The Cloud-Aerosol Transition Zone from Satellite and Ground-Based Lidar Observations

One of the main uncertainties in future climate projections is the cloud and aerosol contribution to the Earth’s radiative budget. The imprecise distinction between these aggregates of particles in suspension, combined with the transition zone within the cloud-aerosol continuum, further complicate the study of their radiative and climatic effects. Despite their importance, observations of the cloud-aerosol transition zone (TZ), particularly its vertical distribution, remain limited.

This study addresses this gap using a Vaisala CL31 ceilometer located at Girona (Spain), and the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument on the CALIPSO satellite. For the ceilometer, we conducted a sensitivity analysis of backscatter and signal-to-noise ratio thresholds used for cloud detection in the Cloudnetpy algorithm from ACTRIS Cloudnet and considered as TZ all those cases where varying those thresholds (from relaxed to a strict situation) changed cloud detection. The CALIOP data was processed by applying several filters to avoid artifacts, and identifying the TZ as the atmospheric layers within the no-confidence range (NCR) of the CAD score, as well as the Cirrus fringes (CAD = 106). Such methodologies enabled the assessment of the vertical distribution and frequency of clouds, aerosols, and the TZ.

Overall, results indicate a gradual transition in backscatter retrievals from cloud to cloud-free, where suspended particles detected near cloud boundaries induced higher backscatter values than those found further away. From the local perspective, we observed a 9.3% (with an uncertainty range of 5.4─20%) variation in cloud occurrence attributed to TZ conditions. When analyzing the whole backscatter profile, we found as many TZ conditions as cloudy values, remarking the importance of TZ vertical frequency. Furthermore, the analysis of TZ occurrence in height and time in Girona revealed that these conditions tend to concentrate below 800 m during night periods. However, annual height-hour distributions showed remarkable seasonal variability. From the global perspective, TZ layers’ optical characteristics showed three main TZ groups:  1. Cluster 1, layers with properties between high-altitude ice clouds and aerosols (e.g. wispy cloud fragments); 2. Cluster 2, layers with properties between water clouds and aerosols at lower altitudes (e.g. hydrated aerosols); 3. Layers classified as Cirrus fringes. The TZ conditions were found worldwide, appearing in 9.5% of all profiles and comprising 6.4% of the filtered layers. The Cluster 1 and Cirrus fringes layers predominate near the ITCZ and in mid-latitudes. In contrast, Cluster 2 is more frequent over the oceans in the central West African and East Asian coasts where elevated smoke and dusty marine aerosols are common. Both ground-based and satellite approaches highlight the significant ubiquity and vertical frequency of the TZ.