Cooling strategies for thermal comfort in cities: a review of key methods in landscape design

Under the climate change scenario, the negative impacts of urban heat island (UHI) will exacerbate due to unsustainable urban planning and human activities. Thermal comfort has close relationships with UHI in urban areas. This paper is based on the studies of urban heat island, thermal comfort, microclimate, and urban planning in cities in the recent decade, combined with a method of research into design. The key topics include vegetation and water conditions, the albedo of materials, and urban morphology. By the comparative case studies in landscape projects, the results further reveal that the density of tree canopies, the natural structure and density of ground cover, the form of water features, the color and texture of materials, and the scale of shading structures have different cooling effect and performance in outdoor thermal comfort improvement with specific features in the landscape design. It is also found that there are some external conditions that can influence design determinations in real practices. The purpose of this study is to provide theoretical research methods and evaluation of thermal comfort landscape design elements and to provide guidance for future sustainable city research and landscape design.


Introduction
In the contemporary world, around half of the human population lives in urban areas. It is expected that the number of urban residents will increase by 70% of the present world urban population by 2030 (Urban Heat Islands 2019). Population growth and the shortage of urban land supply have led to increased pressures on transportation and the environment, which are extremely unsustainable for cities (Rehan 2016). Gradually, the unsustainable urban sprawl, the removal of vegetation, the increasing numbers of cars, the large amounts of energy consumption, impervious surfaces, and air pollution have contributed to the urban heat island (UHI) effect. This negative effect creates warmer and drier conditions in urban areas than in their surrounding rural areas and decreases the level of thermal comfort and quality of life (Grilo et al. 2020). From a wider perspective, to prevent and adapt to the rising temperature in the future, to balance the urban ecosystem and maintain the high quality of the living environment with more acceptable and comfortable temperature for urban citizens, it is necessary to explore, develop, test, and update preventive solutions under feasibility to alleviate urban heat island and improve thermal comfort in cities no matter in the field of urban planning, architect, or landscape.
In the scope of landscape, the purpose of this paper is to provide suggestions for outdoor thermal comfort improvement in landscape designs with effective and adaptable strategies and elements that are explored from thematic research and case studies. It also aims to provide theoretical guidance with a certain reference value and recommendations for future studies in thermal comfort improvement or studies in urban thermal conditions. This paper is started with a brief introduction of the relative definitions and the relationship between the urban heat island and thermal comfort as a general knowledge foundation. The research is based on the "cool city" concept combined with the theoretical basis. Expanded and studied the landscape elements that affect the thermal comfort of the urban environment. The research framework is relatively comprehensive, systematic, and targeted in the landscape field. Furthermore, by the method of research into design, we have analyzed the three successful precedents that reflect the practicality of theoretical knowledge in the thematic scope of thermal comfort, and reveal a visible application and transformation of theoretical strategies in design features in landscape projects. Finally, this study further discussed the problem of effectiveness and adaptability of different strategies for improving thermal comfort from a critical point of view.

The definition and relationship-UHI and thermal comfort
Urban heat island (UHI) The urban heat island (UHI) is defined as those urban areas or metropolitan areas that are significantly warmer than their surrounding rural areas mainly due to human activities, especially the anthropogenic heat produced mainly through the actions of heating and cooling plants and buildings, industrial activities, and the heat mainly produced by CO2 emission from vehicles (Rehan 2016). Additionally, this effect is also caused by impervious surfaces, such as concrete and asphalt instead of natural evaporative surfaces (vegetation), which absorb a large amount of solar heat from radiation during the day (Ahmed Memon et al. 2008).

Thermal comfort
Thermal comfort was defined as people's satisfaction with the thermal environment of the space between buildings (Watkins et al. 2007). There are many urban conditions and factors that could affect the thermal environment, including air temperature, air humidity, wind speed, urban form, urban space configuration, and human activities. Psychological emotions could also influence people's perceptions and subjective feelings about the environment.

Causal relationship and future trend
Many studies and evidence have emphasized and predicted that the most serious impact of climate change is the continuously increasing temperature in the future. As a result, the worry is that the negative impacts of UHI will be exacerbated under the climate change scenario. Furthermore, the constant high temperature in cities during the summer will directly lead to some negative effects, threatening public health, well-being, and quality of life. The persistent summer heat can cause health problems for urban residents, such as heat waves that have led to increased morbidity and mortality (Gabriel and Endlicher 2011). Therefore, from a long-term perspective, improving the urban thermal environment is one of the important steps in the near future to deal with climate change, to prevent and mitigate the negative impact of UHI, to maintain a healthy urban ecology, and to improve people's wellbeing and the quality of life.

