Geoscience Reference
In-Depth Information
Figure 12.1 Left: Profiles of mean wind components in an SBL, in coordinates
aligned with the surface wind, as calculated through LES. Right: The profile of
mean wind speed, and for comparison a CBL mean speed profile calculated from
Wilson's formula,
Eq. (10.18)
, with
L
=0.46ms
−
1
, and
the same mean horizontal pressure gradient. The contrast in surface-layer mean
wind speeds between the CBL and SBL cases is striking. LES data courtesy Peter
Sullivan, NCAR.
=−
15 m,
z
0
=0.01m,
u
∗
of the important features of the quasi-steady SBL - its limiting structure near a
cooled surface and below a capping inversion, its equilibrium height formula, and
its limiting surface heat flux; the generalization of its height formula to a range of
neutral and stableABLs; and LES-based insights into SBL structure. In
Section 12.4
we'll cover some of the important dynamical and structural features of the evolving
SBL, and in
Section 12.5
we'll discuss models of equilibrium SBL depth.
12.1.1 Energetics of stably stratified turbulence
The interfacial layer that caps the CBL
(Chapter 11)
is stably stratified, but its
TKE budget has two gain terms: shear production and turbulent transport from
the CBL below. The first involves the Reynolds stress, which can be extinguished
by sufficiently strong stable stratification. But the roots of the transport term lie
outside the interfacial layer, so we would not expect it to be sensitive to this stable
stratification. By contrast the SBL has TKE gain only through shear production,
and so its turbulence, unlike that in the interfacial layer, tends to be quite sensitive
to stable stratification.
Figures 12.2
and
12.3
show the rapid decay of turbulence
parameters after the onset of surface cooling and stable stratification.
The dynamical balance of shear-driven turbulence is not considered delicate.
Why does stable stratification change this?
The answer lies in the different natures of the TKE loss to viscous and buoyancy
forces. Viscous forces directly impact only the smallest eddies; in equilibrium their
