Urban woodland on intensive green roof improved outdoor thermal comfort in subtropical summer

Global climate change and urban heat island effect have jointly threatened human thermal comfort in cities. Intensive green roof (IGR) could supplement urban green space to alleviate thermal comfort. Aiming at examining the impact of IGR on the thermal comfort of its users, Universal Thermal Climate Index (UTCI) and physiological equivalent temperature (PET) were applied to assess effectiveness of IGR in improving outdoor thermal comfort by IGR. Comprehensive microclimatic monitoring was conducted on urban woodland IGR and an adjacent control bare roof in humid-subtropical Hong Kong. Sunny days (n = 21) and cloudy days (n = 18) were selected from summer in 2016. Insolation, air, and surface temperatures were compared between the two roofs. UTCI and PET were computed for the daytime and nighttime period of each sampled day. On IGR, significantly higher environmental cooling was achieved during daytime in sunny weather, with mean surface and air cooling reaching 4.9 °C and 1.6 °C respectively. Woodland IGR successfully improved thermal sensation by cooling mean UTCI and PET by 5.5 °C and 10.9 °C respectively during daytime in sunny scenario. Woodland IGR reduced the occurrence of extreme and very strong heat stress which struck the control bare roof. Recommendations on about thermal sensation were provided for building owners, town planners, and landscape architects.