Key factors in thermal comfort
Based on the urban context, the microclimate conditions and anthropogenic activities that affect thermal comfort contain lots of elements. Among the urban microclimate conditions, the key factors that are related to the urban thermal environment mainly include air temperature, humidity, and wind conditions. The anthropogenic activities dominated by urban planning and design can be identified as one of the key factors.

Air temperature
Air temperature is the most direct factor affecting thermal comfort because the human body's perception of the ambient temperature is most obvious than other factors. Air temperature is the simplest factor to easily measure and understand (Watkins et al. 2007). In general, people would feel uncomfortable and hot in an environment where the temperature exceeds 25°C. While in an environment where the temperature exceeds 37°C, people would feel extremely uncomfortable and hot. Brager and de Dear also pointed out that even under the climate change scenario, the temperature preferences of people are relatively consistent and similar (Brager and de Dear 1998).

Humidity
The relative humidity of the environment is negatively correlated to the intensity of the urban heat island. A higher relative humidity corresponds to a lower UHI intensity (Santamouris 2015). The influence of air humidity on thermal comfort is mainly related to evaporative cooling. Humidity as a kind of invisible phenomenon can be manipulated by devices through technology or can be promoted by blue and green infrastructure.

Wind condition
The speed at which air moves over the surface of the body is highly influential in the heat balance. Therefore, the wind speed can affect people's sensation of thermal comfort (Watkins et al. 2007). This is because the wind could remove the heat released by the human body through the moves. It is useful that increase the wind speed under proper control allows heat to be quickly removed, making it easier to create a cooler environment. Santamouris had proved this point of view by analyzing and comparing the characteristics and magnitude of the heat island effect in different regions. One of his findings shows that the increase in wind speed makes UHI intensity lower in the same area. Strong winds can change the cooling rate of urban and rural areas (Santamouris 2015).

Urban planning and urban design
The influence of urban planning and urban design on thermal comfort lies mainly in the reflectivity of urban structures to solar radiation. The temperature of the air mainly comes from solar radiation. In cities, due to impervious surfaces and dense urban buildings, solar radiation is reflected multiple times. The total amount of solar radiation absorbed by these urban structures during the day increases, at the same time, the heat released at night also meets an increase (Doulos et al. 2004). This is one of the reasons why city temperatures are generally higher than surrounding rural areas. Besides, the H/W (height/ width) ratio which is related to the urban configuration has a great influence on the temperature distribution in the urban area (Jamei et al. 2017).

Theoretical basis and methodology
The cool city is one of the sustainable urban solutions for the city of tomorrow that depends on the application of the principles of urban heat management. It is the key factor to diminishing the urban heat release, creating solutions for future climate change by reducing the volume of global emissions, and creating smart growth and cool community scenarios (Rehan 2016).
How to reduce heat accumulation in urban areas? Rehan proposed corresponding strategies in the framework of a cool city, and emphasized that cities can alleviate the UHI effect through a series of planning actions. This cool city framework can help improve the urban thermal comfort, the air and water quality, reduce energy consumption, and prepare a sustainable and resilient urban environment for combating future climate change.
This study is based on Rehan's cool city framework as horizontal guidance of thermal comfort improvement in urban planning and landscape design (see Fig. 1). The vertical theoretical lens in this paper comes from landscape theories mentioned in Herrington's book, including forming, material matters, and system logic (Herrington 2017). From a landscape perspective, tree canopy and ground cover are usually used as the design process of green infrastructure, and water features are a kind of engineering infrastructure. The color and form of materials reflect the design formalism. The selection of material texture and consideration of essential performance is a kind of tectonic expression revealing the truth of materiality. After identifying the key factors from theoretical review, the methodology is research into design through precedent studies.

Cooling strategies
Based on the methodology mentioned above, cooling strategies with detailed actions need to be further classified and differentiated in combination with key factors related to thermal comfort. Therefore, the strategies will be divided into five parts to further illustrate each strategy with different urban elements and effective planning and design actions.

Urban greening-vegetation
Increasing urban green space and vegetation coverage is a common and effective method to improve pedestrian thermal comfort and reduce heat islands based on a natural solution. The tree canopy increases the relative humidity of the environment through shades and evapotranspiration to avoid heat accumulation. Green spaces can regulate the microclimate, helping to reduce urban temperatures and increase humidity. Moreover, with higher tree density, green spaces can provide a better cooling effect. In terms of the effective range of green spaces, they can affect the air temperature and humidity within 60 m at the farthest (Grilo et al. 2020) (see Fig. 2). In addition, the natural evaporative surfaces (vegetation) could absorb a large amount of solar heat from radiation during the day (Vieira et al. 2018). The more complex vegetation types have a higher ability to adjust the microclimate. Multi-layer plant communities are one of the most effective approaches in cooling and humidifying (Zhang et al. 2013). In terms of the studies of planting design in recent years, it is recommended to select suitable tree species according to their characteristics in the right places with an effective density (Altunkasa and Uslu 2020; Morakinyo et al. 2020;Hami et al. 2019).

