Evaluating the impact of tree morphologies and planting densities on outdoor thermal comfort in tropical residential precincts in Singapore

Trees play an essential role in improving outdoor thermal comfort amidst rapid urbanization and impending climate change. Physiological Equivalent Temperature (PET) is used as the thermal comfort metric, derived based on a combination of in-situ measured meteorological parameters and numerically simulated pedestrian-level radiant exchange. The numerical model is developed based on surface energy balance coupled with the Monte Carlo ray-tracing technique and is validated with in-situ measurements. We study the thermal comfort enhancement potential of four typical tree morphologies at varied planting densities in the residential precincts in tropical Singapore. Firstly, we examine the temporal evolution and spatial distribution of PET in a matured residential precinct (MRP) on a typical hot day. Trees with large canopies either in diameter or height, i.e., umbrella and oblong trees, are most effective in improving outdoor thermal comfort. In particular, the umbrella trees improve the precinct mean PET by 1 class from "Warm " to "Slightly warm " at 14:00. Amongst different land features, the mean reduced PET is highest on land features near trees, i.e., grass, by up to 4.35 ?. Secondly, we evaluate the effects of varied planting densities in a new residential precinct (NRP), where open grass patches exist between buildings. At 14:00, tree planting densities of 2.20, 12.20, 29.33, and 36.76 trees per 10,000 m(2) are needed per 1 ? of mean reduced PET for umbrella, oblong, round to inverted cone trees, respectively. Our findings provide insights on the choice of tree morphology and planting density to mitigate the thermal stress of high-density living.