Ice formation at moderate supercooling in mixed-phase clouds and its link to precipitation

Mignani, Claudia. Ice formation at moderate supercooling in mixed-phase clouds and its link to precipitation. 2022, Doctoral Thesis, University of Basel, Faculty of Science.

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Official URL: https://edoc.unibas.ch/88056/

Here we applied different approaches to obtain information on primary and secondary ice formation at moderate supercooling. In particular, we present observations of INPs active at around $-$15 °C in more than 120 aerosol samples and 220 individual dendritic ice crystals that were collected and analysed at mountain stations in the Swiss Alps during winter months of 2018 and 2019. Aerosol particle concentrations, air mass origin and precipitation history were combined to parameterise INP concentrations measured at Weissfluhjoch (2671 m a.s.l.). Primary dendritic ice crystals were quantified at Jungfraujoch (3580 m a.s.l.) using an approach that makes use of their particular and narrow growth temperature range. In addition, precipitating snow particles captured at ground level and coinciding radiosonde ascents were analysed to investigate whether mixed-phase clouds were relevant for snowfall at an Arctic site throughout a total of eight cold months in 2019 and 2020.
We found that it is more promising to parameterise atmospheric concentrations of INPs active at $-$15 °C measured at Weissfluhjoch using aerosol particle number concentrations of a size $>$ 2 µm as compared to smaller aerosol particles (Chapter 2). Differentiating between air masses that were precipitating, non-precipitating, and carrying Saharan dust and non-precipitating improved the prediction. The ratio of INP to aerosol particle $>$ 2 µm was larger in precipitating air masses than in non-precipitating air masses. Through freezing assays of individual dendritic ice crystals sampled within clouds at Jungfraujoch, we found that on average one out of eight dendrites contained an INP active at moderate supercooling (Chapter 3). Therefore, the multiplication factor for dendrites was on average only around one order of magnitude. At a site in Northern Finland, observations of often small, unrimed snow particles and the matching relative humidity profiles indicated that probably one quarter of the precipitating clouds were mixed-phase and the remainder were fully glaciated (Chapter 4).