1. Introduction
High-density urban buildings can cause significant changes in a local climate, resulting in lower wind speeds in urban blocks and exacerbating heat island effects and air pollution [
1]. Hu et al. noted that the density (total building area) of an urban area is a decisive parameter affecting the urban heat island (UHI) intensity of that area; the higher the density is, the lower the
SVF value is, which causes higher UHI intensity [
2]. The research of Peng Wang et al. (2021) showed that urbanization significantly increases the frequency of hot extremes occurring in the summer, and that upward trends in human-perceived temperature (HPT) and actual near-surface air temperature (T) are more prominent in areas with higher urbanization levels and denser populations [
3]. Liu (2021) explored the influence of the urban spatial morphology layout on urban heat islands and noted that spatial morphological parameters have become a more important driver of UHI changes than land surface parameters are [
4].
In the past several decades, since Shenzhen was established as a special economic zone in 1980, its permanent population has increased from 330,000 to more than 13 million, and it has become one of the 37 megacities in the world with a population of over 10 million. Shenzhen’s GDP has also increased rapidly, from RMB 196 million to RMB 3066.485 billion, with an average annual growth rate of about 22%, placing its economic aggregate rate among the top five cities in Asia. Furthermore, the urban built-up area rapidly increased from 3.8 km
2 to 960 km
2, and there are nearly 300 high-rise buildings over 150 m tall in Shenzhen, ranking it first in China. The speed of its urbanization is rare. However, due to the low mountains and hills in the territory, the land resources that can be developed and utilized are very scarce. Thus, it is necessary to adopt a relatively high-density development and construction model, which means more buildings and stronger heat emissions per unit area. The research shows that the average annual temperature increase in Shenzhen from 1968 to 2013 was about 1.68 ± 0.18 °C and that the temperature increase rate is about 0.35 ± 0.04/10 a, which are both much higher than the increases of 0.47 ± 0.20 °C and 0.1 ± 0.04/10 a in Hong Kong. The contribution of urbanization to the temperature rise exceeds 80%, which leads to a series of climate and environmental problems, such as decreased environmental comfort, increased energy consumption, increased difficulty with water resource management, frequent occurrences of waterlogging, and deterioration of the ecological environment [
5].
Therefore, research on planning methods and strategies for optimizing the urban wind and heat environment is necessary and important for cities in order to begin adapting to climate change. Giridharan proposed that energy-efficient designs can be achieved by manipulating the surface albedo, sky view factor, and total height-to-floor area ratio (building mass) while maximizing cross-ventilation, which reduces energy consumption and mitigates the heat island effect in Hong Kong [
6,
7]. Kubota performed wind tunnel tests on 22 residential neighborhoods selected from actual Japanese cities, and the results showed that there is a strong relationship between the gross building coverage ratio and mean wind velocity ratio. Next, the wind environment evaluation for case study areas was performed using the wind tunnel results and the climatic conditions of several major Japanese cities. Finally, the development method of guidelines for realizing an acceptable wind environment in residential neighborhoods using the gross building coverage ratio was proposed [
8]. Yuan Leiconducted a quantitative study on the relationship between morphology and environment in the Nanshan District, Shenzhen. They identified the impact of different elements on a city environment and other problems, proposed planning suggestions of a green system and a ventilation system, and provided a reference for sustainable ecological city development [
9]. In Zheng Yingsheng’s study, they first simulated the ventilation conditions of the Tai Po Market in summer using a fluid dynamics simulation software and identified areas with poor ventilation. Second, on the premise of ensuring the existing functional composition and construction density of the Tai Po Market, the block shape and building group layout were adjusted. Next, the sky view factor and wind speed at pedestrian level were compared to verify the possibility of wind environment improvement through urban morphology optimization under the same density. They proved that the urban ventilation strategies proposed in their paper can improve an urban microclimate and enhance human thermal comfort [
10]. However, most of the research to date has focused on using wind tunnel tests or fluid mechanics methods to simulate the local microclimate characteristics of residential quarters with high floor area ratios, and researchers have not yet developed a detailed standard for the quality assessment of the wind and thermal environment in the entire urban environment.
