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

Abstract. For pure ice clouds in the cold-temperature regime (T<235 K), two major formation pathways are possible. 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. For a better investigation of these pathways, we developed a novel microphysics scheme. The new two-moment scheme distinguishes between five ice classes (“ice modes”) each with their own 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, and hail. This scheme was employed to investigate the liquid origin vs. in situ formation in the fully glaciated parts of an idealized convective cloud. 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. We find a vertical layering of ice modes in the cloud. The lowermost layer consists of secondary ice from rime splintering and occurs near the updraft core at temperatures around the Hallett–Mossop zone. At altitudes 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 (INPs) required for immersion freezing were quickly depleted.