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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
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Preprints
https://doi.org/10.5194/acp-2020-772
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2020-772
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  09 Oct 2020

09 Oct 2020

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This preprint is currently under review for the journal ACP.

Microphysical investigation of the seeder and feeder region of an Alpine mixed-phase cloud

Fabiola Ramelli1, Jan Henneberger1, Robert O. David2, Johannes Bühl3, Martin Radenz3, Patric Seifert3, Jörg Wieder1, Annika Lauber1, Julie T. Pasquier1, Ronny Engelmann3, Claudia Mignani4, Maxime Hervo5, and Ulrike Lohmann1 Fabiola Ramelli et al.
  • 1Department of Environmental System Sciences, Institute for Atmospheric and Climate Science, ETH Zurich, Zurich, Switzerland
  • 2Department of Geosciences, University of Oslo, Oslo, Norway
  • 3Leibniz Institute for Tropospheric Research, Leipzig, Germany
  • 4Institute of Environmental Geosciences, University of Basel, Basel, Switzerland
  • 5Federal Office of Meteorology and Climatology MeteoSwiss, Payerne, Switzerland

Abstract. The seeder-feeder mechanism has been observed to enhance orographic precipitation in previous studies. However, the microphysical processes active in the seeder and feeder region are still being understood. In this paper, we investigate the seeder and feeder region of a mixed-phase cloud passing over the Swiss Alps, focusing on (1) fallstreaks of enhanced radar reflectivity originating from cloud top generating cells (seeder region) and (2) a persistent low-level feeder cloud produced by the boundary layer circulation (feeder region). Observations were obtained from a multi-dimensional set of instruments including ground-based remote sensing instrumentation (Ka-band polarimetric cloud radar, microwave radiometer, wind profiler), in situ instrumentation on a tethered balloon system and ground-based aerosol and precipitation measurements.

The cloud radar observations suggest that ice formation and growth was enhanced within cloud top generating cells, which is consistent with previous observational studies. However, uncertainties exist regarding the dominant ice formation mechanism within these cells. Here we propose different mechanisms that potentially enhance ice nucleation and growth in cloud top generating cells (convective overshooting, radiative cooling, droplet shattering) and attempt to estimate their potential contribution from an ice nucleating particle perspective. Once ice formation and growth within the seeder region exceeded a threshold value, the mixed-phase cloud became fully glaciated.

Local flow effects on the lee side of the mountain barrier induced the formation of a persistent low-level feeder cloud over a small-scale topographic feature in the inner-Alpine valley. In situ measurements within the low-level feeder cloud observed the production of secondary ice particles likely due to the Hallett–Mossop process and ice particle fragmentation upon ice–ice collisions. Therefore, secondary ice production may have been partly responsible for the elevated ice crystal number concentrations that have been previously observed in feeder clouds at mountain-top observatories. Secondary ice production in feeder clouds can potentially enhance orographic precipitation.

Fabiola Ramelli et al.

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Fabiola Ramelli et al.

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Short summary
Orographic mixed-phase clouds are an important source of precipitation, but the ice formation processes within them remain uncertain. Here we investigate the origins of ice crystals in a mixed-phase cloud in the Swiss Alps using aerosol and cloud data from in situ and remote sensing observations. We found that ice formation primarily occurs in cloud top generating cells and low-level feeder clouds. Our results indicate that secondary ice processes occur in both of these regions.
Orographic mixed-phase clouds are an important source of precipitation, but the ice formation...
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