Geoscience Reference
In-Depth Information
The solution of this system of equations is given by
u
−
u
g
=
D
v
sin(
ft
)
+
D
u
cos(
ft
),
(2.42)
v
−
v
g
=
D
v
cos(
ft
)
−
D
u
sin(
ft
).
(2.43)
v
) are called ageostrophic wind components,
and
D
u
and
D
v
denote the ageostrophic components at the onset of the inertial
oscillation.
The differences (
u
g
−
u
) and (
v
g
−
2.3 Urban Boundary Layers
Urban agglomerations have recently received special interest in atmospheric bound-
ary layer studies. Nowadays, more than half of mankind is living in cities and the
number of megacities with more than 10 million inhabitants is steadily growing.
Cities are large pollution sources and because the temperature is already higher than
in their surroundings, they are especially prone to the effects of a warming climate.
All these aspects have fostered studies on the structure of the UBL. UBL meteo-
rology has become a special subject in boundary layer meteorology. One aspect of
UBL studies is the analysis of wind profiles and thermally driven secondary circu-
lations over cities (urban heat islands). See for an overview of urban meteorology
and of urban heat islands, e.g. Kanda (
2007
) and Hidalgo et al. (
2008
). The urban
heat island brings about a secondary circulation with winds towards the urban centre
near the ground, uprising motion over the urban centre and compensating outflow
towards the surrounding rural areas aloft (Shreffler
1978
,
1979
).
Urban surfaces are characterized by large roughness elements, wide-spread
sealed areas, reduced moisture availability at the surface, and increased possibili-
ties for heat storage. This leads to higher turbulence intensities in the UBL and to
stronger sensible heat fluxes from the urban surface into the UBL. Both facts induce
a greater depth of the boundary layer (see the urban dome in Fig.
2.5
). Additionally,
towns are often isolated islands featuring these special surface properties surrounded
by rural terrain so that the flow above them is not in equilibrium with the urban
surface. During daytime the reduced moisture availability leads to smaller latent
and thus larger sensible heat fluxes at the urban surface compared to rural surfaces.
The reduced radiative cooling of the urban surface or even still persisting upward
heat fluxes (Velasco et al.
2007
) at night prevents the formation of a stable noc-
turnal boundary layer. Both the increased sensible heat flux during the day and the
reduced cooling during the night cause higher temperatures in the UBL compared to
the surrounding rural boundary layer. This effect is known as the urban heat island
(Atkinson
2003
, Chow and Roth
2006
). The urban heat island is enhanced by the
human energy production (Crutzen
2004
, Kanda
2007
), which with 20-70 Wm
−
2
can be 5-10% of the energy input by solar irradiation.
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