The Diurnal Cycle of Urban Thermal Environment in Scale-model Street Canyon

Building thermal mass is significant to influence urban climate. We performed scale-model field measurement  of urban thermal environment and turbulence over three days in summer of 2015 and 2016 in Guangzhou, China. Two types of north-south street canyons and three aspect ratio (building height H=1.2m, street width W=1.2, 0.6, 0.4m) were investigated. The 'empty' model consists of hollow buildings with wall thickness of 1.5cm and the 'sand model' are made up of buildings which are filled with sand producing bigger heat capacity.

    The turbulence and temperature characteristics in urban roughness layer of such idealized street canyons were investigated by using several instruments such as ultrasonic anemometer, temperature and humidity sensors, four component radiometers, infrared camera. We measured velocity distribution, air temperature and basic characteristics of radiation in and above the idealized street canyon. Especially we emphasized the impact of different building heat storage on urban turbulence and thermal environment.  

    The thermal mass in the empty model is much smaller than the sand model. Therefore, the results show that, during daytime, wall temperature (T_wall) and air temperature in the street canyon (T_air) of empty model reached its peak earlier and this peak value is higher than those of the sand model. After sunset, result is quite contrary. T_wall in the sand model are higher than those in the empty model because the thermal storage of the former is much greater than the latter, but T_air in the street canyon of the 1:1 aspect ratio in these two models are similar, because the measured location of T_air is at the centre of street canyon and a little far from building walls (L=0.5H=0.6m). 

    The street canyons were subjected to differential wall heating and overlying wind during the measurement period. As overlying wind is relatively weak, the heating of building walls may induce thermally driven circulation. For the street canyon of 1:1 aspect ratio, A buoyancy parameter B=gαΔTH/U²[1+(H+L)²] was used to demarcate thermal and inertial circulation regimes. When the street canyon is dominant by the wind, and the canyon velocities scaled by U₀ （z=2H）are approximately constant. When the thermal circulation becomes important, velocity in canyon and buoyancy parameter shows a relationship:U/U₀=γ₁+γ₂B^1/2.