Although the mixed layer may evolve under high wind conditions, more often it
results from the process of thermal impulse. Such processes include heat transfer
from a heated surface that will generate thermal plumes upward. They also include
radiative cooling of the top of the cloudy layer that generates both rising thermal
plumes as well as cold air plumes that may occur simultaneously.
These structures have length scalars equivalent to that of the mixed layer and are
the main vehicles for transport and mixing of both rising and falling scalar and
vector quantities, between the surface and the top layer below the inversion layer.
Vigorous mixing of the atmosphere takes place in the mixed layer (Figs. 1.6 and
1.7a)). The potential temperature shows little vertical variation.
Sensible heat flux shows linear vertical variation until it reaches zero near the
base of the inversion. This flux reverses the direction in which the nocturnal
inversion layer evolves (Figs. 1.6 and 1.7b)). At night, due to thermal stability, the
turbulence of mechanical origin is confined to a lower air layer (h in Fig. 1.6)
without forming a true mixed layer. This weak turbulence together with radiative
cooling gives rise to the night-time potential temperature profile and sensible heat
flux. The vertical profiles of the nocturnal potential temperature characterize the
surface, reverse and neutral layers (Fig. 1.7b).
Tangential stresses that provoke mechanical turbulence are formed at the
interfaces between layers of different densities. These stresses increase until a
threshold is reached, and the flow becomes unstable, with the generation of the
so-called Kelvin–Helmholtz waves. In these waves, the less dense fluid is entrained
in denser fluid, under a mechanism which is statically unstable and discontinuous in
space. The different forms of instability generate turbulence that promotes scalars of
motion transfer, leading to system homogenization and reduction of shear stresses
between the several adjacent fluid layers. All processes in the atmospheric boundary
layer, and especially those concerning the micrometeorological range adjacent to
the ground surface, can be simulated, and compared easily with laboratory experimental results with wind tunnels (Foken 2017).
The wind velocities are subgeostratospheric throughout the mixed layer with the
wind directions crossing the isobars at reduced angles and heading toward
low-pressure zones. The middle zone of the mixed layer has a nearly constant
average wind speed and direction. The speed decreases with height and approaches
zero near the surface (Fig. 1.4). In general, the humidity also decreases with height
within the mixed layer, because of surface evapotranspiration and entrainment of
dry air from the upper levels. Contaminants tend to concentrate more at lower levels
of the mixed layer as thermal plumes cannot penetrate the top of the inversion layer.
During sunset the thermal plumes cease to form, leading to a reduction of
turbulence in the mixed layer. The top layer is maintained but weakened forming a
discontinuous structure with one or more thinner inversion layers. As the thermal
plumes lose their energy near the surface, there is a sudden decrease in turbulent
motions, simultaneously with the beginning of radiative cooling. The resultant air
layer is called the residual layer (Fig. 1.1), with a height of about 1000 m, which
initially has average scalar and vector variables magnitude like that of the mixed
layer from which it evolved. The residual layer is neutrally stratified resulting in the
1 General Characteristics of the Atmospheric Boundary Layer
9
results from the process of thermal impulse. Such processes include heat transfer
from a heated surface that will generate thermal plumes upward. They also include
radiative cooling of the top of the cloudy layer that generates both rising thermal
plumes as well as cold air plumes that may occur simultaneously.
These structures have length scalars equivalent to that of the mixed layer and are
the main vehicles for transport and mixing of both rising and falling scalar and
vector quantities, between the surface and the top layer below the inversion layer.
Vigorous mixing of the atmosphere takes place in the mixed layer (Figs. 1.6 and
1.7a)). The potential temperature shows little vertical variation.
Sensible heat flux shows linear vertical variation until it reaches zero near the
base of the inversion. This flux reverses the direction in which the nocturnal
inversion layer evolves (Figs. 1.6 and 1.7b)). At night, due to thermal stability, the
turbulence of mechanical origin is confined to a lower air layer (h in Fig. 1.6)
without forming a true mixed layer. This weak turbulence together with radiative
cooling gives rise to the night-time potential temperature profile and sensible heat
flux. The vertical profiles of the nocturnal potential temperature characterize the
surface, reverse and neutral layers (Fig. 1.7b).
Tangential stresses that provoke mechanical turbulence are formed at the
interfaces between layers of different densities. These stresses increase until a
threshold is reached, and the flow becomes unstable, with the generation of the
so-called Kelvin–Helmholtz waves. In these waves, the less dense fluid is entrained
in denser fluid, under a mechanism which is statically unstable and discontinuous in
space. The different forms of instability generate turbulence that promotes scalars of
motion transfer, leading to system homogenization and reduction of shear stresses
between the several adjacent fluid layers. All processes in the atmospheric boundary
layer, and especially those concerning the micrometeorological range adjacent to
the ground surface, can be simulated, and compared easily with laboratory experimental results with wind tunnels (Foken 2017).
The wind velocities are subgeostratospheric throughout the mixed layer with the
wind directions crossing the isobars at reduced angles and heading toward
low-pressure zones. The middle zone of the mixed layer has a nearly constant
average wind speed and direction. The speed decreases with height and approaches
zero near the surface (Fig. 1.4). In general, the humidity also decreases with height
within the mixed layer, because of surface evapotranspiration and entrainment of
dry air from the upper levels. Contaminants tend to concentrate more at lower levels
of the mixed layer as thermal plumes cannot penetrate the top of the inversion layer.
During sunset the thermal plumes cease to form, leading to a reduction of
turbulence in the mixed layer. The top layer is maintained but weakened forming a
discontinuous structure with one or more thinner inversion layers. As the thermal
plumes lose their energy near the surface, there is a sudden decrease in turbulent
motions, simultaneously with the beginning of radiative cooling. The resultant air
layer is called the residual layer (Fig. 1.1), with a height of about 1000 m, which
initially has average scalar and vector variables magnitude like that of the mixed
layer from which it evolved. The residual layer is neutrally stratified resulting in the
1 General Characteristics of the Atmospheric Boundary Layer
9