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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/amt-2020-265
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/amt-2020-265
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  26 Aug 2020

26 Aug 2020

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This preprint is currently under review for the journal AMT.

Characterising optical array particle imaging probes: implications for small ice crystal observations

Sebastian O'Shea1, Jonathan Crosier1,2, James Dorsey1,2, Louis Gallagher3, Waldemar Schledewitz1, Keith Bower1, Oliver Schlenczek4,5,a, Stephan Borrmann4,5, Richard Cotton6, Christopher Westbrook7, and Zbigniew Ulanowski1,8,9 Sebastian O'Shea et al.
  • 1School of Earth and Environmental Sciences, University of Manchester, UK
  • 2National Centre for Atmospheric Science, University of Manchester, UK
  • 3Department of Physics and Astronomy, University of Manchester, UK
  • 4Particle Chemistry Department, Max Planck Institute for Chemistry, Germany
  • 5Institute for Atmospheric Physics, Johannes Gutenberg University, Germany
  • 6Met Office, Exeter, UK
  • 7Department of Meteorology, University of Reading, UK
  • 8Centre for Atmospheric and Climate Processes Research, University of Hertfordshire, UK
  • 9British Antarctic Survey, NERC, Cambridge, UK
  • anow at: Max Planck Institute for Dynamics and Self-Organization, Göttingen, Germany

Abstract. The cloud particle concentration, size and shape data from optical array probes (OAPs) are routinely used to parameterise cloud properties and constrain remote sensing retrievals. This paper characterises the optical response of OAPs using a combination of modelling, laboratory and field experiments. Significant uncertainties are found to exist with such probes for ice crystal measurements. We describe and test two independent methods to constrain a probe's sample volume that removes the most severely mis-sized particles: (1) greyscale image analysis and (2) co-location using stereoscopic imaging. These methods are tested using field measurements from three research flights in cirrus. For these cases, the new methodologies significantly improve agreement with a holographic imaging probe compared to conventional data processing protocols, either removing or significantly reducing the concentration of small ice crystals (< 200 µm) in certain conditions. This work suggests that the observational evidence for a ubiquitous mode of small ice particles in ice clouds is likely due to a systematic instrument bias. Size distribution parameterisations based on OAP measurements need to be revisited using these improved methodologies.

Sebastian O'Shea et al.

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Short summary
The number, shape and size of ice crystals in clouds are important properties that influence the Earth's radiation budget, cloud evolution and precipitation formation. This work suggests that one of the most widely used methods for in situ measurements of these properties has significant uncertainties and biases. We suggest methods that dramatically improve these measurements, which can be applied to past and future datasets from these instruments.
The number, shape and size of ice crystals in clouds are important properties that influence the...
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