Download MODELLING THE INTERACTIONS BETWEEN URBAN

B A U J O U R N A L : HEALTH & WELL-BEING, S P E C I A L E D I T I O N :
THEME: Planning Urban Health & Human Wellbeing
MODELLING THE INTERACTIONS BETWEEN URBAN HEAT
ISLAND AND URBAN GEOMETRY
Dina M. Saadallah
1Alexandria
University Faculty Of Engineering Architectural Department / Email: [email protected]
ABSTRACT: Cities are a complex, adaptive, and self-organizing systems, that encounter various challenges due
to rapid urbanization such as socio-demographic problems, air pollution, climate change threats, and urban heat
island consequences (Kim and Han, 2012). Urbanization, is one of the major challenges that cities confront in this
century, as it is strongly affecting the atmosphere and its urban context. In the light of these challenges there is a
crucial need to study the effect of UHI phenomenon, that causes increase in air temperature, in the center of the
cities and decreases on the periphery, it is partly induced by the urban geometry and alter the energy balance of
these cities. UHI is weighted generally as a major problem because it induces many unsatisfactory consequences
to the whole city and to the people, decline in the quality of life due to raised temperature that cause health
problems, more energy consumption and more pollution. According to Bittencourt and Candido (2010) they stated
the crucial need for analyzing the urban aerodynamics phenomenon to persuade wind pressure on urban geometry,
environmental air quality, as understanding the wind dispersion through the city help in improving quality of
urban design thus the quality of life. Highlighting the importance of analyzing the relation between UHI and urban
Geometry. Therefore, this research aims in examining UHI characteristics and suggests some applicable
mitigation strategies to help in reducing the air temperature and save energy through using ENVI- met. Aiming
to adapt this model in Borg El Arab City, Alexandria, Egypt, a real urban context characterized by severe
heatwaves in summer, providing numeric and visual information that can be used as a guideline by urban
designers and planners to support the decision-making process.
KEYWORDS: Urban Heat Island, Urban Geometry, Improving Quality of Life, ENVI- met
1. INTRODUCTION
Urban Heat Island (UHI) has become one of the world’s leading urban environmental issues. Urbanization
has raised concerns about the UHI, especially in terms of human health and a healthy environment, as it has
negative consequences on the environment including air quality, raised temperature and alteration in the
percentage of landscape. The people’s percentage living in urban areas is anticipated to reach by 2050 up to almost
70%, then, the energy consumption in cities is likely to follow that pattern. Therefore, during the next decades, the
urban planners and stakeholders will have to think and act differently towards the serious difficulties in term of
resource flow, traffic and the major concern is the high energy consumption. For instance, the building
configurations, structures and materials are considered from major attributors to the increase in urban temperature,
energy consumption, pollution and in the waste production (Santamouris et. al., 2001). The UHI is triggered by
the explosive population growth and the increasing impact of climate change, which in turn contribute to enormous
health problems. Referring to Oke, 2006 the UHI formation is affected by different meteorological conditions and
urban context in its size and space, while the presence of vegetation and air turbulences help in mitigating its
development.
Bittencourt and Candido (2010) investigated the relation between urban Geometry and UHI, mentioning how
it is essential to analyze the urban aerodynamics phenomenon to persuade wind pressure on urban geometry,
environmental air quality, as understanding the wind dispersion through the city help in enhancing the quality of
urban design thus the quality of life. In this context, the urban geometry is examined as a major the factors that
significantly influence the formation of heat islands, in recent studies, it is determined by the ratio of H/W
(height/width). The higher values of H/W are detected, when the height of the building is higher in comparison to
the width. Experimental simulations have been verified and studied by Oke (1981, 1988). Oke (1981) to
investigate the effect of this relationship on the micro-climate, declaring that the cooling is lowest when the H/W
ratio increases. In addition to the urban geometry , the lack of vegetation, as well as, the high percentage of builtup areas (Buildings and pavements), properties of construction material of buildings , with the low albedo
characterized by low emittance properties of the solar radiations during the night and high absorbance of solar
radiations , this is represented in materials like asphalt and concrete and result in heat stress in the built up areas
with over all increase in the air temperature, thus, all these factors contribute to the UHI development in the cities.
