Abstract
Abstract. This study attempts a new identification of mechanisms of secondary ice production (SIP) based on the observation of small faceted ice crystals (hexagonal plates or columns) with typical sizes smaller than 100 µm. Due to their young age, such small ice crystals can be used as tracers for identifying the conditions for SIP. Observations reported here were 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 that in both MCSs and frontal clouds, SIP was observed right above the melting layer and extended to higher altitudes with colder temperatures. The roles of six possible mechanisms to generate the SIP particles are assessed using additional observations. In most observed SIP cases, small secondary ice particles spatially correlated with liquid-phase, vertical updrafts and aged rimed ice particles. However, in many cases, neither graupel nor liquid drops were observed in the SIP regions, and therefore, the conditions for an active Hallett–Mossop process were not met. In many cases, large concentrations of small pristine ice particles were observed right above the melting layer, starting at temperatures as warm as −0.5 ∘C. It is proposed that the initiation of SIP above the melting layer is stimulated by the recirculation of large liquid drops through the melting layer with convective turbulent updrafts. After re-entering a supercooled environment above the melting layer, they impact with aged ice, freeze, and shatter. The size of the splinters generated during SIP was estimated as 10 µm or less. A principal conclusion of this work is that only the freezing-drop-shattering mechanism could be clearly supported by the airborne in situ observations.
Highlights
Secondary ice production (SIP) has long been acknowledged as a fundamental cloud microphysical process (e.g., Cantrell and Heymsfield, 2005; Field et al, 2017)
In the frame of this study, we explored the microphysics of SIP cloud regions in tropical mesoscale convective systems (MCSs) at the mature stage of their development and midlatitude frontal cloud systems within the temperature range of −15 ◦C < Ta < 0 ◦C
SIP cloud regions were identified based on the presence of numerous small faceted ice crystals with Lmax < 100 μm
Summary
Secondary ice production (SIP) has long been acknowledged as a fundamental cloud microphysical process (e.g., Cantrell and Heymsfield, 2005; Field et al, 2017). Through the modulation of ice particle concentration, SIP can thereby impact precipitation formation, rate of glaciation of mixedphase clouds, the longevity of ice clouds, cloud electrification, and radiative properties of clouds. SIP may significantly impact the hydrological cycle and climate in general. The commonality and precise mechanisms of SIP have remained persistently poorly established. Understanding of mechanisms of SIP is of great importance for developing a parameterization of the ice initiation processes in weather prediction and climate models
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