The effect of various urban design parameter in alleviating urban heat island and improving thermal health—a case study in a built pedestrianized block of China

Increasing urban heat island and global warming have aroused serious thermal environmental problems and even harm people’s thermal health. Because of the importance in people’s daily life, a commercial pedestrianized block represents a symbol of a city or metropolis; therefore, focusing the attention on the thermal environment in such regions is very necessary. Most of the researches on the urban thermal environment are calculated by remote sensing data; limited by the low spatial resolution of remote sensing image, it may not obviously reflect the true thermal environment of the research site, especially in some microscale regions. Based on this, the new software ENVI-met is developed to research the thermal environment and forecast people’s thermal sensation in a microscale region. Therefore, the objective of this study aims at conducting field measurement and numerical simulation to assess the thermal environment of a typical commercial pedestrianized space in southern China and assess the different urban design parameters in ameliorating the urban heat island effect. Our final results demonstrate a quantitative evidence for establishing a comprehensive standard for improving the thermal environment in a microscale region, and this study also can be a supplementary in the research field about improving thermal health.


Introduction
Under the global warming, particular urban heat island (UHI) effect remains a significant issue to tackle, which is a natural phenomenon that refers to the surface and air temperature in cities that are much higher than that in suburban distracts (Oke 1987); this phenomenon can deteriorate urban thermal environment, and what's worse, it may harm people's health. The research about this issue has been conducted for many years and also has been tested for being influenced by many different factors including canyon geometry (Bhargava and Lakmini 2017;Santamouris 2013), anthropogenic heat (Kolokotsa et al. 2009), thermal properties of materials (Wong and Yu 2005), and urban vegetation (Tan and Fwa 1992;Tan et al. 2013;Shashua-Bar and Tsiros 2012). In order to make a deep understanding on the UHI effect, remote sensing is largely used for observing the thermal environment in an urban and rural scale; most of these studies are focused on the macroscale and urban-scale region (Jannatul n.d.; Alobeid et al. 2010;Avtar and Sawada 2012;Buyantuyev and Wu 2009;Arnfield 2003;Grover and Singh 2015;Ramaiah and Avtar 2020) and display the distribution of air temperature and surface temperature in their research sites, thus putting forward some suggestions to decrease the air temperature.
People's thermal sensation in summer will be affected by various parameters including people's clothing and activity, mean radiant temperature, wind speed, relative humidity, and air temperature. The existing studies on macro and urban scale, limited by the low spatial resolution of remote sensing image, cannot fulfill the need for evaluating people's thermal sensation and reflect the real thermal conditions in the microscale region such as residential community, street, and so on. In order to build a suitable living environment for the citizens, the research on thermal environment is changed from the macro-and urban-scale to the microscale region under the development of the technology (Changwang 2019). Different from the observation of the UHI effect on the platform of remote sensing, the study in microscale region mainly relied on the software ENVI-met, which is a grid-based CFD (computer fluid dynamics) three-dimensional (3D) model that can perform a microscale simulation in microclimate with the spatial resolution ranging between 0.5 and 10 m within 10 s time (Huttner and Bruse 2009); also it can forecast people's thermal sensation effectively. Based on the superiority in observing UHI effect in summer, ENVI-met has been widely applied in researching heat stress in microscale region (Yamada and Mellor 1975;Fang et al. 2004;Oliveira et al. 2011;Toparlar et al. 2017;Rosso et al. 2018a, b;Andrade and Alcoforado 2008;Akbari 2014, 2016), and its accuracy has been accepted in different climate zones.
A commercial pedestrianized block is a significant role in cities, which cannot only be a symbol of a city but also a public space for providing the citizen's life and urban tourism (Ma et al. 2019a, b, c). So, it's necessary to assess the inner microclimate and people's thermal sensation and health in such regions. Like other studies in macro and urban scale, the researches about the microclimate and UHI effect in microscale region are focused on urban   The national thermal design specification of civil buildings of China geometry, street orientation, vegetation, and the thermal property of the paving material. In the aspect of urban geometry, the aspect ratio (H/W) and the sky view factor (SVF) are the two main factors; the former describes a proportional correlation between the height of the building and the width of the street, and the latter is an index, changing from 0 to 1, that controls the daytime solar radiation (Fig. 1). A study conducted in Brazil finds that increasing H/W can cool down the ambient air temperature, especially in the canyon with H/W being less than 0.5 (Akbari and Levinson 2008); also the urban canyon with smaller SVF will obtain a higher nighttime air temperature and a lower daytime air temperature (Svensson 2004;Unger 2004).
In the aspect of street orientation, a previous study in the coastal region in Israel proves that the air temperature in the northsouthern-oriented street is 0.64°C cooler than in eastwestern-oriented street (Shashua-Bar et al. 2011). In another study, a study in a semiarid climate zone, Brazil, shows that the northeast-southwestern-oriented street has the most comfortable thermal environment during the daytime (Abreu-Harbich et al. 2013).
In the aspect of vegetation, a previous study has been assessed that the grass can supply a cooler environment than the asphalt surface (Yang et al. 2013). Unlike the simple grass, an urban tree is measured by the size, type, and arrangement of the leaves; this can be expressed by the index LAI (leaf area index)-a dimensionless data of the leaf area per unit of ground area; a series of studies have proved that   Table. 3 The weather conditions during the measured days (Ma et al. 2019a, b, c) J u l y tree with higher LAI will make a contribution, a reduced thermal stress obviously (Lee et al. 2013;Duarte et al. 2015;Wong and Jusuf 2010).
In the aspect of the thermal property of the ground surface, a series of studies show that the impervious paving material and the ground with higher albedo will ameliorate the heat stress effectively (Li 2013;Li et al. 2016a, b).
All mentioned parameters can reduce the heat stress in summer; however, most are discussed separately, lacking a comprehensive standard to assess the effect of various parameters in improving thermal health. Therefore, this study aims at providing a comprehensive standard for evaluating the cooling effect of different parameters in reducing the UHI effect and improving people's thermal comfort and health. This study can be a supplement in the research field of outdoor thermal environment.

