Model description paper 24 Mar 2020
Model description paper  24 Mar 2020
FALL3D8.0: a computational model for atmospheric transport and deposition of particles, aerosols and radionuclides – Part 1: Model physics and numerics
 ^{1}CASE Department, Barcelona Supercomputing Center, Barcelona, Spain
 ^{2}Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Vesuviano, Naples, Italy
 ^{3}Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Bologna, Bologna, Italy
 ^{1}CASE Department, Barcelona Supercomputing Center, Barcelona, Spain
 ^{2}Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Vesuviano, Naples, Italy
 ^{3}Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Bologna, Bologna, Italy
Correspondence: Arnau Folch (afolch@bsc.es)
Hide author detailsCorrespondence: Arnau Folch (afolch@bsc.es)
This paper presents FALL3D8.0, the last version release of an opensource code with 15+ years of track record and a growing number of users in the volcanological and atmospheric communities. The code has been redesigned and rewritten from scratch in the framework of the EU Centre of Excellence for Exascale in Solid Earth (ChEESE) in order to overcome legacy issues and allow for successive optimisations in the preparation of the code towards extremescale 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 centraldifference scheme for a highresolution centralupwind scheme derived from finite volumes, which minimises numerical diffusion even in the presence of sharp concentration gradients and discontinuities. The parallelisation strategy, input/output (I/O), model preprocess workflows and memory management have also been reconsidered, leading to substantial improvements on code scalability, efficiency and overall capability to handle much larger problems. All these new features and improvements have implications on operational model performance and allow, among others, adding data assimilation and ensemble forecast in future releases. This paper details the FALL3D8.0 model physics and the numerical implementation of the code.
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FALL3D is an opensource offline Eulerian model for atmospheric passive transport and deposition based on the socalled advection–diffusion–sedimentation (ADS) equation. The model, originally developed for inert volcanic particles (tephra), has a track record of 50+ publications on different model applications and code validation, as well as an evergrowing community of users worldwide, including academic, private and research institutions and several institutions tasked with the operational forecast of volcanic ash clouds. The first versions of FALL3D (v1.x series) appeared back in 2003 (Costa and Macedonio, 2004); at that time, the code was serial and written in Fortran 77. Code improvements at different levels have been continuously incorporated since then, including relevant milestones leading to code version upgrades, e.g. the coupling with 1D buoyant plume theory (BPT) models to define eruption column sources (v2.x, 2004), the introduction of the Lax–Wendroff (LW) centraldifference scheme for solving the ADS equation (v3.x, 2005) and other algorithmic improvements (Costa et al., 2006), full code rewriting in Fortran 90 and distributed memory parallelisation by means of Message Passing Interface (MPI) (v5.x, 2007), first implementations of operational workflows to forecast ash cloud dispersal and fallout (Folch et al., 2008, 2009), and several other improvements (e.g. de la Cruz et al., 2016) until the v7.3.4 release in 2018.
Along these 15+ years, FALL3D has been used in multiple applications (e.g. Folch, 2012) including, among others, assessment of hazards from tephra fallout at different volcanoes (e.g. Scaini et al., 2012; Selva et al., 2014; Sandri et al., 2016), impacts of ash cloud dispersal on civil aviation (e.g. Sulpizio et al., 2012; Bonasia et al., 2013; Biass et al., 2014; Scaini et al., 2014), obtaining relevant eruption source parameters (e.g. Folch et al., 2012; Parra et al., 2016; Poret et al., 2017), impacts of past supereruptions on climate, environment and humans (e.g. Costa et al., 2012, 2014; Martí et al., 2016), operational forecast of ash clouds and tephra fallout (e.g. BearCrozier et al., 2012; Collini et al., 2013; Poulidis et al., 2019), modelling of ash resuspension events (e.g. Folch et al., 2014; Mingari et al., 2017) or model validation (e.g. Scollo et al., 2010; Corradini et al., 2011; Osores et al., 2013). However, as in other longlived community codes, code legacy limitations have appeared with time on, e.g. lack of code performance and poor scalability on hundreds/thousands of cores, constraints on portability and adaption to emerging hardware architectures and difficulties for code refactoring that is needed in order to widen the spectrum of model applications. With time, properly addressing these aspects required of substantial code refactoring or even code rewriting, a substantially timeconsuming task in terms of human effort. This has recently been possible in the framework of the European Centre of Excellence for Exascale in Solid Earth (ChEESE), which includes FALL3D as one of its flagship codes.
This paper describes FALL3D8.0, a new model version upgrade in which the code has been completely written from scratch, mostly in Fortran 90 but mixed with some Fortran 2003 functionalities. In addition to dramatic improvements on different levels (extended model physics and applications, numerical algorithmic and code performance), v8.0 also provides with a baseline that will allow the incorporation of developments and optimisations. However, the heterogeneity of model users has been considered when rewriting the code, which can still run on platforms spanning from a laptop to a large supercomputer.
This paper starts first outlining what is new in FALL3D8.0 (Sect. 2) with respect to the previous code release (v7.3.4) and then describes the physical model and related governing equations and parameterisations (Sect. 3). The next section focuses on the numerical implementation (Sect. 4), including coordinate mappings and scaling, spatial discretisation and a new solving strategy based on the Kurganov–Tadmor scheme (Kurganov and Tadmor, 2000) that can be combined either with a fourthorder Runge–Kutta or with a firstorder Euler to integrate explicitly in time. These two solver options allow users to choose, respectively, between better solver accuracy or higher computational efficiency. In any case, it will be shown how the Kurganov–Tadmor finitevolumelike formulation is much less diffusive than the previous scheme implemented in v7.x, an important feature when one aims at modelling substances encompassing sharp gradients of concentration. Sections 5 and 6 focus, respectively, on the (preprocess) model workflow and on the new code parallelisation strategy and related memory optimisations, including a comparison of model performance and scalability with respect to v7.x. This paper shows only one illustrative example of FALL3D8.0 model results for ash dispersal from the 2011 Cordón Caulle eruption. A companion paper (Prata et al., 2020) contains the second part, including a detailed FALL3D8.0 model validation for several simulations that are part of the new benchmark suite of the code. Finally, Sect. 7 wraps the conclusions of the paper and outlines which will be the next steps for further model optimisation.
FALL3D8.0 introduces substantial improvements at different levels. From the point of view of model physics, the following points apply:

