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Turbulence
within the stable PBL is often intermittent,
it coexists with gravity waves,
and is strongly affected by the underlying terrain.
Therefore, the statistics in the stable PBL
are much more difficult to characterize.
Using local similarity scaling
to analyze aircraft data from the near-neutral PBL, Grant [1992]
showed that the stress-energy ratio, the ratio between
velocity variances of different components, and
the correlation coefficients of stress and humidity flux
are all nearly constant in the lower half of the near-neutral layer.
He then concluded that turbulence in this lower layer
is locally similar. For the upper half of the neutral
PBL, however, he found significant turbulent transport from below,
and showed that local similarity breaks down.
Smedman [1991] analyzed data of the stable PBL from seven
land sites with differing roughness, and
found three statistics to be site-independent;
the form of the spectra, the ratio of
the standard deviations of the vertical and streamwise wind components,
and the ratio of the vertical to horizontal heat flux.
Nappo [1991] examined temporal behavior of the horizontal wind
speed and temperature covariance at night, and found that stability
breakdowns (defined as sporadic turbulent events
with the wind-temperature covariance larger than 10% of
the corresponding averages) are a common feature and these
breakdowns contribute a significant portion of the nighttime heat flux.
U.S. National Report to IUGG, 1991-1994
Rev. Geophys. Vol. 33
Suppl., © 1995 American Geophysical Union