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Discussion papers | Copyright
https://doi.org/10.5194/acp-2018-836
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 10 Sep 2018

Research article | 10 Sep 2018

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Rapid ice aggregation process revealed through triple-wavelength Doppler spectra radar analysis

Andrew I. Barrett1,2, Christopher D. Westbrook1, John C. Nicol1, and Thorwald H. M. Stein1 Andrew I. Barrett et al.
  • 1Department of Meteorology, University of Reading, Reading, RG6 6BB, UK
  • 2Institute for Meteorology and Climate Research, Karlsruhe Institute of Technology, Karlsruhe, 76131, Germany

Abstract. Rapid aggregation of ice particles has been identified by combining data from three co-located, vertically-pointing radars operating at different frequencies. A new technique has been developed that uses the Doppler spectra from these radars to retrieve the vertical profile of ice particle size distributions.

The ice particles grow rapidly from a maximum size of 0.75mm to 5mm while falling less than 500m and in under 10minutes. This rapid growth is shown to agree well with theoretical estimates of aggregation, with aggregation efficiency close to 1, and is inconsistent with other growth processes, e.g. growth by deposition, riming. The aggregation occurs in the middle of the cloud, and is not present throughout the entire lifetime of the cloud. However, the layer of rapid aggregation is very well defined, at a constant height, where the temperature is −15°C, and lasts for at least 20minutes (approximate horizontal distance: 24km). Immediately above this layer, the radar Doppler spectra is bi-modal, which signals the formation of new small ice particles at that height. We suggest that these newly formed particles, at approximately −15°C, grow dendritic arms, enabling them to easily interlock and accelerate the aggregation process. The estimated aggregation efficiency in the studied cloud is between 0.7 and 1, consistent with recent laboratory studies for dendrites at this temperature.

A newly developed method for retrieving the ice particle size distribution using the Doppler spectra allows these retrievals in a much larger fraction of the cloud than existing DWR methods. Through quantitative comparison of the Doppler spectra from the three radars we are able to estimate the ice particle size distribution at different heights in the cloud. Comparison of these size distributions with those calculated with more basic radar-derived values and more restrictive assumptions agree very well; however, the newly developed method allows size distribution retrieval in a larger fraction of the cloud because it allows us to isolate the signal from the larger (non-Rayleigh scattering) particles in the distribution and allows for deviation from the assumed shape of the distribution.

Andrew I. Barrett et al.
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Status: final response (author comments only)
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Andrew I. Barrett et al.
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Latest update: 12 Nov 2018
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Short summary
We use radars at three wavelengths to study cloud properties. The full Doppler spectra (rather than the processed moments of the spectra) are compared for the radars. This allows us to estimate the size and number of ice particles within the cloud. By following the evolution of the ice particles, we observe a region where particles rapidly and consistently increase in size. The observations suggest that these large particles form through interlocking of branched arms of smaller ice particles.
We use radars at three wavelengths to study cloud properties. The full Doppler spectra (rather...
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