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8329-A/B Laser Ceilometer
- TYPE...Qualimetrics 8329-A/B
- 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
- from 510 to 990 meters.................10
- from 990 to 1515 meters...............15 m
- from 1515 to 3840 meters.............30 m
- 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
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
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
- -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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