Geoscience Reference
In-Depth Information
Chapter 2
Analytical Description and Vertical Structure
of the Atmospheric Boundary Layer
2.1 Introduction
The atmospheric boundary layer (ABL) is the lower part of the troposphere, and by
this the lowest layer of the atmosphere as a whole. In contrast to the free atmosphere
above, the presence of the underlying Earth's surface has a measurable influence
on the ABL. It is the only part of the atmosphere where frictional forces play an
important role, and where the temperature can exhibit a diurnal variation. The ABL
acts as a kind of broker that communicates the transport of energy, momentum, and
other substances between the Earth surface and the free atmosphere, i.e. the ABL
is dominated by vertical fluxes of these variables. These fluxes have their largest
amounts directly at the surface and vanish at the top of the ABL.
This chapter does not offer a complete overview on the ABL, but rather concen-
trates on the vertical structure of the ABL. More general descriptions of the ABL
can be found in Stull (
1988
), Arya (
1995
), Garratt (
1992
), and other topics. Because
the ABL enwraps the whole Earth, it is often also called planetary boundary layer
(PBL).
The Earth's surface is a place where turbulence is generated; it is usually a sink
for atmospheric momentum, and either a source or a sink for heat, moisture, and
other atmospheric compounds such as aerosols and trace gases. Therefore, we find in
the ABL less momentum but more turbulence, and different heat, moisture, aerosol,
and pollutant concentrations than in the free atmosphere above. The detection of the
vertical profiles of the just mentioned atmospheric variables can thus help to identify
the vertical structure and extend of the ABL. This detection of these profiles and
their interpretation is the main topic of this topic.
Three principal types of the ABL can be distinguished: (i) if heat input from
below dominates, we find a convective boundary layer (CBL); (ii) if the atmosphere
is cooled from below, we find a stable boundary layer (SBL); and (iii) if the heat flux
at the lower surface is vanishing and dynamical shear forces are dominating, we find
a neutral or dynamical boundary layer. We will start with the principal description of
the vertical structure of these three ABL types in the subsection on flat terrain below.
The vertical structure of these three ABL types additionally depends to a large
extent on the type and texture of the underlying surface. Its shape, roughness,
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