Urban greening-water
Water bodies can effectively improve human comfort in coastal areas, especially during the hot and hot summer months. Area 10-20 m from the water edge showed the greatest improvement in thermal comfort (Xu et al. 2010). In addition, appropriate planting of vegetation can produce a synergistic effect and make the water body have a more positive and efficient impact on human comfort. Flowing water has a larger cooling effect than stagnant water. Moreover, dispersed water like from a fountain has the biggest cooling effect (Rehan 2016).

Albedo-materials
Materials with high reflectivity obtain less solar heat during the day and therefore have a significant effect on reducing urban heat islands and improving the thermal environment. The reflectivity of the material to solar radiation during the day will affect the thermal balance of the material surface. This depends on the color, construction, and surface texture of materials. The outdoor paving materials characterized by smooth, light-colored, and flat have a more significant cooling effect on improving thermal comfort (Doulos t al., 2004) (see Fig. 3).

Ventilation-urban morphology
The size of the city, the width of the street, the spatial layout, the geometry of buildings, and the materials of the facade which are related to urban planning and design could affect the intensity of UHI and the thermal environment in urban regions. The cooling effect is mainly achieved by establishing urban ventilation corridors and street canyons. Wind penetration in deep canyons can significantly improve pedestrian thermal comfort (Jamei et al. 2017). However, the establishment of urban ventilation corridors needs to consider many planning factors and environmental impacts, such as the aspect ratio of buildings, the sky view factor of public open spaces, the width of streets, the materials of outdoor spaces, and the interaction of these factors with microclimate conditions (Hsieh and Huang 2016).

Other aspects
The methods to improve thermal comfort also include technology, policy, engineering, and energy strategies. Strategies such as new clean energy replace traditional energy, green roof, and green wall engineering facilities; artificial shade structures are useful in thermal comfort improvement. However, all the strategies require related planning and policy support to mitigate the UHI effect.

Landscape practices-comparative precedents
In real projects, thermal comfort improvement usually is not served as the only design goal or intention in any project, and it is often used as an additional benefit in sustainable design. Especially, in tropical cities or cities with extreme atmospheric conditions, improving the urban thermal environment may be considered as one of the initial design goals.
The analysis of precedent studies will compare the design features that are conducive to improving thermal comfort in landscape projects. It aims to find out the application of key factors in thermal comfort improvement and the determinations of design features in different project contexts. Corresponding to the cooling strategies mentioned above, the three precedents will be analyzed in four factors that are related to thermal comfort improvement: vegetation, water, materials, and structure. Because there is no obvious ventilation corridor design in these three projects, the strategy of ventilation will not be analyzed further.
"The Park" in Las Vegas, Nevada "The Park" is located in Las Vegas in America, designed by !MELK in 2014 (see Fig. 4). Las Vegas exists in extremely challenging arid regions, exposed to sunlight, high temperatures, heat, dust storms, and water shortages. One of the design goals is claimed to mitigate these extreme situations and to create an enjoyable urban experience for the public (The !Melk Team 2016).
Overall, most areas of the site can be shaded by the canopy or shading structure at a certain time of the day (see Fig. 5). But there are still some areas exposed to the sun all day long. When looking into these exposed areas, it can be found that water features are distributed in a part of these areas. These potential hot areas are alleviated and cooled to a certain extent due to the presence of water bodies. In addition, it is noticeable that the centralized large-scale art structure provides a large area of shade for the park from 5 AM to 8 PM during the day.

Darling Quarter in Sydney
This project is located in Sydney in Australia. The design goal of this project claimed by ASPECT Studios is to create premium quality and highly sustainable public realm by upgrading the plane materials, lighting, furniture, and plants (ASPECT studio 2011). The landscape design focuses on the playground, providing a safe and cool public space for children and their parents. Different types of play experiences provide children with a rich play environment (including interaction with nature, water, and safe play facilities). Therefore, some of the sustainable design methods which are related to thermal comfort improvement can be found in this area (see Fig. 6).
It can be seen from the analysis map (see Fig. 7) that almost all the visible design features that can effectively improve thermal comfort are concentrated in the playground, making this area the coolest place in the whole project. The street between the buildings is the second comfortable area because of the small water pool and tree canopies along the street; the shadows provided by the buildings are beneficial to cool the environment. However, in other public open spaces on the south the building where are not covered by the shadow from buildings and without water that benefits these areas, the shading zones are inadequate for the daytime in summer. Although large areas of the surface are paved by turf, the direct solar radiation making these areas received more heats than the playground.

