Ceilometer

Ceilometer during transport
Fig. 1: Ceilometer during transport (at the trailer).
Ceilometer measurements (2014-12-03)
Fig. 2: Ceilometer measurements form December 3rd, 2014 during the DESERVE campaign near Massada (Dead Sea).

The ceilometer CHM15k determines cloud base heights of up to three cloud layers as well as the depth of penetration. Additionally, the temporal variability of these quantities as well as the vertical range is determined.

Ceilometer are lidars which emit light vertically and detect the signal returned from backscatter at water drops in clouds. The quantities are determined from the uncalibrated signal. The vertical resolution is 15 m, the temporal resolution is flexible. Typically a temporal resolution of 1 min is applied.

The ceilometer determines the cloud base heights in a frequency range close to the visible frequency range. The visibility of the ceilometer is similar to the visibility a human eye would achieve. The backscatter cross section of a cloud drop is proportional to its area. In contrast, the backscatter cross section of a cloud radar is proportional to the diameter to the power of six. Hence, a few large drops are sufficient to pretend a low cloud base height to a cloud radar.

Figure 2 shows the ceilometer measurements from December 3rd, 2014 during the DESERVE campaign close to Massada at the Dead Sea. The colourbar describes 10 times the 10 logarithm of the backscattered energy. White dots indicate cloud base boundaries.