Methodologies of the current study
It's known to us that the descriptions of urban climate are based on a single or more fixed weather stations in suburban areas; unfortunately, these meteorological data cannot be able to represent the whole city, especially in some microscale regions. In order to evaluate peoples' thermal sensation and UHI effect accurately, recording meteorological information of the whole region simultaneously is very necessary. This work is composed as follows ( Fig. 2): 1) The on-site measurement is conducted for collecting the microclimatic data.
2) The simulated data by ENVI-met is validated against the measured data. 3) Based on the ENVI-met platform, the new strategies for alleviating thermal stress are put forward.

Research site
The Tai Zhou city is a cultural and historical city (Government of Tai Zhou. 2020.), which is located in the southeastern part The different points in this study (Ma et al. 2019a, b, c) of China (Fig. 3). According to the recent meteorological information, this city is in a humid and hot climate zone; in addition, the maximum air temperature in the hottest day can be up to 38°C at daytime. During the hot summer, all the city is in a static-wind region, which will also lead to a worse thermal environment. As a tourism city, Tai Zhou city attracts tourists every year (Tourism organization of Tai Zhou. 2020). The Tai Zhou old block is one of the most tourist attractions of this city, which is a community and consists of some Chinese traditional buildings (Fig. 3).
In accordance with the response of the local citizens and tourists, this region has some shortcomings such as the lack of shading and vegetation, the full use of harden ground, and

Climate conditions in Tai Zhou city
China has a very huge land, and different regions will have various meteorological characteristics in summer. Based on the national thermal design specification of civil buildings (Thermal Design Code for Civil Buildings 1993), the selected city belonged to the hot-summer and cold-winter climate zone (Fig. 4).

On-site measurement
In the measured days, the collected data including wind speed, air temperature, and relative humidity are recorded by the fixed instruments (Table 1). Each selected point is worked in two typical days (hottest time of the year); in addition, all the points are measured during the same time. The detailed principles are as follows: 1. Each instrument is fixed at a 1.5-m height (average pedestrian level) from the ground. No. of y grid 180 No. of z grid 20  2. Each instrument is covered by a shelter to prevent the influence on air temperature by solar radiation. 3. All used instruments are the same in different points.
Based on the different geometry of the research site, this region is divided into six different points ( Fig. 5 and Table. 2) (Ma et al. 2019a, b, c); the measured period is carried out on July 28 and 29, 2016. In accordance with the published weather information by the local meteorological stations, the hottest month of 1 year is on July; thus, we choose the hottest days of this month for evaluation (Table. 3).
The measured SVF is calculated by Ray Man and using a fisheye camera; meanwhile, based on the Google maps and field survey, a simulated model including shading devices and artificial structure is built in ENVI-met to assess the simulated results. The validation process in SVF is guaranteeing the accuracy of the simulated model (Fig. 6).