Gubei Gold Street in Shanghai
The promenade is 700 meters long and 50-80 meters wide, which consists of three different blocks (SWA Group 2009) (see Fig. 8). The project area is blocked by two large open parks perpendicular to the promenade. These parks connect the promenade to adjacent communities. Create an "urban nature" landscape through plants and water features. Due to the exhaust from heating, air-conditioning, and motor vehicles, as well as the sharp rise in the ambient temperature in Shanghai in the past decade, the urban environment is being Fig. 3 Definitions of "cold" and "warm" materials (Doulos et al. 2004) threatened by unsustainable development and climate change. Based on this reason, SWA Group claimed that they take the sustainable strategy as a method to improve the urban ecological environment and thermal environment, aiming to provide a safe, multi-functional, and sustainable continuous walking space for people of all ages.
From the analysis map (see Fig.9), the canopies on the west side of the promenade are larger and denser, and there is a large area of water features. Therefore, compared to the smaller and sparser canopies cover on the east side, the west side of the promenade is cooler. Except for the three intersections, the middle section of the promenade has a large area of hard paving and no shade coverage. The original intention of this part of the design is to provide a public place for people to conduct social activities. However, the thermal environment here is not satisfactory because there is no direct shade to avoid sun radiation, even though the design team added fountain facilities in the area. Therefore, it is recommended to increase green lands or ground covers instead of large areas of hard paving in the central public places where accommodates most of the activities in the promenade. In general, in the scope of vegetation, water, and materials, this landscape project had relatively good considerations in thermal comfort improvement in landscape design.

Comparative results
Comparing the design features and performance of these three precedents in materials, vegetation, water features, and artificial structures (see Table 1), the results show some similarities as well as differences in different public open spaces in these three cases. It could be found that the design determinations of vegetation (especially the tree canopy), artificial shading structures, and water features settings are commonly used for many reasons including to improve the thermal comfort and mitigate the urban heat island in cities. Due to the differences in tree canopy density, the patterns of natural ground covers, the scale of structures, and the form of water features, and the final cooling effects show distinguishes. Generally, the high canopy density, the shading structures with tall and wide size, and the moving or dispersed waterscape have a better cooling performance. The commonly used method in paving materials is a light-colored stone with mosaic texture to prevent the site from absorbing excessive heat during the day.
In general, tree canopies play an important role in thermal comfort improvement. Different densities of tree canopies in these three precedents might result from the different heights of the surrounding buildings and the distribution and allocation of other artificial shading structures. While the selection of plant species always prefers local and ornamental as a priority, for the purpose of better cooling effect, the species, scale, and height of plants should be decided with more care.
The design of the water features has two main purposes, one is to provide viewing and interactive functions, and the other is to provide some cooling effects in hot summer or as a design element to create a cool outdoor environment. Therefore, the demand for water features mainly depends on the microclimate conditions of the site, the site context, and the design goals of the project. These are also the prerequisites for evaluating whether there is a demand for additional artificial shading structures. Once artificial structures are decided to use for shading pedestrian, appropriate scales and forms need to be considered in the shading structure.
Since there are many features that can affect the thermal balance of the material, the selection of materials is more flexible, because each feature can be selected individually or matched together. The superposition of multiple advantageous features can achieve the best cooling effect in terms of materials. But in actual projects, different degrees of thermal balance considerations are often reflected in the use and selection of materials. This could be attributed to differences in project budgets, design goals, and local resources.

