FALL3D-8.0: a computational model for atmospheric transport and deposition of particles, aerosols and radionuclides. Part I: model physics and numerics

This manuscript presents FALL3D-8.0, the last
version release of an open-source code with 15+ years of
track record and a growing number of users in the vol-
canological and atmospheric communities. The code has
been redesigned and rewritten from scratch in the framework
of the EU Center of Excellence for Exascale in Solid Earth
(ChEESE) in order to overcome legacy issues and allow
for successive optimisations that are already planned in the
preparation of the code towards extreme-scale computing.
However, this baseline version already contains substantial
improvements in terms of model physics, solving algorithms,
and code accuracy and performance. The code, originally
conceived for atmospheric dispersal and deposition of tephra
particles, has been extended to model other types of particles, aerosols and radionuclides. The solving strategy has
also been changed, replacing the former central-differences
scheme for a high-resolution central-upwind scheme derived
from finite volumes, which minimises numerical diffusion
even in presence of sharp concentration gradients and discontinuities. The parallelisation strategy, Input/Output (I/O),
model pre-process workflows and memory management have
also been reconsidered, leading to substantial improvements
on code scalability, efficiency, and overall capability to han-
dle much larger problems. This paper details the FALL3D-8.0
model physics and the numerical implementation of the code.