Detection of embedded contrails in airborne lidar measurements

Abstract. Aviation affects the Earth's energy balance through the emission CO₂ and non-CO₂ effects. Contrails mark one of the latter and can occur inside the cirrus clouds where they might affect the clouds' optical and microphysical characteristics as well as their climate impact. In this study, airborne lidar observations with the German research aircraft HALO during the ML-CIRRUS and CIRRUS-HL campaigns are used together with aircraft-location data to detect the occurrence of contrails that have formed within already existing cirrus clouds. Based on manual analysis, we developed (based on ML-CIRRUS) and verified (based on CIRRUS-HL) an automated two-step method for detecting embedded contrails in lidar measurements. In the first, threshold-based step, potential embedded contrail regions are identified by particle backscatter coefficients (β(λ)) larger than 4 Mm⁻¹sr⁻¹ and particle linear depolarization ratios (δ(λ)) smaller than 30 % or 43 % depending on the impact of pollution on the background cloud. The second step assesses the area of the identified objects in a lidar curtain for finding cases that could realistically be associated with an aircraft-related perturbation. Specifically, areas smaller than 10 pixels are dismissed as noisy data, while areas larger than 50 pixels are too homogeneous to be in line with the assumptions of the manual analysis that cloud regions that are perturbed by the passage of an aircraft occur in close vicinity to unperturbed cloud areas. The resulting contrail mask enables the detection and quantification of the occurrence rate of embedded contrails in airborne lidar measurements without the need for auxiliary air-traffic information.