Detailed investigation of vegetation effects on microclimate by means of computational fluid dynamics (CFD) in a tropical urban environment

In light of globally increasing temperatures, accentuated in cities by the urban heat island effect, urban planners and designers are looking for new, quantitative methods to assess the performance of their designs in terms of ecosystem services provided by vegetation. Among these ecosystem services, improved microclimate conditions are particularly important for human thermal comfort and health. In this study, an urban scene in the tropical city of Singapore is numerically investigated with a fully-integrated, three-dimensional urban microclimate model implemented in OpenFOAM. Mass and heat transport in air and storage effect in the urban environment are coupled so that the daily turbulent transport in air using steady Reynolds-averaged Navier-Stokes (RANS) can be solved iteratively with the unsteady heat and moisture transfer from urban surfaces. Vegetation is modeled as a porous medium for the flow of moist air and a leaf energy balance model is used to determine the heat fluxes and transpiration at leaf surfaces. The analysis shows the influence of an urban park upon air temperatures and thermal comfort. Cooling intensity of 1 °C is observed downwind of the park within a region of 27 m for an incoming wind speed of 2.3 m s−1, which reduces to 0.6 °C at a distance of 117 m from the park. The Universal Thermal Comfort Index (UTCI) shows a reduction in thermal stress in and around the park. The approach presented here can provide specific guidelines for urban planners and frame expectations on magnitude and spatial extent of local microclimate modifications generated by an urban park in a tropical city.