Secondary ice production in summer deep convective clouds over New Mexico during the DCMEX campaign

Aircraft observations frequently report ice particle concentrations in deep convective clouds that cannot be explained by primary ice nucleation alone. This discrepancy is commonly attributed to secondary ice production (SIP), yet the dominant mechanisms remain poorly constrained. This study examines deep convective clouds observed during the July–August 2022 DCMEX field campaign using in situ aircraft measurements. We use the University of Manchester bin microphysics parcel model to simulate the development of SIP within these convective systems, and analyse parameterised SIP production rates derived from in situ ice particle measurements. Four SIP mechanisms are systematically analysed: rime splintering, ice–ice collisional breakup, spherical freezing fragmentation of drops (mode 1), and fragmentation during collisions between supercooled droplets and more massive ice particles (mode 2).

 

Our results suggest that the two modes of freezing fragmentation of drops are key to explaining the high ice particle concentrations observed in summer deep convective systems over New Mexico. In contrast, rime splintering appears to be largely inactive across all simulations. We also find that external aerosol entrainment accelerates collision–coalescence under homogeneous mixing, leading to earlier ice enhancement, while having little impact under inhomogeneous mixing. Droplet-dependent SIP mechanisms such as mode 2 show strong sensitivity to entrainment assumptions, underscoring the need for accurate entrainment representation when including SIP processes in large-scale models.