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is higher by a similar amount. The ‘Sand’ land cover class has a very small roughness
length compared to the other classes, and shows the lowest night time UHI intensities.
These results can be compared to those presented in the literature. The UHI intensity has
been found to be a function of the characteristics of the rural surface (Hawkins et al. 2004).
For example Oleson et al. (2008) found a difference of up to 4 K between a rural surface
composed of needle leaf evergreen trees and a grassland surface. The differences found in
this PhD study are a lot smaller, with a maximum difference of just over 1.6 K between the
two extreme examples of the ‘Sand’ and the ‘Heath’ urban class. The difference between
the two main rural classes, ‘Meadows’ and ‘Forest’ are much smaller, and remain small
during night time. For the main simulations presented in Chapter 5, 6 and 7 of this PhD
study the rural classification surrounding London will be driven by the CEH Land Cover
Map 2000 data, and therefore no assumptions need to be made to define the rural
surroundings to the city.
4.3.7 Sensitivity to the geostrophic wind speed
The sensitivity of the model results to different geostrophic wind speed conditions is
analysed in this section. A control simulation was run under the conditions described in
Section 4.2. A further two simulations were run with very low wind speed (2 ms -1 ) and a
very strong wind speed (8 ms -1 ). These wind speeds were chosen in order to span a range
from that used by Martilli (2002) to analyse an example in which thermal effects are
expected to dominate, up to a maximum of 8 ms -1 chosen by Hamdi (2005) as an example
of high wind speed for which mechanical effects will be dominant.
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