Assessing the Role of Aerosol Types and Immersion Freezing Parameterizations on the Ice Crystal Number Concentration in Long-Lived Mixed-Phase Clouds

High-latitude mixed-phase clouds contribute to the uncertainty of the estimated equilibrium climate sensitivity. This uncertainty stems from deficient knowledge of cloud microphysical processes, which determine the supercooled liquid and solid (ice) phase partitioning, and hence, the cloud’s reflectivity on mid-to-large scales. In the case of Arctic mixed-phase clouds, which are ubiquitous and persistent, challenges in estimating the number concentrations of ice-nucleating particles (INPs) from various aerosol populations are still a major source of uncertainty as a result of their impact on cloud lifecycles. In this study, we utilize a minimalistic 1D aerosol-cloud model informed by large-eddy simulations of the Surface Heat Budget of the Arctic campaign to enhance our understanding of the role of various microphysical processes in affecting the cloud’s ice crystal number concentration. This model is employed to prognostically evaluate the evolution of the INP reservoir that ultimately, governs the number of ice crystals formed. Immersion freezing is assumed as the dominant ice nucleation pathway. The model applies two currently debated immersion freezing schemes including the time-independent (singular) and time-dependent (classical nucleation theory) parameterizations. INP number concentrations are derived from polydisperse aerosol particle size distributions. We apply three different aerosol particle types and respective particle size distributions, including mineral dust, organic (humic-like substances), and sea spray aerosol guided by observations. Furthermore, the effect of varying aerosol number concentration and cloud microphysical, dynamical, and radiative parameters such as cloud top radiative cooling rate, cloud top entrainment rate, and ice crystal fall velocity on the INP reservoir and ice crystal number concentrations are assessed. Simulation results demonstrate that among all tested immersion freezing parameterizations, the time-dependent description yields an orders-of-magnitude larger INP reservoir than the singular approaches over the 10 h simulation time, which is the dominant factor for the strength of sustained ice crystal formation. We find that the aerosol types and associated particle size distributions can have the greatest impact on INP and ice crystal number concentrations. Cloud top radiative cooling among all varied parameters has the strongest effect on cloud properties. The production of new ice crystals is mostly controlled by the competition between the radiative cooling and ice sedimentation in classical nucleation theory-based immersion freezing parameterizations, while in singular approaches, cloud top radiative cooling rate, cloud top entrainment rate, and ice crystal fall velocity contribute to the ice production almost equally. These results underline the significance of the choice of applied immersion freezing parameterizations in modeling mixed-phase clouds, and corroborate the need for further in-depth laboratory and field studies to obtain robust and trustworthy immersion freezing parameterizations.