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There is
increasing evidence that coherent eddies (such as
thermals, plumes, or roll vortices)
carry most of the turbulent fluxes of momentum, heat, and scalars.
This importance led to many recent investigations on coherent structures.
Duncan and Schuepp [1992] conditionally sampled aircraft data to
show that a few well-defined `structures' (plumes and thermals)
accounted for most of the flux.
Similar results were found by Chou and Ferguson [1991], who noted that
roll vortices contribute significantly to the net heat and moisture fluxes,
with their relative importance increasing with height.
Based on aircraft data, Mahrt [1991] found that a
change in the phase shift between
horizontal and vertical velocity of the coherent structure,
from shear to buoyancy PBLs, can
explain the observed difference in momentum transport.
This phase shift indicates a strong decrease in
the efficiency of the vertical transport of horizontal momentum
with increasing buoyancy.
Such results suggest that total fluxes might be represented in terms of
coherent structures.
This idea is given support by Schumann and Moeng [1991]
and Moeng et al. [1992]
who conditionally sampled several LES-generated flow fields to show
that mean thermodynamic differences between updrafts and downdrafts
can be used to accurately represent the turbulent flux profiles.
Roll vortices, quasi-two-dimensional coherent eddies that `normally'
scale with the depth of the mixed layer, are the subject of a recent review by
Etling and Brown [1993]. These are typically associated with
weakly convective
layers with moderate to strong winds, but more recent evidence suggests the
roll regime may exist in more convective conditions, e.g.,
Kristovich [1993].
The lateral dimension of most rolls is 2-3 times the
depth of the PBL; however, mesoscale roll-like structures have been
observed whose wavelength is up to 30 to 60 times the PBL depth.
Balaji et al. [1993], using a numerical simulation,
showed how boundary-layer generated tropospheric gravity
waves can impose their scale on the boundary layer and create roll-like
structures with horizontal wavelengths more than 10 times
the boundary layer depth.
Next: Stable or Near-Neutral
Up: Convective PBL
Previous: Top-downbottom-up diffusion.
U.S. National Report to IUGG, 1991-1994
Rev. Geophys. Vol. 33
Suppl., © 1995 American Geophysical Union