Geoscience Reference
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Fig. 3.6 Cut from Fig. 3.1 showing the frequency pairs used for different RASS combinations. The
two combinations to the left are utilized for boundary layer profiling. The acoustic wave lengths
are shifted according to the Bragg condition (see also Fig. 3.7 )
optimal backscatter of the electromagnetic waves from the acoustic waves (Fig. 3.7 ).
The electromagnetic signal is emitted at a fixed frequency, but the emitted sound sig-
nal is a chirp signal with varying frequency f a . From the sound wave length
λ a,B at
which optimal backscatter occurs the propagation speed of the sound signal can be
determined via the following dispersion relation:
c a = λ a , B /
2
f a .
(3.14)
For a VHF windprofiler operating at 50 MHz a sound frequency of about 100 Hz
is used, for a UHF windprofiler operating at 1 GHz a sound frequency around 2 kHz
is most suitable to fulfil the Bragg condition. Because the attenuation of sound
waves in the atmosphere is strongly frequency-dependent, a UHF RASS can detect
temperature profiles up to about 1.5 km height whereas a VHF RASS can observe
temperature profiles throughout the troposphere.
3.4.2 Doppler-(SODAR) RASS
A Doppler-RASS (or SODAR-RASS) is a SODAR with an additional electromag-
netic emitter and receiver (Fig. 3.8 ) operating at a frequency f e, 0 . From the Doppler
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