Integrated Cooling Effects of Blue–Green Spaces in Urban Waterfront Parks: A Field Study in Subtropical Nanjing

Urban blue and green spaces can alleviate extreme heat stress induced by global warming and urban heat islands, yet existing findings on their integrated cooling effects remain inconsistent. This study examined how the integrated cooling effects and relative cooling effectiveness of blue and green spaces differ with vegetation types and diurnal periods, based upon field measurements of two waterfront parks in subtropical Nanjing, China. Hourly air temperature was monitored at 15 sites with distinct blue–green–gray compositions within the two parks over 22 consecutive summer days. Results showed that both parks exhibited a cooling effect relative to the urban reference site (cooling frequency: 90.5% and 95.3%, respectively), whereas nocturnal warming emerged when the suburban meteorological station was used as the reference (warming frequency: 48.3% and 67.0%, respectively). The daytime air temperature was positively associated with gray-space proportions within 25–200 m buffers, whereas the nighttime air temperature was negatively correlated with green-space proportions within 200–500 m buffers. The integrated effect of blue–green spaces in the two parks was highly context-dependent: the tree-covered waterfronts provided sustained daytime cooling, whereas the grass-covered waterfronts cooled mainly at night; water proximity may suppress tree cooling during daytime, with no consistent influences observed for grasses. These observed patterns may offer preliminary empirical reference for climate-adaptive designs in similar subtropical waterfront settings.