New type of evidence for secondary ice formation at around −15 °C in mixed-phase clouds

Claudia Mignani,Jessie M. Creamean,Christine Alewell,Franz Conen,Lukas Zimmermann

doi:10.5194/acp-19-877-2019

Abstract

Abstract. Ice crystal numbers can exceed the numbers of ice-nucleating particles (INPs) observed in mixed-phase clouds (MPCs) by several orders of magnitude, also at temperatures that are colder than −8 ∘C. This disparity provides circumstantial evidence of secondary ice formation, also other than via the Hallett–Mossop process. In a new approach, we made use of the fact that planar, branched ice crystals (e.g. dendrites) grow within a relatively narrow temperature range (i.e. −12 to −17 ∘C) and can be analysed individually for INPs using a field-deployable drop-freezing assay. The novelty of our approach lies in comparing the growth temperature encoded in the habit of an individual crystal with the activation temperature of the most efficient INP contained within the same crystal to tell whether it may be the result of primary ice formation. In February and March 2018, we analysed a total of 190 dendritic crystals (∼3 mm median size) deposited within MPCs at the high-altitude research station Jungfraujoch (3580 m a.s.l.). Overall, one in eight of the analysed crystals contained an INP active at −17 ∘C or warmer, while the remaining seven most likely resulted from secondary ice formation within the clouds. The ice multiplication factor we observed was small (8), but relatively stable throughout the course of documentation. These measurements show that secondary ice can be observed at temperatures around −15 ∘C and thus advance our understanding of the extent of secondary ice formation in MPCs, even where the multiplication factor is smaller than 10.

Highlights

Ice-nucleating particles (INPs) are required to catalyse primary ice formation in clouds at temperatures above −36 ◦C via heterogeneous freezing (e.g. Vali et al, 2015)
The habit of a planar, branched ice crystal, growing exclusively between −12 and −17 ◦C, enables the verification of whether it derived from primary or secondary ice formation based on a number of reasonable assumptions
The required experimental procedure, including refreezing of dendrites using a drop-freezing assay, has a low throughput (∼ 15 min for two ice crystals) it can provide an estimate for the ice multiplication factor around −15 ◦C, even when it is smaller than 10, unlike previous in situ approaches

Summary

Introduction

Ice-nucleating particles (INPs) are required to catalyse primary ice formation in clouds at temperatures above −36 ◦C via heterogeneous freezing (e.g. Vali et al, 2015). Sullivan et al (2018a) have recently studied three of the above-mentioned secondary ice formation processes in terms of their thermodynamic and primary ice requirements in a parcel model They showed that INP concentration can be as low as 2 m−3 (0.002 L−1) to initiate ice multiplication by ice–ice collisional breakup. Ice-nucleation active microbes can be scavenged by raindrops below cloud and alter the spectrum (Hanlon et al, 2017) Another way to separate primary from secondary ice particles could be INP assays on individual hydrometeors collected within MPCs. The first experiment in which individual hydrometeors were analysed for INPs, and the only one to our knowledge, was conducted by Hoffer and Braham (1962).

Location and meteorological conditions

Single crystal selection and analysis

Accounting for riming

Results and discussion

Conclusion