Vertical visibility - CL61

CL61 User Guide

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If there is obscuration in the atmosphere and the cloud signal is not identified in the attenuated backscatter profile, the lidar ceilometer reports vertical visibility instead. This typically occurs in the event of heavy precipitation where the transmitted signal is attenuated by precipitation droplets, or in the event of other ground‑based visibility obscuration like fog or haze. Vertical visibility is used for aviation purposes to report to the pilot at which height they will be able to see the ground.

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 fairly straightforward threshold criteria to determine what cloud is and what it 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.

Vaisala ceilometers use 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.