©2023 International Society of Arboriculture Arboriculture & Urban Forestry 49(5): September 2023 257 inhibited wind flow inside the urban areas, resulting from aerodynamic performance of the trees. This lower velocity led to reduced ventilation and ampli- fied heat accumulation phenomenon below the can- opy zone and windward/downward sides of vegetation and buildings. This decrease was due to the increased roughness of urban surface with the addition of trees and its drag on airflow, which caused a smooth change in wind flow due to the pressure dif- ference created as a result of porous trees (Oke et al. 2017). Tree species with large LAD (TC-2) offered more resistance to the flow, while a tree with a tall trunk (TC-5) offered the least (Figure 6C). This pro- vided better ventilation at all the monitoring locations due to low obstruction to wind flow at the pedestrian height, thus causing a significant improvement in thermal comfort. Tree species with large crown height (TC-3) and crown width (TC-4) provided intermedi- ate ventilation. Vegetation can be favorable for the urban micro- climate since it tends to reduce thermal stress (Yan et al. 2020) due to its ability to reduce radiation and pro- mote air cooling despite decreasing the wind speed. Figure 6D shows the heat stress reduction potential of different tree species in terms of apparent tempera- ture variation. Bauhinia × blakeana (TC-2) was most influential in reducing the actual apparent tempera- ture at the pedestrian height when compared with Guaiacum offinale (TC-1), due to its greater LAD and low height. This was then followed by Aza- dirachta indica (TC-5) and Peltophorum pterocar- pum (TC-3 and TC-4). Thus, irrespective of sky view factor, crown height and width were the least effec- tive for the reduction of heat stress. Figure 7 presents the boxplots for each scenario summarizing the maximum, minimum, median, aver- age, and interquartile ranges of air temperature, flow velocity, and apparent temperature values simulated at 3 PM. This box plot was based on the data of dis- crete points, located only near or on the vegetation zones (Figure 8). The impact or cooling potential of each mitigation scenario on in-canyon apparent tem- perature reductions was expressed as the difference between the reference case and each mitigation sce- nario in the same spot. Figure 7 summarizes the max- imum and average cooling potential for each mitigation scenario with respect to the existing condi- tions. When comparing ΔTMAX, ΔTAVG, and rank orders (based on TAVG) from 1 (coolest) to 5 (warmest), large comfort conditions of the urban area was investi- gated. Five scenarios were modeled to simulate the impact of various tree configuration parameters (trunk height, crown width, crown height, and leaf area density)(Table 4), as these parameters are highly correlated with the cooling capacity of trees (Morak- inyo et al. 2018). The enlisted values of drag coeffi- cient, adopted for these simulation cases, were calculated from the proposed corrective factor of tree species with respect to their relevant canopy shapes. 24-Hour Temperature Distribution The effectiveness of tree vegetation toward regulat- ing thermal environment was presented as 24-hour data (averaged across the circular domain) and discrete data (located on or near vegetation zones in the urban streets) for the studied tree morphological scenarios. Figure 6A establishes that TC-2 (highest LAD) provided the largest reduction in air temperature (1.2 °K) followed by TC-3 (0.8 °K) when compared with TC-1 (smallest LAD), while TC-4 and TC-5 provided the smallest reduction in temperature (0.7 °K). For TC-1, temperatures started rising with the amount of solar irradiance and peaked at time of high solar irra- diance before decreasing. Throughout the day, the tree’s effectiveness also varied similarly with reduced intensity, owing to continuous changes in shading and evapotranspiration rates that varied with solar access. Likewise, Figure 6B displays the surface tempera- ture distribution throughout the 24-hour study. A reduction of up to 4.0 °K in surface temperature occurred with street trees for scenario of highest LAD (TC -2) when compared with the base case scenario (TC-1). This reduction resulted in low heat access and caused lesser accumulation of solar energy inside the urban area. LAD was the most influential in reducing the actual surface temperature due to the interception of large solar radiation rays by tree foli- age, as evident from diurnal variation in Figure 6B. This was then followed by crown height. There was a temperature difference of 3 °K between the trees hav- ing lower LAD and crown height (TC-1) and trees with greater height (TC-3) at 3 PM on 2015 June 19. In reducing the surface temperature, Guaiacum offi- nale appeared to be the least effective. The tree spe- cies having high trunk height (TC-5) and crown width (TC-4) were least effective when compared with Bauhinia × blakeana (TC-2). The distribution of velocity is portrayed in Figure 6C. It was clearly observed that tree vegetation
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