperature is attributed to population growth and urban development. However, regional urban increase in air temperature, commonly described as the Urban Heat Island (UHI), is created due to the expansion of the developed and urbanized areas at the expense of the vegetation cover. This expansion, consequently, leads to impervious warming compared to surrounding and rural areas [4,5]. The Urban Heat Island (UHI) is one of the most influential phenomena in global warming and climate change, as it contributes to climate warming by about 30% [6], and it has both outdoor and indoor environmental impacts. In general, the UHI is caused by the solar radiation absorbed by the structures in developed areas [7]. Therefore, the main reason behind the UHI is the modification of the natural land by development and urbanization, which act as the main causes of this phenomenon [8]. Sustainable urban design and the employment of suitable design strategies can have a direct impact on reducing the effect of the UHI on the built environment. Previous studies showed the importance of urban morphology and urban design elements in reducing the effect of UHI on microclimate conditions [9,10,11]. Che-Ani et al. (2009) [12] highlighted the factors that affect the UHI in two ways: (1) microclimate factors: wind speed and behavior, humidity, and cloud layer; and (2) urban geometry factors: urban pattern, density, built-up areas, Sky View Factor (SVF), urban materials, greenery areas, and landscaped areas.

Global warming and climate change are critical issues that raise attention on a global level. Worldwide, some cities are warming twice as fast as the surrounding rural areas ^[1][2]. Apart from that, a rising temperature has been recorded in Dubai/UAE, with an increase of 2.7 °C between 1975 and 2013 ^[3]. Generally, the increase in air temperature is attributed to population growth and urban development. However, regional urban increase in air temperature, commonly described as the Urban Heat Island (UHI), is created due to the expansion of the developed and urbanized areas at the expense of the vegetation cover. This expansion, consequently, leads to impervious warming compared to surrounding and rural areas ^[4][5]. The Urban Heat Island (UHI) is one of the most influential phenomena in global warming and climate change, as it contributes to climate warming by about 30% ^[6], and it has both outdoor and indoor environmental impacts. In general, the UHI is caused by the solar radiation absorbed by the structures in developed areas ^[7]. Therefore, the main reason behind the UHI is the modification of the natural land by development and urbanization, which act as the main causes of this phenomenon ^[8]. Sustainable urban design and the employment of suitable design strategies can have a direct impact on reducing the effect of the UHI on the built environment. Previous studies showed the importance of urban morphology and urban design elements in reducing the effect of UHI on microclimate conditions ^[9][10][11]. Che-Ani et al. (2009) ^[12] highlighted the factors that affect the UHI in two ways: (1) microclimate factors: wind speed and behavior, humidity, and cloud layer; and (2) urban geometry factors: urban pattern, density, built-up areas, Sky View Factor (SVF), urban materials, greenery areas, and landscaped areas.

2. Urban Heat Island and the Atmospheric Layers

The Earth’s atmosphere layers show that the closest layer to the Earth is the Troposphere layer, which extends 12–20 km from the Earth’s surface into the atmosphere. Microclimate conditions are a part of the Earth’s Troposphere layer, called the Atmospheric Boundary Layer (ABL) [13]. The Atmospheric Boundary Layer (ABL) extends from the Earth’s surface to a range of 200–500 m above the ground surface level [14]. However, the first 100 m of the ABL is the most affected layer by the changes in the Earth’s surface [15,16]. However, this layer is divided into two sub-layers: Urban Boundary Layer (UBL) and Urban Canopy Layer (UCL) [17]. The UBL is above the average height of the buildings, while the UCL is the layer in the space between the buildings. Generally, the UCL microclimate is affected by the morphology, shape, and roughness of the developments [18]. Most of the built environment research is concerned with the UCL, as it has the most significant impact on the thermal performance of the built environment [19,20]. Nevertheless, the microclimate of any area is the climatic conditions of both the UBL and the UCL, and microclimate represents the climatic parameters of a local division from the larger area. It can be the climatic conditions of a small area, such as the canyon between the buildings, or it can also represent the climatic conditions of a larger area, such as neighborhoods or a city with different climatic conditions to the surrounding area [21]. Generally, the microclimate major parameters cover air temperature, solar radiation, humidity, and wind speed and direction. The interaction between the urban morphology and the microclimate conditions has a significant impact on the thermal performance of outdoor and indoor environments. However, it is still difficult to predict every single factor that contributes to the microclimate conditions of the built environment. However, it has been found that some phenomena such as the Urban Heat Island (UHI) have a significant impact on climatic conditions, both on a local and global scale [22]. This research will focus on the interaction of one of the urban environment factors, specifically the greenery, or plants with atmospheric conditions, of the UAE.