(Nakata, Souza, 2013).
ISSN: 2617-1635, © BAU Journal – Health & Well-Being.
Beirut Arab University, Lebanon
Dina M. Saadallah, BAU Journal: Health & Well-Being, special edition, (2018), 155-168
One of major problems in urban design and planning process that most of hot dry countries mostly in developing
world are facing is that, most of the planning regulations used are being examined to be adopted in temperate
climates and therefore, failing to adapt to the local climate. Moreover, they may be unsuitable and inflexible,
restricting the possibility of climate-responsive urban design. Consequently, most of urban areas suffers from un
comfortability toward their users (Johansson 2006). Therefore, the research is concerned with studying UHI
characteristics, factors responsible for its formation and scales of UHI and finally suggesting mitigation and
feasible strategies in order to save energy, reduce the air temperature, and enhance the Micro-climate using ENVImet. Aiming to adapt this model in Borg El Arab City, Alexandria, Egypt, a real urban context characterized by
heatwaves in summer, providing numeric and visual information that can be used as a guideline by urban designers
and planners to support the decision-making process.
FACTORS CAUSING URBAN HEAT ISLAND FORMATION:
An aggregated, composited factors leads to the UHI development, the most influential type of UHIs is the canopy
layer heat island, the one that extends from the ground to the top of roofs at the canopy levels which stimulates
various thermal projections throughout the city. The causes that influence the canopy heat island formation are
listed as follows; Urban design geometry, roads width and directions, existence of canyons, where urban canyons
are designed with tall buildings in narrow streets that change the speed of wind and increase the use of impervious
surfaces that trap the heat. Building materials thermal properties, are from the critical factors that contribute to
UHI phenomenon. Building materials are effective factor in UHI formation, when the reflectance property is low,
then they absorb more and reflect less energy, which lead to increase in temperature at surface level. Commonly
used building materials such as tar, asphalt, brick and concrete are characterized by this property, keeping lare
amounts of solar-energy during the day and discharge it at night, reporting UHI intensity to be stronger at night
time. Anthropogenic factors such as heat waste generated from vehicles are reported to have less effect on the UHI
formation. Existing land use/cover, altitudes and replacing the land surface cover such as porous vegetation with
non-porous materials, thus decreasing the evaporative cooling (Cynthia &et. al., 2005). UHI studies are generally
handled in two major ways reflecting different scales; The Urban Canopy Layer or the Urban Boundary Layer ;
the UBL is conducted in meso-scale in the high altitude, where thermal inversion is predominated through the
daytime, while the first is relevant to the microscale with the lower altitude where thermal reversal is dominant
during night-time at the urban canopy layer , below the roof tops and exist in the spaces between buildings as
clarified in fig (1) .
Fig. 1 illustrating Microscale of UHI, Urban canopy Layer
Reference: Author
2.1 Urban Geometry and Thermal properties of urban Surfaces.
Focusing on two main parameters influencing the urban climate; the urban geometry and the thermal properties of
urban surfaces (Johanssons, 2006). As mentioned before, for the urban geometry analyzing the ratio between the
buildings heights (H), and distance (W) impact the amount of both the outgoing and incoming radiations that affect
the wind speed. Regarding the urban surface materials with high thermal capacity that contributes to the UHI, as
the incoming radiation through the day is stored and then discharged through the night. The air pollution resulted
from waste heat of vehicles also has an impact on both incoming and outgoing radiation but the net effect on air
temperatures has been proven to be small ((Manioğlu & Yılmaz, 2008), compared with the strong influence of the
properties of surface materials and the urban form to the micro-climate around buildings. The negative impact of
UHI can be enhanced through the urban design process, in order to improve the heat stress of outdoor
environments, enhancing the quality of urban environment. However, urban climate and outdoor thermal comfort
implications are generally neglected or obsessed minimal attention in the design and planning processes (Monam,
Ruckert, 2013 & Axarli, Teli, 2008). Accordingly, this highlight the need for analyzing the UHI starting a
Modelling the interactions between Urban Heat Island and Urban Geometry
responsive urban design and planning towards the micro-climate. As it was found that, the compact urban forms
in hot dry regions typically in old city centers are acknowledged to being well adapted to the climate (Toudert,
Mayer, 2006, 2007). Although the small amount of studies applied on the hot dry climates on urban micro-climate,
where the wind and humidity are two main variables that are responsible for controlling the intensity of UHI are
examined or taken into consideration, Meanwhile, the development of UHI phenomenon depends too upon the
varying densities of the population, as the densely populated areas put more challenges on the cities to develop
either horizontally or vertically, and with the implementation of artificial materials in construction process that
causes higher absorption of solar radiation, and ultimately a minimal long-wave emission being reduced to sky
due to the blockage effect of buildings, releasing more anthropogenic heat, that block urban ventilation. (Mirzaei,
2015).
