The energy balance of the built surface is similar to soil surfaces described above, except for the heat production resulting from energy consumption by combustion, which in some cities may even exceed Rn during the winter. Although Rn may not be greatly different from that in nearby rural areas (except during times of significant pollution) heat storage by surfaces is greater (20 to 30 per cent of Rn by day), leading to greater nocturnal values of H; LE is much less in city centres. After long, dry periods, evapotranspiration may be zero in city centres, except for certain industrial operations, and in the case of irrigated parks and gardens, where LE may exceed Rn. This lack of LE means that by day 70 to 80 per cent of Rn may be transferred to the atmosphere as sensible heat (H). Beneath the urban canopy, the effects of elevation and aspect on the energy balance, which may vary strikingly even within one street, determine the microclimates of the streets and 'urban canyons'.
The complex nature of the urban modification of the heat budget is demonstrated by observations made in and around the city of Vancouver, Canada. Figure 12.24 compares the summer diurnal energy balances for rural and suburban locations. Rural areas show considerable consumption of net radiation (Rn) by evapotranspiration (LE) during the day, giving lower temperatures than in the suburbs. While the suburban gain of net radiation is greater by day, the loss is greater during the evening and night due to release of turbulent sensible heat from the suburban fabric (i.e. AS negative). The diurnal energy balance for the dry top of an urban canyon is symmetrical about midday (Figure 12.25C), and two-thirds of the net radiation is transferred into atmospheric sensible heat and one-third into heat storage in the building material (AS). Figure 12.25A-B explains this energy balance symmetry in terms of the behaviour of its components (i.e. canyon floor and east-facing wall); these make up a white, windowless urban canyon in early September aligned north-south and with a canyon height equal to its width. The east-facing
Source: After Clough and Oke, from Oke (1988).
wall receives the first radiation in the early morning, reaching a maximum at 10:00 hours, but being totally in shadow after 12:00 hours. Total R is low because n the east-facing wall is often in shadow. The street level (i.e. canyon floor) is sunlit only in the middle of the day and Rn and H dispositions are symmetrical. The third component of the urban canyon total energy balance is the west-facing wall, which is a mirror image (centred on noon) of that of the east-facing wall. Consequently, the symmetry of the street-level energy balance and the mirror images of the east- and west-facing walls produce the symmetrical diurnal energy balance of Rn, H and AS observed at the canyon top.
The thermal characteristics of urban areas contrast strongly with those of the surrounding countryside; the generally higher urban temperatures are the result of the interaction of the following factors:
2 Changes in the radiation balance due to the albedo and thermal capacity of urban surface materials, and to canyon geometry.
3 The production of heat by buildings, traffic and industry.
4 The reduction of heat diffusion due to changes in airflow patterns caused by urban surface roughness.
5 The reduction in thermal energy required for evaporation and transpiration due to the surface character, rapid drainage and generally lower wind speeds of urban areas.
Consideration of factors 4 and 5 will be left to D.3 (this chapter).
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