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

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 (D_max). In this study, different definitions of D_max 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 D_max ≤ 200 mm and D_max ≥ 2 mm. The large differences for D_max ≤ 200 mm are caused by the strong dependence of sample volume on particle size, whereas differences forD_max ≥ 2 mm are caused by the small number of particles detected. Derived bulk properties can also vary depending on the definitions of D_max because of discrepancies in the definition of D_max 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 D_max 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 (D_S) is recommended as the optimal definition of D_max. If the 3D structure of a particle is observed, then the technique can be extended to determine the minimum enclosing sphere.