Local microclimate above shrub and grass in tropical city: A case study in Singapore

Urban greenery exhibits diverse cooling effect under different conditions. A field experiment was conducted in Singapore to measure the local microclimate above six vegetated surfaces and a concrete surface. Results showed that the average air temperatures above vegetated surfaces were lower than concrete surface all the daytime. Among the four moments from 3:00 am to 21:00 pm, the maximum difference of average air temperature between both surfaces occurred at 15:00 pm (from 0.56 degrees C to 1.72 degrees C), and the minimum difference occurred at 9:00 am (from 0.05 degrees C to 0.29 degrees C). A multilayer hygrothermal transfer model of greenery was developed and validated by the measured data. Sensitivity analysis based on this model shows that leaf area index has different effects on the air temperature within and above plant canopy. The top layer of plant canopy acts as heat source in the daytime and as heat sink at night. Leaf angle affects the air temperature within plant canopy significantly only when solar angle is high, and a more horizontal leaf distribution cools the soil surface better in the daytime. Plants with a higher leaf albedo and transmissivity have stronger cooling effect. The effect of minimum stomatal resistance is negligible.