The code has been generalised to deal with species different from tephra, including other kinds of particles, aerosols and radionuclides. Different types of species have been defined and can be simulated using independent sets of bins.

Weibull and biWeibull particle total grain size distributions (TGSDs) can now be generated in addition to lognormal distributions. On the other hand, FALL3D8.0 can estimate TGSDs of tephra particles directly from magma viscosity (magma composition) and eruption column height following the fit proposed by Costa et al. (2016a).

An “effective bin” flag has been introduced. For a given species, only those bins having this flag “on” are actually simulated, whereas bins tagged as “off” are frozen in terms of transport and used only for source term characterisation. This option has been added because volcanic plume source parameterisations depend on the whole granulometric spectrum of particles but, at the same time, model users are often interested only on a subset of the particle spectra (e.g. fine volcanic ash for longrange tephra dispersal simulations).

For the species “tephra”, several classes of particle aggregates can now be defined in certain aggregation options, differently than the singleclass aggregation option available in v7.x.

Model parameterisations for physics have been revised to include more recent studies. Meteorological drivers have also been updated to add recent datasets (e.g. ERA5) and to remove deprecated options.

Periodic boundary conditions are now possible, permitting simulations on domains spanning from local to semiglobal (pole singularities still remain).

For some species, the initial model condition can be furnished from satellite retrievals. This “data insertion” option is a preliminary step towards a full model data assimilation using ensembles.
From the point of view of numerics and code performance, the following points apply:

The solving strategy has been changed to a highresolution centralupwind scheme (Kurganov and Tadmor, 2000), optionally combined with a fourthorder Runge–Kutta explicit scheme. This replaces the former LW centraldifference scheme, which was known to be overdiffusive in the case of sharp concentration gradients or discontinuities.

New coordinate mappings have been added. These include a new vertical σcoordinate system with linear decay that smooths numerical oscillations over complex terrains (Schar et al., 2002).

A new parallelisation strategy exists based on a full 3D domain decomposition. The former trivial parallelisation on particle bins in v7.x has been removed because, in the case of interaction among bins, it yielded to unnecessary communication penalties, resulting in poor code scalability.

A much more efficient memory management exists to exploit contiguous cache memory positions along each spatial dimension. In some model configurations, this can imply a substantial gain on computing time.

Parallel model I/O using NetCDF and parallel model preprocess. In addition, all the preprocess auxiliary programmes have been embedded within the main code (a single multipurpose executable exists in v8.0). The code can now be run to perform different tasks individually or sequentially through a single parallel workflow. As a result, all the preprocess, modelling and postprocess workflows can now run as a single execution concatenating all tasks and without needing to write/read intermediate files to/from disk. In large problems, this saves substantial disk space and I/O time because the required model input data (e.g. interpolated meteorological fields) are already stored in each processor memory when running the FALL3D model task.

