248 Zeeshan et al: Heat Stress Mitigation in a Hot-Humid Urban Environment Street vegetation is strongly advocated to address current and future global warming due to its attenua- tion of solar radiation absorption (Rahman et al. 2020), shade creation (Massetti et al. 2019), and evapotranspiration production (Gromke et al. 2015; Zhang et al. 2019). However, these studies were per- formed by modeling the tree as a cuboid-shaped porous zone, characterized by the pressure loss and permeability coefficients and aerodynamic turbu- lence, in computational fluid dynamics (CFD) simu- lations due to the challenging nature of discretizing the complex tree canopy shapes. This modeling dif- ference led to an underestimation of actual drag offered by tree canopy, resulting in inaccurate model- ing of its aerodynamic effects. However, Zeeshan et al. (2022a) studied the effects of actual canopy drag of a specialized tree by modeling the actual tree per- formance under representative hot-humid climatic conditions and found a significant difference in air temperature, surface temperature, and apparent tem- perature (0.8 °K, 4 °K, and 0.6 °K, respectively) when compared with simulation results obtained with a tuned value of canopy drag coefficient as employed by previous studies. In reality, the microclimatic benefits of vegetation, such as reduced heat absorption, shade, and transpi- ration rate, are strongly affected by tree species and morphological characteristics (i.e., leaf density, tree height, crown width, tree architecture, and proximity to each other) alongside tree size and shape (Berry et al. 2013). This poses a dire need for research and analysis to move away from the generalized tree to site-specific trees for better thermal conditions in the urban environment. Various studies, both numerical and experimental, have been performed in recent years to observe the impact of species characteristics on ther- mal comfort. Morakinyo et al. (2020) studied the impact of various tree species and their configurational param- eters on thermal comfort in simple canyons and found a varying cooling effect ranging from 0.3 to 1.0 °K in the daytime temperatures and 0 to 2.0 °K in the night- time. This study reported the leaf area density (LAD), tree crown, and trunk height as the most influential parameters. Teshnehdel et al. (2020) studied the effect of different tree species and foliage cover on micro- climate and found a significant reduction of 0.29 °K in air temperature (Ta) with an increase in coverage fraction during the summer. The varying cooling potential by species owes its effectiveness to morphological and structural variability, which has not been extensively studied. Almost all previous studies focused on species performance in open environments, parks, and generic symmetric canyons with fixed sky view factor (SVF) rather than real canyons with variable SVF across asymmetrical canyons and miscellaneous street ori- entations. Such orientation and scenarios strongly affect solar access due to shadowing effects, trapping of longwave radiation, humidity increases, and vari- able wind-flow patterns. Moreover, the cooling effect of tree vegetation with species-specific canopy drags modeled has not yet been investigated numerically in real urban areas with hot-humid climates. Further- more, the comparative analysis of results from simu- lations, based on realistic case studies in an isolated street surrounded by buildings with different height to width ratios, can provide a better understanding of the real contributions and potential limitations of each species to the overall thermal profile of a place (Morakinyo et al. 2017). Such locations are always challenging due to variation over time and space in microclimatic boundary conditions, which affects vegetation effectiveness due to impaired transpira- tion. Thus, a detailed study regarding the evaluation of different tree species in hot-humid climates is essential. In this context, the current study aimed to fill this gap by providing detailed insight into the effective- ness of different tree species, with their actual canopy drag modeled, toward heat stress mitigation under representative hot-humid climatic conditions. This was accomplished through undertaking various CFD simulations employing the finite volume method with unsteady Reynolds-averaged Navier-Stokes (URANS) equations using a well-validated CFD tool (i.e., ANSYS FLUENT). The study results helped in making appropriate selections of the best tree species under representative climatic conditions in a real urban environment (I. I. Chundrigar Road, Karachi, Sindh, Pakistan). In addition, this research suggested more suitable and adaptable tree species for urban settings. Description of Study Area The heat-mitigation potential of street vegetation was evaluated in an urban area of Karachi, Pakistan, the 12th most densely populated urban area in the world (Sajjad et al. 2009; Qureshi 2010; Sajjad et al. 2015).
