Geoscience Reference
In-Depth Information
Figure 12.7 Calculated profiles of eddy diffusivities in the SBL. The horizontal
lines indicate the range of values calculated by a suite of 11 different LES codes
with 6 m vertical resolution; the dashed curves are calculated from the Brost-
Wyngaard (
1978
) algebraic model. Adapted from
Beare
et al
.
(
2006
).
and increased at 0.01 K m
−
1
above that. The geostrophic wind speed was 8 m s
−
1
.
A surface cooling rate of 0.25 K h
−
1
was applied for 9 h to give a quasi-steady,
moderately stable SBL with
h/L
2.
An interesting feature of the mean wind speed profile
(Figure 12.1)
is the “jet” -
wind speed exceeding the geostrophic value by about 20%- at the SBL top
(
Problem
12.20)
. When coupled with the inertial oscillation following the afternoon transition
(Subsection 12.2.2)
this could give quite significant nocturnal wind maxima at the
Wyngaard (
1978
) algebraic model gives eddy-diffusivity profiles within the scatter
of the LES results.
12.4 The evolving SBL
12.4.1 Structure
In their study of nocturnal SBL behavior over a grassland site,
Mahrt
et al
.
(
1998
)
identify two limiting regimes that emerge after transition. The fully turbulent
weakly
stable
regime is typically either an early, quasi-steady phase of the nocturnal SBL or
one present under strong winds and/or cloud cover. The
very stable
regime occurs
under clear skies; it has a larger surface cooling rate and lower mean wind speed. Its
turbulence is weak and intermittent, even near the surface. Presumably its surface
buoyancy flux can approach the limiting value discussed in
Subsection 12.3.4
.
Figure 12.8
shows vertical profiles of turbulence variances and covariances mea-
sured in the clear, early evening runs in the 1973 Minnesota experiments. The
mid-run times averaged only one hour after transition, but the collapse to a similarity
