eTheses Repository - University of Birmingham

presence of the dome of warmer air above the city have been documented in a number of
experimental and numerical studies (e.g. Bornstein et al. 2000; Lin et al. 2008).
Potential temperature [K]
Figure 4.6: Vertical section at y=0 of potential temperature (K) as computed by the
urban_BEP simulation at 12:00 noon of the second day of the simulation.
Figure 4.7 represents a vertical section of the horizontal wind speed at the centre of the
urban domain at 12:00 noon. This shows the convergence of low level winds over the city,
and divergence aloft. The convergence zone is displaced to the east by the geostrophic
wind, as seen in Martilli et al. (2002) and Martilli (2003). This behaviour can be explained
in terms of the three competing forces over the urban area: the drag induced by the urban
surface, the pressure gradient due to higher urban temperatures, and advection due to the
general synoptic flow. As seen in Figure 4.5 low winds are observed near the urban surface
due to the dominance of the drag effects. Aloft (at about 400 m above the ground) however
the pressure gradient and the advection have the same sign, causing a maximum.
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