Sub-cloud aerosol intrusion and microphysical transition in boundary layer clouds revealed by integrated lidar system

Aerosol–cloud interactions are strongly regulated within the narrow transition layer immediately below cloud base, where aerosol activation and boundary-layer restructuring occur. However, continuous observations in this critical altitude range are frequently hindered by geometric overlap effects and severe signal attenuation under optically thick clouds, limiting process-level diagnosis. Here we develop a blind-zone-free extinction retrieval framework that integrates side-scattering CCD measurements with vertically resolved lidar backscatter through a near-end constrained iterative calibration. The approach reconstructs continuous aerosol extinction profiles from the surface to cloud base even under strong cloud attenuation, and enables dual-wavelength lookup-table inversion of effective radius ( R eff ) and particle number concentration (N). Application to a representative cloud evolution event reveals a clear transition from a decoupled boundary-layer structure—characterized by a persistent clean gap beneath cloud base—to a coupled regime associated with enhanced convection. During the recoupling phase, aerosol-rich air masses penetrate the cloud-base layer, accompanied by a rapid increase in N and a simultaneous decrease in R eff , consistent with intensified cloud condensation nuclei activation. ERA5 thermodynamic diagnostics further support the role of boundary-layer destabilization in facilitating vertical aerosol transport. These results highlight the importance of resolving the cloud-base transition layer for identifying aerosol–cloud coupling pathways and provide observational evidence linking boundary-layer restructuring to microphysical cloud responses.