Geoscience Reference
In-Depth Information
Figure 10.5 M-O plots of the rms vertical (upper) and lateral (lower) wind fluc-
tuations at 5.7, 11.3, and 22.6 m height in the convective surface layer of the
1968 Kansas experiment.
σ
w
follows M-O similarity well but
σ
v
does not. From
Wyngaard
(
1988
).
(
1977
).Thedatain
Figures 10.5
-
1
0.7
are consistent with this. But surface-layer
kinematics (Appendix) suggests that
w
2
is only weakly influenced by the large con-
vective eddies and to a good approximation is M-O similar, as seen in
Figure 10.5
.
Businger
(
1973
) has briefly explored some of the impacts of these large
convective eddies on surface-layer structure:
Consider … a large uniform area with free convection extending over the entire planetary
boundary layer up to a height
h
. The mean wind speed
U
0….
Consider now the layer close to the surface over a relatively short time period compared to
the large-scale convection but relatively long compared to the time it takes to develop a local
wind profile. This local temporary wind profile cannot be distinguished in characteristics
from a true mean wind profile. This … leads us to believe that there is, averaged over the
entire horizontal area, mean shear production of turbulence which is not related to a mean
wind but to the convective circulation in the boundary layer.
=
0, consequently
u
∗
=
He then postulated that the local friction velocity generated in this way scales with
w
∗
f (h/z
0
)
, with
f
a decreasing function of
h/z
0
. Zilitinkevich
et al
.(
2006
)have
since studied this problem in detail and confirmed that the surface roughness is an
important parameter for surface-layer structure and heat transfer in free convection.
It is conceivable that these effects extend to weak-wind, very unstable conditions,
causing M-O similarity to fail there.
