eTheses Repository - University of Birmingham

tests also reproduced well documented aspects such as the potential for mitigating urban
temperatures by increasing surface albedo and the vegetation fraction, the sensitivity of the
model to the building morphology and the influence of the geostrophic wind speed on the
mesoscale circulation.
In particular the size of the urban area was found to have an effect on the UHI intensity.
Increasing the urban area from 36 km 2 to 400 km 2 determined an increase of more than 1K
in the UHI intensity. This is more than is documented in the literature, but the major
contribution of this PhD in this comparison is the implementation of an urban canopy
scheme within the mesoscale model METRAS. In Chapter 6 the effect of urban expansion
on the UHI intensity will be investigated further for an urban area representing the
characteristics of London, rather than an idealised domain.
The fraction of vegetation within each urban grid cell was also found to have a significant
effect on the near surface temperature within the urban area, especially during daytime. A
40% increase in the vegetated fraction determined a reduction in the daytime near surface
potential temperature of around 1.5 K. The urban land cover and fraction of vegetation
within an urban area are crucial factors for determining urban climate (Jonsson 2004). A
large number of studies have investigated the sensitivity of the mesoscale atmospheric
models to the presence of vegetation in an urban area, and this was found to have a
significant impact of simulated near surface temperatures and air quality (Taha 1996, 1997;
Civerolo et al. 2000) and the boundary layer structure (Pielke et al. 1998; Seaman 2000). In
Chapter 6 and 7 the fraction of vegetation and urban land cover will be varied in order to
investigate the impact for the London region.
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