PNNL

Abstract

Cloud droplets containing ice-nucleating particles (INPs) may freeze at temperatures above than the homogeneous freezing threshold. This process, referred to as immersion freezing, is one of the major modulator of aerosol-cloud interactions in the Earth’s atmosphere that impacts cloud radiative properties, and thus, climate. In modelling studies, immersion freezing is often implemented using either so-called ”singular” or ”time-dependent” parameterisations. In this paper, we juxtapose both approaches and discuss them in the context of probabilistic particle based cloud microphysics modelling, referred to as the super-particle or super-droplet techniques. These techniques work by populating the particle-attribute phase space with large numbers of computational particles and tracking the evolution of particle attributes. We outline the congruence of the singular and time-dependent approaches with Poissonian process description and point to limitations pertinent to the singular approach. Simulations with both box and two dimensional kinematic models focus on the following aspects of immersion freezing modelling. First, we contrast how both parameterisations respond to different idealised ambient cooling rate profiles. Second, we quantify how the polydispersity of the immersed surface spectrum impacts macroscopic features of the system. Third, we illustrate the implications of applying the singular model in simulations with flow regimes relevant to ambient cloud conditions rather than to the cloud-chamber experiments on which these parameterisations are built upon. We discuss the critical role of the attribute-space sampling strategy for particle-based model simulations in modelling heterogeneous ice nucleation which is contingent on the presence of relatively sparse immersed INPs.