Monthly Archives: December 2018

Ceilometers vs MPL

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There is a decision to be made when deciding between these 2 forms of instrument when considering a monitoring program.

Firstly both Ceilometers and MPL ( Micro pulsed Lidar)  are  instruments based on the LIDAR principle.     MPL uses photon counting detectors,  while Ceilometers use avalanche photodiodes in analog mode.   Beyond that all sampling and data conversion are digital.

  • Cost

Ceilometers are significantly lower in cost than MPL  This is mainly due to the laser technology used.    The lasers in Ceilometers can last up to 6-8 years and the laser replacement cost is  relatively low.

  • Laser Pulse Source.

Most Ceilometers use  a low cost solid-state pulsed laser diode,  while the MPL  uses a frequency doubled NdYag laser.

  • Wavelength

Most Ceilometers emit in the range 905 to 920 nm,  which is not  visible and the launch pulses are reduced in energy so that the sensor is eye safe.   The MPL emits at 532 nm and is able to launch higher energy pulses meaning the range is greater,  or conversely the ability to detect low levels of scattering ( eg MIE )  at low levels is enhanced.   Potential users need to look at the differences between scattering properties of the target at the 2 wavelengths.

  • Polarisation

Ceilometers do not have dual polarisation channels and the discrimination capabilities of MPL may make them more suitable for the planned studies

  • Deployment Costs

Ceilometers are designed to be operated long term in a wide range of environments and are more suited to remote deployment where there is less maintenance available.

Ceilometers are widely used for PBL studies.   Because of increased range/sensitivity MPL is favoured in high cloud studies,  but high range ceilometers are available and might be selected if a multi sensor long term study is being undertaken.

 

Ceilometers for Cloud Amount

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A recent paper by Wagner and Kleiss compares a Ceilometer with Total Sky Imager and Micropulse lidar looking at suitability of ceilometers for  estimation of cloud amount.

https://journals.ametsoc.org/doi/pdf/10.1175/JTECH-D-15-0258.1

In summary they say,  in part:

“Ceilometers will be a mainstay of the operational automated weather observation network for years to come.

They are relatively inexpensive and require little ongoing maintenance, and are capable of 24-h observations.
While TSIs produce automated observations that are qualitatively more similar to human observations of sky cover than the ceilometer, ongoing operational
costs like mirror cleaning and reduced hours of operation limit this instrument’s applicability for deployment in unattended environments.

Two significant sources of error are associated with the automated sky cover observations obtained by single ceilometer ASOS installations: the spot view of the instrument renders it unable to see the entire sky, while the 3660-m height limit renders the instrument incapable of observing high clouds. These errors are not insignificant, and their magnitudes vary depending on the actual cloud coverage. The spatiotemporal averaging error is smallest for clear and overcast conditions as the sky exhibits little variability in these conditions. The high cloud error is at its smallest when skies are dominated by low clouds, and it tends to increase as low cloud coverage lessens, allowing high clouds to peek through.”

The ability to reliably detect upper atmosphere cloud is restricted to a subset of available ceilometers which have ranges in excess of the usual 7,500 to 10,000 metre range. Such ceilometers include the a CL51, the 8600-CHS and the CHM15K