Section “Characteristic Regions of the Flow
and Drivers” presents the underlying concepts
and prediction methods related to the airflow and
meteorology of urban areas that are particularly
relevant for understanding and modelling dispersion from the urban sources. It is noted how spatial and temporal patterns of flow and dispersion
in urban areas greatly depend on their location
(e.g., flat, hilly terrain, coastal, or desert), the
types of surface, prevailing meteorological conditions (e.g., strong/weak geostrophic winds), and
on the patterns and forms of urban building/street/
open space (e.g., isolated buildings or canyons;
high or low range of building heights, etc.). It will
be convenient to describe the flow in terms of
three characteristic ranges of length scale, namely,
mesoscale, neighborhood scale, and building/
street scales, each with their own general features
and where different modelling approaches are
necessary, depending on the requirements for
accuracy
and
speed
of
computation.
Section “Urban Data” presents some details of
data, and sections “Computational Models for
Meteorology and Air Flow in Urban Areas” and
“Examples of Output of Modelling Calculations”
provide details of modelling methods for flow in
urban areas and examples of model calculations.
Characteristic Regions of the Flow and
Drivers
Characteristic Regions of the Flow
In urban areas, there are usually considerable variations in the types, sizes, and layout of buildings
and streets as illustrated in Fig. 1, and over larger
urban areas there are often also significant variations in the natural topography. Both these urban
scale features and the meteorology have general
characteristics over different length scales. These
need to be understood and related to each other
when developing a picture of the urban airflow as
illustrated in Tables 1 and 2.
Over level terrain, except in very light wind
conditions, the direction and speed of the airflow
only changes significantly over the mesoscale
(of order 30 km) or greater scales. These changes
are determined by Coriolis effects, by the effect of
the urban area on convergence/divergence and
turning of the wind, and by temporal variations
in the flow forced by diurnal and synoptic effects.
Where there are sharp variations in topography
the surface air flow may change locally (e.g., at
the coast or edge of an urban area). However, the
whole boundary layer flow does not adjust immediately to the change of surface conditions: this
takes place over a mesoscale distance L M which,
in the atmosphere, is typically of order 30–50 km.
This distance is usually determined by the Rossby
length scale L Ro ¼ hN/f where f is the Coriolis
parameter and N is the buoyancy frequency of the
stable layer at or above the boundary layer of
height h. At midlatitudes f ~ 10
À4 s
À1 , and typically N ~ 10
À3 s
À1 , h ~ 1,000 m. In neutral
conditions, L M is determined dynamically by inertia and turbulence stresses, when L M ~ L f ¼ U/f,
usually a distance of greater than 30 km, while
close to large mountains of height H M ,
L Ro ¼ H M N/f.
Figure 2 shows that in the zone where the
boundary layer is adjusting to the changed surface
conditions, there is an internal layer with depth
l(x, y) above the ground, separating the air flow
and pollutants above and below it. The depth of
this layer tends to increase slowly over the area
where the surface conditions (temperature or surface roughness) have changed; see, for example,
[2, 3]. Over the mesoscale L M , the internal layer
reaches the top of the boundary layer, and then the
vertical profiles of velocity and temperature over
the whole boundary layer are changed.
Over shorter horizontal scales, the internal layer
depth is much less than h. Here the perturbed flow
has separate characteristics over the intermediate
neighborhood scales L N (≲5 km) and over the
shorter building/street scales L BS (≲300 m). The
former scale L N extends over groups of buildings/
streets of similar size and type, and the internal layer
depth l N is greater than the height of the envelope or
canopy of the buildings H C . Note that there is significant mass, momentum and energy transfer
between the flow in and above the canopy on this
scale. Even though the neighborhood scale includes
many buildings, the types of buildings and their
patterns have significant effects on this exchange
process and on the flow within the canopy.
164
Urban Air Quality: Meteorological Processes
and Drivers” presents the underlying concepts
and prediction methods related to the airflow and
meteorology of urban areas that are particularly
relevant for understanding and modelling dispersion from the urban sources. It is noted how spatial and temporal patterns of flow and dispersion
in urban areas greatly depend on their location
(e.g., flat, hilly terrain, coastal, or desert), the
types of surface, prevailing meteorological conditions (e.g., strong/weak geostrophic winds), and
on the patterns and forms of urban building/street/
open space (e.g., isolated buildings or canyons;
high or low range of building heights, etc.). It will
be convenient to describe the flow in terms of
three characteristic ranges of length scale, namely,
mesoscale, neighborhood scale, and building/
street scales, each with their own general features
and where different modelling approaches are
necessary, depending on the requirements for
accuracy
and
speed
of
computation.
Section “Urban Data” presents some details of
data, and sections “Computational Models for
Meteorology and Air Flow in Urban Areas” and
“Examples of Output of Modelling Calculations”
provide details of modelling methods for flow in
urban areas and examples of model calculations.
Characteristic Regions of the Flow and
Drivers
Characteristic Regions of the Flow
In urban areas, there are usually considerable variations in the types, sizes, and layout of buildings
and streets as illustrated in Fig. 1, and over larger
urban areas there are often also significant variations in the natural topography. Both these urban
scale features and the meteorology have general
characteristics over different length scales. These
need to be understood and related to each other
when developing a picture of the urban airflow as
illustrated in Tables 1 and 2.
Over level terrain, except in very light wind
conditions, the direction and speed of the airflow
only changes significantly over the mesoscale
(of order 30 km) or greater scales. These changes
are determined by Coriolis effects, by the effect of
the urban area on convergence/divergence and
turning of the wind, and by temporal variations
in the flow forced by diurnal and synoptic effects.
Where there are sharp variations in topography
the surface air flow may change locally (e.g., at
the coast or edge of an urban area). However, the
whole boundary layer flow does not adjust immediately to the change of surface conditions: this
takes place over a mesoscale distance L M which,
in the atmosphere, is typically of order 30–50 km.
This distance is usually determined by the Rossby
length scale L Ro ¼ hN/f where f is the Coriolis
parameter and N is the buoyancy frequency of the
stable layer at or above the boundary layer of
height h. At midlatitudes f ~ 10
À4 s
À1 , and typically N ~ 10
À3 s
À1 , h ~ 1,000 m. In neutral
conditions, L M is determined dynamically by inertia and turbulence stresses, when L M ~ L f ¼ U/f,
usually a distance of greater than 30 km, while
close to large mountains of height H M ,
L Ro ¼ H M N/f.
Figure 2 shows that in the zone where the
boundary layer is adjusting to the changed surface
conditions, there is an internal layer with depth
l(x, y) above the ground, separating the air flow
and pollutants above and below it. The depth of
this layer tends to increase slowly over the area
where the surface conditions (temperature or surface roughness) have changed; see, for example,
[2, 3]. Over the mesoscale L M , the internal layer
reaches the top of the boundary layer, and then the
vertical profiles of velocity and temperature over
the whole boundary layer are changed.
Over shorter horizontal scales, the internal layer
depth is much less than h. Here the perturbed flow
has separate characteristics over the intermediate
neighborhood scales L N (≲5 km) and over the
shorter building/street scales L BS (≲300 m). The
former scale L N extends over groups of buildings/
streets of similar size and type, and the internal layer
depth l N is greater than the height of the envelope or
canopy of the buildings H C . Note that there is significant mass, momentum and energy transfer
between the flow in and above the canopy on this
scale. Even though the neighborhood scale includes
many buildings, the types of buildings and their
patterns have significant effects on this exchange
process and on the flow within the canopy.
164
Urban Air Quality: Meteorological Processes