Passive scalar diffusion in and above urban-like roughness under weakly stable and unstable thermal stratification conditions

• Passive scalar diffusion in urban geometries was studied in a wind tunnel.
• Thermal stratification without strong secondary circulation was established.
• Even weak thermal stratification affects diffusion behaviors considerably.

Wind-tunnel experiments were conducted to investigate passive scalar diffusion behaviour in and above urban-like roughness under various thermal stratification conditions. Staggered arrays of rectangular blocks with aspect ratios of 0.96 or 1.92 were used to model the urban canopies. Weakly stable and unstable thermal stratification conditions were created by controlling the temperature of the wind-tunnel floor and the airflow. Above the modelled urban roughness, vertical profiles of the velocity and temperature were measured by a laser Doppler anemometer and a cold-wire thermometer, respectively. The estimated Obukhov length was found comparable or larger than the boundary-layer thickness, indicating very weak stratification. Passive-scalar diffusion was studied by emitting ethane gas from a point source on the wind-tunnel floor. Thermal stratification had substantial effect on diffusion even in the vicinity of the source where mechanical mixing by the model blocks was presumed to eliminate the stratification effect.