Flexible implementation of the particle shape and internal inhomogeneity in the invariant imbedding T-matrix method.

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.

Lidar Ratio-Depolarization Ratio Relations of Atmospheric Dust Aerosols: The Super-Spheroid Model and High Spectral Resolution Lidar Observations.

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 ) and depolarization ratio ( δ ) 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 ), roundness parameters ( 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 and α could be used to model almost the entire range of the observed joint distributions of S and δ . The S - δ relation could effectively discriminate among dust particle types. The observed S and δ values with the largest population density were best covered by models with n > 2, especially by those with n varying from 2.4 to 3.0.