Chapter 4 ______________________________________________________ Functional Description
VAISALA_______________________________________________________________________ 47
any backscatter height profile is possible, up to some physical limits. To
distinguish a significant cloud return signal, the attenuation of, for
example, fog or precipitation, has to be taken into account by
normalizing with regard to extinction. The profile thus obtained is
proportional to the extinction coefficient at various heights, and enables
the use of a fairly straightforward threshold criteria to determine what is
cloud and what is not.
By assuming a linear relationship between backscatter and extinction
coefficient according to the previous formula and by assuming that the
ratio, k, is constant over the observation range, it is possible to obtain an
extinction coefficient profile through a mathematical computation. This
is also called inverting the backscatter profile, and it basically answers
the question of what kind of an extinction coefficient profile would
produce the measured backscatter profile.
No assumption as to the absolute value of the ratio, k, needs to be made if
k is constant in terms of height. The assumptions that have to be made
are fairly truthful, and accurate enough for cloud detection purposes.
The backscatter profile inversion is also independent of several
instrumental uncertainties including transmitted power and receiver
sensitivity.
An estimate of vertical visibility can easily be calculated from the
extinction coefficient profile because of the straightforward extinction
coefficient-to-visibility relationship, assuming a constant contrast
threshold. Visibility is simply the height where the integral of the
extinction coefficient profile, starting from the ground, equals the natural
logarithm of the contrast threshold, sign disregarded.
Tests and research have, however, shown that the 5 % contrast threshold
widely used for horizontal measurement is unsuitable for vertical
measurement if values close to those estimated by a ground-based
observer are to be obtained.
Ceilometer CL31 uses a contrast threshold value which, through many
tests, has been found to give vertical visibility values closest to those
reported by ground-based human observers. A safety margin is obtained
with regard to pilots looking down in the same conditions since the
contrast between objects, especially runway lights, is much more distinct
on the ground.
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