The proportion of built- up area is a significant factor in the formation of UHI, as the density of these areas
with the buildings materials are capable of modifying the wind, amount of radiations, and the temperature
conditions near the ground level as the built up area is distinguished by a lower albedo and higher heat capacity
compared to the natural surfaces (Emmanuel, Rosenland, Johansson, 2007). Focusing on Urban geometry how it
can block the radiations then they re-radiate that energy in terms of longwave energy. According to Li, Wang,
Wong (2016) the urban form is more significant than urban geometry on the urban micro- climate. In addition to
that, the combination of landscape features such as the amount of green space, distance to water bodies, and finally
the anthropogenic heat is recommended to be studied to be able to analyze the differences in UHI intensities with
the meteorological conditions. Bosselmann and Arens (2005) examine how the buildings can modify the wind
conditions at street level, and how the sun and wind conditions affect the pedestrian comfort, categorized the
following physical parameters as follows; Urban / Building configuration and Landscape elements. Therefore, for
achieving a climatically responsive sustainable design, these physical settings have to be studied in an attempt to
decrease the effect of Urban heat island and to reduce the energy consumption, using natural resources to provide
comfortability, sustainable and healthy urban environment. In order to be able to Study mitigation strategies of
UHI at neighborhood scale, then it is crucial to differentiate scales of UHI to focus the study on neighborhood
scale.
2. SCALES OF URBAN HEAT ISLAND:
The UHI models are diverse in terms of scale that differs from one research to another according to the
aim of a study, categorized as follows; building-scale known as Building Energy models (BEM), then moving to
bigger scale the Micro Scale Models known as Microclimate models (MCM) for exploration of the UHI impact
on the thermal comfort of the pedestrians to the urban scale for examining the effect of wind on urban ventilation,
finally the City scale model. Thus, the three scales will be introduced as follows in fig 2;
City-scale Models; It is a remote sensing application that test UHI over the scale of cities using the airborne
measurement devices and satellites to provide accurate thermal images that are implemented through relating the
temperature of the surfaces with Land cover and land-use of the city.
Micro scale model; are used to test an area, neighborhood, where the interaction between built up area with open
spaces and surface materials are the basic elements used for the microclimate models (MCM). The building block
configuration, its` height ratio to the street’s width, building orientation, pedestrian sidewalk with the
environmental parameters are studied all together to analyze the airflow pattern.
•
computational fluid dynamics (CFD) technique.
• The urban canopy model (UCM)
Building Energy Model; are developed based on an energy balance applied to a single building, related to energy
consumption of this building neglecting the neighboring buildings, taking into consideration the environmental
parameters to analyze the effect of UHI on building.
Using these models will help in analyzing the factors that contribute to the UHI development and thus implement
suitable policy for urban design and city planning, in order to avoid the severe consequences of UHI on the humans
well -being and the natural environment (Deilami, K., Kamruzzaman, M., Liu, Y., 2018).
