Abstract
Urban trees can significantly improve the outdoor thermal environment, especially in subtropical zones. However, due to the lack of fundamental evaluations of numerical simulation models, design and modification strategies for optimizing the thermal environment in subtropical hot-humid climate zones cannot be proposed accurately. To resolve this issue, this study investigated the physiological parameters (leaf surface temperature and vapor flux) and thermal effects (solar radiation, air temperature, and humidity) of four common tree species (Michelia alba, Mangifera indica, Ficus microcarpa, and Bauhinia blakeana) in both spring and summer in Guangzhou, China. A comprehensive comparison of the observed and modeled data from ENVI-met (v4.2 Science, a three-dimensional microclimate model) was performed. The results show that the most fundamental weakness of ENVI-met is the limitation of input solar radiation, which cannot be input hourly in the current version and may impact the thermal environment in simulation. For the tree model, the discrepancy between modeled and observed microclimate parameters was acceptable. However, for the physiological parameters, ENVI-met tended to overestimate the leaf surface temperature and underestimate the vapor flux, especially at midday in summer. The simplified calculation of the tree model may be one of the main reasons. Furthermore, the thermal effect of trees, meaning the differences between nearby treeless sites and shaded areas, were all underestimated in ENVI-met for each microclimate variable. This study shows that the tree model is suitable in subtropical hot-humid climates, but also needs some improvement.
Highlights
With the continuous advance of the urbanization process, the deterioration of the urban thermal environment has become a big issue, especially in megacities like Guangzhou, in subtropical hot-humid climate zones
The leaf albedo was measured by a spectrophotometer (U-4100, Hitachi, Tokyo, Japan), three leaves of each tree species were measured, and averages were taken
The evaluation of microclimatic parameters in nearby treeless sites found that ENVI-met has a tendency to overestimate the solar radiation and air temperature while underestimating the air humidity, which was foundational work for the evaluation of the tree model in the step
Summary
With the continuous advance of the urbanization process, the deterioration of the urban thermal environment has become a big issue, especially in megacities like Guangzhou, in subtropical hot-humid climate zones. Simon [18] evaluated ENVI-met by comparing the modeled and observed transpiration rate of trees under different weather conditions and showed that for the trees in Germany, the evaluation results were influenced by PAR (photosynthetic active radiation). A comprehensive evaluation of tree models in the subtropical hot-humid climate is urgently needed To solve these pending problems and to evaluate the suitability of ENVI-met vegetation models in subtropical hot-humid climate zones, this study comprehensively compared the modeled and observed physiological parameters (leaf surface temperature and vapor flux) and microclimate variables (solar radiation, air temperature and humidity) of four common tree species Was paid to the vapor flux and the thermal effect of trees (the differences between nearby treeless sites and canopy-shaded areas)
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