An empirical method to correct for temperature-dependent variations in the overlap function of CHM15k ceilometers
Imperfections in a lidar's overlap function lead to artefacts in the background, range and overlap-corrected lidar signals. These artefacts can erroneously be interpreted as an aerosol gradient or, in extreme cases, as a cloud base leading to false cloud detection. A correct specification of th...
| Main Authors: | , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2016-07-01
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| Series: | Atmospheric Measurement Techniques |
| Online Access: | http://www.atmos-meas-tech.net/9/2947/2016/amt-9-2947-2016.pdf |
| Summary: | Imperfections in a lidar's overlap function lead to artefacts in the
background, range and overlap-corrected lidar signals. These artefacts can
erroneously be interpreted as an aerosol gradient or, in extreme cases, as a
cloud base leading to false cloud detection. A correct specification of the
overlap function is hence crucial in the use of automatic elastic lidars
(ceilometers) for the detection of the planetary boundary layer or of low
cloud.
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In this study, an algorithm is presented to correct such artefacts. It is
based on the assumption of a homogeneous boundary layer and a correct
specification of the overlap function down to a minimum range, which must be
situated within the boundary layer. The strength of the algorithm lies in a
sophisticated quality-check scheme which allows the reliable identification
of favourable atmospheric conditions. The algorithm was applied to 2 years of
data from a CHM15k ceilometer from the company Lufft. Backscatter signals
corrected for background, range and overlap were compared using the overlap
function provided by the manufacturer and the one corrected with the
presented algorithm. Differences between corrected and uncorrected signals
reached up to 45 % in the first 300 m above ground.
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The amplitude of the correction turned out to be temperature dependent and
was larger for higher temperatures. A linear model of the correction as a
function of the instrument's internal temperature was derived from the
experimental data. Case studies and a statistical analysis of the strongest
gradient derived from corrected signals reveal that the temperature model is
capable of a high-quality correction of overlap artefacts, in particular those
due to diurnal variations. The presented correction method has the
potential to significantly improve the detection of the boundary layer with
gradient-based methods because it removes false candidates and hence
simplifies the attribution of the detected gradients to the planetary
boundary layer. A particularly significant benefit can be expected for the
detection of shallow stable layers typical of night-time situations.
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The algorithm is completely automatic and does not require any on-site
intervention but requires the definition of an adequate instrument-specific
configuration. It is therefore suited for use in large ceilometer
networks. |
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| ISSN: | 1867-1381 1867-8548 |