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.
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The Cloud Appreciation Society has some great cloud Photos and covers a lot of information about clouds from all over the world.
Cloud Appreciation Society
Here is one of the really good photos .
Cloud Appreciation Society : A classic Cumulonimbus with an arcus feature on the leading edge, spotted near Pipestone, Minnesota, US. © Edward Carberry
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.
There are 2 types of calibration.
- Calibrating the distance measurement: In this case the ceilometer is turned on its side and aimed at a hard target. In this case the ceilometer is aimed at a tree 4450 ft away. The exact distance of the tree was surveyed using Google Earth. The ceilometer was aimed using a telescopic sight. The backscatter profile is an almost perfect replica of the laser pulse, delayed by the time taken for the laser pulse to go to the target and back.
In reality, the calibration of the ceilometer is based on the well known speed of light and if the timing crystal inside the ceilometer is accurate and stable, the distance calibration is stable and accurate and should never need to be checked in the service life of the ceilometer.
Clouds are not solid reflectors, and the backscatter comes from a range of scatterers inside the cloud, so the backscattered laser pulse is broadened and flattened. The height of the cloud is defined as a threshold in the backscatter profile which has been determined will result in a correct reading for most types of cloud.
2. Calibrating the ceilometer constant. For this , there is a clever method as described in a paper by O’connor et al, which utilises the known Lidar Ratio of 18.8 in stratocumulus cloud ( SC).
A technique for autocalibration of cloud lidar
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