Spectral Line Handling in the ArchNEMESIS Radiative Transfer Modelling Tool

archnemesis is a modern Python reimplementation (the original is FORTRAN based) of the widely used NEMESIS radiative transfer and retrieval code. While scientifically robust, the original workflow for handling spectral line data relied on a collection of external helper programs to convert line database outputs into formats readable by NEMESIS. This approach posed both usability and maintenance challenges.

As part of the ongoing development of ArchNEMESIS, we have focused on redesigning the handling of spectral line, continuum, and partition function data to be more flexible, transparent, and performant. Our initial approach leveraged existing APIs provided by major spectroscopic databases, such as the HITRAN API (HAPI), enabling direct programmatic access to line data. Although functional, this method incurred significant performance costs due to repeated parsing and conversion of text-based line list formats.

To address these limitations, we have developed a unified, HDF5-based data format designed for efficient storage and rapid access to spectroscopic inputs. This format supports line data, continua, and partition functions within a single, self-describing structure. Accompanying this, we introduce a dedicated helper tool that simplifies the construction, modification, and maintenance of these HDF5 files, including automated ingestion of data from common spectroscopic databases.

We present ongoing work to accelerate line-by-line calculations within ArchNEMESIS using just-in-time compilation tools such as Numba. Together, the performace plus usability improvements will hopefully lead to easier and more efficient analysis of planetary spectra.