CSM
City Scale
Model
MCM
Microscale
Model
BEM
Building
Energy
Model
Fig. 2 Different models used to test UHI effect
Reference: Author
Dina M. Saadallah, BAU Journal: Health & Well-Being, special edition, (2018), 155-168
According to what is mentioned above, the scale of research is the Micro Scale Models known as Micro-climate
models (MCM) using the Urban Canopy Model (UCM) approach , aiming to investigate the UHI characteristics
and propose some suitable and feasible strategies to help in reducing the air temperature and save energy through
using ENVI- met as a simulation tool. Aiming to adapt this model to a residential compound in Borg El Arab
City, Alexandria, Egypt which is characterized by a hot arid city according to Middleton and Thomas (1997).
According to Ali-Toudert and Mayer (2006) they determined that the thermal comfort in urban environment is
directly influenced by the orientation and the ratio of the urban street canyon, it was tested and documented by
numerical studies in dry and hot climate context using the Envi-met. Furthermore, their simulation showed that
different configurations, requires extra shading and more cooling elements through using vegetation help in
manipulating wind direction providing outdoor comfort within the acceptable ranges and decreases the effect of
UHI. Analyzing the energy pattern in hot arid climate cities highlighted that the highest percentage of energy goes
to domestic use (WHO, 2014), this means that the residential use is responsible for consuming high percentage of
energy to provide cooling needs for the users’ comfort creating more effect of UHI. This high demands for energy
cause severe problems, and is the main reason for UHI formation in urban areas, discomfort to human thermal
feeling and energy inefficiency. Due to the recent challenges of urbanization and overwhelming population growth
rates, this imposes difficult challenges to urban designer in achieving a responsive urban design, in order to control
the micro-climate of neighborhoods and help in reducing the formation of UHI and increase the human thermal
comfort. In Egypt, there is deficiency in applying full environmental studies on the built and the natural
environments (El Araby, M. 2002), while the current situation and the climate change consequences that requires
smart urban design where the buildings and open spaces should be adequately climatic responsive. Therefore, the
urban design must be strongly dependent on climate interactions which can improve or have moderate impacts on
the formation of UHI. This leads to a better understanding and analysis of all the forces affecting the UHI
development in urban environment, and can be used in supporting the urban designers and planners in decision
making. As, the designs will ensure reduced UHI formation, reduced energy consumption, improve human thermal
comfort and improving quality of life.
4-SIMULATION MODEL
ENVI-met was selected as a simulation tool due to its ability to simulate the microclimate of a “complex
urban structure” based on “well-founded physical basis” Ali-Toudert and Mayer (2006). Considering the
environmental parameters, which is mentioned by majority of the scholars investigating the UHI.
▪ Air temperature (Ta measured in °C-Celsius, °F-Fahrenheit, °K-Kelvin)
▪ Solar radiation (SRD measured in MJ/m2)
▪ Wind speed (W measured in m/s, km/h, knots and mph)
▪ Relative humidity (RH expressed in %)
ENVI-met has been used to analyze the urban microclimate in different climatic zones and analyzing the impacts
of typical UHI mitigation strategies on air temperature and thermal comfort in residential neighborhoods.
Moreover, studies have been modelled in analyzing dry/hot cities such as Phoenix, USA, Rome, Italy and in humid
tropical cities for instance Colombo, China, and Shanghai (Roth & Lim, 2017). Therefore, the Studied area will
be modelled using ENVI-met, first, the area input files for the current situation (CS) considering the landscape
layout, position and heights of buildings, materials of buildings, plants, soils types, and Moving to the second part
which will be the mitigation scenarios that will be discussed in detail.
4.1 -STUDY AREA
The chosen site is located in Borg El-Arab City, on the west of Alexandria city, Egypt and it is located in an arid
desert area. It is newly designed city in Egypt was developed and redesigned in the 21st century, it`s climate is
characterized by being extremely hot in summers and cold in winters; this area belongs to a hot arid climatic
region. This neighborhood belongs to the Engineering syndicate presented in fig 3. This site was chosen
specifically as the neighborhood still has new phases to develop so giving recommendations should help them with
the newly developed and to modify the existing one. It is located geographically on 30.891547o N Latitude and
29.571872 o E Longitude. The total area of site is approximately 10.5 acres; the percentage of built-up area is 40%
of the total area to 60 % of open space area. Regarding the configuration of buildings, they are designed of 16 m
height, large blocks using air conditioning systems fixed on most of the buildings, that add extra loading heat to
the open space as represented in fig (4). Concerning the material used in the buildings are concrete, the roads are
covered with asphalt. Concerning Landscaping elements, there are few numbers of trees have been vegetated and
limited green spaces, but they are randomly located which means they are not designed to provide shading, cooling,
Modelling the interactions between Urban Heat Island and Urban Geometry
or wind modification. No shades have been placed in the site causing the overheating for several places in summer.
