Lagrangian Investigation of Secondary Ice Formation in Idealised Deep Convection

Abstract:

Secondary ice production via rime-splintering can be an important process for the rapid glaciation and high ice crystal numbers observed in mixed-phase convective clouds. How rime-splintering is triggered in the relatively short time between cloud formation and observations of high ice crystal numbers is an open question. Here, we use idealised simulations of a deep convective cloud system together with a novel Lagrangian analysis tools to investigate the thermodynamic and cloud microphysical evolution of air parcels, in which the model predicts secondary ice formation. The Lagrangian analysis suggests that the "in-situ'" formation of rimers either by growth of primary ice or rain freezing does not play a major role in triggering secondary ice formation. Instead, rimers are predominantly imported into air parcels through sedimentation form higher altitudes. The exclusion of ice nucleating particles (INPs) initiating heterogeneous freezing of cloud droplets at temperatures warmer than -10°C from the model did not have a discernible impact on the occurrence of secondary ice formation. Excluding Rain freezing slightly delays the initiation of secondary ice processes and shifts them to slightly different places in the cloud, but again there is no major impact on the abundance or spatial distribution of secondary ice in the cloud as a whole. These results suggest that for interpreting and analysing observational data and model experiments regarding cloud glaciation and ice formation it is vital to consider the complex vertical coupling of cloud microphysical processes in deep convective clouds via three-dimensional transport and sedimentation.