On the impacts of different definitions of maximum dimension for nonspherical particles recorded by 2D imaging probes

Wei Wu, Greg M. McFarquhar

Research output: Contribution to journalArticlepeer-review


Knowledge of ice crystal particle size distributions (PSDs) is critical for parameterization schemes for atmospheric models and remote sensing retrieval schemes. Two-dimensional in situ images captured by cloud imaging probes are widely used to derive PSDs in term of maximum particle dimension (Dmax). In this study, different definitions of Dmax for nonspherical particles recorded by 2D probes are compared. It is shown that the derived PSDs can differ by up to a factor of 6 for Dmax ≤ 200 mm and Dmax ≥ 2 mm. The large differences for Dmax ≤ 200 mm are caused by the strong dependence of sample volume on particle size, whereas differences forDmax ≥ 2 mm are caused by the small number of particles detected. Derived bulk properties can also vary depending on the definitions of Dmax because of discrepancies in the definition of Dmax used to characterize the PSDs and that used to describe the properties of individual ice crystals. For example, the massweighted mean diameter can vary by 2 times, the ice water content (IWC) by 3 times, and the mass-weighted terminal velocity by 6 times. Therefore, a consistent definition of Dmax should be used for all data and singleparticle properties. As an invariant measure with respect to the orientation of particles in the imaging plane for 2D probes, the diameter of the smallest circle enclosing the particle (DS) is recommended as the optimal definition of Dmax. If the 3D structure of a particle is observed, then the technique can be extended to determine the minimum enclosing sphere.

Original languageEnglish (US)
Pages (from-to)1057-1072
Number of pages16
JournalJournal of Atmospheric and Oceanic Technology
Issue number5
StatePublished - 2016


  • Aircraft observations
  • Algorithms
  • Cloud microphysics
  • Data processing
  • In situ atmospheric observations
  • Observational techniques and algorithms
  • Physical Meteorology and Climatology

ASJC Scopus subject areas

  • Ocean Engineering
  • Atmospheric Science


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