Secondary ice formation: parsing evidence from observationally constrained simulations
Establishing the occurrence and pathways of secondary ice production (SIP) in mixed-phase and convective clouds faces multiple barriers to progress. Leading observational limitations include reliable measurement of ice mass mixing ratio, liquid drop and ice crystal number size distributions, and ice crystal morphological properties related to mode of growth, temperature and supersaturation. Modeling limitations include knowledge gaps in the representation of various multiplication mechanisms that may lead to efficient SIP, as well as apparent challenges to confidently represent the budget of primary ice nucleating particles (INPs), the formation of warm rain, and ice crystal properties that impact process rates. Evidence continues to indicate that efficient SIP processes do commonly occur with variable strength in mixed-phase and convective clouds, and that the SIP pathways are usually too weak in models. Barriers to progress and resulting biases in model simulations versus measurements are demonstrated from observed case studies.