Geoscience Reference
In-Depth Information
Height
Free atmosphere
h
Entrainment layer
Mixed layer
Surface layer
0
—
v
u
q
c
q
Figure 18.1
Typical height variation in the value of average atmospheric variables through the four main layers which
make up the ABL.
The second main layer of the ABL is the
mixed layer
which is usually so well-
mixed by atmospheric turbulence during the day that the rate of change with
height of mean atmospheric variables is comparatively small. The mixed layer
grows deeper during the day and can reach a height of several kilometers. Above
the mixed layer there is a strong inversion in potential temperature which has the
effect of inhibiting mixing between the well-mixed turbulent ABL and the free
atmosphere above. This layer of air is often called the
inversion layer
and is typically
a few tens to a few hundred meters deep. It is also sometimes called the
entrainment
layer
because it is the entrainment of air from the free atmosphere into the ABL
which causes the mixed layer to grow through the day. In the inversion or
entrainment layer, the mean values of atmospheric variables change rapidly. Above
the inversion layer the free atmosphere then extends upward through the
troposphere. The geostrophic wind speed applies in the free atmosphere and the
air is typically warmer (in terms of potential temperature) and also usually drier
than the air in the mixed layer.
Figure 18.2 shows the typical evolution of the ABL in clear sky conditions
starting at sunrise through the subsequent day and night. The depth of the several
layers in the ABL evolve in response to surface heating by solar radiation during
the day and to surface cooling by longwave radation at night. During the day, heat
and water vapor enter the mixed layer through the surface layer. Some of the air in
the free atmsophere is captured and becomes part of the mixed layer because large-
scale turbulence in the mixed layer can generate temporary breakdowns in the
thermal inversion that otherwise inhibits downward transfer. As a result, the depth
of the mixed layer grows. Its temperature also increases partly as a result of surface