Numerical simulation
On-site measurement in the microscale region lacks experimental control; while examining the using scaled models needs a careful design for similitude and also are too expensive. With the development of computational analysis, the numerical simulation is becoming popular. The simulation of outdoor thermal situation could be evaluated on different scales from a single or several buildings, a block, and a distract to a city. In this study, the numerical model is built by the ENVI-met, which is in accordance with the SVAT model (soil, vegetation, and atmosphere transfer). Also, this tool can simulate the UHI effect and microclimate conditions of the urban space by analyzing all different factors that existed in the atmosphere integrated with buildings, vegetation, paving surface, water body, and pollutant (Bruse 2014). The accuracy of this software has been proved in previous studies (Srivanit and Hokao 2013;Alexandri and Jones 2008;Vidrih and Medved 2013); although the simulated results can account for weather conditions for all scale, it's still necessary that the simulated results be validated against on-site measurement in order to get the reliable results.
In addition, the relationship between the leaf area density (LAD) and leaf area index (LAI) is shown in the following equation: In this equation, h is the height of the vegetation (m) and z is the vertical grid size. A folder of the configurations of the border tree is in accordance with the fisheye images (Fig. 7) (Bowler et al. 2010).
The detailed configuration of the tree is calculated for the vegetation database of ENVI-met to fulfill this study (Table 4). Also, the height of the grass is 0.25 m, and the LAD is 0.25 m 2 /m 3 .
The initial input data and boundary conditions in this study is displayed in Table 5.
The modeled region in ENVI-met is displayed in Fig. 8.

The thermal indices
According to the previous studies, there are a few indices that assess outdoor thermal comfort in an urban environment, including standard effective temperature (SET) (Po 1972), predicted mean vote (PMV) (Gagge et al. 1986), physiological equivalent (PET) (Spagnolo and de Dear 2003), universal thermal climate index (UTCI) (Höppe 1999), effective temperature (ET) (Rosso et al. 2016), and so on. In this study, we choose the index PET for research, which is in accordance with energy balance of humans, acknowledged to researchers as the air temperature that Fig. 10 Correlation between measured and simulated air temperature (Ma et al. 2019a, b, c) makes thermal conditions in the indoor space in balance with the skin and core temperature in the outdoor environment. In addition, it also has been evaluated as an outdoor thermal comfort index by the VDI standard of Germany. The used parameters for calculating the PET include wind velocity, relative humidity, air temperature, mean radiant Fig. 11 Correlation between measured and simulated relative humidity (Ma et al. 2019a, b, c) temperature (MRT), and so on. Because wind velocity, relative humidity, and air temperature can be easily obtained, many studies (Rosso et al. 2016(Rosso et al. , 2018aSantamouris 2013) use mentioned different parameters for a validation criterion. The simulated result can supplement on-site measurements on occasions when the instrument is lacked (Li Fig. 12 Correlation between measured and simulated wind velocity (Ma et al. 2019a, b, c) et al. 2016a, b). According to the previous study, the distribution of the PET values and thermal perceptions in this study is displayed in Table 6 (Li et al. 2016a, b).

Relationship between measured and simulated results
In order to evaluate the outdoor thermal environment, the index RMSE (root-mean-square error) is calculated for checking deviation. This index is normally used in validating the gap between the observed and the predicted values, which is an important factor for testing the simulated results (Willmott 1981(Willmott , 1982. If this index can approach or reach 0, the most accurate results are obtained. Lower RMSE values represent that the simulated data is closed to measured results. Figure 9 shows the index-RMSE between the simulated and measured result. The RMSE of air temperature in Fig. 9a shows the big deviation occurs at point 5, reaching 3.5°C; this may be caused by the position of the measured instruments. In order to consider the peoples' safety, the instruments are not stood in the middle of the road and instead are fixed on the sidewalks. Figure 9b shows the big deviation also occurs at point 5 in the final results of relative humidity. In the field of wind velocity (Fig. 9c), the simulated model shows a favorable correlation with the wind velocity, changing from 0.1 to 0.2 m/s. Besides the calculation of the RMSE, the analysis of coefficient of determination between simulated and measured results is another very important process to assess the accuracy. A very good linear regression is found (Morakinyo et al. 2017), as shown in Figs. 10, 11, 12, where the coefficient of determination (R 2 ) of air temperature is from 0.741 to 0.9582, R 2 values of relative humidity for these points range from 0.7469 to 0.9693, and the values for wind velocity are within 0.7724 and 0.9424. The deviation may be caused by some uncertain factors during the working time. These deviations are smaller than those in previously published studies (Qaid et al. 2016;Park et al. 2012;Syafii et al. 2017). Our final findings proved that ENVI-met tool is an accurate software to finish the task in our work.

