Lufft CHM 15k, User Manual

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Operating Manual CHM 15k R13 / 07-2019
Data Evaluation / Sky Condition Algorithm (SCA) 57
Figure 26 Example of time averaging for determining cloud heights.
9.3 Cloud ceilings / cloud base heights
After successful preprocessing, the averaged backscatter profile is used to identify cloud echoes, rain, fog
and aerosol layers and to distinguish between these events.
Figure 27
shows a day intensity plot where all significant backscatter signals are color coded in black.
The SCA algorithm now identifies precipitation events and aerosol structures and then calculates the cloud
heights and penetration depths.
Figure 27 Cloud detection algorithm.
9.4 Cloud penetration depths
Cloud penetration depth is determined by detecting a cloud bottom and then an upper cloud height using
the signal intensity level that was determined for the cloud base. Subtracting these values results in the
cloud penetration depth.
A threshold and gradient method is used to check the region of the identified values, in order to determine
penetration depth uncertainty. Figure 28 shows how the cloud parameter evaluation process is
performed.
It should be noted that the above-mentioned upper cloud height is generally not the highest point of the
cloud. The penetration depth and the cloud top are similar only when the ceilometer detects another cloud
or aerosol layer above it. In most cases, the laser light in the cloud is scattered and highly attenuated and
the cloud cover can no longer be identified above the cloud.