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urban core, with differences of 300-500 m between simulations with and without an urban
canopy scheme. Sarrat et al. (2006) found a maximum difference of 300 m between the top
of the PBL for a simulation with an urban canopy scheme (that of Masson 2000) and one
without.
Night time
During night time the urban processes are primarily of a thermal origin. Figure 4.10 shows
the vertical section of potential temperature for the urban simulation at 04:00 of the second
day of simulation. Over the surrounding rural area there is a very stable atmosphere due to
the strong cooling, whereas the over the urban area the lower atmosphere becomes unstable
due to the increased turbulent fluxes. The night time UHI is much shallower than the
daytime UHI, but is visible in the lower part of the atmosphere (up to around 100 m). The
higher temperatures above the urban area can also be observed to be shifted downwind of
the centre of the urban area. These results compare well with the numerical modelling
results (e.g. Uno et al. 1989; Atkinson 2003).
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