eTheses Repository - University of Birmingham

(2006) find a spatial displacement of the urban heat island core with height with a shift of
up to 6 km at 50 m.
Figure 4.5 shows the horizontal cross section at the first and second grid levels of the
horizontal wind speed and wind vectors. The wind speed over the city at the first vertical
level of 10 m is reduced by more than 50% compared to the surrounding upwind rural area.
There is also a noted deflection of the wind vectors around the urban boundaries. The same
results are replicated at the second grid level (30 m above the surface) although the area
showing the reduction in wind speed is smaller, and is shifted down wind of the centre of
the domain.
(a) Wind speed [ms -1 ] 10 m (b) Wind speed [ms -1 ] 30 m
Figure 4.5: Horizontal cross section at z=10 m (a) and 30 m (b) of wind speed (ms -1) and
direction as computed by the urban_BEP simulation at 04:00 for the second day of
simulation. The arrows represent the magnitude and direction of the horizontal wind and the
shaded plot represents the magnitude of the horizontal wind speed (ms -1).
Wind speeds over urban areas are expected to be slower due to the drag effect of the
buildings (Roth 2000) and increased roughness of the surface. It has been observed for
example that above approximately 4 ms -1 wind speeds near the centre of New York are
lower than those outside the city (Bornstein et al. 1977). Wind directions can also be
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