Geoscience Reference
In-Depth Information
with
ω
3
the mean vertica
l v
orticity at the reference height. In the northern hemi-
sphere
f
is positive and
ω
3
is negative aroun
d a
high and positive around a low;
in the southern hemisphere the signs of
f
and
ω
3
are reversed. Thus in both hemi-
spheres
(9.34)
indicates that
W (h)
is negative around a high and positive around a
low. Its magnitude is typically on the order of 1 cm s
−
1
, which is often significant
compared to the rate of deepening of the boundary layer due to entrainment. Other
things being equal, ABLs under a high are apt to be shallower and less cloudy as a
result.
Questions on key concepts
9.1 Contrast the nature of the instantaneous tops of laminar and turbulent
boundary layers.
9.2 Contrast the nature of the instantaneous and average tops of a turbulent
boundary layer.
9.3 Explain why the mean flow in the ABL cannot be geostrophic.
9.4 Explain how acoustic sounding is able to detect the instantaneous top of the
ABL.
9.5 Explain why the surface energy balance is one of the most important
influences on the ABL.
9.6 Explain the interpretation of the terms in
Eq. (9.4)
.
9.7 Explain why characterization of the stability state of an ABL through near-
surface measurements alone is apt not to be reliable. How could a stable
ABL be misidentified in this way? An unstable ABL?
9.8 What is subgrid TKE? Why need it not be directly affected by buoyancy?
9.9 Discuss three mechanisms by which an ABL can be made stably stratified.
9.10 Explain why stable stratification affects larger eddies more strongly than
smaller ones.
9.11 Explainwhy and how the difference between average and instantaneous ABL
structure impacts dispersion modeling.
9.12 Discuss why the Ekman solution is generally not applicable to the ABL.
9.13 Why is the mean horizontal pressure gradient in the ABL apt to depend on
height? Is the dependence typically significant? Why does this not occur in
engineering flows?
9.14 Explain the physical mechanism of Ekman pumping.
9.15 The flatness factor of the vertical velocity in the CBL does not differ greatly
from the Gaussian value of 3. Explain why, by the criterion of
Problem 7.16
,
the
∼
1 skewness of vertical velocity there is large.
