Experimental study of urban microclimate on scaled street canyons with various aspect ratios

Urban geometry influences urban microclimate by changing both radiation and ventilation processes. Full-scale experiments provide significant information on urban microclimate but usually experience difficulty from complicated urban morphologies. It is challenging to quantify the effect of urban geometry on radiative exchange and convective ventilation under real-world atmospheric conditions. Therefore, this study conducted a scaled outdoor measurement to investigate the surface temperatures, wind velocities, and radiative fluxes in street canyons with various aspect ratios. The velocity ratio of 0.25H to 2H (V0.25H/V2H) was adopted to assess convective ventilation. The urban canyon albedo (UCA) and net all-wave radiation (Q∗) were used to quantify the radiative exchange. An increase in aspect ratio resulted in reduced convective ventilation and increased trapping radiation. The V0.25H/V2H of H/W = 2and 3were 25% and 55% smaller than that of H/W = 1, respectively. The mean midday (11:00–13:00) UCA of H/W = 1, 2, 3, and 6 was 0.11, 0.06, 0.05, and 0.03, respectively. Compared with H/W = 1, the H/W = 2, 3, and 6 increased daily total Q∗ by 11.41%, 14.41%, and 19.40%, respectively. The surface temperature was negatively correlated with aspect ratio during the daytime but positively at night. Such high-quality experimental data are useful for validating and improving the unsteady numerical simulation.