Geoscience Reference
In-Depth Information
frequency, i.e. as FMCW-RADARs. Usually, triangle-shaped frequency modes are
used. The frequency modulation must lead to larger frequency differences between
emitted and received signal than the expected Doppler shift due to motion of
the scatterers along the line of sight. The advantages of FMCW-RADARs are
continuous observation and less emitted power, which reduces the necessity for
shielding.
3.2.1 Windprofiler
Windprofilers are Doppler RADAR instruments for the detection of the vertical
wind profile working at frequencies of 50 MHz to 1 GHz (VDI 3786 part 17,
2007
).
The respective wavelengths are between 6 and 0.3 m. Windprofiler operating at up
to about 300 MHz is called VHF windprofiler (very high frequency), those operating
at higher frequencies as UHF windprofiler (ultra high frequency) (see Table
3.2
). In
the VHF frequency range we observe usable Bragg backscatter at temperature and
especially at moisture fluctuations in the atmosphere. Water and rain drops do not
absorb in this range; therefore, windprofiler measurements are not disturbed by the
presence of clouds and rain. VHF Windprofiler give profiles from the stratosphere
and mesosphere, but have insufficient lowest range gates of about 1 km. UHF wind-
profiler have a lower range and are used for the observation of the troposphere and
can be used for boundary layer studies as well. The disadvantage of operating a
windprofiler at such high frequencies such as 915 MHz is the enhanced sensitivity
to hydrometeors. This hampers the measurement of vertical velocity in case of rain.
The measurement of horizontal velocity is still possible, if it is assumed that the
rain drops are advected with the mean wind. On the other hand, the vertical range
is enhanced in case of precipitation compared to clear air profiling (Ecklund et al.
1988
). Mainly at lower levels, ground clutter contaminates estimations of the verti-
cal velocity component and thus a quasi-systematic bias of the order of -10 cms
-1
exists (Caccia et al.
2004
).
Due to the high propagation speed of electromagnetic waves (in the order of
10
8
ms
−
1
), the height resolution of windprofilers is only about 100 m. The
range depends on the chosen frequency (from about 1 km at 1 GHz to about 70 km
at 50 MHz) and the emitted power. The size of the antenna is proportional to the
wavelength. For 6 m wavelength an area of about 100 m times 100 m is required,
for shorter wave lengths 10 m by 10 m with yagi elements or even a bowl-shaped
antenna can be sufficient (Strangeways
2003
).
At Lindenberg observatory of the German Weather Service, a 1290 MHz bound-
ary windprofiler is operated (Fig.
3.2
). It has a range of 4-6 km and has been
extended to a RASS (see below) by the addition of an acoustic emitter.
Windprofilers work with the Doppler-beam-swinging (DBS) technology for the
detection of profiles of the three-dimensional wind vector. Here, three to five beams
are emitted one after the other into three to five different directions. One of the
three or five directions is always the vertical direction, the other directions are tilted
by 15-20
◦
3
×
from the vertical. The azimuthal directions of the tilted beams usually
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