Characterizations of tropospheric turbulence and stability
layers from aircraft observations
Velocity, temperature, and specific humidity data collected by
aircraft at 20-Hz resolution are analyzed for stability and turbulence
parameters. Over 100 vertical profiles (mostly over the ocean) with a
total of over 300 km in vertical airspace sampled are used. The
compiled statistics show that anisotropy in the velocity fluctuations
prevail down to the smallest spatial separations measured. A
partitioning of convective versus dynamical instability indicates that
in the free troposphere, the ratio of shear-produced turbulence to
convectively produced turbulence increases from roughly 2:1 for weak
turbulence (epsilon < 10-4 m2 s-3) to
perhaps 3:1 for strong turbulence (epsilon > 10-4 m2
s-3). For the boundary layer, this ratio is close to
1:1 for weak turbulence and roughly 2:1 for strong turbulence. There
is also a correlation between the strength of the vertical shear in
horizontal winds and the turbulence intensity. In the free
troposphere the turbulence intensity is independent of the degree of
static stability, whereas in the boundary layer the turbulence
intensity increases with a fall in static stability. Vertical
humidity gradients correlate with static stability for strong humidity
gradients, which supports the basic notion that stable layers impede
vertical mixing of trace gases and aerosols. Vertical shear
correlates with vertical humidity gradient, so it appears that the
effect of differential advection creating tracer gradients dominates
the effect of differential advection destroying tracer gradients
through shear-induced turbulence.
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