FALL3D in a nutshell
FALL3D is an open-source off-line Eulerian model for atmospheric passive transport and deposition. The model, originally developed for inert volcanic particles (tephra), has a track record of 50+ on different model applications and code validation, as well as an ever-growing community of users worldwide, including academia, private, research, and several institutions tasked with operational forecast of volcanic ash clouds.
The model solves the so-called Advection-Diffusion-Sedimentation (ADS) equation: \[ \frac{\partial c}{\partial t} + \nabla \vec{F} + \nabla \vec{G} + \nabla \vec{H} = S - I \] where \(\vec{F}= c~\vec{u}\) is the advective flux, \(\vec{G}= c~\vec{u_s}\) is the sedimentation flux, \(\vec{H}=-\mathbb{K} \nabla c\) is the diffusive flux, and \(S\) and \(I\) are the source and sink terms respectively. In the expressions above, \(t\) denotes time, \(c\) is concentration, \(\vec{u}\) is the fluid velocity vector (wind velocity), \(\vec{u_s}\) is the terminal settling velocity of the substance, and \(\mathbb{K}\) is the diffusion tensor.
In FALL3D v8.x, the ADS equation has been extended to handle passive transport of other substances different from tephra. In a general sense, substances in FALL3D are grouped in 3 broad categories:
- Category particles: includes any inert substance characterized by a sedimentation velocity.
- Category aerosol: refers to substances potentially non-inert (i.e. with chemical or phase changes mechanisms) and having a negligible sedimentation velocity.
- Category radionuclides: refers to isotope particles subjected to radioactive decay.
Each category admits, in turn, different sub-categories or species, defined internally as structures of data that inherit the parent category properties. For example, particles can be subdivided into tephra or mineral dust; aerosol species can include H2O, SO2, etc.; and radionuclides can include several isotope species. Finally, each sub-category of species is tagged with an attribute name that is used for descriptive purposes only.
