eTheses Repository - University of Birmingham

the UHI intensity is constant, it then decreases rapidly reaching a minimum at 08:00 when
the air above the urban area is cooler than that above the rural area. The UHI intensity then
increases and during the greater parts of the daytime it is between 0 and 0.5 K. After sunset
the UHI increases. Although the Orig simulation does reproduce a UHI, the intensity is, for
all times, lower than those found for example in Sarrat et al. (2006).
For the urban_BEP simulation the UHI intensity continues to increase between 00:00 and
06:00, reaching a peak of 6 K at 06:00. This well developed UHI is expected as conditions
favourable to the formation of well developed heat islands were chosen for the test case,
e.g. clear skies and anticyclonic conditions. Then, like the Orig simulation, the UHI
intensity drops off rapidly until 08:00. This occurs because after sunrise (around 04:00) the
temperature above the rural surroundings increases more rapidly compared to that over the
urban area. The timing of the peak UHI intensity is found to depend on local conditions
(Oleson et al. 2008), for example some studies find the peak occurs a few hours after sunset
(e.g. Oke et al. 1975), whereas others (e.g. Jauregui 1997) observe an increase throughout
the night, reaching a peak around sunrise.
During the day time the UHI is much less intense and shows little variation in the morning
hours. As simulated by Atkinson (2003), after midday there is a further decrease in its
strength, as solar radiation is decreased, until after sunset, when it increases, more rapidly
than the Orig simulation. The increase in the UHI intensity after sunset is due to the release
of heat stored in the urban area during the daytime. For the urban_BEP simulation the UHI
intensity is always positive, in agreement with the simulations by Atkinson (2003) for a
similar geographical location and domain size. The smaller urban-rural temperature
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