The synergistic effect of urban heat and moisture islands in a compact high-rise city

The synergistic effect of urban heat island (UHI) and urban moisture island (UMI) aggravates the heat stress during hot summers. To investigate the mechanisms of UHI and UMI, we developed an advanced urban canopy model with more robust predictability of the urban surface heat and moisture budgets by considering dynamic building-tree-air interactions within an enhanced aerodynamic resistance network. Our results show that in a compact high-rise city, anthropogenic heat (AH) emission from building cooling systems is nonnegligible (up to 400 W/m2, including -60% sensible heat flux and -40% latent heat flux), which can further aggravate UHI and UMI with a 1.64 degrees C increase in air temperature and a 0.89 g/kg increase in air specific humidity, respectively. On the other hand, the impact of building-tree-air interactions on UHI and UMI within compact high-rise street canyons is complicated due to strong shading effect and hindered turbulent transport. In general, planting trees effectively reduces UHI (- 2.90-0 degrees C) but leads to increased UMI (+0-1.66 g/kg), therefore an optimized tree planting strategy is needed to avoid heat stress aggravation due to high humidity. Specifically, tall trees with large and small crown areas are optimal tree types for street canyons with low and high aspect ratios, respectively. In addition to city greening, optimal building air-conditioning (AC) control in an energy-efficient mode can reduce outdoor maximum heat stress index by 1.10 degrees C. Furthermore, synergistic effects of optimal AC operation and optimal tree planting can effectively reduce pedestrians' dangerous hours with heatstroke risks by 11%-35%.