1. Introduction
With the population increase and economic development, the urbanization level of the world has been rising. According to a prediction by the UN, 68% of the world’s population will live in cities by 2050 [
1]. Although cities provide people with many benefits and opportunities, they face serious problems that need to be addressed. One of them is the urban heat island (UHI) effect and the associated deterioration of the urban thermal environment.
A UHI refers to the phenomenon where the temperature in urban areas is higher than that in rural areas [
2]. UHIs may deteriorate the outdoor thermal environment, decrease people’s thermal comfort level and even impact regional atmospheric pollution [
3,
4]. The increase in the indoor cooling load caused by UHIs leads to more consumption of energy and other resources [
5]. Hot weather exacerbated by a UHI can result in heat-related illnesses and even death [
6].
The intensity of a UHI and its temporal–spatial variations are affected by many factors, among which urban morphology is an important one. Urban morphology is constituted by building dimensions and spacing, the characteristics of artificial surfaces and the amount of green space [
7]. Urban morphology affects UHIs in multiple ways. Surfaces of buildings and pavements in urban areas absorb more solar radiation during the daytime. Clustered buildings can reduce the overall wind velocity and trap more longwave radiation due to their geometric and thermal properties. An increase in impervious pavements reduces evaporation and heat dissipation [
8].
UHIs are a multiscale phenomenon [
9]. Cities incorporate a range of scales at which many of the environmental dynamics are constituted [
10]. The study of UHIs and the thermal environment should take into account the influence of climate zones, building forms and parameters affecting shadows and daylight distribution [
11]. The study of urban climates can be classified into different scales. Moudon suggested that the scale of urban morphology can be classified into city/region, street/block and building/plot [
12]. In this study, we defined them as the macroscale, mesoscale and microscale, respectively.
The research on UHIs on the macroscale focuses on the effect of land use and geographical features on the spatial distribution of air temperature. The land use features include the fraction of built-up areas, impervious surface areas, vegetation areas and water areas [
13,
14,
15,
16]. The concept of local climate zones (LCZs) was proposed to classify urban areas into several types of region according to their built forms and thermal performance. Surface cover, structures and materials are uniform within the same LCZ [
17]. Methods such as cluster analysis are used to divide the city region into several urban climate zones (UCZs). Thermal variables within the same UCZ are relatively homogeneous compared with those in other types of UCZ in cities [
18,
19,
20].
On the mesoscale, the relationship between urban form and UHIs is a key research subject. Compared with the macroscale, more three-dimensional urban geometric parameters are incorporated and analyzed. In addition, the turbulent flow and the distribution of air temperature and solar radiation can be determined more precisely by field measurements and numerical simulation. The building footprint area, mean building height, total height to total floor area ratio and sky view factor are frequently reported to influence UHIs or air temperature [
21,
22,
23,
24,
25,
26]. In addition, the complete aspect ratio, frontal area density and mean building volume are found to influence the solar access or wind field [
27,
28,
29,
30].
On the microscale, the differentiation of physical spaces and forms creates pockets of urban microclimates in cities [
8]. Every built element induces its own microclimate around it so that temperature varies in outdoor spaces, even at short distances [
31]. It has been observed that high-rise buildings induce various surrounding microclimates due to the effect of shadow projections [
32]. Temperatures on the leeward side of buildings are generally lower than those on the windward side because of shading and small advective effects [
33].
As discussed above, urban morphology affects UHIs and the urban thermal environment on various scales. As the scale shrinks from the macro- to the microlevel, detailed urban morphological parameters become more influential in determining the UHI intensity and the pattern of the urban thermal environment. Urban morphological parameters and how they affect the urban thermal environment on the macro- and mesoscales have been extensively studied, as evidenced by works such as [
13,
14,
15,
16,
17,
18,
19,
20,
21,
22,
23,
24,
25,
26,
27,
28,
29,
30]. However, fewer studies are available on the relationship between urban morphology and the urban thermal environment on the microscale [
32,
33], which may be explained by the following two reasons.
Firstly, the microscale urban morphological features in modern cities are complex, making it difficult to describe them by spatial parameters. Secondly, thermal variations are relatively inconspicuous in evenly distributed building clusters, which were the main concern of previous research [
34,
35,
36,
37,
38,
39]. However, thermal variations can be more evident in some modern urban fabrics, such as isolated high-rise building clusters [
32]. Although these kinds of urban fabrics have been somewhat studied [
22,
23], there is still a lack of targeted investigations.
