The effect of urbanization and urbanization pattern on the thermal environment and local rainfall is investigated in the tropical coastal city, Singapore. The Weather Research and Forecasting (WRF) model is employed with 5 one‐way nested domains and the highest horizontal resolution is 300 m. The urban effect is taken into account by a single‐layer urban canopy model. Several scenarios with idealized urbanization patterns are designed and simulated for an ensemble of 28 members. In the asymmetric urbanization scenarios, in which either the southern or northern part of Singapore is urbanized while the other part is forest, the magnitude of urban heat island (UHI) intensity is higher than that in the symmetric urbanization scenario, in which the urban and forest land use is homogeneously distributed in Singapore. The anthropogenic heat (AH) associated with the urban areas will exacerbate the UHI intensity. Most of the rainfall in the examined cases occurs from late morning to afternoon when the sea breeze blows northeastward. The results suggest that sea breezes have stronger influence on the rainfall than the urbanization pattern since the downwind part always gets more rainfall than the upwind part. The urbanization and associated AH can have two opposite effects on the rainfall amount: increasing rainfall through increasing buoyancy by AH and decreasing rainfall through reducing evaporation by converting greenery to impervious surfaces. The ultimate effect is dependent on the relative strength of these two influences.

This study employed the Weather Research and Forecasting model with a single‐layer urban canopy model to investigate the urban environment of a tropical city, Singapore. The coupled model was evaluated against available observational data from a sensor network and flux tower. The effects of land use type and anthropogenic heat (AH) on the thermal and wind environment were investigated with a series of sensitivity tests using an ensemble approach for low advection, high convective available potential energy, intermonsoon season cases. The diurnal cycle and spatial pattern of urban heat island (UHI) intensity and planetary boundary layer height were investigated. The mean UHI intensity peaked in the early morning at 2.2°C, reaching 2.4°C in industrial areas. Sea and land breezes developed during daytime and nighttime, respectively, with the former much stronger than the latter. The model predicted that sea breezes from different coastlines of the Malay Peninsula meet and converge, inducing strong updrafts. AH was found to play roles in all the processes studied, while the effect of different land use types was most pronounced during nighttime, and least visible near noon.

Our study employed large-eddy simulation (LES) based on a one-equation subgrid-scale model to investigate the flow field and pollutant dispersion characteristics inside urban street canyons. Unstable thermal stratification was produced by heating the ground of the street canyon. Using the Boussinesq approximation, thermal buoyancy forces were taken into account in both the Navier–Stokes equations and the transport equation for subgrid-scale turbulent kinetic energy (TKE). The LESs were validated against experimental data obtained in wind-tunnel studies before the model was applied to study the detailed turbulence, temperature, and pollutant dispersion characteristics in the street canyon of aspect ratio 1. The effects of different Richardson numbers (Ri) were investigated. The ground heating significantly enhanced mean flow, turbulence, and pollutant flux inside the street canyon, but weakened the shear at the roof level. The mean flow was observed to be no longer isolated from the free stream and fresh air could be entrained into the street canyon at the roof-level leeward corner. Weighed against higher temperature, the ground heating facilitated pollutant removal from the street canyon.