Urban heat island (UHI) refers to the phenomenon that temperatures in the urban area of a city is significantly higher than those in the outer suburbs. With the development of the economy and society, under the influence of climate change and other factors, the UHI gets increasingly obvious, which not only causes extreme weather such as hot weather and heavy rain but also negatively affects the growth of flora and fauna in cities. Medical studies show that high ambient temperature can easily lead to human irritability and mental disorders, and furthermore, it even increases the incidence of heart and respiratory diseases, thus resulting in an increase in mortality (Peng, 2020).

In recent years, scholars have carried out a large number of studies on the UHI, by two major research methods of on-site measurement and estimation based on satellite data (Ouyang et al., 2021) (Amindin et al., 2021). Previous studies have focused on the following facets: the temporal and spatial characteristics, cause of formation, influencing factors, mitigation measures of UHI, the relationship between UHI and air pollution, and the impact of UHI on human health (Tian et al., 2021) (Zhang et al., 2007) (Park et al., 2017) (Li et al., 2014) (Wang et al., 2021) (Laaidi et al., 2011). Studies have found that meteorological characteristics (e.g. wind speed, humidity, etc.) (Zong et al., 2021) (Alekseeva et al., 2019), urban spatial morphology (e.g. building density, floor area ratio, night light intensity, etc.) (Sun, 2020)(Liu et al., 2020)), and land use (Singh et al., 2017) all have a certain degree of impact on the urban heat island intensity (UHII).

The research on strategies of thermal environment improvement focuses on urban ventilation, green space and water and ground materials, rational planning of urban layout and ventilation corridors and optimization of bluegreen space pattern, strategies to improve the quantity and quality of green space and so on ((Peng et al., 2017);(Guo et al., 2015);(Yang et al., 2021)). Among them, green spaces are considered to be able to alleviate the UHII directly and effectively, whose effect is influenced by size, shape and layout (Jonghoon et al,2017;(Li et al., 2014)). Previous studies have shown that the cooling degree and range of green space are positively correlated with the area and negatively correlated with the shape index (Yuan et al, 2017). In further studies, scholars used models (such as CFD model) to quantitatively evaluate the impact of urban spatial morphology, land use, and other factors on urban thermal environment and proposed corresponding planning strategies to provide guidance for urban construction (Yuan et al., 2020) (Iizuka et al., 2020).

In 2018, during a visit to Tianfu New Area of Chengdu, General Secretary Xi Jinping proposed the concept of "Park City" for the first time. The proposal of "Park City" not only reflects the development concept of "Ecological Civilization" and "Human-centered", but also reflects the consideration of the development mode and the pathway of urbanization. In recent years, Chengdu has made persistent efforts to advance the construction of Park City. Related theoretical studies mainly focus on the construction strategies, paths and ecological value of Park Cities, including constructing ecological network systems, promoting park layout optimization, shaping city style, innovating park management mode and so on (Lin et al., 2020). However, the current studies have not yet covered the impact of Park City construction on the urban thermal environment. Although Park City construction helps to increase the urban blue-green space and improve quality, it is still not clear whether the Park City construction can effectively alleviate the UHI. Therefore, it is crucial to discuss how to carry out urban construction in a way to mitigate the increasing urban heat island effect more efficiently.

The purpose of this study is to determine the impact of Park City construction on the urban thermal environment, and discuss how to optimize Park City construction in order to alleviate the urban heat island effect in an even better fashion. Accordingly, we choose the Lantsat 8 satellite image and surface coverage data. Through the spatial statistical analysis and correlation analysis, we try to explore the influence of the Park City construction of Chengdu on alleviating the UHI, reaching the research conclusion on the basis of the parks in Chengdu city urban construction strategy, thus to provide further optimized guidance for the construction of Park City. In addition, we also put forward the urban strategies of Chengdu Park City construction based on the research conclusions, which give further guidance for the construction of Chengdu Park City.

Existing studies have shown that blue-green space is conducive to the mitigation of the heat island effect. The Park City construction aims to build an ecological civilization and create a good living environment, which is considered to have a positive effect on the improvement of urban thermal environment. In the study, we found that the heat island in Chengdu is mostly concentrated in the areas with concentrated construction land and low afforestation rate, especially in the downtown area of the city. After the construction of the Park City, the area of the heat island decreased and the area of MCI increased. Cropland, grassland, and water are beneficial to the mitigation of the heat island effect, while impervious surface and bare land may have the adverse effect. There was a noticeably negative correlation between FVC and LST.

