Tall Building Effects and Their Representation in Fast Urban Dispersion Models

Tall buildings, in isolation or in groups, have a strong and long-ranging impact on flow and pollutant dispersion characteristics within and above the urban canopy layer (UCL). The transport of air-borne contaminants is well known to be affected by enhanced upward flow on the leeward building side ('chimney effect'). The presence of tall structures, however, also changes the flow in neighboring low-rise street-canyons, which can significantly alter initial advection and detrainment characteristics of contaminants released into the urban atmosphere.

Reproducing such complex effects is a challenge for urban dispersion models that rely on parameterizations of urban velocity fields rather than on obstacle-resolving 3D wind fields. Street-network dispersion models like the French model SIRANE (air.ec-lyon.fr/SIRANE), for example, were developed for the case of fully established street-canyon flow with a strong decoupling between external boundary layer und the UCL. While not accounting for the layout of individual buildings, street-network models have an explicit awareness of the street topology, but are currently not equipped for handling dispersion scenarios in cities with tall buildings.

In this study we investigate how to account for tall-building effects in SIRANE with regard to the horizontal advection of pollutants along streets in the UCL and vertical exchange mechanisms between the canopy layer and the external flow. The analysis is based on comprehensive reference data generated in the EPSRC-funded DIPLOS project involving research institutions in the UK and France (‘Dispersion of Localised Releases in a Street Network’; diplos.org). In particular, we exploit data from high-resolution large-eddy simulations and boundary-layer wind-tunnel measurements. The urban test environment is a regular array of rectangular buildings of uniform height H, in which an isolated tall building with a height of 3H was placed. Pollutants were released continuously from localized ground-level sources distributed around the tall building to study the impact of source location on the dispersion behavior. We discuss effects of the tall building on flow structure, characteristics of momentum and scalar exchange processes and pollutant pathways compared to the reference configuration of flow and dispersion in a uniform building array. While still not explicitly resolving the tall building as such in the network model, the effects on horizontal advection velocities in neighboring low-rise street canyons and vertical exchange velocities used in the parameterization of detrainment and re-entrainment processes are quantified and accounted for. The presentation will discuss the performance of the street-network model including tall-building effects and present options and further research needs to arrive at a general representation within the model.