Abstract
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.
Highlights
The summary report of the Intergovernmental Panel on Climate Change (IPCC) predicts that global average temperatures will rise considerably by the end of the twenty-first century, with the exact increase depending upon future emissions of greenhouse gases (GHG) into the atmosphere
As transpiration is proportional to radiation absorbed, the rate is highest at the top of the canopy, while relative humidity is higher by around 2% beneath the canopy
This means that, while air temperature and humidity effects may extend some distance outside the park, any direct benefits for a pedestrian are confined to the park itself. Even with these benefits, people active out of doors in Singapore will experience moderate to strong heat stress during day, reaching 7.4 ◦C heat stress at 15:00 Singapore Local Time (SLT). These results indicate that heat stress during the day is inevitable in the tropical conditions of Singapore, but that the thermal comfort of the pedestrians is improved by the proximity of vegetation
Summary
The summary report of the Intergovernmental Panel on Climate Change (IPCC) predicts that global average temperatures will rise considerably by the end of the twenty-first century, with the exact increase depending upon future emissions of greenhouse gases (GHG) into the atmosphere. More than half of the world’s population lives in cities and it is estimated that 60% of the planet’s future urban area has yet to be built (World Health Organization, 2018). Forty per cent of urban dwellers live in tropical cities, many of which exhibit rapid economic growth and are expected to urbanize faster than in the rest of the world. In these cities, high temperatures are exacerbated by continuously high humidity (Chaiechi and Tavares, 2019), resulting in problems of heat stress that are likely to become even more severe in the future (Mora et al, 2017). Most earlier Computational Fluid Dynamics (CFD) studies (Liu et al, 2015; Botham-Myint et al, 2015; Allegrini et al, 2015) simplified the geometry of buildings, using forms such as cubes and rectangular prisms rather than real urban configurations
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