The Ordinary Nature of Urban Heat Island by Zhenni Zhang

The Ordinary Nature of Urban heat Island s3696906 zhenni zhang

Contents 1.0 Introduction

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1.1 Urban Heat Island 1.2 Melbourne Urban Forest

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2.0 Observation

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3.0 Methods

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4.0 Challenge

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5.0 Conclusion

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6.0 Reference

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The Ordinary Nature of Urban heat Island With the problem of urban heat island and extreme heat in Melbourne, how does these issues affect the street trees in the future?

Melbourne is facing problems including flooding, drought, pollution of urban air, urban heat island effect, stormwater, and summer heat wave. With the increasing population of Melbourne and the trend of future high-rise buildings, the urban heat island and extreme heat in Melbourne are becoming seriously affect human beings health and urban forest. This special section present research that the relationship between urban heat island and urban forest. And also proposed several solutions to help alleviate urban heat island and grows of an urban forest.

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INTRODUCTION

Lineage diagram

1.1 Urban Heat Island 1.1 Description of the Urban Heat Island

In 2014, the State of Climate announced Australian temperature will become warmer. The temperature will be rising 0.6 – 1.5 °C in 2030 compared with the temperature of 1980 to 1990.

The whole world is becoming warmer, particularly in the urban area. In an urban area, this heating called urban heat island. In 1818, the British scientist Luke Howard probably is the first one to propose that air temperatures in cities are higher than suburbs. Urban heat island refers to the phenomenon that when the city develops to a certain scale, the temperature in the urban area is significantly higher than that in the suburbs due to the change of the nature of the surface of the city, air pollution and the discharge of artificial waste heat, forming a high-temperature island. Figure 1 Shows the difference in temperature between cities and suburbs throughout the day. Figure 2 shows the temperature of urban heat island for a different area in Melbourne. From these two figures, we can clarify the different temperature of suburbs and cities.

Figure 1 Shows the difference in temperature between cities and suburbs throughout the day.

Figure 1 ( A. M. Coutts, J. Beringer 2009 )

Figure 2 is a description of the urban heat island in Melbourne

Figure 2(Urban forest strategy 2012-2032) 4

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Four main reasons for the urban heat island effect.

● Impervious hard surfaces: A large number of buildings and roads in the city are mainly made of masonry, cement, and asphalt (Figure 3). These materials have a much higher heat capacity and thermal conductivity than the suburban natural interface, and they are less reflective of sunlight and more absorbent. Therefore, during the day, the surface temperature of the city is much higher than the air temperature. For instance, in summer, when the grass temperature is 32°C, the canopy temperature is about 30°C, the cement ground temperatures can reach 57°C, while the temperature of the asphalt can be as high as 63 °C. Meanwhile, the hard surface of the city is poor water retention, water evaporation dissipation of heat, so the latent heat of the city is a large high temperature. Dense buildings in the urban area(Figure 4), as well as the roads and Bridges, constitute the rough surface of the city, so the wind resistance increases, the wind speed decreases, and the heat are not easy to be lost.

Figure 4 Figure 4 shows Melbourne building height. The darker of the color, the taller of the building The valley between the tallest buildings in the city is also a cause of the heat island effect.

The city’s tall buildings and the middle hard surface provide a lot of reflection and absorption of sunlight. Moreover, these buildings block the flow of wind in the city. When the wind cannot flow freely, convection cooling does not occur and air pollution stagnates and increases. As a result of this combination, city temperatures have risen and ozone levels within the city have increased. Such as buildings around Swanston street and A'beckett streets are mostly Super high-rise apartment. Therefore in this place would have the urban canyon effect.

Figure 3 Figure 3 shows the hard surface in Melbourne, and it can be seen from this picture that the hard surface in Melbourne accounts for a large proportion. This is one of the main reason for the urban heat island effect.

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Human activity: Human activities generate a lot of heat from exhaust gases, such as car energy, factory heat. For example, when people use the air conditioner in the hot summer, they will release a lot of waste heat into the air.

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Air pollution:

In China, air pollution is another big issue. Because air pollution will aggravate the urban heat island effect. And also under the influence of the heat island effect, the clouds and fog above the city will increase, causing the accumulation of harmful gases and smoke and dust over the city and forming air pollution. These absorb radiation from the surface, creating a greenhouse effect that causes the atmosphere to heat up further.

Tree canopies 2018 entire municipal area (Urban Forest)

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1.25

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5 Kilometers

Low vegetation coverage: In Melbourne CBD, people use more land to build roads and

buildings. So there is less vegetation coverage in the city. The loss of plants means that cities have less natural evaporation. In the summer city, when the plant shade is large, it plays a role of cooling and humidifying in the local area. In the plant configuration, the resistance of the multi-vegetation area is small, which contributes to the air flow and thus reaches a cooling and humidification. The role of attracting people to a greater extent in this area.

