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cross sections of the backscatter intensity. The method makes use of the assumption
that turbulence is larger in the mixing layer than in the atmosphere above, and that
this turbulence is depicted in enhanced intensity of the acoustic backscatter. MLH
is analyzed either from the maximum negative slope or from the changing curvature
of the vertical profile of the acoustic backscatter intensity, or it is analyzed from
the height where the backscatter intensity decreases below a certain pre-specified
threshold value.
Horizontal Wind Speed Method
The HWS method requires the analysis of the Doppler shift of the backscattered
acoustic signals. The algorithm is based on the analysis of the shape of hourly-
averaged vertical wind speed profiles using the assumption that wind speed and
wind direction are almost constant within the mixing layer but approach gradually
towards the geostrophic values above the mixing layer. Beyrich (
1997
) listed this
method in his Table 2, but did not discuss it further. The applicability of the method
is probably limited to well-developed convective boundary layers (CBL) due to the
underlying assumptions. Such CBL are often higher than the maximum range of a
SODAR. Even if the CBL height is within the range of the SODAR, the algorithm
for the analysis of the Doppler shift often fails above the inversion topping the CBL
due to too low signal-to-noise ratios.
Vertical Wind Variance Method
The VWV method is also working only for CBLs. It is based on the vertical pro-
file of the variance of the vertical velocity component
σ
w
reaches a
maximum in a height
a
.
z
i
. Typical values for
a
are between 0.35 and 0.4. Thus, in
principle, this is an extrapolation method. It has been tried for SODAR measure-
ments because it permits a detection of MLH up to heights that are 2.5 times above
the limited maximum range (usually between 500 and 1000 m) of the SODAR.
Beyrich (
1997
) classified this method as not reliable. A related method, which is
based on power spectra of the vertical velocity component, is integrated in the
commercial evaluation software of certain SODARs (Contini et al.
2009
).
σ
w
. In a CBL
Enhanced Acoustic Received Echo Method
The EARE algorithm has been proposed by Emeis and Türk (
2004
) and Emeis et al.
(
2007b
). The method is an enhancement of the ARE method in two ways. Firstly, it
includes further variables into the MLH algorithm that are available from Doppler-
SODARs. The benefits of the additional usage of the variance of the vertical velocity
component have been demonstrated by Emeis and Türk (
2004
). Secondly, it deter-
mines not only MLH from SODAR measurements but also the heights of additional
lifted inversions. Especially in orographically complex terrain, the vertical structure
of the ABL can be very complicated. Emeis et al. (
2007a
) have shown that several
persistent inversions one above the other which form in deep Alpine valleys can be
detected from SODAR measurements (Fig.
4.1
).
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