Lufft CHM 15k Nimbus, Manual

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Data Evaluation/ Sky Condition Algorithm (SCA)
Revision R09, November 2016 67
9.4 Cloud ceilings/ cloud base heights
After successful pre-processing the averaged backscatter profile is used to iden-
tify cloud echoes, rain, fog, aerosol layers and discriminate these events from
each other.
Figure 25 shows a daily color coded intensity plot, where all layers with a signifi-
cant backscatter value, marked black.
The algorithm identifies rain and aerosol in these structures and calculates in a
next step cloud bases and penetration depths.
9.5 Cloud Penetration Depth
A cloud penetration depth is established by identifying a cloud base and after-
wards an upper cloud level using the signal intensity level that was determined
for the cloud base. The subtraction of these values result in the cloud penetra-
tion depth. A threshold and gradient method is used to check the surrounding
area of the identified values to determine an uncertainty of the penetration
depth. Figure 26 shows how the evaluation process for the cloud parameters is
done.
Please take into account that the upper cloud level mentioned here is in general
not the top of the cloud. The penetration depth and the cloud top are only simi-
lar in case the ceilometer detects another cloud layer of hard target above it. In
most cases the laser light is scattered in the cloud and strongly attenuated. The
cloud top can not be identified anymore.
Fig. 25: Cloud detection algorithm.