3. Types of Urban Heat Island UHI

The UHI has been investigated on two levels; (a) Surface Urban Heat Island (SUHI), and (b) Atmospheric Urban Heat Island (AUHI) [22]. The SUHI is the average temperature on the horizontal surfaces such as roofs of buildings, outdoor equipment, and canyon pavements. It is a diurnal phenomenon that is affected by the sun’s position and the surface material [22]. The SUHI on a large scale is measured by using the remote sensing data collected from satellite sensors [23]. The other type, AUHI, within the atmospheric boundary layer, is divided into two types depending on the atmospheric layer. The first one is the UHI, within the UBL above the building’s average height; the air temperature of this layer is affected by the interaction with the roughness of the building’s roof. The second type of UHI is the one within the Canopy Layer Heat Island (CLHI), which represents the area within the urban space between the buildings or the canyon space [24]; the Canopy Layer Heat Island (CLHI) is the part of the high-temperature air within the urban canyon space. It extends from the canyon ground to the average height of the buildings. The microclimate within this layer depends on the building’s geometry, canyon height to width ratio, type and percentage of greenery, and canyon material [24]. The CLHI is generally measured by weather-measuring tools used in fixed stations at ground level; the effect of the UHI on microclimate conditions will be explored in the next section.

4. Vegetation and the Impact of the Urban Heat Island UHI on the Microclimate

However, most of the previous studies evaluated greenery according to the percentage of the greenery ratio applied to the total designed area. Other studies indicated the greenery type according to the Leaf Area Index (LAI) [37]. Leaf Area Index (LAI) could be defined as a dimensionless highly variable value; it can range from 0 to more than 6. Some areas have a Leaf Area Index (LAI) less than 1, while tropical forests can have a Leaf Area Index (LAI) that reaches 9, and moderate forests can have LAI values between 3 and 6. LAI is one of the most widely used measurements for describing the plant structure of the canopy layer, as leaf surfaces are the primary border of mass and heat exchange; the important processes such as evapotranspiration and canopy interception are directly proportional to Leaf Area Index (LAI). Other than that, the Leaf Area Density (LAD) is an important structural property of vegetation; it can be measured by using the Leaf Area Index (LAI), which is the sum of the area of all leaves on a plant relative to the projected area of the crown of the same plant on the ground [38,39,40]. Leaf Area Density (LAD) provides valuable insight into the structure of the vegetation that can be used in guidelines for planning proposals [40]. Leaf Area Density (LAD) plays an important role in providing the shaded environment and absorbs the solar radiation to reduce the reflected waves. The Leaf Area Density (LAD) approach also presents the importance of large and high trees in the environment [41].

5. Solar Radiation and Greenery Impact

Urban surfaces of the built environment have a significant impact in modifying solar radiation [45]. It has been found that 90% of direct radiation reaches the Earth in clear sky condition, and diffused radiation reaches the Earth after scattering in the atmosphere, also when reflected from surrounding objects. However, on overcast days 100% of the solar radiation is diffused. The radiation balance or the heat balance depends on the direct radiation and the surface [46]. It is directly related to the solar irradiance of the Earth’s surface, and the surface layer is responsible for the heat transfer between the atmosphere and the ground [47,48]. The physical features of an active earth surface depend on the state and type of the vegetation, soil, water, surface material, and albedo, which are determined for the net radiation [49]. The “Canyon effect” or urban high-density housing has a significant on the spatial diversity of the inflowing solar irradiation intensity and UHI phenomena [50,51]. When studying the radiation and the net radiation fluxes or reflection, most of the previous studies focused on the surface albedo, however, the impact of greenery on the reflected wave surfaces in the urban microclimate needs more investigation. This research will focus on the interaction of greenery or plant cover with atmospheric conditions, specifically solar radiation, and how the greenery affects the reflected shortwave radiation. Furthermore, while most of the previous studies investigated the impact of greenery by adopting the coverage ratio and the simulation method [52], this research will investigate the impact of trees and other types of greenery with respect to the type, height, and Leaf Area Density (LAD).

