Altitude-Dependent Formation of Polar Mesospheric Clouds: Charged Nucleation and In Situ Ice Growth on Zonal and Daily Scales

Abstract. Polar mesospheric clouds (PMCs), composed of ice particles, play a crucial role in mesospheric H₂O redistribution, yet their microphysical formation mechanism – particularly ice nucleation – remains incompletely understood. Using AIM satellite observations, we reveal a previously unreported hemispheric asymmetry: southern hemisphere (SH) PMCs show a significant latitudinal decrease in column ice particle concentration, while their northern hemisphere (NH) counterparts exhibit zero trend. Our further analysis demonstrates that the column-averaged ice particle concentration (N_c) and radius (r_c) are primarily governed by PMC height (h), rather than environment temperature (T_env). To explain these observations, we propose the charged meteoric smoke particle (MSP) nucleation (CMN) scheme, an altitude-dependent framework based on two key postulates: (1) charged-MSPs serve as ubiquitous ice nuclei throughout the PMC layer, and (2) ice particles grow predominantly in situ with negligible sedimentation. The CMN scheme naturally accounts for the observed vertical gradients in ice particle concentration (increasing with altitude due to charged-MSPs distribution) and size (decreasing with altitude due to H₂O competition among ice particles). By eliminating sedimentation, the CMN scheme introduces a novel bottom-up H₂O redistribution mechanism we term the cold-trap effect. This mechanism is driven by summer polar upwelling dynamics: upward H₂O transport induces hydration, while simultaneous ice particle formation (facilitated by upwelling-induced cooling) blocks further H₂O transport, ultimately causing dehydration above PMCs. While the traditional growth-sedimentation (GS) scheme and freeze-drying effect are well-validated, our CMN scheme and cold-trap effect provide an alternative paradigm particularly for understanding zonal and daily-scale PMC variability and associated H₂O redistribution processes.