Preprints
https://doi.org/10.5194/acp-2021-718
https://doi.org/10.5194/acp-2021-718

  27 Sep 2021

27 Sep 2021

Review status: this preprint is currently under review for the journal ACP.

Unveiling atmospheric transport and mixing mechanisms of ice nucleating particles over the Alps

Jörg Wieder1, Claudia Mignani2, Mario Schär1, Lucie Roth1, Michael Sprenger1, Jan Henneberger1, Ulrike Lohmann1, Cyril Brunner1, and Zamin A. Kanji1 Jörg Wieder et al.
  • 1ETH Zurich, Institute for Atmospheric and Climate Science, Zurich, Switzerland
  • 2Department of Environmental Sciences, University of Basel, Basel, Switzerland

Abstract. Precipitation over the mid-latitudes originates mostly from the ice phase within mixed-phase clouds, signifying the importance of initial ice crystal formation. Primary ice crystals are formed on ice nucleating particles (INPs), which are sparsely populated in the troposphere. INPs are emitted by a large number of ground-based sources into the atmosphere, from where they can get lifted up to cloud heights. Therefore, it is vital to understand vertical INP transport mechanisms, which are particularly complex over orographic terrain. We investigate the vertical transport and mixing mechanisms of INPs over orographic terrain during cloudy conditions by simultaneous measurements of in situ INP concentration at a high valley and a mountaintop site in the Swiss Alps in late winter 2019. On the mountaintop, the INP concentrations were on average lower than in the high valley. However, a diurnal cycle in INP concentrations was observed at the mountaintop, which was absent in the high valley. The median mountaintop INP concentration equilibrated to the concentration found in the high valley towards the night. We found that in nearly 70 % of the observed cases INP-rich air masses were orographically lifted from low elevation upstream of the measurement site. In addition, we present evidence that over the course of the day air masses containing high INP concentrations were advected from the Swiss plateau towards the measurement sites, contributing to the diurnal cycle of INPs. Our results the local INP concentration enhancement over the Alps during cloud events.

Jörg Wieder et al.

Status: open (until 08 Nov 2021)

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Jörg Wieder et al.

Jörg Wieder et al.

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Short summary
We investigate the variation of ice nucleating particles (INPs) relevant for primary ice formation in mixed-phased clouds over the Alps based on simultaneous in situ observations at a mountaintop and a nearby high valley (1060 m height difference). In most cases, advection from the surrounding lower regions was responsible for changes in INP concentration, causing a diurnal cycle at the mountaintop. Our study underlines the importance of the planetary boundary layer serving as an INP reservoir.
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