TransitionMatrices.jl: An open-source IITM-based framework for electromagnetic scattering simulation of nonspherical atmospheric particles and remote-sensing forward modeling
Description
High-fidelity electromagnetic scattering computation for nonspherical and internally inhomogeneous atmospheric particles is a key link between particle microphysics and observables in measurement modeling and remote-sensing forward simulation. In practice, however, the use of high-fidelity solvers is often hindered by limited openness, insufficient modularity for large-scale parameter sweeps, high computational cost for multi-orientation reuse, and weak integration with measurement-oriented workflows. We present TransitionMatrices.jl, an open-source Julia package centered on the Invariant Imbedding T-Matrix (IITM) method. The framework provides a modular pipeline covering particle modeling, IITM-based T-matrix computation, and region-wise heterogeneity partitioning acceleration: the inscribed-sphere region is treated as a homogeneous core domain and accelerated using a logarithmic-derivative-based Mie scattering method, while the region between the inscribed and circumscribed spheres is treated as an extended inhomogeneous domain with particle-shape-dependent acceleration strategies. The package further supports validation-oriented Extended Boundary Condition Method (EBCM) cross-checking, far-field scattering products (cross sections, amplitude matrix, scattering/Mueller matrix), and coordinate transformations for efficient orientation handling. We report systematic credibility checks, including Mie consistency in spherical cases, IITM–EBCM cross-validation where applicable, and rotation-related error assessment. Finally, we demonstrate WRF-coupled polarimetric forward simulations to illustrate how particle-shape assumptions propagate into simulated atmospheric observables.