Effects of vertical greenery systems on microclimate in urban blocks with different plan area indices: Scaled outdoor experiments

The thermal performance of vertical greenery systems (VGSs) in indoor/outdoor environments has been extensively studied; nevertheless, spatiotemporal observational experiments of VGSs at block-scale are scarce, with the evapotranspiration and cooling potential of VGSs in response to urban morphology remaining unclear. Therefore, scaled outdoor experiments were conducted to thoroughly investigate the effects of VGSs on the wind, radiation, and thermal parameters in urban blocks with plan area index (λp) of 11% and 25% in the temperate region of Xingtai, China. Additionally, the evapotranspiration effect of VGSs in urban blocks was further quantified. VGSs reduce wind speed in the crossroads by 26% in the block with λp=25%, but no significant effect for λp=11%. Compared with non-VGS cases, VGS cases absorb more shortwave radiation and emit less longwave radiation, resulting in more net radiation capture and lower albedo. VGS cases experience significant temperature reductions in wall (Tw), indoor, canyon air (Ta), and ground (Tg), as well as mean radiant temperature (Tmrt) and physiological equivalent temperature (PET). The south walls in blocks with λp=11% and 25% show the best cooling effect, with maximum reductions of 22.2 and 18.4 °C at 0.1 m height, respectively, while the north walls show weaker cooling. The east and south streets experience better air and ground cooing than the crossroads. In the south street of blocks with λp=11% and 25%, the maximum reductions of Ta are 1.5 and 3.9 °C, and of Tg being 7.4°C and 9.0 °C, respectively. VGSs in urban block with λp=11% have a greater evapotranspiration rate than that with λp=25%. Thus, block with λp=11% achieve more pronounced cooling effects on walls and indoor air, whereas block with λp=25% exhibit better air and ground cooling due to lower wind speed. Moreover, the reductions in Tmrt and PET in block with λp=25% are 36.7 and 20.2 °C, respectively, significantly higher than those with λp=11%.Keywords: Vertical greenery systems; Scaled outdoor experiments; Plan area index; Cooling potential; Evapotranspiration rate