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case of pure free convection (where shear plays no role at all), from dimensional
considerationsa scale velocityis
∝
−
λ
max
w
b
1
/
3
w
∗
F
For forced convection, where both shear and buoyancy driven convection are
present,asinthe LeadExexampleabove,wepostulateamixedscale velocity:
w
∗
=
u
∗
−
λ
max
w
b
1
/
3
;
w
b
<
3
0
(5.8)
007ms
−
1
.
The example illustrates again that even with fairly rapid freezing, the impact of
buoyancy-forced convection in the IOBL is proportionately much less important
thanconvectioninthe atmosphericboundarylayerovera heatedlandsurface.
01ms
−
1
versus
u
∗
≈
UsingaveragevaluesshownifFig.5.14,
w
∗
≈
0
.
0
.
5.5 Summary of IOBL Scales
FromturbulencestudiesintheIOBLexemplifiedbytheexamplesdescribedabove,
a heuristic view of turbulence scales is presented in Fig. 5.17. When melting
or freezing is negligible compared with shear production of TKE (Fig. 5.17a),
Fig. 5.17
Schematic illustrating postulated scales for the IOBL under varying surface flux (mo-
mentum and buoyancy) conditions.
a
Neutral stabilityinthewell mixedlayer.
b
Stabilizingbuoy-
ancy flux from melting.
c
Destabilizingbuoyancy fluxfrom freezing