Moreover, the adjacency of site to a major arterial with heavy vehicular traffic.
Fig. 3 illustrating the whole engineering site in Borg El Arab, with selected area
Reference: Google earth, 2018
Fig. 4 The current situation (CS) materials for the selected site
Reference: Author
5. ADAPTATION AND MITIGATION
Three-dimensional numerical Modelling with ENVI-met, analyzing the factors of temperature, wind speed,
and humidity, was used to study the basic pattern of built environment effects on the microclimatic in the current
situation of the Borg Arab City, as well as under some UHI mitigation strategies. The three-dimensional numerical
model simulates the radiation balance and the aero - thermo dynamics, in complex urban structures. The following
aspects are included in the analysis; building configuration such as width and height dimensions, material of
surface whether for the buildings or hard covering such as concrete and asphalt), type and texture of vegetation
and soil existing in the site. The analysis is carried for typical time-frame of 24 hours and with a horizontal
resolution designed for microscale ranges from 0.5 to 10m, to allow the analysis of small-scale interactions among
individual buildings, plants and surfaces. Different studies have been implementing ENVI-met to analyze the
mitigation of UHI and its effect on the whole urban structure, reflecting the model’s capability. Table 1 shows
the environmental input configuration data input for the model, based upon the analyses of temperature, wind, and
relative humidity obtained from “World Weather & Climate Information “, on the 17th of July 2018 a hot day was
selected for the simulation. The ENVI-met model is broadly used to examine the existing thermal stresses in the
Dina M. Saadallah, BAU Journal: Health & Well-Being, special edition, (2018), 155-168
current situation and investigating how increasing the percentage of greenery or modifying the layout of landscape
elements along with changing the materials of hard cover in the site such as pavements and the streets can be
effective on modifying the wind dispersion all over the site , and enhancing the thermal performance of the
proposed scenarios . (Tukiran, Ariffin, Ghani, 2017). Therefore, the ENVI-met model will be used to examine the
thermal stress of the current situation (CS) in the selected site to configure the current heat stress, and three UHI
mitigation scenarios. Three different UHI mitigation strategies will be applied causing different cooling and
thermal response that will be discussed as follows.
(i) Scenario 1: changing the current materials used from low albedo materials to high albedo materials (HAM).
(ii) Scenario 2: Increase the percentage of greenery in form of trees, shrubs and green roofs. (VEG).
(iii) Scenario 3: Combining the previous two scenarios increasing greenery along with high albedo materials
(HYBRID).
Table 1: Environment Input Configuration data applied in the EVI-met simulations.
Reference: https://weather-andclimate.com
5.1 Current situation analysis (CS):
The total area of the site is 10.5 acers of total area 44,120 m2, the percentage of built up to open spaces is 40 % of
the site is built up using concrete as a building material with an albedo of 0.30. Although the percentage of open
spaces is 60 % but the site suffers from lack of vegetation and greenery that only represent 7%, while the percentage
of pavement 34% with an albedo 0.27, percentage of roads 19% with an albedo material of asphalt of 0.14, 0.16,
as shown in the figure 5 (a,b).
A
B
19%
40 %
7%
34%
Buildings
Pavement
vegetation
Roads
Fig. 5 (a, b): (A) The current situation (CS) material percentage and (B) 3 D building heights in the selected
site
Reference: Author
To examine and analyze the UHI characteristics, it is required to simulate the model for 24 h starting from 7 am
on July 17th till 7 am July 18th .For the analysis two local time zones were chosen to perceive the maximum
temperature through the day and minimum temperature during the night thus, Figure (6 a, b) shows the current
situation of the chosen area (CS) at 15:00 P.m. and 03:00 A.m.
