Lufft CHM 8k, Руководство пользователя, Страница: 57 / 72

Operating Manual CHM 8k
R1.7 / 01-2020
Data Evaluation / Sky Condition Algorithm (SCA) 57
9
Data Evaluation / Sky Condition Algorithm (SCA)
The CHM 8k ceilometer is a laser remote sensing device with embedded algorithm for determining layers
of particles and droplets in the atmosphere. The embedded algorithm is collectively referred to as the Sky
Condition Algorithm (SCA). Ceilometers determine the cloud base and provide information on the
penetration depth into cloud. In case another cloud or aerosol layer can be measured above the lower
cloud the penetration depth can be interpreted as cloud thickness. In addition, the degree of cloud
coverage is determined in terms of eighths of the sky. For visibilities below 2 km the vertical visibility
(VOR) is calculated and output in addition. An aerosol algorithm based on a wavelet algorithm detects
different aerosol layers and transmits those detected within the atmospheric boundary layer. Fog / haze
and precipitation are detected and transmitted in the Sky Condition Index (SCI) parameter.
9.1 Laser remote sensing
A pulsed near infrared laser probes the sky vertically from the top of the instrument up to 10 km altitude.
Targets like aerosol layers and clouds show up as echoes with certain backscatter intensity and signal
extinction. Rayleigh scattering by air molecules is negligible at a laser wavelength of 905 nm. The
distance of the scattering particles to the instrument is calculated from the travelling time of the laser
pulses.
9.2 Preparation of the measured data
Data pre-processing is an important task before the different steps in the SCA algorithm begin. The main
reason for this is to harmonize / normalize the data sets between different
CHM 8k systems to get similar
results, e.g. for cloud bases, even if the sensitivity between instruments varies.
Each single measurement is normalized with the output power
I of the laser, which is measured with the
aid of photodiodes. Also, the detection sensitivity is considered by measuring a reference light pulse on
the detector and using the signal strength p
cal
for normalization. Brightness fluctuations of the reference
light pulse are corrected by an additional photodiode measurement p
ref
. Differences between different
CHM 8k instruments are compensated by a scaling factor c
s
, which is determined using a comparison
measurement with a reference device. Figure 24 shows the profiles of two different devices after
normalization and calibration. A cloud is visible in the signal at a height of approximately 2.5 km.
Figure 24 Normalized and calibrated backscatter signals of two different CHM 8k instruments.
The following formula is used to obtain the normalized backscatter intensity:
𝑃(𝑟) =
𝑃
𝑟𝑎𝑤
(𝑟) − 𝑏
𝑐
𝑠
∙ 𝑂(𝑟) ∙ 𝐼
∙
𝑝
𝑟𝑒𝑓
𝑝
𝑐𝑎𝑙
Here,
P
raw
corresponds to the raw backscatter signal, b to the baseline and O(r) is the overlap function.
The backscatter values in the NetCDF files are stated as attenuated backscatter
𝛽
𝑎𝑡𝑡
= 𝑃(𝑟) ⋅ 𝑟
2
⋅ 𝑐
𝑐𝑎𝑙
in
𝑚
−1
⋅ 𝑠𝑟
−1
. The attenuated backscatter is calculated from the normalized signal P(r) through
multiplication by r
2
and a calibration constant c
cal
.