The new thermal environment under new cases
As mentioned, the index PET is used for assessing people's thermal sensation. Considering the final results of numerical simulation, the thermal environment during the two measured days is shown in Figs. 13 and 14. These two figures both show that the peoples' thermal sensations during the daytime are standing "very hot" and "hot" stages in accordance with the distribution of PET values in the researched climate zone (Liu et al. 2016a, b). Therefore, improving the outdoor thermal environment is very necessary.
Based on the previous studies, it's obvious that a stronger cooling effect will be getting a little higher background air temperature (Shashua- Bar and Hoffman 2000;Shashua-Bar et al. 2010); also it has proved that the positive effect of vegetation in sunny, hot days will be higher than cloudy, cold days (Wong et al. 2003). In addition, the effect of paving material with higher albedo will show a better contribution in reducing temperature on sunny days than that in reducing temperature on cloudy days (Yang et al. 2016). In common sense, the hottest time mainly appears from 2:00 pm to 4:00 pm (Ma et al. 2019a, b, c). In this study, the hottest period appears at 3:00 pm; therefore, the PET on 3:00 pm, July 28, is selected for calculating further analysis. As Fig. 16 shows that all the selected points are suffering from a high heat stress, nearly, the hottest PET of all the points can reach 60°C PET at 3:00 pm (Fig. 15).
Under this situation, it is very necessary for us to ameliorate the UHI effect and improve people's thermal sensation. To be mentioned, the story of the building in the commercial pedestrianized block will not be exceeding three story, and the coverage ratio of the vegetation Fig. 13 The PET of the existing scenario on July 28 should not be less than 25% of the total site in accordance with local design specifications (The commercial building design specification 2019). The coverage ratio of different parameters in the existing scenario (base case) is shown in Fig. 16; the total building coverage ratio occupies 55.37%, in which a three-story building is only 10.35%. In addition, the vegetation coverage ratio just occupies 3.96%. These factors can lead to a worse thermal environment in hot summer.
Based on the existing scenario, new strategies are put forward. The new cases under the scientific hypothesis are shown in Table 7.
In new cases (Fig. 17), case 1 aims at increasing the grass coverage ratio to understand the cooling effect, where the coverage ratio of grass increases from 2.94 to 23.98%. Like case 1, the coverage ratio of tree is increased to 22.06% in case 2, and its cooling effect is evaluated. The third case aims at replacing the ground surface in the existing scenario with a new paving material with higher albedo in improving thermal safety and reducing energy cost. The last case (case 4) is through increasing the three-story building coverage ratio and building height to understand its function, where the coverage ratio of the three-story building is up to 55.37%.
The cooling effect of different parameters is shown as: where PET is peoples' thermal sensation under the existing scenario. PET S is the new thermal sensation under new cases. Upon the new cases, Fig. 18 shows that the improvement of PET appeared in the research site. The new distribution of PETs at peak time (3:00 pm) has shown that the increase in tree coverage ratio (case 2) can largely change the thermal environment at daytime, especially in open space (point 1 and point 2), in which ΔPET ranges from 1.5 to 3.9°C, meanwhile, increasing tree coverage ratio can also make a contribution to reduce PET in canyon space, where ΔPET can be changed within 1.2 Fig. 15 The PET values of the site at 3:00 pm on July 28 and 8.1°C. This effect can be attributed to transpiration and providing shadow of the leaf at daytime. In case 4, increasing the coverage ratio of the three-story building and average building height can obviously improve the thermal environment, which can range from 2.1 to 12.5°C PET in canyon space, but the thermal environment cannot be changed too much (point 1 and point 2). Meanwhile, increasing the grass coverage ratio can also reduce PET, but the result is limited. What's worse is changing the paving material with higher albedo. Case 3 may result in a worse thermal environment in open space (point 2); even paving material with higher albedo may cool down the ground surface, which will also reflect more solar radiation on humans' body, thus leading to Fig. 18. Modification under the new cases at 3:00 pm, a worse PET, and the effect of grass (case 1) are not obvious.
The former results just display the distribution of the improvement of PET through synergistic effect under new cases in general. In order to provide a quantifiable effect of different parameters, a more detailed analysis about the cooling effect is shown in the next part.

