Validation of cloud macrophysical properties from the ATLID Level 2a products A-TC, A-FM, A-CTH using airborne lidar observations during the HALO missions PERCUSION, ASCCI and NAWDIC

The Earth Cloud, Aerosol, and Radiation Explors (EarthCARE) satellite mission aims to improve our understanding of cloud–aerosol–radiation interactions in the Earth's climate system. Its Atmospheric Lidar (ATLID) instrument aboard EarthCARE provides important observational data for determining the properties of clouds and aerosols; therefore, thorough validation of ATLID products is a prerequisite for their reliable scientific use. This study evaluates the reliability of selected parameters applicable to study cloud macrophysical properties of three ATLID single-sensor Level 2 products: The “simple classification” of the ATLID Target Classification (A-TC) product, the “feature mask” of the ATLID Feature Mask (A-FM) product, and the “cloud top height” of the ATLID Cloud Top Height (A-CTH) product. We present an independent validation using a comprehensive multi-campaign dataset of airborne high spectral resolution lidar (HSRL) backscatter ratio (BSR) measurements. During each flight at least one coordinated underpass underneath EarthCARE was conducted ensuring high spatial and temporal matching of the spaceborne and airborne lidars. The dataset spans a wide range of tropical and extratropical cloud regimes, enabling robust assessment under diverse atmospheric conditions. The evaluation addresses two main objectives: (i) assessing how well A-TC and A-FM represent the vertical cloud distribution compared to the airborne HSRL observations, and (ii) quantifying the accuracy of A-CTH cloud top heights for different classes of cloud regimes. In general, the vertical cloud distribution observed from the airborne data is well captured by both A-TC and A-FM. However, we found a systematic overestimation of ice cloud pixels by A-TC in Baseline BA. This overestimation is caused by ice clouds that are horizontally and vertically too large, but also, to a certain extent, by aerosol pixels incorrectly classified as ice clouds (at temperature > 0 °C), and to a spurious occurrence of stratospheric aerosol features near the tropopause. A-TC liquid clouds are in very good agreement with the airborne observations. A-CTH exhibits a consistent positive bias in cloud top height of almost 300 m across different cloud regimes, along with occasional low-altitude cloud top detections, particularly over ocean surfaces, suggesting issues in surface assignment. The presented results primarily reflect Baseline BA, while first comparisons with Baseline BC/BD indicate improvements in future processing baselines, including a reduction of low-cloud artefacts in A-CTH, improved cloud phase representation in A-TC, and a strong reduction of misclassified ice clouds in aerosol layers at temperatures > 0 °C. Altogether, the validated ATLID Level 2 products demonstrate a high ability to realistically represent key macrophysical cloud properties – including cloud cover and cloud top height – which confirms their suitability for scientific applications. The identified limitations in the performance of the BA and BC baselines can help users make optimal use of these datasets, point to expected improvements in the upcoming CA baseline, and may support the further refinement of the ATLID Level 2 products.