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Fig. 3.7
Schematic explanation of the Bragg condition for RASS devices.
Green
: acoustic pulse
(shock wave front) travelling upward
shift
f
e
of the electromagnetic radiation which is backscattered at the density fluc-
tuations caused by the sound waves, the propagation speed
c
a
of the sound waves is
determined,
c
a
=−
0.5
c
∗
f
e
/
f
e
,0
),
(3.15)
where
c
denotes the speed of light. A Doppler-RASS like a Bragg-RASS also emits
a chirp sound signal in order to assure that the Bragg condition is optimally met due
to the varying temperature over the entire height range.
The so determined propagation speed
c
a
is a sum of the speed of sound
c
s
and of
the vertical movement of the air
w
within which the sound waves propagate,
c
a
=
c
s
+
w
.
(3.16)
The vertical air speed component
w
can be determined separately from the
Doppler shift of the backscattered electromagnetic clear-air signal when oper-
ating a Bragg-RASS or from the Doppler shift of the backscattered acoustic
signal when operating a Doppler-RASS. Using the definition of the acoustic tem-
perature, the height profile of
c
s
can then be converted in a height profile of
the acoustic temperature
T
a
. For many purposes this acoustic temperature is a
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