A log-parabolic sensitivity of mixed-phase clouds to ice-nucleating aerosols

We revised and simplified the microphysics of mixed-phase clouds in CESM2, assuming sedimentation and immersion freezing by mineral dust as the only sources of ice crystals. We find that these assumptions produce variability in cloud-top phase that agrees with long-term global satellite observations (Villanueva et al., 2025; Toll et al., 2024, Science). These simulations confirm that the interannual variability of cloud phase is controlled by dust loading.

Furthermore, by probing instantaneous cloud states daily over a 10-year simulation, we propose a simplified theoretical framework that maps the log-normal variability of ice-forming processes onto the observed variability of cloud-top phase. For cold mixed-phase clouds (cloud-top temperatures below −21 °C), we find the cloud climatology is dominated by a reduced set of processes:
        1.        Aerosol-driven droplet freezing,
        2.        Ice depositional growth (WBF),
        3.        Droplet–snow riming in thick clouds, and
        4.        Cloud-top radiative cooling in thin clouds.

As a result, ice-nucleating particles (INPs) can impose a logarithmic control on cloud mass. At sufficiently high INP concentrations, this control becomes reversible (the WBF process is skipped), leading to a log-parabolic cloud response.