3.2 A comparison of the thermal comfort of the six models based on average UTCI values.
It can be seen from the Fig. 3 that the differences in UTCI value at the same time among different models are more obvious during the daytime than during the night. According to Table 2 and Fig. 3, considering the difference in UTCI values among the six models between 12:00 and 19:00, it can be inferred that the maximum difference in UTCI values occurred between Model 2 and Model 3 among the six models at 17:00. The difference in UTCI is 0.76 ℃. From 20:00 to 23:00, there is only a small difference in UTCI values among the models. The maximum difference in UTCI among the six models during this period occurred between model 3 and model 6 at 23:00. The UTCI value of model 3 was 0.06 ℃ higher than that of model 6 at this time. This indicates that different plant characteristics in the street canyon can have a relatively large impact on the thermal comfort of the street canyon between 12:00 and 19:00, while the impact difference between 20:00 and 23:00 is very small. One of the main reasons may be that plantation plays a quite positive role in blocking the sunlight and creating cool areas beneath vegetation during the daytime. Different street canyon vegetation characteristics have different effects in blocking sunlight and intercepting solar radiation and this can have different influences on land surface temperature and air temperature in the street canyon during the daytime (Bowler rt al., 2010; Oke, 1989). The soil in the area where the vegetation is located also has a different effect on absorbing and reflecting solar radiation compared to impermeable underlying surfaces such as asphalt and concrete. Soil can retain water for evaporation and this is beneficial for reducing air temperature. Impervious surfaces can quickly absorb and retain heat when exposed to solar radiation (Oke, 1989). Transpiration transports water from the roots of plants to the top, and then evaporates, taking away a large amount of heat. At night, transpiration is the main way for the plantation to reduce land surface temperature and air temperature in the street canyon. Even if the transpiration effects of different vegetation characteristics are different. The difference in average air temperature at 1.4-meter height in the street canyon area of different models is very small. At night, due to the lack of sunlight, plant stomata are closed and plant transpiration is weak (Caird et al. 2007; Dawson et al. 2007). This is also one of the main reasons why there is a small difference in the average UTCI values among the model street canyon areas at 19:00 and 23:00.
According to Table 3, Model 3 has the lowest average UTCI value during these 12 hours. The other features of Model 3 are the same as Model 1, but Model 3 has a median road divider with many 5 m trees and Model 1 does not. The 12-hour average UTCI value of Model 3 is 0.11 ℃ lower than Model 1. This indicates that the plants in the central divider of the road have played a significant role in improving the thermal comfort of the street canyon. In general, the more vegetations distribute in the street canyon, the better the thermal comfort of the street canyon during the hot day (Dawson et al., 2007), especially tall trees with large canopies can play a significant role in improving thermal comfort. The average UTCI of Model 2 is the highest among the six models. The difference between Model 2 and Model 1 is the size of the plant canopy and the height of the tree.
Tree size and tree canopy characteristics can significantly influence the penetration of solar radiation (Dawson et al., 2007). In Model 1, the 15-meter camphor tree and 10-meter Osmanthus Fragr can block more sunlight and form more shadow areas than the 10-meter camphor tree and 5-meter Osmanthus Fragr in Model 2. Having more leaves in a larger canopy can also better utilize the transpiration of plants to lower temperatures. Model 2 has an average UTCI value 0.2 ℃ higher than Model 1 during these 12 hours. Hedges can also affect solar radiation, wind speed and wind direction (de Quadros and Mizgier, 2023). The average UTCI value of Model 4 is only 0.05 ℃ lower than that of Model 2. This indicates that adding a larger area of 1 meter of hedge (Ligustrum lucidum) based on Model 2 has a weak effect on improving the thermal comfort of the street canyon. Vegetation applied to the building envelope can also influence radiant and convective energy exchange. This can also influence human comfort in the built environment (de Quadros and Mizgier, 2023). The average UTCI value of Model 5 is only 0.03 ℃ lower than Model 1. This indicates that adding ivy to the roof of the building has a weak effect on improving the overall thermal comfort of the street canyon. The average UTCI value in the street canyon area of Model 6 is 0.08 ℃ higher than Model 1. This indicates that when the LAI of the plant canopy is two-thirds of its original value, the sparse canopy has a significantly weaker improvement effect on street canyon thermal comfort compared to the dense canopy. This mainly due to tree canopy can reduce the penetration of shortwave solar radiation and intercepts long-wave radiation from the ground surface to atmosphere and desnse canopy can have stronger effect.
