An empirical method to correct for temperature-dependent variations in the overlap function of CHM15k ceilometers

Abstract

Abstract. 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.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.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 gradientbased 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.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.

Mixing layer height as an indicator for urban air quality?

“…From summer observation over 5 years in Paris, France, Pal and Haeffelin (2015) found larger values (H ml = 1.95 ± 0.38 km), whereas maxima less than 0.8 km were observed during 2 years at Vancouver, Canada (van der Kamp and McKendry, 2010), and Santiago, Chile (Muñoz and Undurraga, 2010). The mean H ml,c at night is in the range of 0.2 km, underlining the need of ceilometers with a very low overlap (or a reliable overlap correction function; see, e.g., Hervo et al (2016) for a CHM15k ceilometer) for investigations of the mixing layer. The most prominent differences between BL-VIEW and COBOLT are the larger H ml,v during the night and the rapid changes of H ml,v around noon.…”

“…The output signals are range corrected consistently for the whole measurement range; i.e., the "H2on" parameter was set to 1 as discussed by Kotthaus et al (2016). To improve the detection of aerosol layers close to the ground, an additional overlap correction function, similar to a concept outlined by Hervo et al (2016), was applied.…”

PathfinderTURB: an automatic boundary layer algorithm. Development, validation and application to study the impact on in situ measurements at the Jungfraujoch

“…The retrieval method STRAT+ (Pal et al, 2013), based on STRAT2D, uses the Canny edge detection applied to gradient profiles along with the information brought by the variance profiles and by the radiosoundings to detect the main BL height and internal boundaries as well as the growth rate. Hervo et al, 2016). The standard instrument output is the background-, rangeand overlap-corrected, normalized signal S defined at range r and time t as follows:…”

“…At both sites, PAY and KSE, the overlap function O CHM15k has been corrected for temperature variations following Hervo et al (2016).…”

Recommendations for processing atmospheric attenuated backscatter profiles from Vaisala CL31 ceilometers

“…Recent research shows, for example, that the overlap function of a Lufft CHM15K is slightly temperature dependent (Hervo et al, 2016). Due to the co-axial beam design, the full optical overlap for the CL31 is reached at low ranges , which can be beneficial when studying meteorological processes in the lowest part of the atmosphere, such as fog, haze, or aerosols emitted at the Earth's surface.…”