Aircraft In Situ Observations of Cloud Microphysics During Icing Events over Southwestern China in Spring 2025

Aircraft icing associated with supercooled large droplets (SLD) remains a critical hazard to flight safety, yet direct in situ observations over China are scarce. This study presents the cloud microphysical characteristics observed during two flight missions of the Aircraft Icing Research Flight Experiment (AIRFEx) conducted in March 2025 over eastern Sichuan Province, southwestern China. A Cloud, Aerosol and Precipitation Spectrometer (CAPS), a Nevzorov total water content probe, and an icing detector were deployed to obtain particle size distributions from 0.6–1550 μm, liquid and ice water contents. The 17 March case featured a mixed-phase, ice-dominated cloud with enhanced large irregular particles near an elevated inversion, suggesting active secondary ice production. In contrast, the 29 March case exhibited a liquid-dominated cloud with persistent supercooled droplets and episodic SLD, accompanied by pronounced ice accretion on the airframe. Icing periods Among icing environmente extracted to compare microphysical conditions between icing and non-icing periods at subfreezing temperatures and to evaluate consistency with the FAR Part 25 Appendix O freezing-drizzle envelopes. Icing periods were characterized by systematically higher liquid water content and slightly larger droplet sizes than non-icing periods, while most observed conditions fell within the Appendix O envelopes, with one out-of-envelope event indicating locally enhanced severity. These first in situ observations of SLD-related icing over southwestern China provide a process-based reference for validating icing hazard assessments and support future development of region-specific SLD climatologies and icing certification criteria.