Previousstudies on the cooling of urban vegetation mainly focused on its transpiration or shading effect separately, neglecting to explore the combined evapotranspiration-shading cooling. Further, accurate quantification of evapotranspiration-shading cooling remains challenging due to heterogeneity of urban landscapes, which limits understanding of its high-resolution spatiotemporal patterns. Here, we integrate high-precision remote sensing data and the Soil-Canopy-Observations of Photosynthesis and Energy Fluxes (SCOPE) model to propose an optimized quantitative approach. The approach was used to investigate changes in evapotranspiration-shade cooling during extreme heat.Taking Shanghai metropolitan as case, the results show: (1) The cooling capacity of urban vegetation in nighttime (18:00–6:00) is enhanced during extreme heat, which is attributed to accumulated effect of shading and enhanced evapotranspiration due to elevated vapor-pressure deficit. (2) In densely built-up areas with limited vegetation, there is a significant lack of thermal regulation, especially in the early morning (7:00) and late evening (17:00), thus exacerbating thermal stress. (3) At midday (11:00–13:00) there was a slight decrease in evaporative cooling, probably caused by the behaviour of the stomatal closure at high temperatures. Concurrently, high radiation causes the shading effect of vegetation to become more prominent, amplifying the cooling contrast between areas with dense and sparse vegetation cover. Moreover, the study also highlights that grassland with >50% cover can provide cooling effects similar to that of forest land. Overall, our study not only enhances the understanding of urban vegetation's cooling effects but also underscores the importance of strategic urban vegetation planning in mitigating urban heat, particularly under the escalating frequency and intensity of heat waves.

中文翻译：

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优化的土壤-冠层-光合作用和能量通量模型的增强观测揭示了极端高温期间城市植被的蒸散-遮荫冷却动态

以往对城市植被降温的研究主要集中于其单独的蒸腾或遮荫效应，而忽视了对蒸散-遮荫联合降温的探索。此外，由于城市景观的异质性，准确量化蒸散遮荫冷却仍然具有挑战性，这限制了对其高分辨率时空模式的理解。在这里，我们整合了高精度遥感数据和光合作用和能量通量的土壤冠层观测（SCOPE）模型，提出了一种优化的定量方法。利用该方法研究了极端高温期间蒸散发-遮阳降温的变化。以上海大都市为例，结果表明：（1）极端高温期间城市植被夜间（18:00~6:00）降温能力增强。热量，这是由于遮阳的累积效应和由于蒸气压不足而导致的蒸发量增加所致。 (2)在植被有限的建筑密集区，热调节明显缺乏，特别是在清晨(7:00)和傍晚(17:00)，热应激加剧。 (3) 中午(11:00~13:00)蒸发冷却略有减少，可能是高温下气孔关闭行为造成的。同时，高辐射导致植被的遮荫效应更加突出，放大了植被茂密和稀疏区域之间的降温对比。此外，研究还强调，覆盖率>50%的草地可以提供与林地类似的降温效果。总体而言，我们的研究不仅增强了对城市植被降温效应的理解，而且强调了战略性城市植被规划在缓解城市热量方面的重要性，特别是在热浪频率和强度不断升级的情况下。