Abstract
Secondary ice production via rime-splintering is considered to be an important process for rapid glaciation and high ice crystal numbers observed in mixed-phase convective clouds. An open question is how rime-splintering is triggered in the relatively short time between cloud formation and observations of high ice crystal numbers. We use idealised simulations of a deep convective cloud system 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. While ice nucleating particles (INPs) initiating heterogeneous freezing of cloud droplets at temperatures warmer than −10 °C have no discernible impact of the occurrence of secondary ice formation, in a scenario with rain freezing secondary ice production is initiated slightly earlier in the cloud evolution and at slightly different places, although with 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.
Highlights
In many clouds ice and liquid particles co-exist.Ice particles in the atmosphere are formed by homogeneous nucleation or homogeneous freezing of cloud droplets in the upper troposphere at temperatures colder than −38 ◦C and by heterogeneous freezing of cloud droplets triggered by aerosol particles at all temperatures below 0 ◦C
Idealised simulations based on the Weisman and Klemp [33] case are used to investigate the initiation of secondary ice production (SIP) via rime-splintering and the spread of secondary ice crystals through the cloud system
The thermodynamic and cloud microphysical evolution of air parcels prior to rime-splintering events is considered by using trajectory data at very high time resolution
Summary
In many clouds ice and liquid particles co-exist (e.g., reference [1], and reference therein). Based on conceptual considerations Beard [17] suggests primary ice crystal number concentration in the order of 0.01 L−1 may suffice to initiate SIP via rime-splintering, which is supported by the idealised 2D simulations by Huang et al [18]. The 2D sensitivity simulations by Huang et al [18] allow for such interactions, but they do not provide detailed insight into the impact of primary ice nucleation at different temperatures for the initiation of SIP. This question is vital for assessing the impact of high-temperature INP (e.g., reference [24]).
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