Geoscience Reference
In-Depth Information
EARE determines three different types of heights based on acoustic backscatter
intensity and the variance of the vertical velocity component. Because the horizon-
tal wind information above the inversion is not regularly available from SODAR
measurements, horizontal wind data have not been included into this scheme. In the
following, a letter “H” and an attached number will denote certain derived heights
that are related to inversions and the MLH, while the variable z is used to denote the
normal vertical coordinate. The EARE algorithm detects
the height ( H 1) of a turbulent layer characterized by high acoustic backscatter
intensities R ( z ) due to thermal fluctuations (therefore having a high variance of
the vertical velocity component
σ w ),
several lifted inversions ( H 2 _n ) characterized by secondary maxima of acoustic
backscatter due to a sharp increase of temperature with height and simultaneously
low
σ w (like those depicted in the left-hand frame of Fig. 4.1 ), and
the height of a surface-based stable layer ( H 3) characterized 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.
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:
The height H 1 corresponds to a sharp decrease
R
/∂
z
<
H 1
z ,
if R ( z )
=
(4.1)
<
R c and R ( z
+
1)
<
R ( z )
+
zDR 1 and R ( z
+
2)
<
R ( z )
+
2 zDR 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
R c
=
88 dB and DR 1
=−
=
H 2 and a decrease above
is stipulated while the turbulence intensity is low:
z + 1 <
z 1 <
z ,if
R
DR 2 and
R
0.7 ms 1 ,
H 2_ n
=
DR 2 and
σ w ( z )
<
(4.2)
z
z
for n
=
1,
...
, N . In Emeis et al. ( 2007b ) N was chosen to be five. A threshold
value DR 2 =
0.08 dB/m has proven suitable. But again, an application-dependent
tuning may be advisable.
The determination of the height of the stable surface layer H 3 is started if the
backscatter intensity in the lowest range gates is above 105 dB while
σ w is smaller
 
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