A case study of urban heat island dynamics in metro-integrated urban canyons in Chennai, India

Urban Heat Island (UHI) effects are worsening in tropical cities due to rapid urbanization, increased impervious surfaces, and the shrinking of green spaces. Chennai, a densely populated coastal city in Southern India, exemplifies this trend, where expanding metro infrastructure is significantly changing the urban landscape. This study explores the combined effects of metro structures, building heights, and roadside vegetation on the thermal behavior of urban canyons. Using a two-phase approach, the research combines 56 days of field measurements across seven types of urban canyons with 21 detailed ENVI-met simulations. Key factors include metro heights (12 m and 24 m), building heights (18 m, 27 m, and 36 m), and vegetation. Field data show that metro corridors can reduce peak daytime temperatures by up to 5.6°C compared to nearby non-metro areas. Simulation results indicate that, for wider roads, a setup with a 12 m-high metro line bordered by 36 m-tall buildings with vegetation provides the greatest thermal benefit—reducing peak temperatures by 2.78°C and eliminating localized hotspots. Vegetation also provides moderate cooling during the night. These insights offer practical guidance for climate-sensitive, transit-oriented urban planning, highlighting how integrating metro design, vertical urban structures, and greenery can help reduce urban heat stress. This strategy supports the development of more sustainable and livable tropical cities, aligning with the United Nations Sustainable Development Goals.