Another research gap in the field is the lack of cross-scale studies of the correlation between urban morphology and the urban thermal environment. Most research works, if not all, focus on one scale, be it the macro-, meso- or microlevel. A common effort is to establish a quantitative relationship between the temperature at measurement points and their surrounding urban morphology on a specific scale. However, the environmental context of cities constitutes a multidimensional issue, so the effects of various factors on different scales should be considered in urban climate studies [
11].
The heterogeneous features of urban spaces make the cross-scale approach more necessary. Although it is impossible to find perfectly homogeneous urban spaces in real cities, residential districts and some traditional urban areas spanning hundreds to thousands of meters are fairly homogeneous in that the buildings are evenly distributed and their heights are uniform, both in a relative sense. However, in many modern cities, especially their central business districts, urban spaces exhibit strong heterogeneous features horizontally, vertically and functionally. The spatial distribution of outdoor air temperature in homogeneous urban spaces is relatively uniform, which lowers the thermal contrast on different scales. This phenomenon makes it easier to establish the correlation between the urban thermal environment and urban morphology on a mono scale. Meanwhile, in heterogeneous urban spaces, even the morphological features of adjacent plots can be dramatically different, and outdoor air temperatures exhibit more significant variations. In this situation, the cross-scale approach can be more suitable for investigating the correlation between air temperature and urban morphology.
Considering the background and research gaps discussed above, we have conducted a cross-scale analysis, both theoretically and experimentally, of the thermal environment of the central business district in Shenyang, a major city located in northern China. The studied area represents urban spaces with strong heterogeneity and, therefore, a complex thermal environment. The focus of the research is on determining and understanding the pattern of the thermal environment on different scales and establishing its correlation with the urban form.
The remainder of the paper is organized as follows.
Section 2 presents the methodology, including the studied urban district, the model simulation, the scale of analysis and the selection of urban morphological indicators.
Section 3 presents the results of the model simulation and curve estimation, including the air temperature patterns on the mesoscale and microscale and their correlations with urban morphological indicators.
Section 4 discusses the effect of morphological heterogeneity and scale differences on the correlations.
Section 5 concludes the paper.
5. Conclusions
To investigate the relationship between spatial temperature variation and urban morphology in a heterogeneous city district, a numerical simulation on the summer thermal environment of a central business district in Shenyang was conducted. It was found that the temperature distribution changed with the time because of the variation of the solar azimuth. At 14:00, the air temperatures differed by 2.12 ℃ in the studied district, while the temperature differences ranged from 1.03 to 1.70 ℃ within the regions on the microscale.
The results of the curve estimation indicate that the BFR, Com, ED and HA were significantly correlated to the air temperature on the mesoscale. The Com and BFR were the most influential morphological indicators, explaining 59% and 49% of the temperature variation, respectively. The explanatory powers of the ED and HA were relatively weaker, explaining 44% and 40% of the temperature variation, respectively. On the microscale, only the ED and BFR had relatively steady correlations with the air temperature, and the ED was more influential on the microscale than the BFR. The distinctions in the correlation between urban morphology and air temperature on two scales were caused by the different ranges of spatial temperature variation and different distances for heat exchange between the upwind and underlying surfaces.
The heterogeneity indicators SDH and SDF were both not influential indicators in this study, while heterogeneity did affect the correlation between the air temperature and some morphological indicators. The dramatic distinction in heights and footprints of the buildings weakened the effects of the MH, FAR and Cex on the air temperature. The heterogeneity of the horizontal building layout could enlarge the distinction in the inflow temperature of each plot unit and weaken the statistical correlation between urban morphology and air temperature.
The results of this study can help to get a deeper understanding about the correlation between urban morphology and the air temperature in a heterogeneous urban district. However, the current study may be limited by the fixed wind conditions of the model simulation and limited scale of the studied district. The setting of the nesting area could not consider the impact of the buildings around the model domain. The effect of urban morphology on nocturnal air temperature was not analyzed because of the relatively short simulation period, and the effect of urban green infrastructure was excluded. In the future, measurements will be conducted in the wider urban area to further investigate the effect of urban morphology on the air temperature across different scales. Meanwhile, morphological indicators correlated to the thermal properties of heterogeneous urban districts are supposed to be proposed in the next stage. The thermal performance of a vegetation scenario will be compared with that of a vegetation-free scenario in a future study.