Characterizing the thermal effects of vegetation on urban surface temperature

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 (Tr, b − Tr, 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 (Ti, r − Ti, v) is excess temperature of pixel i. The ratio of (Ti, r − Ti, v) to (Tr, b − Tr, 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 (Ti, r − Ti, v)/(Tr, b − Tr, v). Based on the high spatial resolution data, the Ti, v and Ti, rwithin 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.