Green roof cooling and carbon mitigation benefits in a subtropical city

Green roofs are promoted as an effective nature-based urban heat island mitigation strategy. Green roof cooling and energy-saving benefits have been simulated for various climatic zones, but mainly at the building scale. Due to a lack of fact-based information on neighborhood cooling benefits, green roof construction lags and has rarely been incorporated into urban planning actions. This study investigated the thermal benefits and energy savings of green roofs for the central area of the Xianlin Campus of Nanjing University at the neighborhood scale. Three scenarios were simulated for a hot summer day using a validated ENVI-met model: a base case (S0), extensive green roofs (EGRs) (S1), and intensive green roofs (IGRs) (S2). The air temperature cooling benefit from green roofs extended downwards to the pedestrian level. The EGR scenario achieved a maximum 0.29 °C air temperature reduction at the pedestrian level and 0.37 °C at the rooftop level. The IGR scenario achieved a maximum 0.35 °C air temperature reduction at the pedestrian level and 0.45 °C at the rooftop level. EGRs and IGRs reduced energy demands for air-conditioning by 0.39 kWh·m⁻²·d⁻¹ and 0.56 kWh·m⁻²·d⁻¹ and CO₂ emissions by 31,997 kg·d⁻¹ and 45,967 kg·d⁻¹, respectively. These results confirm that green roofs yield substantial cooling and carbon mitigation benefits. Our study provides essential data to establish green roofs as mainstream cooling technology in subtropical cities. The results also imply that urban planners and policymakers may need to embrace the implementation of green roofs in long-term planning and building design practices to improve urban thermal environments, reduce building energy demand, and curb carbon emissions.     

Maximum extent of human heat stress reduction on building areas due to urban greening

Based on the ENVI-met model v4.0 BETA, numerical simulations were carried out for five different sized building areas in the city of Stuttgart (Southwest Germany) on the heat wave day 4 August 2003. Human heat stress is primarily quantified by the physiologically equivalent temperature (PET). Additional background information is provided by both near-surface air temperature (T_a) and mean radiant temperature (T_mrt). The simulations concern five urban land use scenarios. Related to differences of simulation results between a scenario that only consists of asphalt surfaces and a green scenario only showing grasslands and trees, the resulting ΔT_a, ΔT_mrt and ΔPET values are interpreted as maximum extent of human heat stress reduction on the building areas by urban greening. To achieve a higher reliability for urban planning, the results are averaged over the period 10-16 CET. Exemplarily for one building area the results are presented in terms of grid-related absolute values. Besides mean absolute values for each building area whose magnitudes depend on the meteorological conditions of the simulation day, the results include mean relative ΔT_a, ΔT_mrt and ΔPET values. As verified by additional simulations for a current typical summer day, they can be regarded as representative for summer in Central Europe. Averaged over the five building areas mean ΔT_a amounts to 1.1 °C (4%), mean ΔT_mrt to 17.6 °C (26%) and mean ΔPET to 7.5 °C (16%). The results of further simulations point to the increase of human heat stress by the planning variants for the building areas compared to the maximum extent of human heat stress reduction by the green scenario. It reaches 0.4 °C (1%) for mean ΔT_a, 4.9 °C (9%) for mean ΔT_mrt, and 3.7 °C (9%) for mean ΔPET.     

Effects of different vertical façade greenery systems on pedestrian thermal comfort in deep street canyons

Vertical greenery systems (VGSs) have been adopted in city planning operations to mitigate excess heat in hot and humid subtropical cities. This study focused on the influence of different arrangements of vertical greening on pedestrian thermal comfort and particulate matter with a diameter of 10 µm (PM₁₀) in street canyons. In this paper, the ENVI-met computational fluid dynamics (CFD) method was used to investigate the effects of different façade greenery arrangements with the same amount of greenery in the Nan Hai Yi Ku (NHYK) industrial district. On-site measurements were used to validate the simulation results in a transition season. The results showed that greening façades could improve pedestrians' thermal comfort with physiological equivalent temperature (PET) value reductions varying from 0.17 °C to 1.4 °C. Under a certain amount of greenery, the critical factor determining pedestrians' thermal comfort was the coverage rate of the greening façade near the pedestrian level. Specifically, increasing the greening façade coverage near the lower parts of street canyons could enhance the pedestrian-level cooling effect. In addition, the VGSs positively affected the pedestrian-level air quality in the street canyons. Nevertheless, the changes in pedestrian-level PM₁₀ concentration induced by the presence of VGSs were not very obvious under the building-parallel wind direction.