Revamping FLEXPART for the next generation of simulations

The Lagrangian atmospheric transport model, FLEXPART, is used to investigate a broad spectrum of topics within the atmospheric sciences, from the propagation of particles emitted during nuclear accidents to global moisture transport. Since its inception in 1998, FLEXPART has undergone many changes, with its last official release (version 10.4) published in 2019. At the same time, numerous versions have been developed across institutes to cater to various needs. To make it easier to modify FLEXPART, while not having to diverge from the main version and its updates, we introduce a more modular way of organising the source code. Running times are improved by consistent OpenMP parallelisation in all parts of the code, resulting in reasonable scaling behaviour. Alongside the restructuring of FLEXPART, other improvements have been made. For instance, the interpolation errors have been reduced by replacing the traditional FLEXPART-internal terrain-following coordinate system with the option of doing all calculations on the native ECMWF ETA coordinate systems. Accuracy improvements are being investigated by quantifying the conservation quantities of dynamical tracers.