Synergistic effects of wind direction, walkway position, and tree canopy characteristics on pedestrian thermal comfort in urban riverside greenways

Abstract

In high-density cities with hot-humid climates, Urban Riverside Greenways (URGs) serve as critical cooling corridors, vital for mitigating urban heat island (UHI) effects and improving pedestrian thermal comfort. However, the synergistic cooling mechanisms between water bodies and vegetation, and how they interact with airflow within URGs, remain insufficiently quantified. This study presents a parametric analysis integrating validated ENVI-met simulations and field measurements, based on a case study in Shanghai. Through 75 factorial scenarios, it investigates the synergistic effects of prevailing wind direction, walkway position, and tree canopy characteristics —specifically Leaf Area Density (LAD) and foliage albedo—on pedestrian thermal comfort within URGs. Critically, this study dissects the trade-offs between water-based cooling (driven by high specific heat capacity and evaporative latent heat exchange) and vegetative cooling (driven by radiative shading and transpiration), and reveals how airflow redistributes these effects across the URG profile. The results show that: (1) Convective heat transfer (governed by prevailing wind direction) is the dominant factor regulating air temperature ( T a ), whereas radiative shading (determined by tree canopy characteristics) is the primary driver for Physiological Equivalent Temperature (PET). (2) The spatial distribution pattern of thermal comfort exhibits a tipping point governed by the equivalence in cooling capability between vegetation and waterbody. In the context of this study, this threshold occurred at LAD = 1.0 and foliage albedo =0.2 under full canopy cover. (3) Optimal walkway position is determined by the airflow-driven redistribution of water-based and vegetative cooling: upwind waterbodies favor water-adjacent locations. Conversely, downwind scenarios trigger a trade-off: street-side placement is superior under dense canopies (shading-dominant), whereas water-side remains optimal under sparse canopies. By synthesizing these micro-physical interactions, this study proposes a climate-adaptive decision framework, including decision trees and a parametric tree species selection matrix, providing actionable design strategies for thermally resilient URGs in hot-humid high-density cities.