Geoscience Reference
In-Depth Information
(a)
Daytime profiles of turbulent fluxes
Free atmosphere
2
Entrainment layer
1
Mixed layer
Surface layer
0
0
0
0
Heat flux (
w
q
)
Momentum flux (
w
u
)
Moisture flux (
w
q
)
(b)
Nighttime profiles of turbulent fluxes
2
Free atmosphere
Capping inversion
1
Residual layer
Stable boundary layer
0
Surface layer
0
0
0
Heat flux (
w
q
)
Momentum flux (
w
u
)
Moisture flux (
w
q
)
Figure 15.12
Idealized profiles of the turbulent fluxes of sensible heat, momentum and moisture as a function of height
(a) in daytime conditions through the convective mixed layer, and (b) in nighttime conditions through the stable boundary
layer. The typical range of variability in momentum and moisture fluxes is shown in gray.
Important points in this chapter
●
Turbulent eddies
:
fast response sensors of atmospheric variables reveal
quasi-random fluctuations around a mean value whose variance changes with
time, which are not regular and not wave-like, and which suggest there are
structures in the air flow that are sometimes referred to as
turbulent eddies
.
●
The spectral gap
:
Fourier analysis of observed atmospheric fluctuations
shows the variability associated with time scales of hours to days to months
is separated from variability at higher frequencies by a
spectral gap
corresponding to time scales of 30 to 90 minutes in which there is limited
variability.
Decomposed variables
:
because there is a spectral gap, atmospheric variables
can be written as being a slowly varying 20-60 minute average (described by
equations based on physical principles) with superimposed haphazard
turbulent fluctuation at higher frequencies that have zero mean value.
●
