Effect of hilly urban morphology on dispersion in the urban boundary layer

Air flow and dispersion in the atmospheric surface layer are strongly affected by terrain and buildings. Through Large-eddy simulation (LES) with a three-dimensional immersed boundary method (IBM) atmospheric boundary layer flow in a hilly urban area was simulated to study turbulence and dispersion properties in and above the urban canopy. Five different domains were designed to simulate flow over an infinite sequence of hills (defined by the Witch of Agnesi and having a maximum slope of 0.26), buildings on flat terrain and buildings on the Witch of Agnesi hills (hill height to building height ratios 3/2 and 9/4). Shear stress and velocity variance above the urban canopy were smaller for the small hill with buildings compared to building array on flat terrain. Shear stress increased with the hill height for hills with buildings. For hills with buildings turbulence kinetic energy (TKE) in the urban canopy increased dramatically upwind of the hillcrest and fell below the canopy level TKE for the flat urban case in the lee of the hill. Canyon ventilation at the sub-canopy level was two to three times larger for the hilly urban case compared to the flat case, but air exchange through the top of urban canyons was not greatly affected by the hill. Our study demonstrates that urban dispersion models with the ability to handle terrain and bluff obstacles in the domain are necessary to simulate important flow features and dispersion in hilly urban environments.