| 21 Aug 2026
Mixed-Phase Microphysical Evolution in Large Eddy Simulations of Tropical Cumulus Congestus: Developing and Evaluating a Laboratory-based Ice Multiplication Parameterization of Freezing Drops
Abstract. Ice microphysical processes modulate cloud structure, evolution, and Earth's radiative balance, yet secondary ice production (SIP)—whereby fragmentation enhances ice number concentrations (Nice) beyond what ice-nucleating particle (INP) populations alone can explain—remains poorly constrained. We develop a drop-shattering parameterization based on laboratory-observed pressure release event frequencies during drop freezing and evaluate it, alongside a water-activity-based immersion-freezing model for primary ice formation, in large eddy simulations (LES) of a tropical cumulus congestus case from NASA CAMP2Ex—the second of a two-part study extending liquid-phase results from Part I into the mixed-phase region. Bin and double-moment bulk simulations are evaluated against in situ aircraft observations from 0 to -15 °C. The baseline parameterization negligibly enhances Nice; a 10× multiplier on per-event splinter numbers—reflecting substantial production uncertainty—increases Nice by 1–2 orders of magnitude. The bulk scheme reaches localized maxima near 103 L-1, while the bin scheme reaches 10–20 L-1, reflecting fundamentally different collision-kernel structures between the schemes. A primary-secondary ice feedback emerges exclusively in the bin scheme, driven by INP enrichment of precipitation-sized drops through collision-coalescence and INP accumulation; this feedback is absent in the bulk scheme due to its lack of aerosol core mass tracking. The 10× parameterization partially reconciles a 1–2 order-of-magnitude deficit in simulated concentrations for sizes > 200 μm relative to observed particle size distributions, with turbulence-induced collision enhancement essential for conditioning SIP efficiency. Together, these bin and bulk implementations provide a foundation for improving SIP representation in large-scale models.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
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