A hierarchy of MPI communicators has been defined. This is actually not active yet in v8.0 but gives flexibility to extend some code functionalities in a near future with little refactoring effort. For example, plans for future versions include ensemble modelling using the Parallel Data Assimilation Framework (PDAF) or model nesting. These will require of teams of processors associated with different ensemble members or model grids, respectively.
3.1 Model governing equations
In continuum mechanics, the general form describing the passive transport of a substance mixed within a fluid (air) in a domain Ω derives from mass conservation which, in conservative flux form and using the Eulerian specification, reads (see Tables 1 and 2 for list of symbols)
where F=c u is the advective flux, G=c u_{s} is the sedimentation flux, $\mathit{H}=\mathbb{K}\mathrm{\nabla}c$ 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 (in kg m^{−3}), u is the fluid velocity vector (i.e. wind velocity), u_{s} is the terminal settling velocity of the substance, and 𝕂 is the diffusion tensor (in m^{2} s^{−1}). Note that the definition of the diffusive flux H explicitly assumes Fick's first law of diffusion.
Boundary conditions are imposed at the Dirichlet Γ_{D} (inflow), Neumann Γ_{N} (outflow) and Robin Γ_{R} (ground) parts of the boundary of the computational domain Γ (with $\mathrm{\Gamma}={\mathrm{\Gamma}}_{\text{D}}\cup {\mathrm{\Gamma}}_{\text{N}}\cup {\mathrm{\Gamma}}_{\text{R}}$ and ${\mathrm{\Gamma}}_{\text{D}}\cap {\mathrm{\Gamma}}_{\text{N}}\cap {\mathrm{\Gamma}}_{\text{R}}=\mathrm{0}$) as
where $\stackrel{\mathrm{\u203e}}{c}$ is the concentration prescribed at inflow (typically $\stackrel{\mathrm{\u203e}}{c}=\mathrm{0}$), n is the outwards unit normal vector, and D=cu_{d} is the ground deposition flux (u_{d} is the ground deposition velocity). Note that when the deposition flux D coincides with the sedimentation flux G (i.e. when u_{s}=u_{d}), the boundary condition at ground reduces to the standard freeflow condition imposed at Γ_{N}.
Equation (1) is the socalled ADS equation and, in FALL3D8.0, has been extended to handle other substances different from tephra. Note that the passive transport equation (Eq. 1) neglects inertial terms and, consequently, assumes a low (≤ 1) particle Stokes number. In a general sense, substances in FALL3D8.0 are grouped in three broad categories: particles, aerosols and radionuclides. The category “particles” includes any inert substance characterised by a sedimentation velocity. In the context of FALL3D8.0, the category “aerosol” refers to substances potentially noninert (i.e. with chemical or phase changes mechanisms) and having a negligible sedimentation velocity. Finally, the category “radionuclides” refers to isotope particles subject to radioactive decay. Each of these categories admits, in turn, different subcategories or “species”, defined internally as structures of data that inherit the parent category properties (see Table 3). For example, particles can be subdivided into tephra or mineral dust; aerosol species can include H_{2}O, SO_{2}, etc.; and radionuclides can include several isotope species. Finally, each subcategory of species is tagged with an attribute name that is used for descriptive purposes only.
