Sensitivity of ice multiplication mechanisms among natural clouds

Secondary ice production explains why clouds often contain ice particle concentrations that are orders of magnitude higher than the number of ice-nucleating particles available to initiate freezing. Accurate prediction of concentrations of ice in atmospheric clouds necessitates an understanding of SIP mechanisms. A fundamental challenge is determining how modeled SIP mechanisms depend on cloud properties and environmental conditions.

In this study, we used the Aerosol–Cloud (AC) model, which incorporates four secondary ice production mechanisms: ice–ice collisional breakup, fragmentation during raindrop freezing, the Hallett–Mossop process, and sublimational breakup. The various numerical simulations for sensitivity studies are conducted with the AC model and evaluated using a control simulation. 

Ice multiplication is driven by positive feedback mechanisms formed by interconnected microphysical processes. Investigating the potential for ice enhancement in natural clouds therefore requires consideration of the full range of microphysical interactions that may either suppress or amplify its influence under varying environmental conditions. The objective is to assess and determine the synergy among the secondary ice mechanisms.

In tropical deep convective clouds with very warm cloud bases, lower environmental CCN concentrations led to higher simulated ice concentrations in the lower mixed-phase region. In contrast, plausible variations in environmental IN concentrations had little effect on total simulated ice at any altitude. Overall, ice multiplication acted to dampen the simulated sensitivity to changes in both IN and CCN aerosol loadings.