the vertical direction which is of the order of a few mm or cm/sec. Friction reduces the
horizontal flow and decreases the mean wind speed so that it is slowest near the ground.
The atmospheric turbulence of thermal origin coexists with the mechanical turbulence generated by tangential stresses. The atmospheric turbulence is characterized by these thermals or wake-type eddies, which are formed because of forces such
as buoyancy or the retarding effect of surface frictional drag. All processes in the
atmospheric boundary layer and especially those concerning the micrometeorological range are adjacent to the ground surface and can be simulated and compared
easily with laboratory experimental results with wind tunnels (Foken 2017).
Atmospheric waves are often seen in the stable night-time boundary layer and are
important for the transport of energy and momentum and have no bearing on a convective transport of scalars such as sensible heat, humidity, and pollutants. Atmospheric
waves are caused, for example, by the interaction between the mean flow and surface
obstacles or are generated from far away sources such as lightning or explosions (Stull
1994). Atmospheric turbulence can be analyzed by breaking down the main variables
into mean components and fluctuations or perturbations. The mean component describes
the effects of the mean wind and temperature, while the disturbances or fluctuations
relate to the effects of atmospheric waves or to the overlapping turbulent effects.
Atmospheric turbulence is composed of eddies of various sizes, ranging between
millimeters and kilometers, and these interact so that the smaller feed on the larger
ones, resulting in mass and energy exchange. The larger eddies scale to the depth of
the boundary layer varies between 100 and 2000 m in diameter. An approach used
to quantify the relative contributions of each eddy in heat and mass exchange
processes is the analysis of the turbulence spectrum. Larger eddies with shorter
frequencies of 1 Hz or less are highly efficient in heat and mass transport processes
in the atmosphere. The smaller eddies of higher frequencies (about the order of
10 Hz) are much weaker because of the dissipating effects of molecular viscosity.
The boundary layer is, in general, thinner in high-pressure regions than in
low-pressure regions. The convergence or subsidence and low-level horizontal
divergence associated with high-pressure move boundary layer air from high- to
low-pressure zones (Fig. 1.3). The shallow depths are often associated with regions
that typically lack clouds.
(a)
(b)
(c)
0
t
t
t
0
2
-2
0
2
-2
10
U
Fig. 1.2 Schematic of
a mean wind, b waves, and
c turbulence (after Stull 1994)
1 General Characteristics of the Atmospheric Boundary Layer
3
horizontal flow and decreases the mean wind speed so that it is slowest near the ground.
The atmospheric turbulence of thermal origin coexists with the mechanical turbulence generated by tangential stresses. The atmospheric turbulence is characterized by these thermals or wake-type eddies, which are formed because of forces such
as buoyancy or the retarding effect of surface frictional drag. All processes in the
atmospheric boundary layer and especially those concerning the micrometeorological range are adjacent to the ground surface and can be simulated and compared
easily with laboratory experimental results with wind tunnels (Foken 2017).
Atmospheric waves are often seen in the stable night-time boundary layer and are
important for the transport of energy and momentum and have no bearing on a convective transport of scalars such as sensible heat, humidity, and pollutants. Atmospheric
waves are caused, for example, by the interaction between the mean flow and surface
obstacles or are generated from far away sources such as lightning or explosions (Stull
1994). Atmospheric turbulence can be analyzed by breaking down the main variables
into mean components and fluctuations or perturbations. The mean component describes
the effects of the mean wind and temperature, while the disturbances or fluctuations
relate to the effects of atmospheric waves or to the overlapping turbulent effects.
Atmospheric turbulence is composed of eddies of various sizes, ranging between
millimeters and kilometers, and these interact so that the smaller feed on the larger
ones, resulting in mass and energy exchange. The larger eddies scale to the depth of
the boundary layer varies between 100 and 2000 m in diameter. An approach used
to quantify the relative contributions of each eddy in heat and mass exchange
processes is the analysis of the turbulence spectrum. Larger eddies with shorter
frequencies of 1 Hz or less are highly efficient in heat and mass transport processes
in the atmosphere. The smaller eddies of higher frequencies (about the order of
10 Hz) are much weaker because of the dissipating effects of molecular viscosity.
The boundary layer is, in general, thinner in high-pressure regions than in
low-pressure regions. The convergence or subsidence and low-level horizontal
divergence associated with high-pressure move boundary layer air from high- to
low-pressure zones (Fig. 1.3). The shallow depths are often associated with regions
that typically lack clouds.
(a)
(b)
(c)
0
t
t
t
0
2
-2
0
2
-2
10
U
Fig. 1.2 Schematic of
a mean wind, b waves, and
c turbulence (after Stull 1994)
1 General Characteristics of the Atmospheric Boundary Layer
3