• Atmosphere
• /
• v.30 no.1
• /
• pp.75-89
• /
• 2020

Current in-situ airborne probes that measure the sizes of ice crystals smaller than 50 ㎛ are based on the concept that the measured intensity of light scattered by a particle in the forward and/or backward direction can be converted to particle size. The relationship between particle size and scattered light used in forward scattering probes is based on Mie theory, which assumes the refractive index of particle is known and all particles are spherical. Not only are small crystals not spherical, but also there are a wide variety of non-spherical shapes. Although it is well known that the scattering properties of non-spherical ice crystals differ from those of spherical shapes, the impacts of non-sphericity on derived in-situ particle size distributions are unknown. Thus, precise relationships between the intensity of scattered light and particle size and shape are required, as based on accurate calculations of scattering properties of ice crystals. In this study, single-scattering properties of ice crystals smaller than 50 ㎛ are calculated at a wavelength of 0.55 ㎛ using a numerically exact method (i.e., discrete dipole approximation). For these calculations, hexagonal ice crystals with varying aspect ratios are used to represent the shapes of natural small ice crystals to determine the errors caused by non-spherical ice crystals measured by forward scattering probes. It is shown that the calculated errors in sizing nonspherical ice crystals are at least 13% and 26% in forward (4~12°) and backward (168~176°) directions, respectively, and maximum errors are up to 120% and 132%.