Impacts of entrainment on secondary ice production in deep convective clouds

Abstract. The accurate representation of secondary ice production (SIP) is essential for describing the microphysics of deep convective clouds, yet the exact mechanisms and efficiencies of SIP are still uncertain. In this study, we used the University of Manchester bin microphysics parcel model to investigate four SIP parameterisations: rime splintering, ice-ice collisional breakup, spherical freezing fragmentation of drops (mode 1), and fragmentation between supercooled droplets and more massive ice particles (mode 2). We simulated air trajectories through deep convective clouds observed during the Deep Convective Microphysics EXperiment (DCMEX) field campaign. Our results show that mode 2, as currently understood and represented in the model, is the key mechanism for explaining the high ice particle concentrations observed. We also present the first systematic study of how different entrainment conditions (homogeneous and inhomogeneous mixing) affect SIP mechanisms. In our simulations, homogeneous and inhomogeneous mixing with aerosol entrainment provide reasonable agreement with cloud-core and cloud-edge microphysical properties observed during DCMEX, respectively. In addition, the peak liquid water content values in the adiabatic simulations was approximately 300 % of that in the entraining simulations. The entrainment of external aerosols was also found to accelerate the collision-coalescence process under homogeneous mixing, leading to earlier ice enhancement. Our results show that SIP mechanisms, which are dependent on large droplets, such as mode 2, are highly sensitive to different entrainment conditions, emphasising the importance of representing entrainment correctly when including SIP processes in large-scale models.