According to Table 3, if the 12 hours from 12:00 to 23:00 are divided into two periods: 12:00 to 19:00 and 20:00 to 23:00, and the UTCI means of the 6 models are compared in these two periods, it can be seen that the UTCI means of each model between 12:00 and 19:00, from highest to lowest, are Model 2 > Model 4 > Model 6 > Model 1 > Model 5 > Model 3; The average UTCI of each model between 20:00 and 23:00, in descending order, is Model 3 > Model 2 > Model 1 > Model 5 > Model 4 = Model 6. This shows that although the model 3, which has tall street trees, large canopies and a large number of trees, has a good cooling effect from 12:00 to 19:00, the UTCI value of the whole model is the lowest and the comfort level is the best, but this model has the highest average UTCI value and the worst comfort level at 20:00 to 23:00 at night. Tree canopies have been demonstrated as being able to retain heat at night (de Quadros and Mizgier, 2023). According to Wang et al., (2021) tree cover can reduce heat dissipation at night and aggravate the urban heat island in certain situations.Thus, planting more trees may not be reasonable measure to reduce urban heat island at night. Model 2 showed relatively poor thermal comfort in both periods, which further indicates that short trees with small crowns have poor effects on improving thermal comfort in street canyons during both day and night. Although the improvement effect Model 4 and 6 on street canyon thermal comfort is poor from 12:00 to 19:00. Model 6, which has sparse canopies, and Model 4, which has shorter crown diameter but a larger area of hedges, have the best improvement effect on street canyon thermal comfort from 20:00 to 23:00. One plausible explanation for this result is that, at model 6, tree leaves play a important role in preventing heat transfer and slowing down heat loss at night (Akbari, 2002). When the canopy is sparse, the effect become weak; At model 4, the hedges are 1-meter tall, the height which we analyze the UTCI vallue is 1.4 meter. The area we analyze is located above the hedges. Hedges may worse the thermal comfort of the area under the leaves of hedges because leaves can attenuate solar radiation and prevent the night’s heat flow from the surface to the sky at night (Wang et al., 2015). The areas where shrubs and soil are located have better thermal comfort compared to impermeable underlying surfaces (Oke, 1989). Model 4 has a larger area of underlying soil and shrubs compared to Model 2. This provides an explanation for Model 4 having lower PET values than Model 2 during both day and night. In addition, at night, hedges may also be able to have weak transpiration effect (Dawson et al., 2007). The weak transpiration effect of hedges also be able to improve thermal comfort of the area above or near the hedges weakly. This accords with Meili et al., (2021) that tree coving is more efficient in decreasing UTCI during daytime and vegetated ground fraction provides more cooling during night. Model 1 (constructed based on the actual situation of the street canyon) has a median UTCI value in both periods, indicating that there are still many ways to further improve the thermal comfort of the street canyon based on the actual situation. The significant differences in the improvement of street canyon thermal comfort by different models during the two different time periods mentioned above also indicate the importance of studying the different effects of plants on improving street canyon thermal comfort during the day and at night. Although Model 5, which increases roof greening, has a weak effect on improving the thermal comfort of the street canyon, this method slightly reduces the mean UTCI of the street canyon in both periods mentioned above. Therefore, when considering both the daytime and nighttime thermal comfort of the street canyon, increasing roof greening can slightly improve both the daytime and nighttime thermal comfort of the street canyon. However, the effect of this method on improving the thermal comfort of the street canyon is relatively weak, with an average decrease of only around 0.04 ℃ in UTCI value in the street canyon UTCI values from 12:00 to 19:00, and only approximately 0.01 ℃ decrease in UTCI value in street canyon UTCI values from 20:00 to 23:00. However, we choose ivy as the roof vegetation, if we chose trees or hedges as building wall and roof vegetation, the improvement effect may be more obvious. We only consider the impact of street greening on the thermal comfort of the street canyon at 1.4 m height outdoor. Green walls and roofs can have more obvious influences on the indoor thermal comfort of roadside buildings (Hao et al., 2020) and deserve to be considered as a measure to improve summer thermal comfort.
