What is the benefit of ceilometers for aerosol remote sensing? An answer from EARLINET

Abstract

Abstract. With the establishment of ceilometer networks by national weather services, a discussion commenced to which extent these simple backscatter lidars can be used for aerosol research. Though primarily designed for the detection of clouds it was shown that at least observations of the vertical structure of the boundary layer might be possible. However, an assessment of the potential of ceilometers for the quantitative retrieval of aerosol properties is still missing. In this paper we discuss different retrieval methods to derive the aerosol backscatter coefficient β p , with special focus on the calibration of the ceilometers. Different options based on forward and backward integration methods are compared with respect to their accuracy and applicability. It is shown that advanced lidar systems such as those being operated in the framework of the European Aerosol Research Lidar Network (EARLINET) are excellent tools for the calibration, and thus β p retrievals based on forward integration can readily be implemented and used for real-time applications. Furthermore, we discuss uncertainties introduced by incomplete overlap, the unknown lidar ratio, and water vapor absorption. The latter is relevant for the very large number of ceilometers operating in the spectral range around λ = 905-910 nm. The accuracy of the retrieved β p mainly depends on the accuracy of the calibration and the long-term stability of the ceilometer. Under favorable conditions, a relative error of β p on the order of 10 % seems feasible. In the case of water vapor absorption, corrections assuming a realistic water vapor distribution and laser spectrum are indispensable; otherwise errors on the order of 20 % could occur. From case studies it is shown that ceilometers can be used for the reliable detection of elevated aerosol layers below 5 km, and can contribute to the validation of chemistry transport models, e.g., the height of the boundary layer. However, the exploitation of ceilometer measurements is still in its infancy, so more studies are urgently needed to consolidate the present state of knowledge, which is based on a limited number of case studies.

Ceilometer aerosol profiling versus Raman lidar in the frame of the INTERACT campaign of ACTRIS

“…This effect is strongly related to the presence of a water vapor absorption band at 905-910 nm, while the effect of water vapor is limited at 1064 nm. This effect is considered in detail in Wiegner et al (2014), where the relative error affecting CC is estimated to be on the order of less than 20 %. For INTERACT, the correlation between the variability of the calibration constant and the variability of the integrated water vapor (IWV) content over the time was evaluated and is reported in Fig.…”

“…As clarified in Wiegner et al (2014), for the retrieval of aerosol properties, incomplete overlap is not a severe issue. At typical ceilometer wavelengths and correction functions, incomplete overlap generally contributes a few percent of the uncertainty in the full overlap region.…”

“…Indeed the use of the Rayleigh calibration technique, based on the normalization of a raw ceilometer signal (if available) on a molecular profile, is often challenging (Binietoglou et al, 2011;Wiegner et al, 2014). Moreover, when raw ceilometer signals are not available, the use of the outputs of manufacturer software can provide large discrepancies with respect to advanced or elastic lidar profiles.…”

“…Since the 1064 nm wavelength is not significantly influenced by water vapor absorption as it occurs for the 905-910 nm band, T 2 H20 (z) for MUSA at 1064 nm has been set to 1. Wiegner et al (2014) have shown that, if water vapor absorption is excluded, the uncertainty in the retrieved backscatter profile should be lower than 10 %. This accuracy depends on both the ceilometer type and the meteorological condition.…”

“…Compared to most of the lidar systems, ceilometers have the important advantage of being designed to be deployed unattended in the field for many years with minimal maintenance. Further information on how to discriminate a lidar, basic or advanced, from a ceilometer is provided in Wiegner et al (2014).…”