Effect of optimal allocation of urban trees on the outdoor thermal environment in hot and humid areas: A case study of a university campus in Guangzhou, China

The process of urbanization in developing countries has had an ecological impact, destroying the outdoor thermal environment, especially in hot and humid areas where high summer temperatures are a nuisance for human outdoor activities. The role of urban greening in improving the regional microclimate has been confirmed by many studies. To improve the outdoor thermal environment, taking a university campus in Guangzhou as an example, the tree species and tree geometric parameters were analyzed, the ENVI-met model was established, and a simulation platform was calibrated based on the measured data, which improved the accuracy of the simulation platform. In this simulation platform, the maximum root mean square errors of air temperature and humidity were as low as 1.49 degrees C and 5.63 %, respectively. To reveal the influence of the physical morphology of tree species and their community characteristics on the outdoor thermal environment, six typical greening tree species and 37 green space configurations were established, and the influence of each tree species and their spatial configurations on the cooling effect of the outdoor environment was simulated. The results showed that the leaf area index, crown width, and tree height had a great influence on the ground temperature and sensible heat flux under the tree canopy, and the cooling effect of the tree group was improved when the height of the trees exceeded 10 m, with an ideal canopy coverage of 30 %. Optimizing the green space configuration reduced the temperature at pedestrian height (1.5 m) in the study area by up to 1.5 degrees C, and a reduction of 0.5 degrees C was observed over approximately 50 % of the green area during the peak summer temperatures (14:00-16:00), mitigating the microclimate heat environment. This study can enable urban planners to determine the type, quantity, and spatial allocation of green vegetation to improve the urban thermal environment.