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

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.

Microphysics and dynamics of snowfall associated with a warm conveyor belt over Korea

“…Using the values described in Table A1, we find N tot r tot = 1.7 µm cm −3 , which we can use in Fig. 10 of Korolev and Mazin (2003) to read U * z at about −13 • C and find 0.1 m s −1 .…”

“…(temperature above −8 • C) can contribute to maintain high K dp values. Finally, Korolev et al (2020) recently suggested that secondary ice produced by shattering of freezing droplets transported above the melting layer could be lifted to higher levels. This may enhance the concentration of secondary ice in regions of strong updraughts.…”

“…In this appendix we show details on the estimation of the critical vertical velocity U * z needed to form and maintain liquid water in the presence of both ice crystals and snow. Korolev and Mazin (2003) showed that U * z can be expressed as a function of pressure, temperature, number concentration and mean size of ice crystals. We use Fig.…”

“…We use Fig. 10 in Korolev and Mazin (2003), which gives the relation between U * z and the product of the number concentration of ice particles N i and the characteristic size of ice particles r i for temperatures between −35 and −5 • C. Since in our case we have both ice and snow particles, we compute N i and the number concentration of snow particles N s separately such that the total number concentration is N tot = N i +N s . We also compute the characteristic size of the mixture of ice and snow particles r tot .…”

On the ice-nucleating potential of warm hydrometeors in mixed-phase clouds

“…The present work aims to revisit the details of the supersaturation field around an idealized falling hydrometeor and to quantify the volume of air which is affected by the presence of hydrometeors. Furthermore, it is yet to be evaluated how significant meteor-induced enhancement of ice nucleation is in clouds (Korolev et al, 2020), and therefore, we attempt to link our results to heterogeneous nucleation of aerosol particles.…”