Chapter 5 ______________________________________________________ Functional Description
VAISALA _______________________________________________________________________ 77
Extinction Normalization and
Vertical Visibility
Any fog, precipitation, or similar obstruction to vision between the
ground and the cloud base may attenuate the cloud base signal and
produce backscatter peaks that far exceed that from the cloud.
Virtually 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 range observed, it is possible to obtain an
extinction coefficient profile through a mathematical computation.
This is also called inverting the backscatter profile to obtain the
extinction coefficient profile, and answers the question of what kind
of extinction coefficient profile would produce the backscatter profile
measured.
No assumption as to the absolute value of the ratio, k, needs to be
made if k is constant with height. The assumptions that have to be
made are fairly truthful, and in any case accurate enough for the
purpose of cloud detection.
Likewise, the 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, provided that a constant contrast
threshold is assumed. Visibility will simply be 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.
