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Figure 12.8 Vertical profiles of dimensionless variances and covariances measured
in the early-eveningMinnesota runs shown in Figure 12.2 . The SBL depth h ranged
from 30 m to 400 m over the seven runs. The curves are visual fits to the data.
From Caughey et al . ( 1979 ).
structure is evident. The boundary-layer depth followed the equilibrium expression
Eq. (12.50) , but with a proportionality constant about double the equilibrium value
( Caughey et al . , 1979 ).
This state was transient, however, and the runs were terminated about five hours
after transition. In some there was an extreme shift in mean wind direction caused
by the very slight (0.0014) terrain slope or by baroclinity; others entered the very
stable regime and turbulence at 4 m height was extinguished.
In the very stable regime an internal intermittency mechanism discussed by
Businger ( 1973 ) can modulate the turbulence. The extinguishing of the turbulence
eliminates the turbulent friction term in the momentum equation, and the unbal-
anced horizontal pressure gradient then accelerates the flow until the Richardson
number decreases enough to regenerate turbulence. Va n d e Wi e l et al . ( 2002 )show
that surface vegetation can have a strong influence on this intermittency dynamics.
Nocturnal SBLs can differ from those we have discussed here. Mahrt and Vickers
( 2003 ) have described the contrasting vertical structure of nocturnal boundary layers
observed in the CASES99 experiment in relatively flat grassland in the USMidwest.
In addition to the “traditional” stable ABLs of varying depth, they observed what
they call an “upside-down” variety in which the TKE increases with height and
 
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