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. This study attempts a new identification of mechanisms ofsecondary ice production (SIP) based on the observation of small faceted icecrystals (hexagonal plates or columns) with typical sizes smaller than 100 µm. Due to their young age, such small ice crystals can be used astracers for identifying the conditions for SIP. Observations reported herewere conducted in oceanic tropical mesoscale convective systems (MCSs) and midlatitude frontal clouds in the temperature range from 0 to−15 ∘C and heavily seeded by aged ice particles. It was found thatin both MCSs and frontal clouds, SIP was observed right above the meltinglayer and extended to higher altitudes with colder temperatures. The rolesof six possible mechanisms to generate the SIP particles are assessed usingadditional observations. In most observed SIP cases, small secondary iceparticles spatially correlated with liquid-phase, vertical updrafts and agedrimed ice particles. However, in many cases, neither graupel nor liquiddrops were observed in the SIP regions, and therefore, the conditions for anactive Hallett–Mossop process were not met. In many cases, largeconcentrations of small pristine ice particles were observed right above the melting layer, starting at temperatures as warm as −0.5 ∘C. It isproposed that the initiation of SIP above the melting layer is stimulated bythe recirculation of large liquid drops through the melting layer withconvective turbulent updrafts. After re-entering a supercooled environmentabove the melting layer, they impact with aged ice, freeze, and shatter. The size of the splinters generated during SIP was estimated as 10 µm orless. A principal conclusion of this work is that only the freezing-drop-shattering mechanism could be clearly supported by the airborne in situobservations.

A new look at the environmental conditions favorable to secondary ice production