Author Archives: Mike

Ceilometers. Who supplies them ?

There are very few Ceilometer manufacturers in the world.   Ceilometers have advanced technical requirements and the cost of development is  high.   In the production phase ceilometers are assembled from  many special optical and electronic parts.   During testing they require specialised pulsed laser power meters,  spectrophotometers and advanced electronic test equipment, together with a cloudy climate to enable regular testing and continuous product improvement.

Sensor Range Class

  • 12500 ft range, ( 3800m)  an example of which was the original CT12K.  This range is now encompassed by the 25,000 ft range sensors.
  • 25,000 ft range ( 7600m) These are the main sensors on the market since  in the main application there is little operational need to go beyond even 12500 ft.  These sensors also find application in Planetary Boundary Layer ( PBL) studies.
  •  50,000 ft  range (15,200m) Special instruments that find more application in volcanic ash warning in aviation and upper atmosphere studies in atmospheric science.   Although in theory only requiring a modest increase in signal to noise ratio,  the cloud species above 20,000 ft are most often comprised of ice crystals and have much lower volume back-scatter coefficients than water cloud so the reliable detection of thin layers of cirrus cloud becomes very difficult while maintaining the laser eye safety mandate.  Ceilometers in this range generally achieve the necessary signal to noise ratio improvement by a range of techniques including increasing laser pulse energy ( while still remaining eye safe ), using a different laser wavelength,  and or reducing the telescope field of view and laser beam divergence.

25,000 ft range for Aviation and PBL studies

CL31     :  http://www.vaisala.com/en/products/ceilometers/Pages/default.aspx

8200-CHS :   http://www.ceilometers.com/

CS135   :   https://www.campbellsci.com.au/cs135

50000 ft Range for Atmospheric Science

CL51     : http://www.vaisala.com/en/products/ceilometers/Pages/default.aspx

8500-CHS    http://www.ceilometers.com/

 

 

Ceilometers and Snow

Like Rain, Snow produces quite high levels of backscatter.   But the raindrops and snow flakes have very different shapes, velocities and surface area to mass ratios.  More expensive ceilometers may have the ability to discriminate between snow and rain.

A typical LIDAR curtain plot for cloud appears below:

snow plot

Ref: University of Utah Atmospheric Science

The snow is coming from a cloud at around 500m .  The cloud and snow appear to extinguish the returns from higher layers,  if any.   Some of the snow is light and evaporates before it gets to ground level.  ( low level green return )

Work has been done to try to determine snowfall rate from Lidar returns.  According to Ed Eloranta of the University of Wisconsin Madison,  the technique requires radar and  does not require any knowledge of the snowflake shape.

Download Poster

http://lidar.ssec.wisc.edu/papers/conferences/arm2009/arm_09.pdf

 

 

Ceilometer Siting

 

In the past ICAO Annex 3 SARPS recommended that a ceilometer be placed at the middle marker site 900-1200 m from the touch down zone for instrumented runways .

This had an advantage that power and comms were generally  already established at that site or were planned at that site.

This gave a reading of cloud base height at a crucial decision height on the glidepath

With addition of ILS and co-located DME, more and more aerodromes have no middle marker. The piece of land located 900 to 1200 m from the landing threshold may be outside the aerodrome airfield and a ceilometer installation may be impracticable, or very costly.

As a consequence in more and more cases an alternative location must be found.

A typical recommendation for siting the ceilometer would be :

“When instrumented systems are used for the measurement of the cloud amount and the height of cloud base, representative observations should be obtained by the use of sensors appropriately sited. For local routine and special reports, in the case of aerodromes with precision approach runways, sensors for cloud amount and height of cloud base should be sited to give the best practicable indications of the height of cloud base and cloud amount at the runway threshold in use. For that purpose, a sensor should be installed at a distance less than 500 m from the threshold. This distance can be extended up to 900-1200 m from the landing threshold in the axis of the approach end of the runway”

http://www.icao.int/safety/meteorology/amofsg/AMOFSG%20Meeting%20Material/AMOFSG.10.SN.012.5.en.pdf

The user should refer to the latest edition of Annex 3 to ensure compliance.

Ceilometers for Dust Storm Profiling

Sydney Dust Storm 2009

A wall of dust stretched from northern Queensland to the southern tip of eastern Australia on the morning of September 23, 2009, The storm, the worst in 70 years, led to cancelled or delayed flights, traffic problems, and health issues,  The concentration of particles in the air reached 15,000 micrograms per cubic meter in New South Wales during the storm, A normal day sees a particle concentration 10-20 micrograms per cubic meter.

Work on the use of Ceilometers for analysis of  that  Dust Storm is decribed in the paper:

Laser ceilometer measurements of Australian dust storm highlight need for reassessment of atmospheric dust plume loads      By Hamish McGowan and Joshua Soderholm

Among the more interesting information in this paper was the curtain plot showing the increase  in backscatter when the wall of the duststorm hit,  the very high concentration around ground level and the vertical extent of the dust.  The maximum vertical extent of this plot is 1500 metres ,  or approx 5000 ft.

dust-storm

(Curtain Plot showing onset of the Dust Storm and estimated particle Concentration from paper: Laser ceilometer measurements of Australian dust storm highlight need for reassessment of atmospheric dust plume loads      By Hamish McGowan and Joshua Soderholm )

Ceilometers like the 8200-CHS are suitable for this type of work,  where dust storms are experienced regularly,  such as the Harmattan in sub saharan Africa,  the Churgui in Morocco,  the Khamasin in Egypt, the Shamal in Iraq or the Kali Andhi in India