Vegetation is important for urban heat mitigation. The cooling intensity of vegetation is affected by background climate and urban design. How to evaluate vegetation cooling efficiency under different climate conditions is still an issue open to discussion. In this study, a normalized indicator of urban vegetation cooling efficiency (NVCE) is proposed as a metric of urban vegetation cooling efficiency applicable and comparable under different climate and urban conditions. When surfaces are only covered by vegetation, the cooling effects should be highest than other pixels at the local climate scale. The difference of surface temperature between the pure vegetation surfaces and surfaces without vegetations (T_{r, b} − T_{r, v}) is the range of the vegetation cooling intensity at the same local climate conditions. Difference between radiometric surface temperature of a mixed pixel and the vegetation temperature within the mixed pixel (T_{i, r} − T_{i, v}) is excess temperature of pixel i. The ratio of (T_{i, r} − T_{i, v}) to (T_{r, b} − T_{r, v}) can indicate how much percent of existed excess temperature after vegetation cooling effects for pixel i under such local climate condition. Thus, the NVCE is defined as (T_{i, r} − T_{i, v})/(T_{r, b} − T_{r, v}). Based on the high spatial resolution data, the T_{i, v} and T_{i, r}within each 30 m × 30 m grid are derived to calculate the NVCE and the relationships between NVCE and fractional vegetation cover were studied under different conditions. Results showed that NVCE can reduce the differences caused by background climate in the assessment of vegetation cooling efficiency, i.e. making vegetation cooling efficiency under different climate conditions comparable. The NVCE is also sensitive to the vegetation fraction. When vegetation fraction is smaller than 0.2, the mean value of NVCE is about 0.5 and no obvious change. This means that the vegetation has no obvious cooling effects when vegetation fraction is smaller than 0.2. When the vegetation fraction is higher than 0.2, NVCE decreases linearly with increasing vegetation fraction. When the vegetation fraction is higher than 0.9, NVCE tends to 0. This indicates that 0.2 for vegetation fraction is the threshold of vegetation cooling effects. This study can provide information for evaluating the vegetation cooling efficiency under different climate and geometric conditions. This study also can provide useful information for urban green infrastructure design and planning, e.g. the vegetation fraction should be higher than 0.2 for urban cooling and the vegetation cooling efficiency can reach maximum when SVF is about 0.5 to 0.6.

植被对于缓解城市热量很重要。植被的降温强度受背景气候和城市设计的影响。如何评价不同气候条件下的植被降温效率仍是一个有待讨论的问题。在这项研究中，提出了城市植被冷却效率（NVCE）的标准化指标，作为在不同气候和城市条件下适用和可比较的城市植被冷却效率的度量。当表面仅被植被覆盖时，降温效果应高于当地气候尺度的其他像素。纯植被表面与无植被表面的地表温度差 ( T _{r , b}  -  T _{r , v}) 是当地相同气候条件下植被降温强度的范围。混合像元的辐射表面温度与混合像元内的植被温度 ( T _{i , r}  -  T _{i , v} ) 之间的差值是像元i的超温。( T_{我, r}  -  T_{我, v} ) 与 ( T _{r , b}  -  T _{r , v} ) 的比率可以指示像素i在植被冷却效应后存在的超温百分比在这样的当地气候条件下。因此，NVCE 定义为 ( T _{i , r}  -  T _{i , v} )/( T _{r , b}  -  T _{r , v} )。基于高空间分辨率数据，T _{i , v}和T _{i , r}在每个 30 m × 30 m 网格内推导出 NVCE 并研究不同条件下 NVCE 与植被覆盖率之间的关系。结果表明，NVCE在植被降温效率评估中可以减少背景气候造成的差异，即使不同气候条件下的植被降温效率具有可比性。NVCE 对植被分数也很敏感。当植被分数小于 0.2 时，NVCE 的平均值约为 0.5，无明显变化。这意味着当植被分数小于 0.2 时，植被没有明显的降温效果。当植被分数高于 0.2 时，NVCE 随着植被分数的增加呈线性下降。当植被分数高于 0.9 时，NVCE 趋于 0。这表明为 0。2 植被分数是植被降温效应的阈值。本研究可为评估不同气候和几何条件下的植被降温效率提供信息。本研究还可以为城市绿色基础设施的设计和规划提供有用的信息，例如城市降温的植被分数应高于0.2，当SVF约为0.5至0.6时，植被降温效率可以达到最大值。