Geoscience Reference
In-Depth Information
turbulent velocity gradients strengthens its temperature gradients, accelerating the
removal of this temperature excess by molecular diffusion (
Chapter 3
,
Section 3.3
).
The deformation rate of the eddies most effective in distorting a parcel of spatial
scale
r
is
u(r)/r
, which according to Kolmogorov scaling is of order
r
−
2
/
3
1
/
3
;
the time scale for parcel distortion is the inverse,
r
2
/
3
−
1
/
3
. Thus we expect that
the largest buoyant parcels retain their temperature excess the longest. The largest
eddies in the convective boundary layer (CBL) extend from the surface to the
inversion base.
Another prominent feature of the convective ABL is the skewness of its vertical
velocity fluctuations, defined by
w
3
(w
2
)
3
/
2
.
S
w
=
(9.11)
Skewness is a statistical measure of the differences between positive and negative
fluctuations - here, updrafts vs. downdrafts.
S
w
0
.
4-1.0 in the CBL driven by
surface heating. The updrafts tend to be stronger than downdrafts, but since
w
averages to zero they are also rarer. The most likely value of
w
is slightly negative -
a very weak downdraft.
S
w
is negative in a cloud-capped CBL driven by radiative
cooling of cloud top (
Moeng and Rotunno
,
1990
). We shall see in
Chapter 11
that
its
w
-skewness gives the CBL some unusual diffusion properties.
Since buoyancy forces tend to make intense, flow-filling turbulent eddies, we
expect the convective ABL to be particularly diffusive. We see this in the dif-
fering behavior of an instantaneous smokestack plume in the ABL on a sunny
day and an overcast day. On an overcast day with primarily
mechanical
turbu-
lence (turbulence generated by shear production) the plume can remain intact for
some distance downwind, but on a sunny day it loops and twists wildly under
the influence of the large, convectively driven eddies (
Figure 9.6
)
. The ensemble-
average plume grows most rapidly with downstream distance under convective
conditions.
9.3.4 The stable ABL
We can recognize at least three types of stably stratified ABLs. One occurs when
ABL air encounters a cooler surface downwind (as in
Figure 9.7
,
upper). A second is
a boundary layer that is entraining higher potential temperature air from the capping
inversion and has zero surface heat flux
(
Figure 9.7
,
lower). This might better be
called the “inversion-capped neutral” case, since
neutral
traditionally means zero
surface heat flux. The third and perhaps most common type is that over cooler land
at night in clear weather. Each of these stable ABLs is typically shallow (as much as
