Urban impacts on climate - The Global Climate System: Patterns, Processes, and Teleconnections - page 235

city, reducing the urban-rural temperature gradient, but creating an equilibrium
between the city and its rural surroundings that allows the UHIC to continue for
several hours (Haeger-Eugensson and Holmer 1999 ).
Once developed the UHIC can interact with, and interfere or support, other
local airflows in very complex ways (Oke 1995 ; Haeger-Eugensson and Holmer
1999 , and others). Cold air drainage from higher elevations around a city,
channeled though valleys, may reduce UHI and UHIC development. If airflows
from valleys are weak, the UHI may prevent these airflows from entering the
city, creating airflow stagnation and high pollution episodes. If the sides of the
valleys extend above the inversion, the valleys may create UHIC channeling,
influencing cooling rates and wind direction. In coastal areas, the UHIC can act
against sea breeze development, retarding the strength and depth of its inland
flow. However, when the convective zone of the UHIC and the sea breeze merge
together, the sea breeze can be strengthened.
Light winds can also carry the urban influence to the atmosphere downwind,
over the countryside (Figure 7.4a ). The resulting urban ''plume'' may extend
over the rural boundary layer, creating a series of complex meso-scale inversions
with height. The extent of urban plume transport and development depends on
the regional terrain and the atmospheric conditions.
7.5 Urban canyons
An urban canyon is defined as a street or flat area bounded on two sides by
buildings, with an, at least partially, open top to the sky. The building sides are
normally defined as vertical with minimal variation. The top of the urban canyon
usually defines the boundary between the UCL and the lowest part of the UBL.
The exchange between the UCL and UBL of water vapor, heat, and pollutants,
etc., occurs mainly through turbulence associated with wind interactions and
canyon structure (Arnfield and Mills 1994a ). Combinations of canyons are a
micro-scale structure, and can create local-scale climates which influence the
UHI, energy balance fluxes, and wind, depending on their orientation, depth, and
materials (Arnfield and Mills 1994a , b ; Bonan 2002 ).
Solar energy entering the canyon though the open top may be absorbed and
reflected several times, depending on the angle of entry, the geometry of the
buildings, and the building surface type. Urban canyons tend to trap solar energy
during the day, and release this energy as longwave radiation and sensible heat
over the diurnal cycle. There is a direct relationship between the amount of sky
viewed at the top of the canyon and the amount of energy trapped (Oke 1981 ).
The sky view factor (Ys) is defined as the geometric relationship between the
building height (H ) and the street width (W ), which determines the angle of
view (a) from the bottom to the top of the canyon. If an infinitely long street is
assumed, tan(a) ¼ 2H/W and Ys ¼ cos(a). The smaller the central city Ys, the
stronger will be the maximum air UHI. Figure 7.9 shows little difference in