Geoscience Reference
In-Depth Information
R
f
R
∗
w
e
w
∗
=
.
(11.48)
Thus with
R
∗
=
30 and
R
f
0
.
3, for example,
w
e
∼
0
.
01
w
∗
.Thisisverysmall
indeed.
In the interfacial layer the intensity of turbulent fluctuations decreases from the
mixed-layer values to the much lower values typical of the essentially nonturbulent
free atmosphere above. Thus, we expect the eddy diffusivity in the interfacial layer
to be less than in the mixed layer, and vertical mean gradients to be larger. If we
know the value of amean property -mean velocity, temperature, water vapor mixing
ratio, say - in the free atmosphere just above the ABL and the value at the surface,
it is not clear a priori how the changes in this mean property are distributed across
the surface layer, the mixed layer, and the interfacial layer. One can model each
layer, however, and estimate the profile from the resulting coupled set of equations.
In this way one finds that the baroclinic, convective ABL tends to concentrate its
mean wind shear within the interfacial layer. The “jump conditions” of
Eq. (11.43)
imply that the turbulent kinetic energy budget there will have positive-definite shear
production terms,
(U)
2
(V )
2
.
uw
∂U
vw
∂V
w
e
h
−
∂z
−
∂z
+
(11.49)
Even in barotropic cases we saw there is a mean wind jump across the interfacial
layer in order to meet the geostrophic upper boundary condition on the mean wind.
We conclude that there typically is non-negligible shear production in the interfacial
layer. If so, the concept that
R
f
a
in the interfacial layer implies that the ratio
of the negative flux of entrainment and the surface flux can exceed
a
.
→
Questions on key concepts
11.1 Discuss the three-layer structure of the CBL.
11.2 Discuss the notion of mixed-layer similarity, giving examples of statistics
that do and do not follow it.
11.3 Discuss the behavior of the TKE budget in the CBL.
11.4 What is
K
-closure? Why is it so difficult to assess its reliability in the mixed
layer? Why could it be easier to evaluate with numerically computed fields?
11.5 Explain the shape of the mean profiles in the CBL as represented in
11.6 Explain why it is so difficult to evaluate the mean-momentum balance from
observations in the ABL.
11.7 Explain how the budgets of Reynolds stress and scalar flux can be used to
produce expressions for eddy diffusivities.
