eTheses Repository - University of Birmingham

Fraction of urban land cover [%]
Figure 5.2: Percentage of urban land use in the domain (taken from the CEH Land Cover
2000 data).
The simulations were 48-72 hours long, depending on the cases. Periods were chosen with
anti-cyclonic meteorological conditions characterised by relatively clear skies and low
winds, since these are found to be favourable to the development of a strong UHI (GLA
2006). In particular the heat island is found to be small when cloud cover exceeds 4 oktas,
although cloud cover alone cannot be used as a predictor of strong heat island events (GLA
2006). This allows the cloud and rain parameterisation schemes in the METRAS model to
be switched off, saving computational time.
In order to select simulation periods characterised by these conditions, the Lamb Weather
classification (Hulme et al. 1997) was obtained for each day for the summer (June, July and
August - JJA) and winter months (December, January and February - DJF) of years 1995-
2000. The Lamb classification is based on surface synoptic charts, which represent the state
of the atmospheric circulation close to the ground, and charts at the 500 hPa level. There
are three non-directional types, anti-cyclonic (A), cyclonic (C), which are dependent on
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