Spatiotemporal distribution of cooling effects from urban-scale rooftop mitigation strategies during high-temperature weather—A case study of the coastal Metropolis Tokyo

Rooftop mitigation strategies (RMSs) can alleviate extreme heat in highly urbanized cities under global warming. In this study, gridded urban canopy parameters for Tokyo, a coastal metropolis, were integrated into the Weather Research and Forecasting model, coupled with a multilayer urban canopy model. The effects of rooftop photovoltaic (PV) panels, sedum green roofs (GRs), and grass GRs on the urban thermal environment during high-temperature days were then analyzed. PV panels, sedum GR, and grass GR with 50 % coverage achieved the most significant reductions in regional average pedestrian-level air temperatures at 2 PM, 10 AM, and 12 PM, respectively, with reductions of 0.10 °C, 0.11 °C, and 0.13 °C. The daily peak of regional maximum cooling effects for all RMSs were observed around 2 PM, reaching 0.25 °C, 0.25 °C, and 0.30 °C, respectively. Despite the daytime cooling effect, both GRs increased heat stress, which was particularly noticeable in grass GRs due to their stronger evaporative capacity, resulting in an increase in WBGT of up to 0.15 °C in approximately 75 % of the study areas. Additionally, the impacts of RMSs in coastal areas extended to the northwestern hinterland, indicating a non-local pattern influenced by sea-breeze advection. Furthermore, under high-temperature conditions, the cooling effect of GRs at midday increased linearly with background temperature.