order of centimeters in urban areas with low houses and of the order of meters in
more modern areas with tall buildings.
Under mild winds and clear skies, the atmospheric flow due to urban island
heating prevails to dominate over the effects of increased roughness. Thermal
induced circulation vortices are therefore formed, rising towards the tallest buildings, and descending with subsidence motion over the lower buildings. During the
day, this circulation can reach the top inversion of the atmospheric boundary layer
which can then acquire a dome shape. In this dome, accumulation of emissions such
as dust, smoke, particles, and haze, takes place under conditions of little or no wind.
In the daytime mixed layer, vertical profiles of variables such as temperature, air
velocity or specific humidity vary considerably among surface locations, following
the rule of vertical homogeneity up to the inversion at about 1000 m (Arya 1988).
Vertical fluxes of sensible heat and water vapor were reported by Ching (1985) to
vary two to fourfold, as did Bowen ratios. Maximum Bowen ratios of 1.8 and 0.2
were found in areas with tall buildings and in non-urban areas and lower housing,
respectively.
At night due to strong stability, the urban boundary layer decreases to a height of
a few hundred meters and is higher in zones with taller buildings compared to the
non-urban areas. This reduction in height is due to the stability of the nighttime
airflow, which suppresses vertical turbulent mixing (Arya 1988).
In mid-sized urban areas, the combined heat island effect and higher roughness
can disrupt the inversion of the surface night layer, by changing the velocity and
temperature vertical profiles of the boundary layer moving towards the inner-city
canopy. A similar situation may occur in temperate zones during sunrise in winter,
in which, the initial thermal stability can attenuate vertical turbulent mixing due to
warmer urban areas (Oke and East 1971). When moving over urban areas, the lower
air layer becomes unstable, while the layers above remain neutral or slightly stable.
The atmosphere above the top of the inversion maintains the initial characteristics
of the surrounding boundary layer of flat non-urban areas.
The daytime variations in atmospheric stability of the urban boundary layer are
much smaller than for the boundary layer above the surrounding non-urban areas,
despite daytime variations in height (Arya 1988). This urban boundary layer is
mixed intensively by convective processes throughout the daytime cycle, in contrast
with the boundary layer in surrounding non-urban areas. This accounts for the
higher intensity of nighttime surface winds in the urban surface.
5.4 Flow in Urban Areas
Man-made obstacles to the atmospheric flow have in general simple well-defined
geometries arising from combinations of cubic or hemispherical shapes. Flow
through these obstacles has three main features that distinguish it from flow on flat
surfaces: the accelerated flow resulting from the concentration of streamlines,
separation and recirculation, and higher eddy concentration (Rohatgi and Nelson
138
5 Flow Over Modified Surfaces
more modern areas with tall buildings.
Under mild winds and clear skies, the atmospheric flow due to urban island
heating prevails to dominate over the effects of increased roughness. Thermal
induced circulation vortices are therefore formed, rising towards the tallest buildings, and descending with subsidence motion over the lower buildings. During the
day, this circulation can reach the top inversion of the atmospheric boundary layer
which can then acquire a dome shape. In this dome, accumulation of emissions such
as dust, smoke, particles, and haze, takes place under conditions of little or no wind.
In the daytime mixed layer, vertical profiles of variables such as temperature, air
velocity or specific humidity vary considerably among surface locations, following
the rule of vertical homogeneity up to the inversion at about 1000 m (Arya 1988).
Vertical fluxes of sensible heat and water vapor were reported by Ching (1985) to
vary two to fourfold, as did Bowen ratios. Maximum Bowen ratios of 1.8 and 0.2
were found in areas with tall buildings and in non-urban areas and lower housing,
respectively.
At night due to strong stability, the urban boundary layer decreases to a height of
a few hundred meters and is higher in zones with taller buildings compared to the
non-urban areas. This reduction in height is due to the stability of the nighttime
airflow, which suppresses vertical turbulent mixing (Arya 1988).
In mid-sized urban areas, the combined heat island effect and higher roughness
can disrupt the inversion of the surface night layer, by changing the velocity and
temperature vertical profiles of the boundary layer moving towards the inner-city
canopy. A similar situation may occur in temperate zones during sunrise in winter,
in which, the initial thermal stability can attenuate vertical turbulent mixing due to
warmer urban areas (Oke and East 1971). When moving over urban areas, the lower
air layer becomes unstable, while the layers above remain neutral or slightly stable.
The atmosphere above the top of the inversion maintains the initial characteristics
of the surrounding boundary layer of flat non-urban areas.
The daytime variations in atmospheric stability of the urban boundary layer are
much smaller than for the boundary layer above the surrounding non-urban areas,
despite daytime variations in height (Arya 1988). This urban boundary layer is
mixed intensively by convective processes throughout the daytime cycle, in contrast
with the boundary layer in surrounding non-urban areas. This accounts for the
higher intensity of nighttime surface winds in the urban surface.
5.4 Flow in Urban Areas
Man-made obstacles to the atmospheric flow have in general simple well-defined
geometries arising from combinations of cubic or hemispherical shapes. Flow
through these obstacles has three main features that distinguish it from flow on flat
surfaces: the accelerated flow resulting from the concentration of streamlines,
separation and recirculation, and higher eddy concentration (Rohatgi and Nelson
138
5 Flow Over Modified Surfaces