The Characterization of the Cloud-Aerosol Transition Zone Using Ground-Based and Spaceborne LiDAR

Aerosol-Cloud Interactions (ACI) contribution to the Earth’s radiative budget remains as a major uncertainty in future climate projections. Clouds constantly interact with the surrounding non-saturated environment, forming cloud-aerosol transition zones (TZs). These suspensions are not fully assessed by cloud-cloudless distinction methodologies and have a non-negligible role in the radiative budget, making the lack of large-scale TZ observations a challenge for full comprehension of the climate system.

In this study, two complementary ground-based and spaceborne lidar TZ observation techniques are integrated to enhance knowledge on TZ conditions, including their detection occurrence, distribution, and optical characteristics, while highlighting the advantages and limitations of each methodology used. 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 coordinates 30º–80ºN and 7ºW–35ºE, covering Europe. The ALC method relies on varying the set of thresholds for cloud detection of the Cloudnetpy algorithm from ACTRIS Cloudnet. In contrast, the method for the CALIOP data applies several filters to avoid artifacts, and uses the CAD score values to identify clouds, aerosols, and TZ conditions.

Results show that the transition from cloud to cloud-free is gradual, and cloud detection depends on the thresholds used in the methods, as well as the local climatology. To properly assess the synergy between the methods, case studies of coincidental observations are presented, where the distance between the CALIOP overpass and the ALC site is less than 4 km. These cases represent various atmospheric patterns, such as cloud-free and boundary layer aerosols, Cirrus, low-level clouds, dense tropospheric clouds, and multi-layer cloud structures. Overall, ground-based ALC provide high temporal and vertical resolution, and are particularly effective at detecting TZ at low altitudes. In contrast, CALIOP offers global coverage and is especially useful for detecting TZ located at high altitudes. Although each approach has individual limitations, integrating spaceborne downward-looking and ground-based upward-looking lidar observations can provide a more comprehensive characterization of cloud-TZ-aerosol distribution.