3.3.2.4 Urban Areas

As noted in Section 3.2.2.2 (see also Box 7.2), the microclimates in cities are clearly different than in neighbouring rural areas. The presence of a city affects runoff, moisture availability and precipitation. Crutzen (2004) pointed out that while human energy production is relatively small globally compared with the Sun, it is locally important in cities, where it can reach 20 to 70 W m^–2. Urban effects can lead to increased precipitation (5 to 25% over background values) during the summer months within and 50 to 75 km downwind of the city (Changnon et al., 1981). More frequent or intense storms have been linked to city growth in Phoenix, Arizona (Balling and Brazel, 1987) and Mexico City (Jauregui and Romales, 1996). More recent observational studies (Bornstein and Lin, 2000; Changnon and Westcott, 2002; Shepherd et al., 2002; Diem and Brown, 2003; Dixon and Mote, 2003; Fujibe, 2003; Shepherd and Burian, 2003; Inoue and Kimura, 2004; Shepherd et al., 2004; Burian and Shepherd, 2005) have continued to link urban-induced dynamic processes to precipitation anomalies. Nor is land use change confined to urban areas (see Section 7.2). Other changes in land use also affect precipitation. A notable example arises from deforestation in the Amazon, where Chagnon and Bras (2005) found large changes in local rainfall with increases in deforested areas, associated with local atmospheric circulations that are changed by gradients in vegetation, and also found changes in seasonality.

Suggested mechanisms for urban-induced rainfall include: (1) enhanced convergence due to increased surface roughness in the urban environment (e.g., Changnon et al., 1981; Bornstein and Lin, 2000; Thielen et al., 2000); (2) destabilisation due to UHI thermal perturbation of the boundary layer and resulting downstream translation of the UHI circulation or UHI-generated convective clouds (e.g., Shepherd et al., 2002; Shepherd and Burian, 2003); (3) enhanced aerosols in the urban environment for cloud condensation nuclei sources (e.g., Diem and Brown, 2003; Molders and Olson, 2004); or (4) bifurcating or diverting of precipitating systems by the urban canopy or related processes (e.g., Bornstein and Lin, 2000). The ‘weekend effect’ noted in Section 3.2.2.2 likely arises from some of these mechanisms. The diurnal cycle in precipitation, which varies over the USA from late afternoon maxima in the Southeast to nocturnal maxima in the Great Plains (Dai and Trenberth, 2004), may be modified in some regions by urban environments. Dixon and Mote (2003) found that a growing UHI effect in Atlanta, Georgia (USA) enhanced and possibly initiated thunderstorms, especially in July (summer) just after midnight. Low-level moisture was found to be a key factor.