Investigating the cooling impacts of Green Hearts in summer: A case study in the Changsha-Zhuzhou-Xiangtan urban agglomeration based on Weather Research Forecasting (WRF) simulations

Numerous studies have established that green spaces are crucial in combating urban heat islands (UHI). As ecological Green Hearts have been adopted in many multipolar urban agglomerations, this study questions their long-term impact on urban heat mitigation. Taking the “Green Heart” area of the Changsha-Zhuzhou-Xiangtan urban agglomeration, and utilizing land use datasets between 1990 and 2020, numerical simulations of near-ground air temperatures were conducted to analyze the temperature variations induced by the spatial dynamics and the land-use change inside the Green Heart. The findings reveal a 0.4 °C increase in urban temperatures caused by urban expansion. A “cool island effect” by the Green Heart is observed but the temperature difference achieved (2 °C to 4 °C) is no more significant than in city-scale parks and the cooling extent beyond the Green Heart's perimeter was insignificant. Doubling the construction land inside the Green Heart increased daytime and nighttime temperatures by 1.5 °C and 2.6 °C, respectively, suggesting the importance of maintaining forests. The long-term urban temperature fluctuations are arguably influenced more by the spatial transformations inside the cities than changes inside the Green Heart. These results imply that combating urban heat should prioritize city-centered cooling strategies and land-use control. • Compared to the cooling intensity of city level vegetation, the regional Green Heart does not perform particularly better. • The land use dynamics within the Green Heart have little effect on air temperature variation in the surrounding cities. • Inside the Green Heart, forests achieve the best Cool Island Effect, with a 2°C day and 4°C night difference compared to the urban areas. • Doubling construction land in the Green Heart raised daytime and nighttime temperatures by 1.5°C and 2.6°C, respectively.