Detailed correlation between the new case and people's thermal sensation
The whole selected block is composed of canyon space and open space. The detailed effect of new cases in open space is displayed in Fig. 19. The index R 2 between different parameters and ΔPET demonstrates the proportion that can be interpreted by various regression analysis. In this figure, we can find a strong positive correlation  Fig. 16 The coverage ratio of different parameters in the existing scenario between tree coverage ratio and ΔPET, where it can be observed that a 5% increase will reduce 0.4°C PET. Meanwhile, it's obvious that an invalid correlation is found between the three-story building height and ΔPET, with an irregular R 2 = 0.4844; in addition, an ascension in the average building height will contribute to a lower SVF and higher H/W; it's shown that the relationship between the H/W and ΔPET tends to be irregular (R 2 = 0.2971). What's worse, as SVF develops, a negative correlation between these two parameters will appear. After increasing the percentage of grass, it's found that a 5% increase in it will reduce 0.15°C PET. But a 3% changing with the new paving material will lead to a 0.2°C PET increase. Different from the thermal environment in open space, the most essential strategy (Fig. 20) in reducing PET in canyon Fig. 17 The configuration of different parameters in new cases space is increasing coverage ratio of the three-story building, in which a 10% increase in the percentage of the three-story building will reduce 0.5°C PET; in addition, a 0.1 increase in SVF will lead to increase 0.11°CPET, while an ascension of 0.1 in H/W can reduce 0.15°C PET at peak time. Like open space, increasing the coverage ratio of tree can obviously Fig. 18 Modification under the new cases at 3:00 pm reduce PET largely, and a 5% increase will contribute to reduce 0.25°C PET at peak time. According to the final correlation analysis of the other two cases (Grass and paving material), it's observed that the cooling effect of these two cases is limited.
In order to help the local manager and policy makers understand the cooling effect of a different strategy briefly, a new figure (Fig. 21) is presented to assess the comprehensive standard.

Conclusions
Remote sensing can be used for researching the UHI effect, but it cannot provide a platform for observing the thermal environment in a microscale region, and it also cannot assess people's thermal sensation effectively. Within this study, we have finished a systematic research about the UHI effect on affecting thermal environment in a commercial pedestrianized block in hot-summer and cold-winter climate zone of China. This paper is the first to display the comprehensive cooling effect of different parameters (grass, tree, pavement material, building) together. As we all know, a commercial pedestrianized block is an effective factor for increasing local tourism income; a better thermal environment in it will boost the vitality of the cities and attract tourists. Quantitative ENVI-met validation shows that the most significant correlation between thermal sensation and strategy is increasing the tree coverage ratio (R 2 = 0.851), where ΔPET can be changed within 1.5 and 3.9°C. Meanwhile, the most essential strategy in improving outdoor thermal safety in canyon space is increasing the coverage ratio of the threestory building (R 2 = 0.9787), in which ΔPET can be changed from 2.1 to 12.5°C. Thus, the final results can provide a quantifiable standard for future research. In accordance with the final results of our study, we put forward the following suggestions for designing the commercial pedestrianized block:

1.
In an open space, it is recommended that increasing the vegetation coverage ratio is very necessary. Vegetation cannot only provide shading for humans but also block solar radiation and reduce air temperature through transpiration. It is worth mentioning that the moderate coverage ratio of paving material with higher albedo may improve outdoor thermal safety but an excessive coverage ratio may cause a worse thermal environment. 2. n a canyon space, based on the local design specification, increasing the coverage ratio of the three-story building can increase aspect ratio (H/W) and reduce SVF. Compact canyon provides a more suitable thermal environment for human; even the wind velocity in the compact region is weaker than in open space, and the effect of blocking solar radiation exceeds the effect of reducing wind.