A New Look at the Environmental Conditions Favorable to Secondary Ice Production: The Melting Layer As a Potential Source

The role of environmental conditions associated with secondary ice production is analyzed based on the in-situ measurements collected during the High Ice Water Content project (French Guiana, 2015) and the Weather Radar Validation Experiment (Canada/US, 2016/17) from the Canadian National Research Council Convair-580 research aircraft. A detailed analysis of the SPEC Cloud Particle Imager (CPI) imagery was conducted with the help of neural net image recognition, fine-tuned for the identification of facetted ice particles. The results of this analysis showed the existence of spatially compact cloud regions with high concentrations of small (40-80um) pristine ice crystals in the vicinity of the melting layer. These small ice particle clusters were found both in mesoscale convective systems and in mid-latitude frontal stratiform clouds. The observed concentration of small ice particles was too large to be explained by ice initiation on ice nucleating particles (INP) and is instead, likely a result of secondary ice production. Importantly, the occurrence of such small ice crystals was found to be associated with the presence of: (a) precipitation size liquid drops (80-300um), and (b) vertical updrafts. The absence of any significant amounts of liquid in cloud layers above the observation level indicates that the large drops could not originate as a result of the collision-coalescence process. Rather, these drops form due to recirculation of melted ice crystals, which have passed through the melting layer. In the presence of regular or turbulent updrafts these large drops can be lifted to the -10<T<-3C levels favorable for ice multiplication. Furthermore, observations of deformed and fragmented frozen drops in the cloud regions with high concentrations of small ice, suggest that secondary ice production occurs due to breakup, bubble bursting, jetting or cracking of large drops during their freezing. These mechanisms of secondary ice production were documented in laboratory experiments by the Karlsruhe Institute of Technology group (Germany). Images of frozen drops attached to other ice particles, from the current study, indicate that large droplet freezing is facilitated by their impact with preexisting and newly generated ice particles. When all of these pieces of the puzzle are put together it suggests that the melting layer serves as a universal and efficient facilitator of secondary ice production. The spatial and microphysical characteristics of the cloud regions with secondary ice production are discussed.