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Ceilometer

Ceilometer CHM15k determines up to three layers the cloud base and penetration. In addition, the temporal variability of these parameters and a vertical perspective were determined. 

All this data were determinded from the uncalibrated signal of a vertically aligned lidar. The spatial resolution amounts to 15 m, the time can be selected freely, normally we use 1 minute. 

The ceilometer determines the cloud base edges in a frequency range close to the visible one, so that it achieves the same result as it would have a human observer. The cross-section of backscattered cloud droplets are proportional to their surface. In contrast, the backscatter cross-section at the frequencies of (cloud-)radars is proportional to the diameter d6. Therefore a few large droplets suffice to feign a lower cloud base to the radar.

The graph below shows a day´s measurement, which was executed in Massada, near to the dead sea in isreal. The coloured code corresponds the 10-times of 10th-logarithm of backscattered energie. White points show the cloud base edges.

 

Fig. 1: Ceilometer during transport Fig. 2: Measurement of Ceilometer

Technische Parameter:

 

Range: 30 m to 15000 m (realistic minimum level rather at 150 m or 300 m)
Spatial resolution: 15 m
Test time: 5 s to 60 min, mostly 60 s
Measure: Backscatter, deduced: Cloud heights up to 3 layers, penetration/cloud thickness, vertical visibility
Measurement principle: Lidar
Wavelenght: 1064 nm
Spread: 0,1 nm
Pulse lenght: ca 1 ns
Pulse rate: 5-7 kHz
Beam diameter, widened: 90 mm
Laser divergency: 100 μrad
Energy per pulse: 8 μJ
Maximal power consumption: 0,8 kW