Geoscience Reference
In-Depth Information
(d)
Figure 7.5 (cont.)
S
N
S
N
1 420-1430
u c = 1.07 ms -1
θ c =
Ri = -0.45
1 450-1500
u c = 0.49 ms -1
θ c = 178°
Ri = -2.64
34.0
34.0
34.0
34.0
34.5
34.5
35.0
35.5
36.5
35.0
33.5
36.0
34.0
for example, may generate ''cool islands,'' caused by longwave radiative
cooling (because of more open geometry) and enhanced latent heat flux.
Industrial or dense residential areas often generate enhanced heat islands. At
the micro scale, the scale at which most people directly experience urban
climate effects, differences in air temperature within an urban canyon
(Figure 7.5c and d ) or across a park may be greater than the variability across
the entire urban area.
Different UHI phenomena can be distinguished vertically (Figure 7.6 ). Air
temperature sensors at weather-screen level (UCL) record something differ-
ent from those at higher elevation above the surface, in the UBL, and from
that derived from remotely sensed surface temperatures. Increasingly, UHIs
are being studied using remotely sensed surface temperature data. These
satellite-derived ground temperatures do not always reveal the same spatial
and temporal patterns as air temperature surveys: UHI intensities tend to be
greatest by day and smallest at night (almost the reverse of the UCL tem-
perature patterns); and spatial temperature patterns tend to be linked closely
to land use (parks, parking lots, etc.) by day but not at night (Roth et al. 1989 ).
Some of these differences can be linked to the difference between a radiative
surface temperature and a near-surface (UCL) air temperature, where the
latter depends on surface energy exchanges as well as heating and cooling of
an air volume (see Section 7.8 for more detail).
Even when surface temperatures are all that is of interest, depending on
their view angle, satellites may preferentially measure horizontal surfaces
(roofs or roads) or wall facets with a preferred aspect. Thus the geometry of
urban areas will complicate interpretation of remotely sensed surface tem-
perature data due to the presence of vertical surfaces and shadows. Work by
Voogt and Oke ( 1997 ), using ground and airborne infrared radiometers, has
taken into account the total active surface area of an urban environment (all
the roofs, walls, and road surfaces), to document the complete surface
temperature. Comparisons between complete surface temperatures and
those derived from satellite images with different view angles are significant.
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