Rapid urbanization has changed the structure of urban surfaces. According to statistics, more than 50% of the global population is urban; it is estimated that by 2050, the proportion of the urban population will exceed 66% [1
]. Urban areas are the center of human activities, energy consumption, and greenhouse gas emissions, which contribute to global climate change. Most of the cities are located in plains at lower elevations [2
]. Due to the composition of the underlying surfaces of the city (urban roughness) such as impervious surfaces, buildings, and municipal facilities [3
], have led to a change in the local climate that has resulted in problems such as urban heat island effects [4
]. Local climate change is caused by two different but related reasons. One of them includes surface cover, building materials and building forms [5
]. The other one is anthropogenic activities, such as industrialization, transportation, solid waste generation, and excess waste-water generation, which have also been reported to influence the natural structure of cities [6
]. The urban form also has an impact on urban heat island, as many researchers have demonstrated this [7
]. Due to the lack of consideration on the relations between urban forms and urban ventilation in city planning, the urban ventilation environment is getting worse and worse, hence, increasing the intensity of the heat island effect. This drawback forms the backbone of enormous research interest in the adjustment of urban form and urban function, especially adjusting the microclimate to mitigate the heat island effect.
Urban heat island (UHI) was first discovered by Luke Howard in 1818, which refers to the phenomenon that cities are warmer than surrounding rural areas [9
]. From then, UHI effects have been studied over the last two centuries. Generally, the traditional heat island measurement is called “canopy layer urban heat island”, which exists in the layer where people live, from the ground to below the tops of trees and roofs [10
]. This shows that the urban heat island is obtained from the measurement of the air temperature in the city. Because the air is flowing and transparent, there is no effective recording method to indicate its spatial state. Most scholars study the urban heat island effect by using satellites [11
]. The surface temperature obtained from the image needs to be calibrated. Although there is a certain relationship between the surface temperature and the air temperature, this relationship becomes extremely uncertain due to the heterogeneity of the surface. Furthermore, human thermal comfort is related to air temperature, mean radiant temperature, wind speed and relative humidity [17
], thus, studying the characteristics of microclimate at the city scale is more conducive to interfacing with urban planning. From the thermal conduction studies, the simulation method was commonly used to investigate with a different scale. For example, the Urban Weather Generator (UWG) model was mainly applied to simulate the local microclimatic phenomena in the city scale [19
]. In the mesoscale weather simulation, the Computational Fluid Dynamics (CFD) model was widely used to estimate the thermal conduction and heat flux in city areas such as buildings and street levels [20
Urban cool island (UCI) usually refers to the areas that have a lower temperature compared with their surroundings such as vegetation areas and water bodies in cities [22
]. Vegetation changes the three-dimensional space of the city seasonally and also changes the incident and reflected energy of the sun. Vegetation cools down the air and surfaces through evapotranspiration and shadows. Soil and water use their own absorption and high heat capacity to achieve the cooling effect [24
]. In addition to the physical properties of the above-mentioned and other urban constituent materials, the impact of green space layout is also proven [25
]. At present, there are two main scales for studying the cooling effect of urban green space (UGS): one is large-scale research based on satellite imagery and meteorological data [27
]; these studies applied indicators such as park cooling intensity (PCI) and green space cooling intensity (GCI) to quantify the cooling effect of UGS [22
]. The other one is small-scale research through field observation and application of models [17
]. Field observation data can more directly and accurately characterize the dynamic relationship between urban green space and atmospheric temperature than other spatial datasets derived from satellite imagery. This approach, at this scale, has an important practical guiding significance for the rational planning and layout of urban green space. Compared with the large-scale landscape design, the small-scale design is easier to manage. The SGSs like cells in the city play an important role in regulating the microclimate [34
]. Specifically, they play a significant role in improving the environmental quality of local microclimate. However, the role played by SGSs has often been neglected in research.
