Lufft CHM 15k Nimbus, Manual

Page: 65 / 101

Data Evaluation/ Sky Condition Algorithm (SCA)
Revision R09, November 2016 65
9 Data Evaluation/ Sky Condition Algorithm (SCA)
The ceilometer CHM 15k is a laser remote sensing instrument with a build in sky
condition algorithm (SCA) generating cloud ceilings, cloud penetration depths/
thickness values. Moreover, the cloud cover (cloud amount in eights) for the
total coverage in any altitude and for the base layer is determined and the verti-
cal visibility is delivered. An aerosol layer algorithm based on wavelet algorithm
detects different aerosol layer heights. Haze and precipitation type output is
encountered and transmitted within the sky condition index parameter.
This chapter does not contain the mathematical description of our SCA
algorithm. You may ask for a more complete description.
9.1 Laser remote sensing
One near infrared laser probes the sky vertically from top of the instrument up to
15 km altitude. Targets like aerosol layers and clouds show up as echoes with
certain backscatter intensity and signal extinction. Molecular absorption as well
as Rayleigh scattering by air molecules is negligible at a laser wavelength of
1064 nm. The distance from ground is calculated from the travelling time of the
laser pulses.
9.2 Data preprocessing and averaging
Data pre-processing is a major task before the different steps in SCA algorithm
starts. The major reason to do it is to harmonize / normalize the data sets
between different CHM 15k systems to get similar results, e.g. for cloud base
heights, even if the sensitivity between instruments varies.
In figure 23 two profiles of a reference unit and a test unit are illustrated after
the normalization process.
The following formula is used to receive the normalized backscattered power:
with, P
raw
as raw backscatter profile (photon counts), b baseline, p
calc
: average
test pulse intensity, O(r) overlap function, c
s
: calibration constant and P(r) is the
Fig. 23: Normalized signals P(r) for reference unit (blue) and a test unit (red). A
horizontal path is used with a hard target in 9.4 km distance for this
method. At 16 km distance a test pulses is added to the signal.
P r
 
P
raw
r
 
b
–
c
s
O r
 
p
calc
 
-------------------------------------- =