Edge Effect Correction to the Physical Geometric Optics Method in the Case of a Hexagonal Column

Nonspherical particles are commonly observed in the atmosphere and ocean. Accurate quantification of the radiative effects of nonspherical particles on our climate energy budget requires a better characterization of their light scattering and absorption properties. Understanding of the single-scattering properties of nonspherical particles is improved by the successful implementation of a numerically exact method called the invariant imbedded T-matrix (II-TM) and an approximate method called the physical geometric optics method (PGOM). A previous study by Yang et al. (2019) found that applicable size parameter ranges of II-TM and PGOM overlap for ice particle shapes such as hexagonal columns and aggregates. Their preliminary results have shown that II-TM cannot be applied to very large size parameters due to the huge computational burden, and PGOM cannot be applied for very small size parameters due to the geometric optics approximation. In addition, PGOM does not include the edge effect contributions to the extinction and absorption efficiencies. The most efficient way to implement the edge effect corrections in PGOM is to develop a formula for the edge effect efficiency for non-spherical particles. In this study, we will develop an empirical formula for the edge effect contribution to the extinction and absorption efficiency based on the Debye series and shape data. We will add the edge effect correction formula to PGOM in the hopes of improving the accuracy of the extinction and absorption efficiency in PGOM for moderate size parameters. This allows more accurate use of the II-TM and PGOM combination to calculate the single-scattering properties of nonspherical particles that covers all size parameter ranges.