eTheses Repository - University of Birmingham

epresentatively sample the underlying surface measurement towers should be located in the
inertial sub-layer.
Both the city scale (represented by the average city diameter) and the regional scale take into
account the modification of the atmospheric boundary layer by the urban surface, i.e. the urban
boundary layer (UBL) (see Figure 2.3 overleaf). Stull (1988) defines the atmospheric boundary
layer as “that part of the atmosphere that is directly influenced by the presence of the Earth’s
surface, and responds to surface forcing with a time scale of about an hour or less”. Surface
forcing are for example pollutant emission, heat transfer, frictional drag and terrain induced flow
modifications. Turbulence is the main process which defines the boundary layer, and is
responsible for the vertical transport of quantities such as heat, moisture, momentum and
pollutants. The kinetic energy of the flow can be partitioned into that associated with the mean
wind, and that associated with the turbulence. This defines the turbulent kinetic energy (TKE),
which is one of the most important quantities used to study the turbulent boundary layer. The
horizontal transport in the boundary layer (advection) is on the other hand dominated by the
mean wind.
At the city and regional scale (see Figure 2.3 overleaf), variations in flow and dispersion around
individual buildings are averaged out and the urban surface represents a perturbation to the mean
flow. Urban climate effects such as the urban heat island and urban pollution plume are apparent.
As cities grow it is possible that regional wind and temperature circulations will be influenced,
especially if general wind patterns are weak, although it is unlikely that these will be significant
on the global scale. In terms of pollutant emission, large amounts of pollutants are emitted at the
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