eTheses Repository - University of Birmingham

5.2 Model evaluation
It was difficult to evaluate the model results due to the lack of detailed field data and the
extreme heterogeneity of the urban surface, which cannot be captured by a mesoscale
model with 1 km horizontal resolution. Point measurements such as the MIDAS surface
stations are influenced by their fetch, which may include high and complicated building
structures which are not represented in the mean characteristics in the urban scheme. The
surface stations cannot capture the spatial variability within the METRAS grid cells, and
ideally a high sampling density of measurements would exist which could be used to
calculate spatial averages to evaluate the performance of the model against (Otte et al.
2004). Another way of improving model validation is to run a CFD for a small portion of
the urban area and validate it against point measurements; these results once validated
could be spatially averaged and used for a comparison with mesoscale model results
(Martilli 2007). For this reason the initial focus was on reproducing common features from
urban observations for an idealised domain (see Chapter 4).
The diurnal cycle of the air temperature, wind speed and wind direction for the simulations
with and without the urban scheme were compared with three MIDAS weather stations
within the domain. These were chosen to be the urban station of the London Weather
Centre (LWC), the peri-urban station at Heathrow airport (LHR) and the urban park station
located at St James’ Park (SJP). These are quality-assured UK Met Office sites with hourly
observations of air temperature, wind speed and wind direction. In order to evaluate the
performance of the model over rural land use, where the implementation of BEP is
expected to have little effect, it is necessary to use a rural station which is not influenced by
the presence of the city. The station at Wisley, 32 km to the South-West of London, is
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