A scaled outdoor experimental study of the urban thermal environment in street canyons with green walls under various weather conditions

Scaled outdoor experiments were conducted to examine the thermal effects of west-facing green walls (GWs) in street canyons with different street aspect ratios (building height/street width H/W=AR=1, 1.5, and 2, H=1.2 m) under different weather conditions in humid-subtropical Guangzhou, China. Compared with the control cases (AR=1, 1.5, and 2), the wind speed at the height of 0.3 m (0.25H) was reduced by 15%-34% in GW cases under three weather conditions (sunny, cloudy and rainy). On a typical sunny day, significant wall temperature (Twall) reduction in GW cases was observed due to the thermal effects of GWs, with maximum values at 1.1 m (above the ground) reached 15.9, 21.2, and 22.1°C for AR=1, 1.5, and 2, respectively. The maximum ground temperature (Tground) reductions in GW cases were 1.5, 5.5, and 5.7°C as AR=1, 1.5, and 2, respectively. Moreover, GWs maximally reduced air temperature (Tair) at 0.1 m height (equal to the pedestrian level in real cities) by 0.5, 1.0, and 1.0°C when AR=1, 1.5, and 2, respectively. Compared with sunny days, the weaker solar radiation and higher relative humidity on cloudy and rainy days suppressed the thermal effects of GWs on Twall, Tground, and Tair in street canyons.