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
Search WWH ::
Custom Search