eTheses Repository - University of Birmingham

2.1 Urban climate modification
Urban areas have well documented effects on air quality and weather, such as the urban heat
island (e.g. Landsberg 1981; Oke 1982), reduced winds (Lee 1979) and increased atmospheric
turbulence (e.g. Roth et al. 1993). There might also be less obvious effects such as changes in
cloud cover (Changnon et al. 1971; Rabin et al. 1996; Inoue et al. 2004), enhancement of storms
and changes in precipitation (Palumbo et al. 1980; Oke 1987; Jauregui et al. 1996; Bornstein et
al. 2000; Shepherd et al. 2002; Shepherd 2005), and changes in humidity and atmospheric
electricity (Changnon et al. 1971). All these effects result from mechanical and thermal
modifications induced by the urban surface.
2.1.1 Mechanical effects
Urban areas mechanically affect air flow in the following ways: increased drag force due to the
presence of buildings; the enhancement of the transformation of mean kinetic energy into
turbulent kinetic energy in the shear layer at the top of the canopy; turbulent wakes generated by
roughness elements which serve to mix pollutants, momentum, heat and moisture; and enhanced
drag force by thermal plumes from street canyons at night time and roof tops in the day time
(Roth 2000). These factors have the effect of increasing turbulent mixing, and modifying
boundary layer wind fields. There are also changes in wind speed and direction induced when air
blows across the boundary between two surfaces of different roughness (Klaic et al. 2002), and a
thermal circulation with the convergence of horizontal wind into the city, and vertical motions
over the city, caused by the urban heat island phenomenon (Britter et al. 2003). Most urban areas
have a warm urban plume aloft which can extend to 100 km downwind of the city (Britter et al.
2003).
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