Investigating ice formation pathways using a novel two-moment multi-class cloud microphysics scheme

Abstract. We developed a novel microphysics scheme to investigate the formation pathways of ice crystals in the atmosphere. The new two-moment scheme distinguishes between five ice classes (’ice modes’) each with their unique formation mechanism: homogeneous freezing of solution droplets, deposition nucleation, homogeneous freezing of cloud droplets and raindrops, immersion freezing and secondary ice from rime splintering. The ice modes interact with each other, e.g. in competition for growth by deposition of water vapor and aggregation, but also with the other cloud particle classes, i.e., cloud droplets, rain, snow, graupel, hail.

This scheme was employed to investigate the liquid origin vs in-situ formation in the fully glaciated parts of an idealised convective cloud. Liquid origin ice clouds stem from droplets that freeze close to water saturation. In-situ formed ice clouds form directly from the vapor phase below water saturation. The majority of the cloud ice in the deep convection cloud consisted of frozen droplets (liquid origin). This was caused by the high number concentration of cloud droplets available for freezing. In-situ formed ice was only relevant for the overshoot where ice from both formation pathways mixed.

The new scheme is also useful for investigation of the ice formation in the mixed-phase parts of the convective cloud. There is a vertical layering of ice modes in the cloud. The lower most layer consists of secondary ice from rime splintering and occurred near the updraft core at temperatures around the Hallet-Mossop zone. In an altitude between 6 and 9 km ice mostly stems from immersion freezing. We find a correlation between the abundance of ice from immersion freezing and snow. The majority of ice crystals above 9 km stems from homogeneously frozen cloud droplets since ice nucleating particles (INP) required for immersion freezing where quickly depleted.