Global Assessment of the Cloud-Aerosol Transition Zone Using CALIPSO

The interactions between clouds and aerosols represent one of the largest uncertainties in assessing the Earth’s radiation budget, highlighting the importance of research on the transition zone (TZ) within the cloud-aerosol continuum. This study assesses the global distribution of TZ conditions, analyzes its optical characteristics, and determines the cloud or aerosol types most commonly associated with them, using the cloud-aerosol discrimination (CAD) score of the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument on the CALIPSO satellite. The CAD score classifies clouds and aerosols by the probability density functions of attenuated backscatter, total color ratio, volume depolarization ratio, altitude, and latitude. After applying several filters to avoid artifacts, the TZ was identified as those atmospheric layers that cannot be clearly classified as clouds or aerosols, layers within the no-confidence range (NCR) of the CAD score, and cirrus fringes. The optical characteristics of NCR layers exhibit two main clusters: Cluster 1, with properties between high-altitude ice clouds and aerosols (e.g., wispy cloud fragments), and Cluster 2, with properties between water clouds and aerosols at lower altitudes (e.g., large hydrated aerosols). Our results highlight the significant ubiquity of TZ conditions, which appear in 9.5% of all profiles and comprise 6.4% of the detected layers. Cluster 1 and cirrus-fringe layers predominate near the ITCZ and in mid-latitudes, whereas Cluster 2 layers are more frequent over the oceans along the central West African and East Asian coasts, where elevated smoke and dusty marine aerosols are common.