Ceilometers vs MPL

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.

 

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Ceilometers for Cloud Amount

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

 

 

ToPROF: Ceilometers and Lidar Applications

 As part of the EU “COST” programmes,  the ToPROF group is working on  operational ground based profiling with ceilometers, doppler lidars and microwave radiometers for improving weather forecasts.
Work started in 2014,  and is progressing towards a nominal completion date in 2020.
For overall details refer to the ToPROF Home Page
Within the To PROF group,  a Ceilometer Working group has been established.
Here are the details of this working group  This group would be of interest to  those within the following End-Users
  • National weather services (incl. COPERNICUS/MACC): calibrated attenuated
    backscatter profiles to evaluate NWP models (through forward operators); Cloud
    base height for NWP evaluation and weather monitoring.
  • Agencies in charge of atmospheric surveillance for air traffic: occurrence, height and  mass concentrations of ash layers; diagnostic and short-term forecast of fog and other low visibility events.
  • Agencies in charge of Air Quality monitoring: boundary layer height; freetropospheric aerosol transport.
  • Networks in charge of GHG monitoring: boundary layer height to quantify GHG
    dilution effects.
  • EUMETSAT: European-wide validation of cloud-base height and fog
  • Renewable energy industry: Photovoltaic ReN – cloud/fog fraction and evolution for nowcasting applications (combined with geostationary satellite); Concentrated solar power: aerosol vertical distribution; Wind ReN – wind profiles from Doppler Lidars.

Ceilometer Network detects Dust Plume

Title: Dust storm from Syria- a potential new aerosol source in the E. Mediterranean- A ceilometer and synoptic study
Authors: Alpert, Pinhas; Egert, Smadar; Uzan, Leenes

Abstract

On 7 Sep 2015 an unprecedented huge dust plume approached the SE Mediterranean basin from the northeast- Syria region. According to the Israeli meteorological service it is the first time in 75 years of measurements, that a dust storm reaches Israel early September, lasts several days and dust concentrations reach values 100 times the normal (1700µg/m3). Dust storms are normally monitored in the east Mediterranean using satellites and surface PM data. Obviously, these cannot show the vertical evolution of the dust including penetration, sinking and cleaning since vertical profiles are not available. High-resolution, micro Lidar Ceilometer network is gradually established in Israel. A few instruments of this network were already operational during the dust storm. The most crucial vertical information, monitored by these Ceilometers with 10m resolution vertically, every 16s, is analyzed. The difference in the cloud-layers allow the investigation of the high altitude of 1000m dust penetration, its sinking into the complex structured 250-500m mixed layer and the gradual 3D cleaning. This finding contradicts the conventional understanding that cleaning is due to gradual descent and shows not only the vertical fluctuation during the entire event but also the vertical rise to 2000m at the end of the event. The vertical information showed that the actual event period duration was 7 days, compared to only 90 hours based on traditional detectors. Is it a new dust source in the E. Mediterranean-long and short term trends?

International Commission on Clouds and Precipitation (ICCP)

The International Commission on Clouds and Precipitation (ICCP) is a Commission of the International Association of Meteorology and Atmospheric Sciences (IAMAS)
The IAMAS is one of the associations of the International Union of Geodesy and Geophysics (IUGG)

The ICCP holds a conference every 4 years.   The last conference was at Manchester University in 2016, The next is due in 2020.

Typical subjects in calls for papers are theoretical, observational and numerical modelling studies of cloud and precipitation physics, cloud chemistry and cloud dynamics.  

For instance the following subjects are commonly covered at the conferences

  • Basic cloud and precipitation physics
  • Warm boundary layer clouds
  • Convective clouds (including cloud electrification)
  • Mixed phase clouds (including Arctic/Antarctic stratus, mid-level clouds)
  • Cirrus clouds
  • Orographic clouds
  • Fog and fog layers
  • Mesoscale cloud systems (including severe storms)
  • Tropical clouds
  • Southern Ocean clouds
  • Polar stratospheric clouds and noctilucent clouds
  • Aerosol-cloud-precipitation-interactions and processing
  • Clouds and climate (including radiative properties of clouds)
  • Ice nuclei and cloud condensation nuclei
  • Cloud and precipitation chemistry
  • Measurement techniques (of cloud and precipitation properties) and uncertainties
  • Applications of cloud and precipitation physics

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

8400-CHS    http://www.ceilometers.com/