Discussion and recommendations
The effectiveness and adaptability of different strategies for improving thermal comfort have occurred some consensus A few studies starting from the two key factors of microclimate conditions and urban planning are often presented in two experimental models. One is to study the relationship and changes of local microclimate conditions and heat intensity under the same scenario (single or unique urban context) by manipulating atmospheric parameters. The other is to observe and deduce the general relationship between microclimate conditions and heat intensity through the manipulation of atmospheric parameters under different scenarios (multiple urban contexts). The former aims to formulate the most effective strategies for certain specific areas or regions, while the latter aims to summarize and develop more adaptable methods in cities. To cope with the climate change crisis soon, the effectiveness of thermal comfort improvement has received more attention and development. However, in the face of potential crises in the far future, the applicability of the strategy requires more support and longerterm research to repeatedly test, modified, and update.
Based on the discussion about the effectiveness of related strategies for improving urban thermal comfort, the consensus is that natural-based solutions (such as green spaces dominated by trees) should be given priority because they have a more effective cooling intensity and wider cooling range. However, plant strategy, as one of the key strategies to improve thermal comfort, needs to be carefully considered and selected, because the plant species, sizes, and locations can largely influence the cooling effectiveness. And also, the changes in daily shade from surrounding buildings could affect the growth of Another effective method is to use reflective materials to replace impervious outdoor paving to avoid solar heat storage and release. Additionally, "shade plays an important role in designing pedestrian-friendly spaces in cities because it has a major impact in improving the thermal comfort and that artificial and natural shades are equally efficient." (Klok et al. 2018). Based on the microclimate conditions and built form of the target site, it is necessary to further test and simulate the summer sunshine time and shadow coverage area of the site in order to obtain a more accurate assessment and identification of areas that need to improve thermal comfort.
On the other hand, due to the subjectivity of thermal comfort, it is necessary to comprehensively consider human physiological comfort and psychological needs, and further study other factors that can affect human perception, such as acoustic environment, air quality, and urban landscape appreciation (Jusuf et al. 2018). According to the acoustic study written by Patón et al. in 2020, there is a close relationship between sound parameters and comfort. Sound has a significant effect on people's mental health and comfort (Patón et al. 2020).
It is also worth noting that in some of the practical research, strategies are complementary and have a synergistic effect (see Table.2). On the opposite side, some of the authors show that there are also negative results when overlapping these strategies. In open locations exposed to direct sunlight, when the climatic condition is extremely hot, high humidity and low wind speed will exacerbate discomfort (Vanos et al. 2019). Therefore, in general, comprehensively consider the microclimate in urban planning and the selection of design technology to achieve a better cooling effect. When adopting multiple measures, it is necessary to further consider the geographical and atmospheric conditions, the built forms, resources, policies in the unique urban context at a local level with professionals when developing urban planning or landscape design proposals.

Conclusions
In conclusion, the significance of this research is to provide theoretical research methods and practical applications as -large areas of light-colored paving; -dark-colored paving surround the water feature; -smooth and flat surface; -paving stone, concrete, wood; NO structure for shading Use of evaporative cooling from lakes may increase humidity and decrease thermal comfort efficient guidance for future thermal comfort landscape design. In theory, the analysis of vertical and horizontal theoretical lens will help improve the accuracy, breadth, and systematicity of future related research. In terms of practicality, it also gives clear guidance for the future urban thermal comfort landscape design, that is, through the combined planning of materials, vegetation, water features, and artificial structures, and other considerations to achieve a better cooling effect.
In the past five years, under the global trend of sustainable development goals, the planning and design patterns of urban greening on the nature-based solution have become the dominant aspect of studying outdoor thermal comfort. There are also many related studies and applications in material performance evaluation and environmental engineering focus on surface heat balance. However, relatively speaking, although the cooling effect of water bodies is generally recognized, the studies review in the recent decade is often limited to the existence of water bodies and their forms rather than multiple modes of practical use. In addition, the synergy effects and interactions between different cooling strategies and design factors have not received much attention.
The close relationships between negative effects provide opportunities for professionals and experts to further consider and explore mitigation measures that could benefit in diverse aspects because of interactions. For example, improving the thermal comfort of the public open spaces not only enables the outdoor temperature to maintain an acceptable level in summer, but also encourages urban citizens to touch nature or conduct social activities in the public domain with a comfortable environment at any time instead of relying on the constant indoor environment maintained by air conditioners. To some extent, this is also beneficial in the reduction of energy consumption.
On the other hand, the high-temperature conditions are predicted to deteriorate in many cities. Overreliance on refrigeration devices and increased demand for irrigation by plants in high-temperature conditions has accelerated energy consumption. This situation is particularly serious in dry and hot tropical cities as well as in developing countries because the continuous longterm high temperature, water shortage, and the use of traditional fuels (such as coal and petrol) have brought greater environmental burdens.
Finally, for future thermal comfort studies in landscape design, it is recommended that the improvement of urban outdoor thermal comfort is not only to maintain people's physical bodies to a comfortable state, but also need to pay attention to those outdoor landscape factors that can affect people's psychological perception, such as sound and beauty. Therefore, how to create a psychologically and sensory outdoor environment from those invisible factors is another important angle for future research on urban thermal comfort.