By: Denise Hertwig
Based on current UN estimates, by 2050 over 6.6 billion people (68% of the total population) will be living in cities. Across the world, tall (> 50 m height) and super-tall (> 300 m) buildings already define the skylines of many large cities and will become increasingly more common outside of city centres to accommodate growing urban populations, especially when horizontal urban sprawl is geographically limited. For London, 2019 was declared the “year of the tall building” (NLA London Tall Buildings Survey 2019). At the moment, 541 buildings over 20 storeys (approx. 60 m in height) are planned or already under construction in the UK capital. Tall buildings are currently being built in 22 out of the 33 London boroughs and 76 of them are expected to be completed this year.
Tall buildings, in isolation or as clusters, affect the urban micro-climate of the local surroundings and the neighbouring region. The impact on aerodynamics (e.g. local flow distortions, long-range wake effects), radiation budget (e.g. shadowing, radiative trapping) and components of the surface energy balance (e.g. storage of heat in building materials, anthropogenic heat emissions) can be large compared to low-rise buildings. Such modifications challenge current modelling frameworks for urban areas. Urban land-surface models used in numerical weather prediction, for example, typically do not account for building-height variations. They also rely on the concept that the flow within the urban canopy is sufficiently decoupled from the flow aloft, which is not the case if tall buildings protrude deep into the urban boundary layer.
Figure 1: Normalised pollutant concentrations (a,b) in an idealised building array. Pollutants are released from point sources located (a) in the street canyon behind a tall building, (b) in an intersection upwind of the tall building. (c) Mean-flow streamlines near the tall building with colours showing the mean vertical velocity. The black arrow indicates the upwind flow direction. Data are results from large-eddy simulations by Fuka et al. (2018) for the DIPLOS project.
Similarly, operational urban air quality and dispersion models do not usually account for tall-building effects (Hertwig et al. 2018). Tall buildings strongly change pedestrian-level winds in the surrounding streets and the flow field above the roofs of the low-lying buildings. This affects pollutant pathways and the overall ventilation potential of cities. Pollutants released near the ground in a street canyon on the leeward side of a tall building (Fig. 1a) can be rapidly lifted out of the building canopy by updrafts (Fig. 1c). Although the pollutants are emitted at the ground, the tall building causes a large proportion of the released mass to be transported above the roofs of the low-rise neighbourhoods, thereby reducing street-level pollution. A pollutant source located in an upwind intersection leads to drastically different results (Fig. 1b). The downdrafts on the windward side of the tall building result in strong horizontal flow out of the upwind street canyon (Fig. 1c). This outflow shifts the pollutants away from their release point in the intersection, creating a virtual source location in the adjacent street canyon and deteriorating air quality in the streets downwind.
Figure 2: (a) Building heights and (b) wind-tunnel model buildings of the neighbourhood between Waterloo station and Elephant & Castle in London (MAGIC project study area). Wind-tunnel measurements of the wake behind the central tall building (81 m height) in isolation and together with the low-rise building canopy shown in terms of (c) height profiles of flow speeds at several sites downwind of the tall building, (d) velocity differences to the ambient (undisturbed) flow with downwind distance at several heights. Details in Hertwig et al. (2019).
Flow interactions between tall and low-rise buildings also change the structure of the momentum deficit region (wake) that forms behind tall buildings. Wake models used for local air-quality predictions currently do not account for such interactions as they were derived for isolated buildings. Wind-tunnel experiments in a realistic scale model of the area between the Waterloo and Elephant & Castle stations in London (Fig. 2a,b) documented the strong impact of the canopy on tall-building wakes (Hertwig et al. 2019). Compared to tall buildings in isolation, the presence of a low-rise canopy displaces the wake vertically (Fig. 2c), so that flow speeds are reduced over longer distances downwind well-above the canopy (Fig. 2d). In the case shown, the wake extends over distances larger than 5 times the height of the tall building (i.e. > 400 m). The increasing spatial resolution (of the order of 100 m) of mesoscale and microscale atmospheric models means that tall-building wakes no longer are subgrid-scale phenomena, but have an impact at the grid-scale. Understanding and quantifying tall-building impacts on the boundary layer over cities is essential to identify needs for model refinements.