turbulence of equal intensity in all directions. In this way, plumes containing
chemical agents are evenly dispersed and these may interact and form aerosols or
particles that eventually precipitate out (Stull 1994).
Shortly after sunset, along with the formation of the residual layer, the cold
night-time air adjacent to the surface is transported upwards by mechanical turbulence, forming a calm and stable boundary layer, which at midnight extends to a
height of 100–200 m. A thin and narrow surface layer is found below the nocturnal
boundary layer (Fig. 1.1). The flow within the night-time boundary layer is characterized by strong shear stress, small scale turbulent eddies, and activity in the
atmospheric waves, mentioned before.
The residual layer, located above the nocturnal boundary layer, is not in a strict
sense directly affected by surface effects that lead to turbulence. However, it is
usually considered one of the atmospheric boundary layer component, as it lies
below the bottom of the inversion layer. The nocturnal residual and stable boundary
layers are quickly destroyed after sunrise with the formation of the mixed layer as
mentioned above.
Fast super-geostrophic winds or nocturnal jets are formed above the nocturnal
boundary layer due to the Earth's rotation. These winds move downward at night.
Thus, while the stable air in the nocturnal boundary layer serves to suppress turbulence, nocturnal jets exert an opposite effect in which intense and rapid phenomena promote mixing.
The stable atmospheric boundary layer rarely reaches an equilibrium comparable
to that of the convective boundary layer. Wind profiles and average temperatures
evolve throughout the night. Also, drainage or katabatic winds from near the surface (adjacent to the ground) caused by the colder air flowing down under the
influence of gravity. Thus, at a height of a meter, wind speeds of 1 ms
−1 occur.
Turbulence in the nocturnal boundary layer gradually decreases with height
(Kaimal and Finnigan 1994), as it is dampened by thermal stability while
decreasing tangential stresses.
The nocturnal boundary layer does not have a sharply defined top, in contrast
with the diurnal mixed layer which ends at the top inversion layer. The upper limit
of the nocturnal layer is defined as the height at which the turbulence intensity is a
small fraction of its surface value. As a rule of thumb, the height of the night-time
layer may be defined as that at which turbulence intensity decreases to about 5% of
the value at the surface. Alternately, it may be defined as the average height of the
inversion layers (Fig. 1.6). At sunrise, these night-time flux perturbations begin to
settle, and in this way, the diurnal cycle begins again leading to the formation of the
atmospheric boundary layer.
References
Foken, T. (2017). Micrometeorology, 2nd ed., Springer, Berlin, 362 pp.
Kaimal, J. C., & Finnigan, J. J. (1994). Atmospheric boundary layer flows (p. 289). Their Structure
and Measurement: Oxford University Press.
10
1 General Characteristics of the Atmospheric Boundary Layer
chemical agents are evenly dispersed and these may interact and form aerosols or
particles that eventually precipitate out (Stull 1994).
Shortly after sunset, along with the formation of the residual layer, the cold
night-time air adjacent to the surface is transported upwards by mechanical turbulence, forming a calm and stable boundary layer, which at midnight extends to a
height of 100–200 m. A thin and narrow surface layer is found below the nocturnal
boundary layer (Fig. 1.1). The flow within the night-time boundary layer is characterized by strong shear stress, small scale turbulent eddies, and activity in the
atmospheric waves, mentioned before.
The residual layer, located above the nocturnal boundary layer, is not in a strict
sense directly affected by surface effects that lead to turbulence. However, it is
usually considered one of the atmospheric boundary layer component, as it lies
below the bottom of the inversion layer. The nocturnal residual and stable boundary
layers are quickly destroyed after sunrise with the formation of the mixed layer as
mentioned above.
Fast super-geostrophic winds or nocturnal jets are formed above the nocturnal
boundary layer due to the Earth's rotation. These winds move downward at night.
Thus, while the stable air in the nocturnal boundary layer serves to suppress turbulence, nocturnal jets exert an opposite effect in which intense and rapid phenomena promote mixing.
The stable atmospheric boundary layer rarely reaches an equilibrium comparable
to that of the convective boundary layer. Wind profiles and average temperatures
evolve throughout the night. Also, drainage or katabatic winds from near the surface (adjacent to the ground) caused by the colder air flowing down under the
influence of gravity. Thus, at a height of a meter, wind speeds of 1 ms
−1 occur.
Turbulence in the nocturnal boundary layer gradually decreases with height
(Kaimal and Finnigan 1994), as it is dampened by thermal stability while
decreasing tangential stresses.
The nocturnal boundary layer does not have a sharply defined top, in contrast
with the diurnal mixed layer which ends at the top inversion layer. The upper limit
of the nocturnal layer is defined as the height at which the turbulence intensity is a
small fraction of its surface value. As a rule of thumb, the height of the night-time
layer may be defined as that at which turbulence intensity decreases to about 5% of
the value at the surface. Alternately, it may be defined as the average height of the
inversion layers (Fig. 1.6). At sunrise, these night-time flux perturbations begin to
settle, and in this way, the diurnal cycle begins again leading to the formation of the
atmospheric boundary layer.
References
Foken, T. (2017). Micrometeorology, 2nd ed., Springer, Berlin, 362 pp.
Kaimal, J. C., & Finnigan, J. J. (1994). Atmospheric boundary layer flows (p. 289). Their Structure
and Measurement: Oxford University Press.
10
1 General Characteristics of the Atmospheric Boundary Layer