Numerical simulation of cooling effect of vegetation enhancement in a subtropical urban park

Vegetation covers in urban parks are very useful for providing a cool microclimate which mitigates urban heat islands (UHIs). The objectives of this investigation are to therefore conduct on-site measurements and computational fluid dynamic simulations to evaluate the cooling efficacy from vegetation planted in a public park in Taipei, which is a subtropical city in Taiwan. The thermo-flow characteristics are predicted and compared with the measured air velocity and temperature data by using ultrasonic anemometers and an infrared camera to validate the computer modeling, including the sophisticated configurations of trees. Computations are also conducted to resolve the physiological equivalent temperature (PET) profiles for assessing the thermal comfort state at the pedestrian level of the outdoor environment. To investigate the impacts of park renewal on the urban microclimate, three pavilions and supplementary green areas are added to the simulation, and the results reveal that there is a better cooling effect in the park with a higher green coverage ratio (GCR). Moreover, the simulations find that the increased tree coverage ratio can more than compensate for loss of coverage of grasses, resulting in an overall decrease in average temperature. The relationship between thermal comfortable area and green coverage ratio tends to be nonlinear in nature. However, it would be more convenient for applications to adopt the linear regression analysis for determining the correlation between the GCR and PET for the percentage of areas that are comfortable (Cf). It shows that Cf = 0.96 × GCR + 5.08, with a reasonable R2 value of 0.91, for GCR ranged from 54.5 to 71.6%, indicating that there is a significant reduced UHI effect with increases in the GCR. In addition, the correlation implies that a completely thermally comfortable condition will prevail in the park if it has full green coverage. This correlation thus serves as an important guideline for urban planners and managers when designing and managing public urban spaces, such as parks.