Influence of vertical greenery systems with different greenery coverage ratios on microclimate improvement in street canyons by scaled outdoor experiments

Extensive research has emphasized indoor/outdoor thermal environments of building-scale vertical greenery systems (VGSs), yet comprehensive assessments of effects of street-scale VGSs on urban microclimates are scarce, with the relationship between greenery coverage ratios (GCRs) and cooling effects remaining unclear. This study determined the influence of VGSs on microclimate parameters in street canyons under different GCRs in a humid subtropical climate through scaled outdoor experiments. Key parameters included wind velocity (U_0.25H), net radiation (Rn), urban canyon albedo (UCA), temperatures across various surfaces, namely west-facing walls (T_w-west), east-facing walls (T_w-east), and ground (T_g), as well as air temperatures (T_a). Compared with the reference street canyon without VGSs, those with GCRs of 25%, 50%, and 100% experienced reductions in U_0.25H by 10%, 20%, and 22%, respectively. Increased GCR was associated with higher Rn trapping and decreased UCA. Additionally, the reductions of T_w-west, T_w-east, and T_a increased with decreasing street canyon height. Higher GCRs tended to provide greater temperature reductions, longer cooling durations, and lower daily temperature range in street canyons. The reductions of T_w-east, T_a, and the central T_g within street canyons exhibited higher values with increasing GCR, reaching maximum reduction values of 15.7, 2.5, and 5.1 °C for a 100% GCR, respectively. Notably, the reduction of T_w-west in the lower level and of T_g adjacent to the west-facing wall of street canyon with a 50% GCR reached maximum values of 15.1 and 10.2 °C, respectively, exceeding the data with 25% and 100% GCRs.