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.
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.
Most Ceilometers use a low cost solid-state pulsed laser diode, while the MPL uses a frequency doubled NdYag laser.
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.
Ceilometers do not have dual polarisation channels and the discrimination capabilities of MPL may make them more suitable for the planned studies
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.
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.
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
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.
Within the To PROF group, a Ceilometer Working group has been established.
- 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
- 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.
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
The WMO has released a new Cloud Atlas. The release was timed to coincide with the World Meteorological day. 23/3/2017
Here is the Press Release
Here is the Home page for the new Cloud Atlas.
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:
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.
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.
(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