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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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