The backscattering optical properties of an ensemble of randomly oriented dust particles at a wavelength of 355 nm were comprehensively studied by examining the invariant imbedding T‐matrix results of the super‐spheroid dust model. In particular, we focused on the lidar ratio (S $S$) and depolarization ratio (δ $\delta $) relations of dust aerosols to aid interpretation of data from the Atmospheric Lidar (ATLID) instrument that will be onboard the Earth Cloud, Aerosol and Radiation Explorer (EarthCARE) satellite. Super‐spheroid models with various aspect ratios (α $\alpha $), roundness parameters (n) $(n)$, and refractive indices were investigated over a wide range of particle sizes and compared to the observation data of the National Aeronautics and Space Administration (NASA) Langley 355‐nm airborne high spectral resolution lidar. We found that super‐spheroid dust particles with different sets of n $n$ and α $\alpha $ could be used to model almost the entire range of the observed joint distributions of S $S$ and δ $\delta $. The S−δ $S-\delta $ relation could effectively discriminate among dust particle types. The observed S $S$ and δ $\delta $ values with the largest population density were best covered by models with n > 2, especially by those with n $n$ varying from 2.4 to 3.0.
We report a new implementation of the invariant imbedding T-matrix (IITM) method based on a discrete spherical grid approach for representing the particle shape and internal inhomogeneity. The new version of the IITM (referred to as the IITM-discrete) improves the flexibility of the IITM—especially for inhomogeneous particles. It is much more convenient for specifying the particle morphology in the electromagnetic wave scattering simulations. Particle shape is represented by a series of discrete spherical layers ranging from the inscribed sphere to the circumscribed sphere. Spherical layers are discretized by the centroidal Voronoi tessellation (CVT) approach. The procedure of computing the U-matrix (the only shape-dependent module in the T-matrix program) is simplified upon using the gridded particle shape and refractive index information saved in an external file. The grid resolution is a key factor that determines the numerical accuracy and computational cost. Numerical tests of IITM-discrete show its compatibility with other light scattering methods. Using IITM-discrete, we found that the internal inhomogeneity could have large impact on dust optical properties.
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