CLOUD SENSOR

< Qualimetrics 8329-A/B Laser Ceilometer

TYPE...Qualimetrics 8329-A/B Laser Ceilometer
PERIPHERAL INTERFACE...CL
PI ALGORITHM VERSION...Version 5.2

SENSOR FUNCTIONAL DESCRIPTION
The ceilometer uses a gallium arsenide laser working at 9000 Angstrom. The laser emits a 50 ns pulse upward, part of which is reflected back to the sensor by any intervening cloud. At the same time a reference pulse is sent to the receiver, telling it to look for a returned pulse at a specified time. The receiver is capable of detection only at this time. One detection cycle through the lower atmosphere requires up to 50,000 laser pulses and can take from 15 secs to two minutes, depending on the pulse repetition frequency. The time taken for an emitted pulse to be detected at the receiver varies directly as to cloud height. The returns detected at each level are binned into a memory array according to height.

Sensor Accuracy and Resolution
from 10 to 510 meters...................5 m
from 510 to 990 meters.................10 m
from 990 to 1515 meters...............15 m
from 1515 to 3840 meters.............30 m

OUTPUT
Once the cycle is complete (i.e. all heights have been surveyed), the cloud algorithm searches through the bins, determining cloud layers ( bases and tops). Bases are reported separately until the top of the lower and base of the higher are within a predetermined distance of each other, at which point only one base is reported. Tops are not reported. The maximum height reported is 10,000 ft. If the cloud layers are thick enough, attenuation of the pulses becomes such that the maximum height reported is lowered.
In an attempt to estimate extent of cloud cover, the algorithm keeps track of the time a layer is present over the station during the preceding hour. It assesses each level for SCT, BKN or OVC conditions for a length of time that corresponds to its height above the ground...one minute for each hundred feet. e.g. cloud at 600 ft is assessed for 6 minutes, 2000 ft for 20 minutes, 6000 ft or higher for the entire hour. In the case of 2000 ft, if cloud is detected for 18 to 20 minutes, it is labelled OVC. If detected for between 10 and 17 of the 20 minutes...BKN, and if for less than 10 minutes...SCT. Six minutes is the minimum evaluation time, 60 minutes the maximum.
If a lower layer moves in, obscuring a previously reported higher layer, or if a layer is detected through a lower layer, two or more OVC layers can be reported. For cloud no longer sensed, a layer is retained as SCT one minute for every 100 ft of thickness except for cloud based below 2000 ft which is carried as SCT for 20 minutes beyond the last "hit". For surface-based layers (layers detected below 100 ft) that are less than 600 ft thick -X is reported. For layers greater than 600 ft thick, E0 OVC or E1 OVC will be reported. The sensor records vertical visibility, but it is not reported.

SPECIAL CRITERIA
When ceiling drops below, or rises to 2500, 1500, 1000, 500, 400, 300 and 200 feet.

KNOWN PROBLEMS...
- i. Extended periods of CLR BLO 100 during light snow due to a total scattering of the laser beam, and no reflected pulses being received.
- ii. Overly sensitive to ice crystals, resulting in false reports of low cloud.
- iii. Underestimates layer amounts, especially as cloud layers are moving in. The ceilometer measures cloud only once it has passed directly over the station, and thus, is evaluating downwind cloud cover.
- iv Will pick the lowest "hit" for a layer as the base, and thus be somewhat pessimistic in it's cloud heights.
- v. Virga often reported as cloud resulting in underestimated cloud heights.
- vi. Cloud types are not reported

PLANNED UPGRADES...
Problem i.At the present time, all AWOS sensors operate independently. Version 6.1of the cloud algorithm is ready for Beta testing and this version brings a new level of sophistication to AWOS processing. With this new algorithm, the cloud PI will consult with some of the other sensors in order to make some decisions and come up with a better report. To explain in very simple terms, V6.1 will monitor for a sensor report of no cloud. If this condition is reported, the cloud PI will consult the POSS PI to determine if snow is falling. If not, the clear report will be allowed to pass. If, however, an S or a P (for precipitation of undetermined type) with an intensity of at least light is being reported, the cloud algorithm will then consult the horizontal visibility sensor, and using the following table, insert a surface based layer with the indicated vertical visibility height into the cloud array for further processing.
- 6 to 9+.....SM....................Unlimited
- 3 to 6.......SM.......................2500 ft
- 1 to 3.......SM.......................1500 ft
- 0.7 to 1....SM.......................1000 ft
- 0.5 to 0.7 SM........................500 ft
- 0.3 to 0.5 SM........................300 ft
- 0.2 to 0.3 SM........................200 ft
- 0 to 0.2....SM..........................0 ft
The layer will be reported as -X until such time as the coverage is 90% or greater, at which time the layer will be reported as an obscured layer e.g. P10 X. As well, the vertical visibility as reported by the ceilometer will now be included as a cloud layer for processing. Testing of this multi-sensor algorithm is a very important part of the AWOS Performance Evaluation.
Problem ii. This oversensitivity to ice crystals has proven to be a limitation of the current sensor. Recently, a performance qualification of a number of sensors has been conducted in Toronto and indications are that new technology is available that solves this particular problem. The tendering process for the next procurement is underway, and if a new ceilometer is selected that is superior to the current one, the feasibility of retrofitting sites that have displayed this problem will be investigated.
Problems iii to vi These problems are inherent limitations of the technology of which users must be aware.

NOTES...
- -Additional information is available from the RA report. Summation opacities are carried for each reported layer, including -X. Note that the amount for -X in the case of a surface based layer less than 600 ft thick could easily be 10/10, something that would have been recorded as an obscured ceiling in a manned report. As mentioned above, processing is being beta tested to change the way AWOS reports -X.
- -Amounts are a time average of cloud passing over the sensor, not a snapshot of conditions.

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