Lufft CHM 8k, Руководство пользователя, Страница: 58 / 72

Operating Manual CHM 8k
R1.7 / 01-2020
Data Evaluation / Sky Condition Algorithm (SCA) 58
A further processing step is performed to determine cloud heights and aerosol layers. To compensate for
the reducing signal-to-noise ratio at higher altitudes, the signal is averaged with a height-depending
averaging time as shown in Figure 25. At different altitudes, the time averaging varies from 15 seconds
below 3 km to 300 seconds above 6 km.
Figure 25 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 26
shows a day intensity plot
where all significant backscatter signals are coded in black.
The SCA algorithm now identifies precipitation events and aerosol structures and then calculates the
cloud heights and penetration depths.
Figure 26 Cloud detection algorithm.
9.4 Cloud penetration depths
Cloud penetration depth is determined by detecting a cloud base and then an upper cloud height using
the signal intensity level that was determined for the cloud base. Subtracting these values gives 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 27 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 cover are similar only if the ceilometer detects another cloud
layer with a firm target 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.