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The top-down
and bottom-up concept describes the asymmetrical transport of scalars
released from the surface and the top of the PBL.
The mechanisms for this transport asymmetry were investigated by
Wyngaard and Weil [1991], using a kinematic model (for
small-time-scale, homogeneous turbulence with a
skewed vertical velocity field) and
a Lagrangian particle model (for large-time-scale, inhomogeneous
turbulence with a skewed vertical velocity field).
This study suggested that transport asymmetry
is a fundamental consequence of the interaction of a skewed
vertical velocity field with a scalar flux gradient.
A simple updraft-downdraft model was developed by
Schumann [1993] to further investigate transport asymmetry.
The model results indicate that nonstationarity of the concentration field,
vertical-velocity skewness of the flow field, and the inhomogeneous
manner by which the scalars are introduced into the updrafts and
downdrafts all contribute to the transport asymmetry.
In spite of advances in understanding transport
asymmetry, characterizing the
entrainment near the top of the PBL remains problematical.
Some results from the First ISLSCP (International Satellite Land Surface
Climatology Project) Field Experiment (FIFE) illustrate this.
Betts et al. [1992]
found that the ratio of entrainment to surface buoyancy flux
in the FIFE PBLs is about --0.4,
about twice the value assumed by most mixed-layer modelers, but close
to the --0.5 value found by Moeng and Sullivan [1994] in
two simulated PBLs where both shear wind
and buoyancy forcing are important in driving turbulent motions.
The variation of this flux ratio could result from the effects
of wind shear at the surface and at the PBL top, as pointed out by Tennekes
[1973] and others, but may also depend on surface inhomogeneity.
U.S. National Report to IUGG, 1991-1994
Rev. Geophys. Vol. 33
Suppl., © 1995 American Geophysical Union