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• the height (H1) of a turbulent layer characterised by high acoustic backscatter
intensities R(z) due to thermal fluctuations (therefore having a high variance of
the vertical velocity component r
w
),
• several lifted inversions (H2_n) characterized by secondary maxima of acoustic
backscatter
due
to
a
sharp
increase
of
temperature
with
height
and
simultaneously low r
w
, and
• the height of a surface-based stable layer (H3) characterised by high backscatter
intensities due to a large mean vertical temperature gradient starting directly at
the ground and having a low variance of the vertical velocity component.
The height H1 corresponds to a sharp decrease qR/qz \ DR
1
of the acoustic
backscatter intensity R(z) below a threshold value R
c
with height z usually
indicating the top of a turbulent layer. R
c
= 88 dB and DR
1
=-0.16 dB/m have
proven to be meaningful values in the abovementioned studies. R
c
is somewhat
arbitrary because the received acoustic backscatter intensities from a SODAR
cannot be absolutely calibrated. An absolute calibration would require the
knowledge of temperature and humidity distributions along the sound paths for a
precise calculation of the sound attenuation in the air. DR
1
is, at least for smaller
vertical distances, independent from the absolute value of R
c
. An application-
dependent fine-tuning of R
c
and DR
1
may be necessary.
Elevated inversions are diagnosed from secondary maxima of the backscatter
intensity that are not related to high turbulence intensities. For elevated inversions
increase in backscatter intensity below a certain height z = H2 and a decrease above
is stipulated while the turbulence intensity is low. The determination of the height of
the stable surface layer H3 is started if the backscatter intensity in the lowest range
gates is above 105 dB while r
w
is smaller than 0.3 ms
-1
. The top of the stable layer
H3 is at the height where either the backscatter intensity sinks below 105 dB or r
w
increases above 0.3 ms
-1
. The threshold values for r
w
have been determined by
optimizing the automatic application of the detection algorithm. In doing so it turned
out that no lifted inversions occurred with a variance r
w
higher than 0.7 ms
-1
and
that the variance r
w
in nocturnal stable surface layers was below 0.3 ms
-1
. The first
r
w
threshold made it possible to distinguish between inversions and elevated layers
of enhanced turbulence. The latter r
w
threshold made it possible to differentiate
between nocturnal stable surface layers and daytime super-adiabatic surface layers
although both types of surface layers yield more or less the same level of backscatter
intensity. Finally MLH from the acoustic remote sensing is determined as the
minimum of H1, H2_1, and H3.
B.2 Optical Detection Methods
Usually the particle content of the mixed layer is higher than in the free
troposphere above (Fig.
B.1
), because the emission sources for aerosol particles
are in most cases at the ground. Particle formation from precursors mainly takes
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