Modelling the interactions between Urban Heat Island and Urban Geometry
A
B
Fig. 6 (a, b): (A) The simulated air temperature at 15:00 P.m. and (B) simulated air temperature at 3:00 A.m.
Reference: Author
5.2 Scenario 1 (HAM): changing the current materials used from low albedo materials to high albedo
materials.
In this scenario it is intended to change the Albedo properties of the materials from low to high solar
reflectance and from low to high thermal emittance used in the site whether on the roof tops of buildings or for the
hardcovers such as the roads, and pavements. Taha, Chang, Akbari, (2000) stated that improving thermal stress
and improve the quality of life through reduced temperature can be achieved through using high albedo materials
for both the roof tops and pavement. Therefore, this mitigation strategy is used to be examined due to its urgency
and feasibility as the area is characterized by hot weather in the summer in order to reduce the amount of incoming
solar radiation absorbed through building tops and through the hardscape thus, keeping their surfaces cooler.
Therefore, in this scenario, low albedo materials were changed to high albedo ones, such as asphalt is changed to
bright asphalt with albedo of 0.55, concrete was covered with white coating with albedo of 0.85. Using high albedo
materials. Hardscape elements such as Pavements and other elements represented in roads, pedestrian walkway,
bicycle path, parking area, gathering nodes are considered to be the main contributors to UHI development
(Tukiran, Ariffin, Ghani, 2017). Several studies have stated that 29% to 45% of the urban fabric are designed to
roads and pavements (Rose, Akbari, Taha, 2003), in the chosen case pavements covers 34 % and roads 19%. Heat
islands can be mitigated by using materials having a high albedo material fig 7 (a, b).
A
B
Figure 1: Simulation
Engineering area 15:00:01
17.07.2018
120.00
110.00
x/y Cut at k=3 (z=1.4000 m)
100.00
90.00
80.00
Air Temperature
< 32.71 °C
70.00
32.88 °C
Y (m)
33.05 °C
33.22 °C
60.00
33.39 °C
33.56 °C
33.74 °C
50.00
33.91 °C
34.08 °C
40.00
> 34.25 °C
Min: 32.71 °C
Max: 34.42 °C
30.00
20.00
10.00
0.00
0.00
10.00
20.00
30.00
40.00
50.00
60.00
X (m)
ENVI_met
70.00
80.00
90.00
100.00 110.00 120.00
N
<Right foot>
Fig. 7 (a, b): (A) The simulated air temperature at 15:00 P.M and (B) simulated air temperature at 3:00 A.M
for First scenario HAM
Reference: Author
Dina M. Saadallah, BAU Journal: Health & Well-Being, special edition, (2018), 155-168
5.3 Scenario 2(VEG): Increase the percentage of greenery in form of trees, shrubs and green roofs.
The effect of vegetation on urban microclimate have been broadly examined in various researches (Skelhorn,
Lindley, Levermore, 2014). Sododui, (2014) stated that greenery is the most effective and applicable mitigation
strategy that has a great impact on energy- saving through providing shade decreases the temperature of urban
area. The benefits of using shade trees are providing ecological solution such as reducing the noise levels, provide
shading, improving air through evapotranspiration and moreover, changing the wind pattern, also providing
pedestrians with pleasure visual scenes Accordingly, in this scenario, 60 % of the chosen site is covered by
vegetation whether in the form of landcover as grass, or shade trees with middle density canopies (Buxus hyrcana
hedges), shrubs around the buildings, finally green roofs for buildings top as fig8(a, b). Shade trees are not
considered as an ecological solution but also one of the important solutions that can be used to decrease UHI effect
and improve the urban environment and enhance inhabitants’ comfort.
A
B
Fig. 8 (a, b): (A) The simulated air temperature at 15:00 P.M and (B) simulated air temperature at 3:00 A.M
for second scenario VEG
Reference: Author
5.4 Scenario 3(HYBRID): Combining the previous two scenarios increasing greenery along with high
albedo materials.