2. Urban Heat Island and the Atmospheric Layers

The Earth’s atmosphere layers show that the closest layer to the Earth is the Troposphere layer, which extends 12–20 km from the Earth’s surface into the atmosphere. Microclimate conditions are a part of the Earth’s Troposphere layer, called the Atmospheric Boundary Layer (ABL) ^[13]. The Atmospheric Boundary Layer (ABL) extends from the Earth’s surface to a range of 200–500 m above the ground surface level ^[14]. However, the first 100 m of the ABL is the most affected layer by the changes in the Earth’s surface ^[15][16]. However, this layer is divided into two sub-layers: Urban Boundary Layer (UBL) and Urban Canopy Layer (UCL) ^[17]. The UBL is above the average height of the buildings, while the UCL is the layer in the space between the buildings. Generally, the UCL microclimate is affected by the morphology, shape, and roughness of the developments ^[18]. Most of the built environment one is concerned with the UCL, as it has the most significant impact on the thermal performance of the built environment ^[19][20]. Nevertheless, the microclimate of any area is the climatic conditions of both the UBL and the UCL, and microclimate represents the climatic parameters of a local division from the larger area. It can be the climatic conditions of a small area, such as the canyon between the buildings, or it can also represent the climatic conditions of a larger area, such as neighborhoods or a city with different climatic conditions to the surrounding area ^[21]. Generally, the microclimate major parameters cover air temperature, solar radiation, humidity, and wind speed and direction. The interaction between the urban morphology and the microclimate conditions has a significant impact on the thermal performance of outdoor and indoor environments. However, it is still difficult to predict every single factor that contributes to the microclimate conditions of the built environment. However, it has been found that some phenomena such as the Urban Heat Island (UHI) have a significant impact on climatic conditions, both on a local and global scale ^[22]. It will focus on the interaction of one of the urban environment factors, specifically the greenery, or plants with atmospheric conditions, of the UAE.

3. Types of Urban Heat Island UHI

The UHI has been investigated on two levels; (a) Surface Urban Heat Island (SUHI), and (b) Atmospheric Urban Heat Island (AUHI) ^[22]. The SUHI is the average temperature on the horizontal surfaces such as roofs of buildings, outdoor equipment, and canyon pavements. It is a diurnal phenomenon that is affected by the sun’s position and the surface material ^[22]. The SUHI on a large scale is measured by using the remote sensing data collected from satellite sensors ^[23]. The other type, AUHI, within the atmospheric boundary layer, is divided into two types depending on the atmospheric layer. The first one is the UHI, within the UBL above the building’s average height; the air temperature of this layer is affected by the interaction with the roughness of the building’s roof. The second type of UHI is the one within the Canopy Layer Heat Island (CLHI), which represents the area within the urban space between the buildings or the canyon space ^[24]; the Canopy Layer Heat Island (CLHI) is the part of the high-temperature air within the urban canyon space. It extends from the canyon ground to the average height of the buildings. The microclimate within this layer depends on the building’s geometry, canyon height to width ratio, type and percentage of greenery, and canyon material ^[24]. The CLHI is generally measured by weather-measuring tools used in fixed stations at ground level; the effect of the UHI on microclimate conditions will be explored in the next section.