This article is in the context of the rapid spread of global urbanization. A series of urban homogenization symptoms continue to be unraveled [35
], for instance, a study shows that urban plant communities from 35 Chinese cities had lower dissimilarities of species composition between urban areas than these of plant communities in natural areas. More specifically, plant species from families like Prunus, Populus, and Magnolia have contributed to the homogenization of urban woody plants, due to their wide use in landscaping during the rapid urbanization [38
]. Another study also showed that China’s urban plant communities are becoming homogenized, as urban communities of different cities are highly similar to each other despite the geographical separation [39
]. Green spaces in different cities often exist with the same plant communities, hence, it is of universal significance to study a certain type of green space. Traditional studies on urban thermal environments focused on large-scale ranges [40
], these studies used satellite images with low resolution which were unable to identify small-scale complex factors, and could not reflect the hourly changes in urban thermal environment. From this perspective, this article employed SGS as study areas and applied related instruments to quantify and explore the impact of different coverage types on the thermal environment, with an outlook towards future small-scale landscape design.
In this study, the time was selected in the hot and dry summer of August 2019, and the study area was selected in the green spaces of a university campus. The research object includes four differently coverage surface types in four green spaces, canopy and vegetation parameters in 16 spots were quantified by instruments. The thermal performance of the coverage type and its impact on the meteorological parameters were analyzed and compared. The intrinsic influencing factors for the regulation effect of green spaces on microclimate were also conducted. This is also a topic of practical significance in the context of reducing UHI.
The objectives of this study were:
To study the spatiotemporal microclimatic characteristics of different types of green spaces types on hot and dry summer days.
To analyze and compare different surface coverage types of SGS on microclimate.
To analyze the relationship between microclimatic and coverage characteristics (vegetation structure, coverage attributes, leaf area index, leaf angle, photosynthetic radiation) of the green space.
The research results in this paper provide a scientific basis for characterizing the microclimate changes of green space. In this study, the field observation and measurement method was used to study the relationship between temperature and humidity among types of small green space (SGS). By selecting four green spaces in the university campus as sample spots, using meteorological data to analyze spatial and temporal characteristics of the SGSs, we compared the effects of four coverage types (1—impervious surface; 2—shrub-grass; 3—tree-grass; 4—tree-shrub-grass) on microclimate. Finally, we analyzed the four impact factors (PAR, CD, MLA, LAI) of the SGSs among all 16 spots. The research results in this paper provide a scientific basis for characterizing the microclimate of green space. The conclusions of this article were as follows:
There were evident differences in temperature between the four types in SGSs. The largest difference was concentrated in the noon period when solar radiation was strongest during the day, but the difference between the types at night was small. Specifically, the difference in temperature and humidity between the four types during the day was large, and the temperature was expressed as AT1 > AT2 > AT3 > AT4. At noon, the difference reached the maximum, and the relative humidity order was the opposite RH4 > RH3 > RH2 > RH1. The four coverage types showed that the temperature and humidity values were relatively close at night.
The four coverage types of four gardens essentially showed the same trend. Type 1 (impervious surface) had the highest temperature and the lowest relative humidity, while the type 4 (tree-shrub-grass) multi-layer vegetation structure had the lowest temperature and the highest humidity. This type had the highest temperature difference as well, that can reach 8.9 ℃ (Garden B, B1, and B4, 09/08/2019, 10:45 a.m.). The maximum relative humidity difference was 28.5% (Garden B, B1 and B4). Those results showed that tree cover types were cooler and more humid than no tree-cover types, which reveals that tree cover was the core factor affecting the temperature.
There was a close correlation between surface coverage types and plant community characteristics. Canopy density (CD) and leaf area index (LAI) had a positive effect on cooling and relative humidity, while photosynthetically active radiation (PAR) and mean leaf angle (MLA) had a negative effect on cooling and relative humidity.
In order to better understand all the factors that explain the impact of green areas in their surrounding environment, further research is needed to take into account the specific characteristics of urban green space. The results can provide recommendations for green space management and future landscape design, which can alleviate urban heat island effects, and enhance and improve the ecological benefits of urban green spaces.