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Useful life expectancy of Melbourne’s trees

‘Useful life expectancy is an estimate of how long a tree is likely to remain in the landscape based on health, amenity, environmental services contribution and risk to the community’ (Urban forest strategy 2012-2032).

The urban heat island effect ● People: People who live in the high-density areas of the heat island for a long time

will show emotional restlessness, listlessness, depression, and depression, memory decline, insomnia, appetite loss, dyspepsia, increased ulcers, gastrointestinal disease recurrence, etc, which bring troubles to people's work and life in the city. The black Saturday heat wave of 2009 brought the number of deaths in the city center to 374, significantly higher than normal and almost double the number of deaths from fires. So the heat wave in Melbourne is threatening people's lives. The urban heat island not only affects human health but also killing urban trees. ● Plants: The urban heat island effect seriously confuses the dynamic process of air flow. First of all, the urban ecosystem is a special ecosystem for the plants in the city, which is obviously different from the forest ecosystem. Secondly, the local mean temperature rise caused by the urban heat island effect is more obvious than that caused by global warming.

Therefore, plants living in cities for a long time must respond to the urban heat island effect.

Useful life expectancy estimates could lead to better planning for Melbourne's urban forests.

A Useful life expectancy an evaluation of 35,000 trees in Melbourne in 2011-2012 showed that 23% of the trees would end their landscape life in 10 years, increasing to 39% in 20 years and 16% in 10 years. From the data of urban-forest-strategy, there are 55% of Melbourne’s elm trees have less than 1 year to 10 years useful life expectancy, 21% elm trees have 11 years to 20 years useful life expectancy, 11% elm trees have 21 years to 30 years useful life expectancy, 9% elm trees have 31 years to 60 years useful life expectancy and 4% elm trees have over more than 61 years useful life expectancy. So as we can see, there are a lot of old trees in Melbourne CBD. And the urban heat island would hasten the death of these trees.

As can be seen from figure 5, the lighter of color, the less tree useful life expectancy. So most of the trees in Melbourne CBD are already quite old. Meanwhile, they are less tolerant of extreme weather. The urban heat island effect can exacerbate these tree deaths.

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OBSERVATION claim ground ‘Many of Melbourne’s aging trees, including those in our iconic streets and parks, are well over 100 years old and approaching the end of their useful life. Elms planted in the late 1800s such as those in Fitzroy Gardens, Royal Parade, Flemington Road, Fawkner Park, Alexandra Avenue and St Kilda Road were planted in socially, culturally and environmentally different times’(Urban forest strategy 2012-2032).

Nowadays, 74% of trees in melbourne are elms and in the future it will decrease to 25%. So Melbourne urban forest is increasing the diversity of street trees. Such as adding some species that are grow fast, have fewer pests and diseases, less maintenance, and are highly tolerant to the environment.Such as Lilly-Pilly, London Plane, and Kurrajong.

This 3D point cloud data shows the context of building, surface and vegetation at the Russell and Bourke streets

Image 1 shows the thermal image of Melbourne, through the top view, it shows the heat absorption and storage degree of the road surface, roof and green space in the city.

. Image 2: A thermal image at the corner of Russell and Bourke Streets, through the human view showing the heat storage and reflection of the facade of the road and the building, it can be seen that plants are the best way to solve the heat island effect.

(Photograph: Jason Dowling, courtesy City of Melbourne). These two pictures directly show the difference between the ground temperature and the surrounding temperature.

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As can be seen from the figures, buildings and roads account for a large proportion, and the number of green plants is completely insufficient to improve the urban heat island effect.It can be seen from the figure that the green space covers a very small area in the whole area. So during summer, the area will be very hot. This is the case in most of Melbourne CBD.

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METHODS 1. Urban vegetation, water, and wetland are important components of the urban ecosystem, which can alleviate urban environmental pressure, reduce heat island effect, and finally realize the virtuous cycle of an urban ecosystem. The urban heat island is damaging the urban forest, however, the most effective methods to mitigating heat in the urban area are an urban forest. So there are still a lot of challenges to the urban forest, including In Melbourne city, most of the old trees need to remove in the future.

From the Urban forest strategy 2012-2032, it will take 20 years for a tree to reach the size that would have an impact on the urban heat island effect. Urban heat island effect leads to a decrease in soil water content, which makes street trees have insufficient water to survive. Therefore, artificial watering should be increased in the hot season or dry season to ensure the health of trees. For newly planted trees, some tree species with fast growth rate and high environmental tolerance should be selected.

