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 formation mechanism remains incompletely understood. Using the Aeronomy of Ice in the Mesosphere (AIM) satellite observations, we reveal a previously unreported hemispheric asymmetry: southern hemisphere PMCs show a significant latitudinal decrease in column ice particle concentration, while their northern hemisphere counterparts exhibit zero trend. Our further analysis demonstrates that the column-averaged ice particle concentration (Nc) and radius (rc) are primarily governed by PMC height (h), rather than environmental temperature (Tenv). 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.