The (Hybrid) scenario combines the both solutions together; increasing the percentage of greenery along
with changing materials characterized with high albedo factor, in order to examine the effectivity of both strategies
on reducing the temperature and accordingly decrease the UHI effect fig 9 (a, b).
A
B
Fig. 9 (a, b): (A) The simulated air temperature at 15:00 P.M and (B) simulated air temperature at 3:00 A.M
for third scenario Hybrid
Reference: Author
Modelling the interactions between Urban Heat Island and Urban Geometry
It is obvious, based on the Comparison between maximum and minimum air temperature for the mitigated
scenarios and current situation as resulted from simulation illustrated in figure 10, comparing the temperature of
current situation with the one achieved after applying the strategy and changing the albedo of built up areas, roads
and pavement. The albedo factor increased by 0.4 by changing asphalt to bright asphalt, absorbing only 20 % of
the sun’s energy, this is reflected in the cooling effect all over the site as the maximum and minimum temperatures
decreased by 0.39 ℃ and 0.51 ℃, respectively. Thus, overall temperature was decreased by 0.51 ℃ at 15.00 P.m
beak of heat stress, While, decreased by over all of 0.03 ℃ at 03:00 A.m., as temperature started to cool slowly
the temperature decreased by 0.03 ℃ and 0.01 ℃.
Fig. 10: Comparison between current situation and three mitigation scenarios in simulated air temperature ℃
Reference: Author
Studying the temperature pattern between street canyons (H/W) in three different sections represented in fig 11,
Shows contrasting patterns in the stress heat generated between the deep and shallow urban street canyons, in zone
A, for instance section 1, 2 showing a peak difference in air temperature of 1.54 ℃ and 1.2 ℃ higher than section
3 , where the canyons with H/W = 1 , while in zone 2 the street canyons doesn`t have much influence in temperature
pattern.
Figure 1: Simulation
Engineering area 15:00:01
17.07.2018
120.00
110.00
x/y Cut at k=3 (z=1.4000 m)
1
100.00
1
3
2
Zone A
90.00
80.00
Air Temperature
70.00
32.88 °C
< 32.71 °C
Y (m)
33.05 °C
2
33.22 °C
60.00
33.39 °C
33.56 °C
33.74 °C
50.00
33.91 °C
H
H
> 34.25 °C
Min: 32.71 °C
Max: 34.42 °C
30.00
Zone B
20.00
3
10.00
0.00
0.00
10.00
20.00
30.00
40.00
50.00
60.00
X (m)
ENVI_met
H
34.08 °C
40.00
70.00
80.00
90.00
W
2
W
0.5
100.00 110.00 120.00
N
<Right foot>
Fig. 11: Analyzing the temperature pattern between street canyons (H/W) in three different sections
Reference: Author
W
1
Dina M. Saadallah, BAU Journal: Health & Well-Being, special edition, (2018), 155-168
In Scenario 2, The greenery increased the cooling effect of the overall site due to shading and
evapotranspiration resulting in decrease in the temperature by 0.74 ℃ at 15:00 P.m. but high temperature was
noticed on the roads, as temperature decreased only by 0.24 ℃. Simulation reported slow cooling effect at 03:00
A.m., by overall decrease of 0.33 ℃, with min and max of by 0.08 ℃ and 0.33 ℃.
The most successful scenario is the one applying the HYBRID strategy in scenario 3, as the overall temperature
decreased by 1.17 ℃ at peak heat stress 15:00 P.m., reporting min and max temperatures decreased by 0.54 ℃ to
1.17 ℃, respectively. At 03:00 A.m., over all decreased temperature 0.34 ℃, with min and max of by 0.09 ℃ and
0.34 ℃. Table 2 shows reduction in temperature in the three scenarios.
Table 2: Comparison between reduced air Temperature in three scenarios
Reference: Author
Analyzing wind speed between different scenarios and current situation represented in fig 12, shows a significant
increase in wind speed between hybrid and current situation by 0.96 and 0.98 m/s respectively. Also, in second
scenario there is an increase in wind speed, then current situation. Table 3 shows differences in wind speed
between three scenarios in 15:00 P.m. and 3:00 A.m. This is a good indication that the existence of trees and
vegetation in site improved the movement of wind in the whole area.