4.Install green roofs and cool roofs are also a good way to solve the urban heat island effect. The green roof is not only the roof is planting, but also for balcony, wall, at the top of the underground garage, overpasses. That means the special space not all connected to ground, natural, soil. Green roof can increase the urban green space and improve the deteriorating living environment, to improve the standing of tall buildings in cities and the status quo of natural land and plants replaced by hard pavement in many roads; to improve the excessive deforestation of natural forests, various waste gas pollution and the formation of urban heat island effect, dust storms and other hazards to human beings.

It is of great significance to open up human green space, build the garden city, improve people's living condition and quality of life, beautify the urban environment and improve the ecological effect.

The cool roof is a new roofing system that uses special materials to increase the reflection of sunlight and reduce the absorption of energy by the building, thereby reducing the temperature inside the building. Cool roof is a relatively fast and effective way to reduce urban heat island effect.

2. The use of reflective glass should be reduced on the facades of buildings.

The facades of a large number of high-rise buildings in the city are made of glass, which will cause a local temperature rise. And trees need sunlight for photosynthesis, not the reflection of glass from tall buildings. Therefore, the facades of buildings in the city are made of low reflective materials, which can also be improved by adding vertial greenery.

3. Artificial heat reduction The government can control the green coverage in public areas, but not in private courtyards. So the government should take some measures to encourage people to plant green lands. It is necessary to indoctrinate urban residents with the concept of environmental protection. It is advocated that everyone and every family should integrate the concept of environmental protection into every aspect of their daily life and make it a conscious act. The change of residents' daily lifestyle will be long-term and meaningful work, which can directly reduce the waste heat in transportation, air conditioning, cooking, and industrial production. In addition, improve the efficiency of energy utilization, implement clean production, or develop and utilize new high-efficiency and environmentfriendly energy. For example, the impact of urban heat island effect is vigorously publicized to make people feel the seriousness of the problem, thus increasing the green coverage of private land.

Figure 5 shows the cool roof in Melbourne. As the figure shows, Melbourne is starting the plan of cool roof.

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Long term and short term strategy in Melbourne

PRECEDENT STUDY PHOENIX ROOFTOP Retro-fit Green Roof _ Melbourne CBD VIC. Australia _ Completed 2017 This is a successful project to retrofit the roof. In this project not only creat a new place for human activity, but also creat a situation for plants.

Melbourne has a lot of opportunities to retrofit. So we still can do a lot in future.

For short term strategy, the government should promote the construction of green roof and cool roofs to achieve a reduction in heat in a short period of time.For long term strategy, the trees on both sides of the street should be maintained and the number of trees on the street should be increased. It is also possible to establish some vertical greening to increase plant coverage.

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CHALLENGE According to ABC news, Melbourne water suppliers are preparing for a possible water crisis after a decade of running out of the water, according to a new report by authorities. Melbourne's water consumption has risen rapidly in recent years as a result of its rapidly growing population, and the likelihood of a drought has been exacerbated by climate change in recent years. So in the future, Melbourne may do not have enough water to support people's life. And whether there is enough water to irrigate the street trees is also a challenge.

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CONCLUSION Impervious surface, human activities, low vegetation coverage, air pollution and other causes of urban heat island effect. The urban heat island leads to the accelerated death of plants as the trees in the city become aging and less tolerant of the environment. Moreover, new plants need to grow in a cycle and cannot alleviate the urban heat island efficiently. To respond to these methods put forward a series of mitigation measures: 1. The old trees need more maintenance to ensure health 2.Increase green space, increase the roof garden and cool roof 3. Reduce the building's facade reflex, increase vertical greening 4. new urban planning, increase the permeability material used 5. enhance the human environmental protection consciousness, control the release of the heat.

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REFERENCE Urban forest strategy making a great city greener 2012-2030, City of Melbourne, viewed 25 March 2019,<https://www.melbourne.vic.gov.au/SiteCollectionDocuments/urban-forest-strategy.pdf> Urban forest Diversity Guidelines , 2011 tree species selection strategy for the city of Melbourne A. M. Coutts, J. Beringer, S. Jimi2 and N. J. Tapper 2009, The urban heat island in Melbourne: drivers, spatial and temporal variability, and the vital role of stormwater, viewed 25 March 2019,<https://www. researchgate.net/publication/266267164_The_urban_heat_island_in_Melbourne_drivers_spatial_and_ temporal_variability_and_the_vital_role_of_stormwater> Australian Government Bureau of Meteorology,State of the Climate 2014, viewed 27 March 2019,<http:// www.bom.gov.au/state-of-the-climate/2014/> central city urban forest precinct plan 2013-2023,City of Melbourne, viewed 23 March 2019,<https:// www.melbourne.vic.gov.au/SiteCollectionDocuments/UFPP_Central_City_Precinct.pdf> United States Environmental Protection Agency, What You Can Do to Reduce Heat Islands, viewed 1 May 2019, <https://www.epa.gov/heat-islands/what-you-can-do-reduce-heat-islands>

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