eTheses Repository - University of Birmingham

educed near surface temperatures of more than 1 K. Moreover these temperature changes
were not always confined to the grid cells for which the land cover classification had been
modified. Dhakal et al. (2002) found the maximum reduction in average noon temperatures
due to greening the area surrounding the buildings to be 0.47 K. Another benefit of urban
vegetation, and especially of trees, is the effect on air quality. For example Nowak et al.
(2000) showed that increased tree cover can help reduce high ozone concentrations and
those of other pollutants. Simulations in Taha (1996) indicated that the net effect of
increased urban vegetation was a decrease in ozone concentrations, if the trees are low
emitters.
In the control simulation the urban area had no vegetated fraction at all. This is obviously
not a very realistic situation, although some Mediterranean cities have a very low fraction
of vegetation, but the choice was made in order to ease the interpretation of the model
results for the test case. Two further simulations were carried out:
• veg1: 10% vegetation evenly distributed within the urban area
• veg2: 30% vegetation evenly distributed within the urban area
In particular, 30% is a realistic assumption for the vegetated fraction and is a good
approximation for the city of London, where the vegetation coverage is estimated at 20-
30% (www.english-heritage.org.uk). The vegetated area was treated as the METRAS land
cover class of ‘Meadows’, the rest of the urban area was treated with the BEP urban
scheme.
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