Operating Manual CHM 15k
R13 / 07-2019
Data Evaluation / Sky Condition Algorithm (SCA)
59
9.8 Precipitation and fog
Fog and various types of precipitation are detected by multiple scattering. Typically, only single scattering
processes are considered as the signal source. Fog and high particle density correspondingly generate
a stronger signal than usual close to the device. An integral over the signal in certain ranges is used to
evaluate fog and precipitation.
9.9 Aerosol and mixing layer height
Aerosols that are detected near the ground spread in the lower air layer, where the upper limit can be
defined as a planetary boundary layer (onshore) and maritime boundary layer (offshore). The lowest
aerosol layer that can be identified within the boundary layer can interpreted as mixing layer height (MXL).
The MXL like all aerosol layer structures in the boundary layer depends on atmospheric conditions and
on sunny days mostly on time of day.
Aerosol layer heights can be identified by analyzing gradient signatures in the backscatter signal. The
quality of the detected aerosol layers depends very much on the local conditions and time. Table 18
shows an index describing the quality of the identified aerosol layers in terms of high accuracy and low
uncertainty.
Q-Index
Description
/ (telegram)
-1 (NetCDF)
There is insufficient raw data for a calculation
- (telegram)
-2 /NetCDF)
Hardware error or system is not ready to measure
(telegram)
-3 (NetCDF)
Algorithm cannot detect values
0
No particle layer detected
(Index was not calculated in older firmware versions)
1
Particle layer detected with high accuracy (< 50 m)
9
Particle layer detected, but with low accuracy
Table 18 Q-Index description of aerosol layer height.
9.10 Cloud cover (BCC / TCC)
The cloud coverage is determined statistically from the behavior of the lower cloud bases over time. Here,
a distinction is made between the coverage of the lowest cloud layer (BCC: base cloud cover) and the
total of all cloud layers (TCC: total cloud cover). The values for these parameters are also stored in the
NetCDF files.
The time interval considered is height-dependent (Figure 29). The frequency of the occurring cloud layers
is determined for each height interval. This histogram is smoothed with a height-dependent weighting
function. Within this smoothed frequency distribution, the peaks are separated. All cloud bases within a
peak are bundled into a cloud layer.
Sections that contain cloud bases are counted against the total number of cone sections. The cloud cover
values are expressed as a percentage from this comparison. The final degree of coverage is reported in
eighths. Table 19 lists the WMO code 2700 for the cloud cover index.
Summary of Contents for CHM 15k
Page 1: ...General Information 1 Titelseite...
Page 73: ...Index of Tables 73...
