The Cloud-Aerosol Transition Zone Derived from Ground-Based and Satellite Lidars

Abstract. The contribution of Aerosol-Cloud Interactions (ACI) to the Earth’s radiative budget remains a major source of uncertainty in future climate projections. Clouds continuously interact with the surrounding non-saturated environment, forming cloud-aerosol transition zones (TZs). The suspensions in these regions are not fully assessed by cloud-cloudless distinction methodologies and have a non-negligible role in the Earth’s radiative budget, making the lack of large-scale TZ observations a challenge for full comprehension of the climate system. This study assesses TZ distributions using ground-based and spaceborne lidar observations to improve understanding of TZ conditions and evaluate the respective instrument capabilities. Ground-based Automatic low-power Lidars and Ceilometers (ALC) located at Burjassot (Spain), Gruenow (Germany), Girona (Spain) and the Cloudnet network are used, along with CALIOP observations over the region between 30°–80° N and 10° W–35° E, covering Europe. Results show that cloud-to-clear transitions are gradual, depend on detection thresholds and local climatology. Coincidental ALC and CALIOP observations are presented to assess the potential complementarity between the methods. Overall, ground-based ALCs provide high temporal and vertical resolution and are particularly effective at detecting TZs at low altitudes. In contrast, CALIOP offers global coverage and is especially useful for detecting TZs at high altitudes. Both methods show good agreement for cloud classification. However, there are some differences for aerosol and TZ detections due to instrument capabilities, altitude sensitivity, and the classification techniques. Although each approach has its individual limitations, integrating spaceborne downward-looking and ground-based upward-looking lidar observations provides a more comprehensive characterization of cloud-TZ-aerosol distribution.