According to the analysis on the impact of land cover and FVC on the UHII, we propose the following strategies for Park City construction of Chengdu:

This study shows that the existence of blue-green space reduces LST, while the existence of impervious surface and bareland renders it higher. UHII increases when converted to impervious surface and bareland, while UHII decreases when converted to farmland, grassland and water. Therefore, in the process of construction, attention should be paid to the protection of the ecological environment and to increase the area of urban blue-green space. Furthermore, the expansion of impervious surface and the increase of construction intensity shall be controlled, and bare land shall be eliminated and rationally utilized.

However, in the process of urban development and construction, the expansion of impervious surface is inevitable, which will inevitably compress the area of urban blue-green space. Existing studies have shown that the layout of urban green space has a notable impact on the urban meteorological environment. The cooling effect of green space on the surrounding environment is limited to a certain spatial range, yet the overall cooling effect of the decentralized green space on the surrounding environment is better than that of the centralized large green space (Effect of urban green land on thermal environment of surroundings based on remote sensing: A case study in Beijing, 2014) (Miao et al., 2013). It can be perceived that in the process of alleviating the urban heat island effect, it is also necessary to improve the quality of the blue-green space and adjust the spatial layout appropriately in addition to ensuring the urban blue-green space. During the construction, attention should be paid to increasing the amount of green space, making full use of the existing blue-green space, and optimizing the urban blue-green space network to enhance its cooling effect.

In the downtown area of the city, the impervious surface is large but the land resources are limited, so it is not suitable to increase the large scale of urban green space. In the constructions, limited space can be used to increase the thermal comfort of a small area by appropriately adding spot-like green space or small bodies of water, which is conducive to breaking the heat island effect area and eliminating the scale effect and superposition effect of the heat island. At the same time, it can also convert the existing bare land into green space to improve the environmental quality.

Increasing air motion is one of the crucial ways to mitigate the UHII. In this study, we found that the LST of the medium low FVC zone in 2019 was slightly lower than that of the moderate FVC zone, which may be related to the presence of plants that hindered the flow of wind, resulting in agglomeration and increase of temperature. In the study of the impact of urban structure on the urban thermal environment, we can see that UHII is directly proportional to the urban building density, that is, the greater the density of urban building, the lower the air circulation efficiency, resulting in the severer heat island effect (Peng et al., 2017). In the process of Park City construction, the protection of wind sources should be strengthened and attention should be paid to creating open spaces to introduce cold air from the periphery of the city into the city. At the same time, the temperature difference between the cold and hot islands in the city should be fully utilized, and the spatial layout should be reasonably arranged to form local air circulation in the city and accelerate air motion.

-Reasonable control of FVC Although both vegetation coverage and surface temperature showed a noticeably negative correlation, for alleviating the heat island effect, higher vegetation coverage is not always better. The quantitative analysis also reveals that the presence of forest may increase LST. Excessively high FVC will hinder air flow, cause heat accumulation and increase regional temperature, which is also one of the causes for the high temperature in Chengdu in summer. Existing studies have shown that, for different floras, the range of the effect of FVC on temperature is different (Li et al., 2021). In the process of Park City construction, plant species should be reasonably selected according to the needs, and FVC should be classified and controlled, to give full play to the cooling effect of different plants and alleviate the UHII.

The results indicated that water have a good cooling effect, and its cooling effect may be even superior to that of green space. In actual construction, water can be added to the existing green space to make full use of its mitigation effect on the heat island effect, and to reduce the accumulation of vegetation in the green space, so as to accelerate the air flow in the urban green spaces.

This study contributes to the understanding of the contribution of Park City construction to the enhancement of the urban environment, and also provides a new perspective for further optimizing Park City construction. In this paper, we used Landsat8 data and land use data to clarify the impact of Park City construction on the urban heat island effect, and found out the direction for the further improvement of urban thermal environment. The results show that there is no doubt about the Park City construction serving as the aid to the mitigation of the heat island effect, even though its effect is affected by land use and FVC. The existence of blue-green spaces is beneficial, while the impervious surface and bareland have the adverse effect; an increase in FVC is helpful in general, but in some cases, may not be. According to the results, we propose optimization strategies, including optimizing urban space layout, integrating construction with urban ventilation corridor planning and controlling FVC reasonably.

This study is expected to provide guidance for further construction given that urban heat island effect may be exacerbated later. The conclusions and the proposed strategies are useful for optimizing Park City construction and improving the quality of urban thermal environment.