eTheses Repository - University of Birmingham

Wind speed (URB_BASE-NOURB) (ms -1 )
Figure 6.4: Horizontal wind speed difference (ms -1 ) between the base case of current urbanised land
cover (URB_BASE) and rural domain (NOURB) at z = 10 m at 04:00 (left) and 12:00 noon (right) of
the second day of simulation.
The mean wind speed was spatially averaged across the whole domain for the urbanised
and rural model runs. The domain averaged change in the mean wind speed is (-0.66 ±
0.49) ms -1 at 04:00, and (-0.93 ± 0.64) ms -1 at 12:00. At 04:00 56% of the model grid cells
are affected by a reduction in mean speed of more than 0.5 ms -1 , compared to 65% at noon.
The maximum reduction is wind speed in the domain is 2.03 ms -1 at 04:00, compared to 2.6
ms -1 at noon. The largest differences in wind speed were found over the urbanised cells,
and in their vicinity, corroborating results in Klaic et al. (2002).
Figure 6.5 shows the wind speed and direction vectors at z = 10 m for the URB_BASE
simulation and the NOURB simulation. For the NOURB case no deflection in the wind
vectors is observed, whereas in the URB_BASE case there is a small deflection as the flow
bends around the urban area (Britter and Hanna 2003). As found in Klaic et al. (2002),
urbanisation does not have a significant effect on wind direction.
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