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