Geoscience Reference
In-Depth Information
12
The stable boundary layer
12.1 Introduction
The stable boundary layer (SBL) is as different from the convective boundary layer
(CBL) as night is from day. The SBL is typically much thinner and much less
diffusive; chimney plumes in the stable air just after sunrise can travel intact for
long distances, quite unlike those in midday. The decrease in surface wind speed
around sunset on a clear day, which is prominent inwind climatologies (
Arya
,
2001
),
is also evident to casual observers.
Figure 12.1
shows the root cause: in a given
mean horizontal pressure gradient the surface mean wind speed is lower in stable
stratification than in neutral or unstable stratification. Thus the lower nocturnal wind
speeds tend to persist until the initiation of a CBL at sunrise.
We gave examples of SBLs in
Chapter 9
. The two sketched in
Figure 9.7
are
made stably stratified by flow over a cooler surface and by entrainment of warmer
air aloft. Perhaps the most common example is the nocturnal SBL over land in
fair weather. Another is the “long-lived” SBL at the South Pole; it is caused by
a combination of a temperature inversion over a cooled, sloped surface and the
typically downslope orientation of the mean pressure gradient (
Neff
et al
.
,
2008
).
As we'll see, turbulence in the SBL tends to be in a delicate dynamical balance,
and so SBL structure tends to be more difficult to study and to parameterize than
that in the CBL. But through analytical, computational, and observational studies
it is gradually yielding to understanding.
The material in this chapter is arranged as follows. We'll begin with an overview
of the energetics of stably stratified turbulence and the notion of a degree of stability
past which turbulence cannot exist. We'll then discuss some applications of second-
order closure and LES to the SBL.
Section 12.2
surveys the familiar late-afternoon
transition of the ABL over land - the response of the flow near the surface, the
inertial oscillation aloft in what was the previous afternoon's CBL, and treating the
effects of sloping terrain and gravity waves. In
Section 12.3
we'll discuss some
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