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Trees can effectively regulate the urban microclimate, while the change of microclimate conditions in turn affect the physiological state of trees. In this paper, CFD simulations are performed by coupling canopy energy balance (CEB) model to study the effects of trees on outdoor microclimate. The results show that the tree has different effects on outdoor microclimate under different wind speeds, air conditions, solar radiation and stomatal resistance. Increasing the wind speed will weaken the cooling effect of the tree. This weakening effect can extend to a distance of about 3.2 times of the tree canopy width behind the tree when the inflow air temperature is 30 °C. However, the influence range at high wind speeds is greater than that at low wind speeds. Air temperature and relative humidity have opposite effects on the sensible and latent heat fluxes of the tree, while they both have negligible effects on the net radiation flux. The humidification effect of the tree will be weakened as the relative humidity increases. Solar radiation has a greater effect on leaf surface temperature (LST) than on air temperature. The net radiation flux is high at the top and bottom of the tree and comparatively low at the central section, which is related to the vertical distribution of leaf area density (LAD). Trees can mitigate the effects of environmental changes on the LST by regulating stomatal resistance, and the reduction of stomatal resistance leads to a greater cooling effect. • CFD study trees effect on urban microclimate under different influencing factors. • Trees have different effects on urban microclimate in different environments. • Increasing wind speed weakens cooling effect of trees under the same conditions. • Air parameters have little effect on the net radiation flux absorbed by trees. • The distribution of net radiation flux is highly correlated with leaf area density. [ABSTRACT FROM AUTHOR]

Modifying urban morphology, defined as mass, density, and orientation of building stock in cities, are well-known heat mitigation strategies addressing urban heat islands (UHI) at various scales and consequent thermal discomfort. However, varying morphological aspects may have divergent effects on Outdoor Thermal Comfort (OTC) in cities. Unlike UHI, which is derived from urban-rural temperature differences, OTC can be quantified by thermal comfort indices considering the objective assessment of microclimatic variables including air temperature (T a), relative humidity (RH), mean radiant temperature (T MRT), and wind speed (V a), as well as a subjective assessment of individual perception. In Singapore and other tropical cities, thermally uncomfortable conditions prevail year-round due to higher T a and RH coupled with high solar irradiance from its equatorial location. To better understand the relationship between density related morphological variables, microclimate conditions and OTC in Singapore, we first conduct a systematic literature review to identify existing research gaps and uncertainties. We subsequently analyse prominent building bylaws and urban planning codes of Singapore to understand the potential comfort implications of existing urban morphological norms. Finally, we propose a methodological framework on how to address the gaps and uncertainties in mainstream urban design and urban planning process keeping into consideration microclimatic, comfort and socio-economic variables. • Systematic literature review to assess link between urban morphology and microclimate • Analyzing building guidelines of Singapore to assess its climate potentials • Proposing frameworks for future research directions in tropical cities [ABSTRACT FROM AUTHOR]