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https://doi.org/10.5194/acp-2020-1145
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2020-1145
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  19 Nov 2020

19 Nov 2020

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This preprint is currently under review for the journal ACP.

How frequent is natural cloud seeding over Switzerland?

Ulrike Proske, Verena Bessenbacher, Zane Dedekind, Ulrike Lohmann, and David Neubauer Ulrike Proske et al.
  • Institute for Atmospheric and Climate Science, ETH Zürich, Zürich, Switzerland

Abstract. Clouds and cloud feedbacks represent one of the largest uncertainties in climate projections. As the ice phase influences many key cloud properties and their lifetime, its formation needs to be better understood in order to improve climate and weather prediction models. Ice crystals sedimenting out of a cloud do not sublimate immediately but can survive certain distances and eventually fall into a cloud below. This natural cloud seeding can trigger glaciation and has been shown to enhance precipitation formation. However, up to date an estimate of its occurrence frequency is lacking. In this study, we estimate the occurrence frequency of natural cloud seeding over Switzerland from satellite data and sublimation calculations.

We use the DARDAR satellite product between April 2006 and October 2017 to estimate the occurrence frequency of multilayer cloud situations, where a cirrus cloud at T < −35 °C can provide seeds to a lower lying feeder cloud. These situations are found to occur in 31 % of the observations. Of these 42 % have a cirrus cloud above another cloud, separated, while in 58 % the cirrus is part of a thicker cloud, with a potential for in-cloud seeding. Vertical distances between the cirrus and the lower-lying cloud are distributed uniformly between 100 m and 10 km. They are found to not vary with topography. Seasonally, winter nights have the most multilayer cloud occurrences, in 38 % of the measurements. Additionally, in situ and liquid origin cirrus cloud size modes can be identified according to the ice crystal mean effective radius in the DARDAR data. Using sublimation calculations we show that in a significant number of cases the seeding ice crystals do not sublimate before reaching the lower lying feeder cloud. Depending on whether bullet rosette, plate like or spherical crystals were assumed, 10 %, 11 % or 20 % of the crystals, respectively, could provide seeds after sedimenting 2 km.

The high occurrence frequency of seeding situations and the survival of the ice crystals indicate that the seeder-feeder process and natural cloud seeding are widespread phenomena over Switzerland. This hints to a large potential for natural cloud seeding to influence cloud properties and thereby the Earth's radiative budget and water cycle, which should be studied globally. Further investigations of the magnitude of the seeding ice crystals' effect on lower lying clouds are necessary to estimate the contribution of natural cloud seeding to precipitation.

Ulrike Proske et al.

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Ulrike Proske et al.

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Data for the publication "How frequent is natural cloud seeding?" Ulrike Proske, Verena Bessenbacher, Zane Dedekind, David Neubauer, and Ulrike Lohmann https://doi.org/10.5281/zenodo.3987757

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Scripts for the publication "How frequent is natural cloud seeding?" Ulrike Proske, Verena Bessenbacher, Zane Dedekind, David Neubauer, and Ulrike Lohmann https://doi.org/10.5281/zenodo.3987754

Ulrike Proske et al.

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Latest update: 24 Nov 2020
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Short summary
Ice crystals falling out of one cloud can initiate freezing in a second cloud below. We estimate the occurrence frequency of this natural cloud seeding over Switzerland from satellite data and sublimation calculations. We find that such situations with an ice cloud above another cloud are frequent and that the falling crystals survive the fall between two clouds in a significant number of cases, suggesting that natural cloud seeding is an important phenomenon over Switzerland.
Ice crystals falling out of one cloud can initiate freezing in a second cloud below. We estimate...
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