Urban impacts on climate - The Global Climate System: Patterns, Processes, and Teleconnections - page 228

Background Box 7.2
The energy balance in Mexico City (Oke et al. 1999 )
The structure of a city has a major impact on the energy balance. Measurements in
the dry season (early December 1993) over a rooftop in a densely built-up location,
provide results vastly different from those in rural areas. During the day, absorption of
incoming shortwave radiation and anthropogenic heat sources (industry, traffic, etc.)
are critical to the spatial variation of the energy budget. Daytime net radiation (Q*)
peaks at about 400Wm 2 , a reduced value due to urban pollution and also lower
diffuse radiation in a thinner winter atmosphere. Outgoing longwave radiation under
clear skies is enhanced. The buildings act as a major sink, storing 58% of average
daytime Q* (on the order of 155Wm 2 ). The afternoon convective (sensible) heat
flux is reduced to 38% of Q*, where in other cities it would dominate the daytime
energy budget.
At night, Q* is strongly negative, on the order of 120Wm 2 after sunset, and
90Wm 2 near sunrise. However, the release of daytime heat from storage in the
urban surfaces overcomes this deficit, releasing up to 1.3 times the Q* deficit, and
is often the only source of energy at night. Spatial variations depend on urban
geometry, building density, and the level of stored energy release. A weak convective
heat flux is thus maintained overnight.
The dominance of the storage and sensible fluxes and the small amount of
vegetation cover mean that evaporation is very small, on the order of 4% on Q*. At
night, evaporation is virtually zero. This creates a Bowen ratio of 0.58, considerably
larger than the 0.19 to 0.40 range determined for other mid-latitude cities.
The dominance of storage in the energy budget enhances the development of
the UHI, especially at night. Nighttime UHIs average 4-5 8C and can occur in
both seasons, although maximum UHIs > 7 8C only occur during the calm clear
conditions of the dry season. The highest UHIs occur at daybreak, associated with
maximum longwave radiation loss and a cleaner atmosphere, whereas in many other
cities maximums occur before midnight.
compared to the countryside, it can be less if the city is irrigated, or if urban
heat storage is particularly large in a city center (e.g. Mexico City, see
Background Box 7.2 ; Oke et al. 1999 ). Evaporation is usually reduced by
the ''waterproofing'' of the urban surface but urban-rural comparisons can be
complicated by urban irrigation, stronger surface-mixing layer coupling in
the city, intra-urban advection, and the spatial variability of rural moisture
(see the example of Sacramento conditions described above) (Figure 7.7b ).
Any such reversals of expected patterns have the potential to produce counter-
intuitive urban-rural effects on parameters such as stability and mixing depth,