Dispersion in an array of buildings in stable and convective atmospheric conditions

Wind tunnel experiments were conducted to study the impact of atmospheric stratification on flow and dispersion within and over a regular array of rectangular buildings. Three stable and two convective incoming boundary layers were tested with a Richardson number ranging from −1.5 to 0.29. Dispersion measurements were carried using a fast response flame ionisation detector. The results show that the stratification effect on the plume width is significantly lower than the effect on the vertical profiles. Stable stratification did not affect the plume central axis inside the canopy, but in the unstable case the axis appeared to deviate from the neutral case direction. Above the canopy both stratification types caused an increase in the plume deflection angle compared to the neutral case. Measured mean concentrations in stable stratification were up to two times larger in the canopy compared to the neutral case, while in convective conditions they were to three times smaller. The proportionality between the vertical turbulent fluxes and the vertical mean concentration gradient was also confirmed in the stratified cases. The high-quality experimental data produced during this work may help developing new mathematical models and parametrisation for non-neutral stratified conditions, as well as validating existing and future numerical simulations. •Non-neutral stratification is common in urban areas.•Wind tunnel experiments on an urban array with non-neutral stratifications.•Vertical dispersion strongly affected by stratification.•Concentrations in stable conditions (0.14≤Ri≤0.29) up to twice larger.•In convective conditions (−1.5≤Ri≤−0.5) they are up to 3 times smaller.