For a high-density city like Shenzhen, the problem is how to optimize the urban form to reasonably use the remaining open space and increase green land in the process of urban renewal within the currently developed high-density built-up environment in order to revitalize the stock space by focusing on adjusting the structures, improving quality, and mitigating the heat island effect, as well as improving human comfort. This is a fundamental requirement in order to improve the living environment, and it is also an necessity for building an ecologically civilized city and developing it sustainably. Therefore, this study considered the characteristics of wind and thermal environments combined with the overall shape of the urban layout in Shenzhen, which is long and narrow in the east–west direction and short in the north–south direction, and classified climate quality, identified climate-sensitive areas, and analyzed the relationship between the urban form and heat island and ventilation. The whole city of Shenzhen is urbanized, in the context of limited open space resources and a large population. Using climate statistics, remote sensing inversion, and GIS spatial calculation to propose planning and control strategies, such as urban ventilation corridors and locally zoned land layout based on suggestions according to wind and heat environment comfort, can be useful for providing technical support for urban planning and for improving the quality of urban living.
4. Urban Spatial Wind and Thermal Environment Optimization Strategy
4.1. Thermal Environment Optimization Strategy
We used a new method of categorizing urban climatic zones, called “Climatopes”, based on the research of Yonghong Liu et al. [
17]. Taking into account the background wind environments and the
VPC and SUHI indexes, Climatopes can further divide spaces into four classes of “sensitivity”, the climate environment, and the spatial distribution characteristics of SUHI. Suggestions for the thermal environment are given in
Figure 16.
The green areas are those that need to increase the proportion of green land to ensure vegetation coverage. Since forest land is a low-temperature center throughout the year, it is the place where fresh and cold air is generated, and its climate quality is generally high. Either the wind or thermal environment can provide good compensation to nearby areas. If the vegetation coverage of the forest land is low, it cannot have a significant cooling effect on the surroundings. Therefore, based on the terrain slope and vegetation coverage, the areas that need to increase the proportion of green land were extracted. If the slope was greater than 5 and the vegetation coverage was less than 0.6, we regarded that area as the corresponding layer.
The red areas are those that need to reduce the impervious surface ratio, mainly for climate sensitive areas, and we believe it necessary to reduce the impervious ratio for expansion within 100 m of sensitive areas.
The yellow areas are where high-density buildings are prohibited. We recommend avoiding large building components or tall trees and keeping the space open.
The blue areas are where urban development intensity needs to be slowed down; generally, new buildings within 300 m of areas close to ecological green sources need to be strictly controlled to avoid hindering climate resources.
4.2. Wind Environment Optimization Strategy
Good natural ventilation requires greater ventilation potential and that the wind passes through as easily as possible. Considering the distribution of ventilation potential in Shenzhen (
Figure 15), combined with the background wind environment (
Figure 3), we outlined seven level one corridors and nine level two corridors (
Figure 17), along with suggestions on planning and control.
Since wind in the northern area is dominated by northerly wind, the direction of the corridor is mainly north–south. We recommend that development should mainly focus on the north–south direction in order to avoid hindering dominant winds in the vertical direction.
In Longgang and Pingshan Districts, we recommend building level one corridors in open areas with high ventilation potential in the suburbs. Full use should be made of the Bijiashan, Honghu, Weiling, and OCT Wetland Parks and other green spaces as climate compensation, as well as Shenzhen River, Xili Reservoir, Dasha River, and other fresh and cool air sources. The green areas and vegetation coverage of open spaces should be maintained, and level one and two corridors should be better connected with each other to guide air flow direction and allow fresh air to transfer to climate-sensitive and urban-barrier areas in order to improve the ventilation environment.
Since the southern part of Longhua District, the western part of Longgang District, and the northern part of Nanshan District are located in the LCQA, we recommend that in LVQA zones, more ventilation corridors should be set up to increase the penetration of the dominant wind and achieve good air circulation.
The development intensity should be limited in HCQA and potential high-ventilation areas as ecological conservation areas, in order to protect land with high ecological value so that it can easily and continuously provide fresh and cool air to the main urban areas.