Immersion Freezing in Particle‐Based Aerosol‐Cloud Microphysics: A Probabilistic Perspective on Singular and Time‐Dependent Models

Abstract Cloud droplets containing immersed ice‐nucleating particles (INPs) may freeze at temperatures above the homogeneous freezing threshold temperature in a process referred to as immersion freezing. In modeling studies, immersion freezing is often described using either so‐called “singular” or “time‐dependent” parameterizations. Here, we compare both approaches and discuss them in the context of probabilistic particle‐based (super‐droplet) cloud microphysics modeling. First, using a box model, we contrast how both parameterizations respond to idealized ambient cooling rate profiles and quantify the impact of the polydispersity of the immersed surface spectrum on the frozen fraction evolution. Presented simulations highlight that the singular approach, constituting a time‐integrated form of a more general time‐dependent approach, is only accurate under a limited range of ambient cooling rates. The time‐dependent approach is free from this limitation. Second, using a prescribed‐flow two‐dimensional cloud model, we illustrate the macroscopic differences in the evolution in time of ice particle concentrations in simulations with flow regimes relevant to ambient cloud conditions. The flow‐coupled aerosol‐budget‐resolving simulations highlight the benefits and challenges of modeling cloud condensation nuclei activation and immersion freezing on insoluble ice nuclei with super‐particle methods. The challenges stem, on the one hand, from heterogeneous ice nucleation being contingent on the presence of relatively sparse immersed INPs, and on the other hand, from the need to represent a vast population of particles with relatively few so‐called super particles (each representing a multiplicity of real particles). We discuss the critical role of the sampling strategy for particle attributes, including the INP size, the freezing temperature (for singular scheme) and the multiplicity.