Environmental Engineering Reference
In-Depth Information
Table 1: The distance between the three beams at a number of heights [9].
Distance (m)
Altitude (m)
Tilt-vertical
Tilt-tilt
40
11.7
16.5
80
23.4
33.1
120
35.1
49.6
160
46.8
66.2
200
58.5
82.7
for example from animals, can have an adverse effect. A particularly critical issue
is the increased background noise due to high wind speeds [9]. False echoes from
the Sodar's enclosure or nearby obstacles can also lead to a falsifi ed signal. Other
parameters which may infl uence the Sodar measurement are errors in the vertical
alignment of the instrument, temperature changes at the antenna and, especially
for three-beam Sodars, changes in wind direction [8].
The measurement accuracy of Sodar systems cannot match that of cup ane-
mometry and unless high acoustical powers are used, their availability falls in
high wind speeds. A hybrid system comprising a moderately tall mast (say 40 m)
and a relatively low power (30-100 W electrical) Sodar has many attractive
features - the high absolute accuracy and high availability of the cup anemom-
eter complements the less accurate but highly relevant vertical resolution
obtained from the Sodar [10].
2.2.5.2. Lidar
Until recently, making wind measurements using Lidars was prohibitively
expensive and essentially limited to the aerospace and military domain. Most
limitations were swept aside by the emergence of coherent lasers at wave-
lengths compliant with fi bre optic components (so-called 'fi bre lasers'). Since
light can be much more precisely focused and spreads in the atmosphere much
less than sound, Lidar systems have an inherently higher accuracy and better
signal to noise ratio than a Sodar. The Lidar works by focusing at a specifi c
distance and it measures the scattering from aerosols that takes place within
the focal volume.
The operation of the Lidar is infl uenced by atmospheric conditions (e.g. fog,
density of particles in the air). Lack of particles infl uences its response, some-
times prohibiting measurement while fog can severely attenuate the beam
before it reaches the measurement height. Rain also reduces the Lidar's ability
to measure, as scattering from the falling droplets can result in errors in the
wind speed, particularly its vertical components. Other parameters that infl u-
ence the measurement are, as in the case of the Sodar: errors in the vertical
alignment of the instrument and uncertainties in the focusing height. The Lidar,
 
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