Secondary Ice Formation in Cumulus Congestus Clouds: Insights from Observations and Aerosol-Aware Large-Eddy Simulations

Abstract. Secondary ice production (SIP) was investigated in a cumulus congestus system observed during the Secondary Production of Ice in Cumulus Experiment (SPICULE) campaign. Large-eddy simulations were performed using UCLALES–SALSA, a model that explicitly resolves aerosol–hydrometeor interactions through a sectional representation of aerosols, cloud droplets, rain droplets, and ice crystals. Two scenarios were compared: one including only immersion freezing as an ice formation process, and another incorporating additional SIP mechanisms – namely droplet shattering, rime splintering, and ice–ice collisional breakup.

The SIP-inclusive simulation reproduced the evolution of the observed cloud microphysical structure in both warm and mixed-phase regions. Ice–ice collisional breakup generated substantially more secondary ice particles than droplet shattering; however, it was only initiated after droplet shattering provided a sufficient initial ice particle population to meet the SIP triggering conditions. Droplet shattering was triggered by the presence of large supercooled droplets, defined by an integral raindrop diameter exceeding 3.5 mm L^-1 at temperatures below 265 K. Once formed, secondary ice particles enhanced riming and accretion, leading to auto-catalytic amplification of SIP through ice–ice breakup. This feedback rapidly depleted cloud liquid water within approximately 10 min.

Enhanced updrafts were identified in SIP-active regions, suggesting invigoration in the upper mixed-phase levels. SIP also intensified precipitation via the ice phase, resulting in a 26 % increase in domain-mean cumulative precipitation. The simulations reproduced key aspects of the observed ice multiplication, supporting the adequacy of the SIP representation in the model framework.