Thermal Influence of a Large Green Space on a Hot Urban Environment

City-scale warming is becoming a serious problem in terms of human health. Urban green spaces are expected to act as a countermeasure for urban warming, and therefore better understanding of the micro-climate benefits of urban green is needed. This study quantified the thermal influence of a large green park in Tokyo, Japan on the surrounding urban area by collecting long-term measurements. Apparent variations in the temperature difference between the park and surrounding town were found at both the diurnal and seasonal scales. Advection by regional-scale wind and turbulent mixing transfers colder air from the park to urban areas in its vicinity. The extent of the park's thermal influence on the town was greater on the downwind side of the park (450 m) than on the upwind side (65 m). The extent was also greater in an area where the terrain slopes down toward the town. Even on calm nights, the extent of the thermal influence extended by the park breeze to an average of 200 m from the park boundary. The park breeze was characterized by its divergent flow in a horizontal plane, which was found to develop well in calm conditions late at night (regional scale wind <1.5 m s−1 and after 02:00 LST). The average magnitude of the cooling effect of the park breeze was estimated at 39 Wm−2. This green space tempered the hot summer nights on a city block scale. These findings can help urban planners in designing a heat-adapted city. Core Ideas The park is 1.5 to 3 K cooler than the surrounding town in summer daytime. The park cooler air was advected to the urban area until a maximum extent of 450 m. The extent of thermal influence gets larger on the slope down to the town. The park breeze occurred more frequently in calm conditions late at night. Amount of town cooling by the park corresponds to 2600 room air-conditioning units.