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.
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.
Click to access arm_09.pdf
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
High Altitude clouds fall into 2 categories,
- Those with a low base and vertical development are Cumulonimbus and Towering Cumulus. At the base these are water clouds, so the ceilometer only “sees” a few hundred feet into the cloud.
- Those with a high base, including Cirrus, Cirrostratus and Cirrocumulus and Altostratus
The Cirrus cloud family are composed of Ice Crystals, and are very often “optically thin”and they have low backscatter coefficients, so are difficult to detect with ceilometers, because the laser pulse energy is limited to eye safe levels.
Altostratus may be composed of ice crystals. In some ice crystal altostratus, very thin, rapidly disappearing horizontal sheets of water droplets appear at random. The sizes of the ice crystals in the cloud tended to increase as altitude decreased. However, close to the bottom of the cloud, the particles decreased in size again
Altostratus cloud with a water phase may have a strong backscatter signal and can be picked up as in the case below
Alto stratus Cloud at 17,000 ft
Ref 1 : Wikipedia : Alto Stratus entry.
Ref 2. Wikipedia Cirrus Cloud entry.
All ceilometers which are set up for long range cloud height measurement are “far sighted”, having a blind region in front of the unit. This is shown in the diagram below , and the height of the blind spot Rio is heavily dependent on the axial separation d , the beam divergence and the telescope angle of acceptance. The signal is maximised at the full overlap distance Rovf as shown below.
Since most ceilometers are designed for the best acheivable signal to noise ratio, the telescope angle of acceptance is set to the limit of focal length, sensor active area and lens aberration.
The single lens designs, such as the CL51 and 8200-CHS feature a low value of d and thus a much reduced overlap height
Single lens overlap geometry
There are a number of different optical arrangements to enable the reduction of d to zero or to a small value to minimise the overlap height.
One form of “single lens” Ceilometer, using a “split lens ” approach (reference (Vande Hey, J. ; Coupland, J. ; Richards, J. ; Sandford, A. )
It is worth noting that earlier designs of dual lens ceilometers actually utilised the blind spot to reduce the required dynamic range to prevent overload of the return signal processing channel, and greatly reduce optical crosstalk in the instrument itself ( known as To crosstalk)
Later ceilometers using the single lens optics, such as the MTECH SYSTEMS 8200-CHS feature special techniques to minimise optical crosstalk and very high dynamic range analog to digital converters to enable detection of fog close to the ground without saturation of the signal
A slight tilting of the ceilometer gives better performance in rain.
Rain drops tend to flatten as they fall. See the NASA explanation for this
Consequently, when aligned vertically the backscatter from raindrops may be sufficiently high to cause difficult in resolving the cloud base above the rain.
However when tilted, the backscatter of laser pulses by the raindrops is reduced .
In heavy rain even a tilted ceilometer cannot resolve the cloud base since the integrated backscatter quickly dominates and prevents further penetration up to the cloudbase, while extinction in the return path also starts to extinguish the return signal..
Please refer to the screen below.
These tests were carried out during light rain. The tilted unit shows resolvable cloud base while the untilted unit reverts to vertical visibility.
Stratus clouds, irrespective of altitude have a more or less constant and solid base , indicating stable layers in the atmosphere and lack of thermal activity.
By contrast the Cumulo Nimbus cloud betrays a high degree of vertical motion of the atmosphere and thermal activity. Cumulus congestus and Towering cumulus display a moderate degree of vertical extent
In the curtain plot below, the classic rapid change of cloud level characteristic of Cb cloud can be seen between 19.00 and 23.00 hours. After that time the cloud base solidifies and rises to 6500 ft
Cb cloud gives way to Ac cloud
There are a great many factors effecting the performance of ceilometers, but the key 2 issues for any given cloud volume backscatter coefficient are:
- The eye safe limit of the ceilometer . Infra Red Ceilometers must operate as Class 1M lasers which limits the energy density of the beam.
- The noise level. The inherent noise level of the ceilometer is the ultimate determinant of the signal to noise ratio which enables the ceilometer to discriminate cloud boundary.
The 8200-CHS has an extremely low level of inherent noise, which is tested for each ceilometer and is recorded as per the backscatter profile below.
The external source of noise is the shot noise of the scattered and or direct solar radiation within the spectral acceptance of the sensor . The laser operates around 910 nm and the filtering can only limit the “out of band” component of the solar noise spectrum. The ceilometer will thus t\detect cloud at higher altitudes at night.
The 8200-CHS has the capability of detecting up to 4 simultaneous layers but certain types of standard messages display only 3 layers .
If the lowest layer is optically dense, the returns from beyond the first layer are extinguished , and the layers above it are not detected until a gap in the lower layer is detected. If the cloud is not optically dense, the return from a higher level cloud is not significantly attenuated when passing back through the lower layer, so both layers can be detected.
Here is a case where multiple layers are at 6620 and 7290 are both detected.
Clouds at 6620 and 7290 ft