Geoscience Reference
In-Depth Information
11
The convective boundary layer
11.1 Introduction
The mean structure of the convective boundary layer (CBL) is sketched in
Figure 11.1
.
The
surface layer
(the lowest 10%, say) is the most accessible to
observation and therefore the best understood. Above it lies the
mixed layer
(not
“mixing” layer;
†
that is the turbulent shear layer between parallel streams of differ-
ent speeds). Here the turbulent diffusivity tends to be largest and mean gradients of
wind and conserved scalars smallest. The
interfacial layer
buffers the mixed layer
from the free atmosphere. Its top at mean height
h
2
can be thought of as the greatest
height reached by the surface-driven convective elements, and its bottom at
h
1
the
deepest penetrations of the nonturbulent free atmosphere. The mean CBL depth
z
i
lies between these two; it is often taken as the height at which the vertical turbulent
flux of virtual potential temperature has its negative maximum.
11.2 The mixed layer: velocity fields
11.2.1 Mixed-layer similarity
A mimimal set of governing parameters for the quasi-steady mixed layer is the
M-O group
u
∗
,z,g/θ
0
,Q
0
(the surface flux of virtual temperature),
cw
s
(the
surface flux of a conserved scalar) plus the boundary-layer depth
z
i
. The dimen-
sional analysis
(Chapter 10)
here has
m
−
1
=
6 governing parameters and
n
=
4
dimensions, so there are
m
3 independent dimensionless quantities. One is
the dimensionless dependent variable; it is conventional to take the other two as
z/z
i
and
z
i
/L
. The latter is a stability parameter for the CBL as a whole.
−
n
=
†
Tennekes
(
1974
) admired the semantic precision of
mixed
rather than
mixing
layer.
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