Overall, there is a relatively large difference in UTCI values among the models between 12:00 and 19:00, and a relatively small difference in UTCI values between 20:00 and 23:00. From 12:00 to 19:00, Model 2 with the highest UTCI mean was 0.48 ℃ higher than Model 3 with the lowest UTCI mean, while from 20:00 to 23:00, Model 3 with the highest UTCI mean was only 0.05 ℃ higher than Model 4 and Model 6 with the lowest UTCI mean. Therefore, optimizing the landscape pattern of the green space of the street canyon can have a noticeable effect on improving the daytime thermal comfort of the street canyon, but the effect is not obvious at night. Given this situation, we believe that considering both the daytime and nighttime thermal comfort of the street canyon. Street canyons with a large number of tall trees, large crown width, high leaf area index, wide shrub coverage area, and vertical greening, may bring greater benefits overall after weighing the pros and cons of their influence on the thermal comfort of the street canyon during a hot summer day and night. In addition, the planting of street trees needs to comply with the minimum distance between street trees, meet the convenience of daily traffic and pedestrian walking, and maintain the overall aesthetics of the street canyon. Therefore, when optimizing the landscape pattern of the green space of a street canyon, it is not feasible to blindly add too much greening to have a significant negative impact on the street canyon from many other aspects.
Table 2
The UTCI values of different models during day and night.
Time | Model 1 (℃) | Model 2 (℃) | Model 3 (℃) | Model 4 (℃) | Model 5 (℃) | Model 6 (℃) |
12:00 | 39.72 | 40.04 | 39.64 | 40.00 | 39.66 | 39.86 |
13:00 | 41.47 | 41.70 | 41.40 | 41.63 | 41.44 | 41.59 |
14:00 | 42.94 | 43.20 | 42.85 | 43.13 | 42.90 | 43.08 |
15:00 | 43.45 | 43.81 | 43.27 | 43.75 | 43.41 | 43.61 |
16:00 | 42.64 | 43.04 | 42.36 | 42.98 | 42.59 | 42.82 |
17:00 | 40.34 | 40.73 | 39.97 | 40.66 | 40.30 | 40.53 |
18:00 | 36.49 | 36.81 | 36.07 | 36.75 | 36.42 | 36.58 |
19:00 | 32.40 | 32.45 | 32.38 | 32.41 | 32.38 | 32.41 |
20:00 | 30.53 | 30.55 | 30.55 | 30.51 | 30.52 | 30.52 |
21:00 | 29.87 | 29.88 | 29.90 | 29.85 | 29.86 | 29.85 |
22:00 | 29.24 | 29.24 | 29.27 | 29.22 | 29.23 | 29.22 |
23:00 | 28.80 | 28.80 | 28.84 | 28.79 | 28.80 | 28.78 |
Table 3
The average UTCI values of the six models.
| Model 1 (℃) | Model 2 (℃) | Model 3 (℃) | Model 4 (℃) | Model 5 (℃) | Model 6 (℃) |
Average UTCI value from 12:00 to 23:00 | 36.49 | 36.69 | 36.38 | 36.64 | 36.46 | 36.57 |
Average UTCI value between 12:00 and 19:00 | 39.93 | 40.22 | 39.74 | 40.16 | 39.89 | 40.06 |
Average UTCI value between 20:00 and 23:00 | 29.61 | 29.62 | 29.64 | 29.59 | 29.60 | 29.59 |
3.3 A comparison of the thermal comfort of the six models based on the UTCI values at 13:00 and 20:00.
Figure 4 compares the UTCI values of six models at 13:00. From the comparison between Model 1 and Model 2, it can be seen that during the daytime, the UTCI value of the area where the street trees are located in Model 1 is relatively low, indicating that tall trees with larger crown width form a larger shaded area that blocks sunlight. In theory, roadside trees with different heights and crown widths have varying degrees of impact on the wind environment, which in turn affects the thermal comfort of local spaces (Hsieh et al., 2016). However, due to the low wind speed during this period and the dense construction in the area, the difference in UTCI values measured by Model 1 and Model 2 in this study is relatively small. From Model 1 and Model 3, it can be seen that under the same conditions, the increase of the median divider with trees in the middle of the road will reduce the UTCI value through the influence of sunlight obstruction and transpiration. Trees can reduce wind speed and this can have negative impact on thermal comfort (Hsieh et al., 2016). By comparing the UTCI values around the central reservation of the Model 3 and the corresponding part in Model 1, it can be seen that the trees in the central reservation of the road has very limited impact on the thermal comfort of the surrounding area through wind blockage and transpiration. Except for the significant difference in UTCI values between the shaded area of trees in the central reservation of the road in Model 3 and the corresponding part in Model 1, there is no significant difference in UTCI values between the other areas where the vehicle lane is located in the two models. Although the underlying surface absorb and re-emit the sun’s heat more than natural landscape (United States Environmental Protection Agency, 2023), Model 4 has a larger soil coverage area and a smaller area of impermeable underlying surfaces such as asphalt and concrete compared to Model 2. This is beneficial for reducing the air temperature rise caused by absorbing and emitting sun’s heat from the ground (United States Environmental Protection Agency, 2023). Compared Model 2 with Model 4, the difference in the effect of adding soil covered area and hedges on the illustrated results is relatively small, and the difference in