4. Vegetation and the Impact of the Urban Heat Island UHI on the Microclimate

In 2008, the United States Environmental Protection Agency stated that the impact of the UHI is felt in an increase in the developed area’s air temperature compared to the rural surrounding area air temperature of between 1–3 °C during the day ^[24]. This increase in air temperature will consequently increase electricity demand and, accordingly, GHG emission and air pollution. The UHI is affected by day and night time; the variation between surface temperature and air temperature at night is less compared to the variation in the daytime. This result reflects the fact that during the daytime the developed surfaces absorb the solar radiation more than reflecting this radiation, while at night time the release of the surface heat and the balance between the surface and air temperature at night occur. Therefore, increasing the surfaces that absorb the solar radiation will consequently reduce the impact of the reflected waves. Wanphen and Nagano (2009) ^[25] found that the variation in air temperature within the urban context can reach 5–15 °C because of the pockets of small islands. However, the impact of the UHI is varied according to the location of the area and weather conditions. Hirano and Fujita (2012) ^[26] conducted a field measurement to find the effect of UHI in Tokyo, Japan. It was stated that UHI is an accumulation of high-temperature areas, and it has a more positive effect by reducing heating consumption in winter, compared to the negative effect of increasing cooling consumption in summer. In spite of the positive effect of reducing energy consumption in the winter, it will not recommend stopping the effort of mitigating the UHI and its impact on the global environment. However, urban design variables that affect this phenomenon, such as building configuration, Sky View Factor (SVF), materials, vegetation, and landscaped green areas, have been investigated in these ^{[27][28][29][30]}. The UHI was explored and illustrated in it in order to know the reasons behind these phenomena and their impact on climatic conditions. In addition, it was also sought to find solutions to reduce and mitigate its negative effect on the built environment. Different strategies have been proven and used for reducing the impact of UHI and enhancing the microclimate of urban areas. Apart from other strategies, urban greenery, especially trees, and vegetation areas have been proven to be one of the most effective measures for enhancing the microclimate and mitigating UHI environmental impacts ^{[31][32][33][34]}. Furthermore, it was listed a number of benefits that can be achieved by increasing greenery, such as reducing air pollutants and particulates, energy-saving and conservation through cooling benefits, noise buffering, improving psychological wellbeing, human interaction, and social life, and increasing property value ^[35][36]. However, most of the previous ones evaluated greenery according to the percentage of the greenery ratio applied to the total designed area. Other was indicated the greenery type according to the Leaf Area Index (LAI) ^[37]. Leaf Area Index (LAI) could be defined as a dimensionless highly variable value; it can range from 0 to more than 6. Some areas have a Leaf Area Index (LAI) less than 1, while tropical forests can have a Leaf Area Index (LAI) that reaches 9, and moderate forests can have LAI values between 3 and 6. LAI is one of the most widely used measurements for describing the plant structure of the canopy layer, as leaf surfaces are the primary border of mass and heat exchange; the important processes such as evapotranspiration and canopy interception are directly proportional to Leaf Area Index (LAI). Other than that, the Leaf Area Density (LAD) is an important structural property of vegetation; it can be measured by using the Leaf Area Index (LAI), which is the sum of the area of all leaves on a plant relative to the projected area of the crown of the same plant on the ground ^[38][39][40]. Leaf Area Density (LAD) provides valuable insight into the structure of the vegetation that can be used in guidelines for planning proposals ^[40]. Leaf Area Density (LAD) plays an important role in providing the shaded environment and absorbs the solar radiation to reduce the reflected waves. The Leaf Area Density (LAD) approach also presents the importance of large and high trees in the environment ^[41].

5. Solar Radiation and Greenery Impact

Urban surfaces of the built environment have a significant impact in modifying solar radiation ^[42]. It has been found that 90% of direct radiation reaches the Earth in clear sky condition, and diffused radiation reaches the Earth after scattering in the atmosphere, also when reflected from surrounding objects. However, on overcast days 100% of the solar radiation is diffused. The radiation balance or the heat balance depends on the direct radiation and the surface ^[43]. It is directly related to the solar irradiance of the Earth’s surface, and the surface layer is responsible for the heat transfer between the atmosphere and the ground ^[44][45]. The physical features of an active earth surface depend on the state and type of the vegetation, soil, water, surface material, and albedo, which are determined for the net radiation ^[46]. The “Canyon effect” or urban high-density housing has a significant on the spatial diversity of the inflowing solar irradiation intensity and UHI phenomena ^[47][48]. When knowing the radiation and the net radiation fluxes or reflection, most of the previous were focused on the surface albedo, however, the impact of greenery on the reflected wave surfaces in the urban microclimate needs more investigation. It will focus on the interaction of greenery or plant cover with atmospheric conditions, specifically solar radiation, and how the greenery affects the reflected shortwave radiation. Furthermore, while most of the previous ones investigated the impact of greenery by adopting the coverage ratio and the simulation method ^[49], it will investigate the impact of trees and other types of greenery with respect to the type, height, and Leaf Area Density (LAD).