eTheses Repository - University of Birmingham

5.3 Summary and discussion of the evaluation
To summarise the results of the evaluation of the METRAS+BEP model for London, it is
found that there is a cold bias during the daytime for all the air temperature results for the
detailed cases; for both the METRAS+BEP model and the METRAS model alone. This
affects the comparison for all weather stations and therefore is likely to be caused by a
modelling issue rather than the specific local conditions of the stations. The cold bias could
be investigated further by refining the simplistic METRAS radiation scheme and analysing
the sensitivity to the thermal parameters used to define the METRAS urban land class and
the BEP urban materials.
Despite the cold bias the simulation with METRAS+BEP performed better than the
traditional simulation. This is due to the treatment of urban radiation sources which are
neglected in the traditional simulation. During night time the comparison between
METRAS+BEP and the measurements was best for the highly urbanised location (LWC),
whereas there is a warm bias for the other locations. This could be due to too much heat
storage in the urban area.
Although the additional five case studies only confirmed the difficulties in model
evaluation for urban areas, they did prove that the results presented for the more detailed
cases were not just specific to those particular dates, but could be replicated for other
summertime periods.
It is questionable however whether daytime measurements taken over open spaces exposed
to direct sunlight can be used to validate the temperatures computed by an urban canopy
scheme, since the model results are aggregated over a heterogeneous area which will
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