UTCI values between the two models in the same spatial area is not obvious. Increasing roadside trees can produce a more significant cooling effect than hedges during daytime. In the street canyon area, after meeting the needs of both traffic and pedestrian flow, the remaining road greening area is very limited and needs to be fully utilized. Improving the coverage of trees is more beneficial for improving thermal comfort at 1.4 meters than increasing the coverage of hedges during the daytime. From the simulation results of Model 1 and Model 5, it can be seen that increasing roof greening has very limited effect on the UTCI value of local space. Greenery on the roof can help reduce the mean radiation temperature and improve the thermal comfort of the surrounding area due to it emit less short-wave radiance than hard surface (D'SOUZA, 2013). The analysis area is located 1.4 meters above the ground in the street canyon. It is far from the roof of buildings. The changing the underlying surface did not cause noticeable differences in thermal comfort in different areas of the street canyon. Comparing Model 1 and Model 6, it can be found that Model 1 with higher leaf area density has a relatively lower UTCI value at the tree shadow area, which is mainly due to more leaves blocking sunlight and transpiration. Even if the layout of street trees is thought as having significant impact on the thermal comfort of the surrounding environment (Zhao et al., 2018), it is hard to change the spatial layout of street trees in street canyon because the area designated for road green space is very limited, and plants such as roadside trees need to meet the requirements of the minimum distance between roadside trees in the city, maintain good permeability, and improve the aesthetic level of the street (Hunan municipal government, 2020). Under the premise of difficulty in improving the layout of roadside trees, selecting appropriate tree species and providing favorable conditions for the growth of roadside tree branches and leaves to increase the leaf area density and leaf area index of roadside trees will be a good method to improve the daytime thermal comfort during summer in the street canyons.
Figure 5 shows the UTCI values of six models at 20:00. Figures 4 and 5 can be used to compare the impacts of plants on thermal comfort during both daytime and nighttime, respectively. It can be seen that compared to the impacts of plants on thermal comfort during daytime, the impacts of plants on the nighttime thermal comfort of the respective areas are very small. At night, urban temperature can often be much higher than those in rural areas (United States Environmental Protection Agency, 2023). The variance of air temperature between urban and rural area can be higher during nighttime than daytime in some cases (Peng et al., 2012; Xu et al., 2022). Thus, it is also important to reduce nighttime heat island effect for the street canyon area. It can be observed from Fig. 5 that the six models exhibit minimal differences in street canyon thermal comfort due to varying characteristics of roadside trees. The corresponding UTCI values for the same area in all six models show no significant variation. Optimizing the road greening characteristics such as street tree height, crown width, road green divider, roof greening, and leaf area density have very limited impact on nighttime thermal comfort. The areas with better thermal comfort during the day and night are also different. During the day, they are more influenced by green space and distributed in areas with highly concentrated plantations. At night, areas with better thermal comfort are mainly concentrated in well-ventilated areas or areas far away from buildings. Thus, improving the ventilation condition is a good way to improve thermal comfort of the street canyon. Road pavement has the capacity to absorb a large amount of heat during the day. They release heat slowly in to the atmosphere at night (Katzschner, 2009; Ibrahim et al., 2018). Good ventilation environment plays a quite active role in reducing the heat release by the road pavement. During the daytime, shaded areas created by buildings typically offer good thermal comfort by blocking sunlight, thereby creating a cooler environment. However, at night, due to buildings slowly release heat absorbed during the day (Ahmed et al., 2014) and the building's ability to provide wind protection, areas with poor thermal comfort are formed around the building. Although the simulation results of the six models at 20:00 are not significantly different, road greening and soil underlying surfaces undoubtedly do not absorb large amounts of heat during the day and gradually release it at night, like impervious underlying surfaces such as asphalt and cement. From this perspective, this can still have a positive impact on nighttime thermal comfort. Meili et al., (2021) found that ground vegetation plays an active role in decreasing UTCI at night. This shows that soil and low-growing plants can be more beneficial to street canyon nighttime thermal comfort than impervious underlying surfaces during hot summer days.