Extension of interferometric particle imaging to small ice-crystal sizes using the discrete dipole approximation

Interferometric Particle Imaging (IPI) is a powerful technique to characterize aerosol particles, which so far has been applied only to irregular rough particles larger than about 100 wavelengths. We extend its applicability to smaller ice crystals, combining rigorous modelling of particle shapes with the Phase-Field Modelling and light-scattering simulations with the Discrete Dipole Approximation (DDA). Even for particles with the largest dimension of 11.5 wavelengths (and the smallest one comparable to the wavelength), the 2D Fourier transform of the interferometric image remains linked to the 2D autocorrelation of the particle shape at various viewing angles, validating the general measurement principle. However, the sensor must necessarily have wide viewing angle, which complicates interpretation of apparent particle shape, when such particles are observed from the edge. IPI is thus shown to be a powerful optical technology for characterizing ice particles down to a few micrometers in the atmosphere. Meanwhile, DDA is a versatile method for such synthetic experiments and can further supply large datasets for development of various inversion methods.