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|
|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|
|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|