Fig. 12: Comparison between current situation and three mitigation scenarios in simulated wind speed m/s
Reference: Author
Table 3: Comparison between increased wind speed in three scenarios and current situation
Reference: Author
Modelling the interactions between Urban Heat Island and Urban Geometry
Accordingly, to what has been mentioned and Summarizing the results, the three scenarios has a cooling effect all
over the site with different percentages the most cooling effect was noticed between 12:00 to 15:00 P.m., after that
time the cooling effect was minimal it was very obvious in tracking the temperature decrease at 3.00 A.m. The
most effective Scenario that has a noticeable decrease in temperature is the HYBRID one with on average decrease
of 1.17℃ during daytime, then the VEG scenario with 0.74℃ on average, finally the HAM Scenario with only
0.51 ℃ but this scenario reported unnoticeable decrease in night temperature by only 0.01℃. Therefore, the
HYBRID and VEG scenarios has the ability to a better mitigated UHI and nocturnal cooling effects for the chosen
site. Indicating that the lack of vegetation along the site with the used material with low reflecting and emission
ability are the main reason behind the high temperature, low relative humidity and high energy consumption by
air conditioning through the day and night time.
CONCLUSIONS
The mentioned results showed the capabilities of Envi-met in analytical modeling the different behaviors of
various surfaces from hard covers to preamble and how their effect on the air temperature, humidity and wind
pattern. In the proposed scenarios only the material properties and the percentage of vegetation were tested to
achieve results and recommendations that could be applied to the existing situation. As, analyzing Urban Geometry
and effect of H/W in the studied area didn`t show much effect on change in temperature pattern. This study showed
that all elements of urban street canyons influence the heat stress in different pattern through the street in the center
and the one on the edges, showing that shading is the dominant factor for achieving thermal comfort in dry/ hot
climate. Therefore, it should be taken in consideration more dynamic changes such as examining the effects of
changing the urban configuration from general layout of the buildings, orientation and their heights, to give
recommendations for the newly designed neighborhoods.
The Envi-met simulation showed consistency in the pattern of air temperature and humidity, the following results
were noticed as follows in each scenario , the first one (HAM) showed slight improvement in temperature of the
chosen site during the night and cannot be used as an effective mitigation scenario, although, using material with
high reflectance , materials that can save cooling energy use by reducing the heat gain and by lowering air
temperature in the neighborhood is of a crucial need due high sunshine per day in Borg El Arab. However, it
lowered the temperature by average of 0.51 ℃ during the day, and with 0.01 ℃ decrease in temperature during
the in the night. Then, in (VEG) scenario the percentage of vegetation is increased to provide more shading effect
and increase the cooling effect. The results showed decrease in the air temperature by average of 0.74℃ during
the day. These results showed the need to test the third scenario that combines both strategies which showed the
greatest cooling effect to the whole neighborhood. The average decrease in air temperature is 1.17℃ during the
day and 0.34 during the night and a very good modification of wind dispersion along the site as shown in figure
13.
Fig. 13: Comparison between current situation and HYBRID scenarios illustrating the wind dispersion
Reference: Author
Dina M. Saadallah, BAU Journal: Health & Well-Being, special edition, (2018), 155-168
This paper validate the benefits of combining two mitigation strategies which is increasing the percentage of
vegetation with changing the albedo characteristics of the used materials, as by numerical value it was proved that
each strategy could help in mitigating the urban heat island effect with a small percentage while combining both
had a higher influence on improving UHI in Borg El Arab city influence of greenery compared to high albedo
materials to mitigate the urban heat island intensity. The paper authenticate the crucial need for urban designers/
planners to develop their designs towards urban green planning and creating more green spaces , considering the
materials used all over the site using more sustainable ones and focus on responsive urban design towards microclimate , in order to save energy consumption, as each decrease by 1℃ can reduce energy demand by about 3-6
% (Akbari et al,. 2001), and reduce air pollution and prevents the overheating of sunlit surfaces, this would achieve
better Microclimate and thermal comfort that lead to better conditions of human comfort and enhancing the quality
of life.
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