On the representativeness of the ground-based lidar observations for satellite calibration/validation – the example of the archipelago of Cabo Verde

Abstract. Ground-based lidar stations play a vital role in the validation of spaceborne lidar products. While ground-based measurements have a high temporal resolution, they have limited spatial coverage, which potentially imposes implications for the Calibration and Validation (Cal/Val) of the satellite products. Therefore, in this study, we assess the representativeness of a remote ground-based ACTRIS (Aerosol, Clouds, and Trace Gases Research Infrastructure) station, in Mindelo, Cabo Verde by utilizing the continuous observations of a ground-based PollyNET multiwavelength polarization Raman lidar. This station was selected since Cabo Verde has been a key location for the validation of two recent Earth Explorer missions of the European Space Agency (ESA), namely Aeolus and the Earth Cloud, Aerosol and Radiation Explorer (EarthCARE). The islands are located in the Atlantic Ocean, in the outflow region of the African continent with frequent dust outbreaks, but also smoke advection and, thus, along with the local (marine) boundary layer provide an excellent atmospheric laboratory. Continuous, vertically-resolved aerosol measurements are being conducted with the state-of-the-art multiwavelength polarization Raman lidar PollyXT at Mindelo since June 2021. Based on these observations and in combination with the LIVAS (LIdar climatology of Vertical Aerosol Structure for space-based lidar simulation studies) products available at different radii around Mindelo, a statistical analysis of the optical properties was performed to evaluate the representativeness of the station in the context of aerosol profiling Cal/Val activities. Additionally, three case studies, focusing on different distances from the ground-based station, have been closely examined for a more complete and detailed comparison. Our study results indicate that overall the ground-based station in Mindelo can be considered conditionally representative. According to the monthly analysis, at altitudes where the lofted aerosol (dust) layers occur, lidar observations were very representative for radii up to 300 km around the island, while the boundary-layer characteristics varied. Case studies confirmed the long-term results and revealed that lidar observations of lofted aerosol layers can be representative for radii up to 100 km around Mindelo and at the same time highlighted the importance of spatiotemporal homogeneity of the target. From our findings and especially for the Cabo Verde region, we conclude that it is better to use monthly averaged aerosol profiles for the validation of spaceborne profiles over long times rather than using single overpasses, as representativeness cannot be guaranteed for the latter without additional measures. Thus, using fixed radii around a certain ground site (as e.g., the frequently used 100 km) for validation activities seems to be inappropriate for profile-to-profile comparison without any further considerations. However, we show in our case studies that if representativeness can be guaranteed, also single-profile validation is possible and has its own valuable potential. Additionally, the proposed study can serve as a calibration/validation tool for the remote sensing facilities of the European Aerosol Research Lidar Network (EARLINET).