Depending on the species under consideration, the mass source S and sink I terms in Eq. (1) can be decomposed as
where the superscripts denote emission source terms (S^{e}; see Sect. 3.2.3), wet deposition sinks (I^{w}; see Sect. 3.2.4), particle aggregation source and sinks (S^{a} and I^{a}, respectively; see Sect. 3.2.6), radioactive decay source and sinks (S^{r} and I^{r}, respectively; see Sect. 3.2.7), and chemical reactions' source and sinks (S^{c} and I^{c}). Note that FALL3D8.0 does not account for aerosol chemistry yet. However, the code has been designed to allow incorporating this functionality in future versions in a straightforward manner.
^{1} For any species in the category particles or radionuclides, users can specify the number of effective bins from a grain size distribution. ^{2} For tephra, the Φ number is defined as $d={\mathrm{2}}^{\mathrm{\Phi}}$, where d is the particle diameter in millimetres. ^{3} SO_{2} chemistry not included yet in v8.0.
^{1} Only for volcanic aerosols. ^{2} Applies only to particles/aerosols smaller than 100 µm. ^{3} Cutoff at 100 and 1 µm assumed for below and incloud scavenging, respectively. ^{4} Not included yet in v8.0.
When the ADS equation (Eq. 1) is discretised, species in the mixture are binned in n_{b} discrete “classes” or bins, so that the total concentration c at any point of the domain decomposes as the sum of bin concentrations, i.e. $c={\sum}_{i=\mathrm{1}}^{{n}_{\text{b}}}{c}_{i}$. Substitution of bin discretisation in Eq. (1) yields to n_{b} equations (one per discrete bin), each formally identical to Eq. (1):
Note that the n_{b} equations for bins can be coupled by means of the source and sink terms, which define the eventual transfer of mass among different bins, e.g. in the case of particle aggregation/disegregation, chemical reactions and formation of child radionuclides.
3.2 Model parameterisations
Parameterisations in FALL3D have been revised and updated, removing deprecated options and adding new options available from more recent studies. Table 4 shows the parameterisations implemented in FALL3D8.0 that are described in the following subsections.
3.2.1 Diffusion tensor
The atmospheric flow is characterised by large horizontaltovertical aspect ratios of wind velocities and length scales, as well as by an anisotropic momentum diffusion with the horizontal diffusion coefficient being typically much larger than the vertical one (e.g. SchaeferRolffs and Becker, 2013). For this reason, model diffusion that accounts for subgridscale atmospheric eddies is typically assumed anisotropic, with two distinct eddy diffusion coefficients along the horizontal and vertical dimensions:
In the case of FALL3D8.0, the horizontal coefficient k_{h} can be either assumed constant or parameterised as in Byun and Schere (2006):
where
In the equations above, Δ_{g} is a characteristic grid cell measure (e.g. the equivalent area length), α≅0.28 denotes the Smagorinsky constant, S_{Γ} and S_{Λ} are the stretching and shear strength (i.e. the two components of the bidimensional wind deformation), and k_{ref} is a reference horizontal diffusion for a reference grid cell size Δ_{ref} (FALL3D8.0 considers k_{ref}=8000 m^{2} s^{−1} for Δ_{ref}=4 km). Equation (6) was proposed by Byun and Schere (2006) to overcome the dependency of horizontal diffusion on grid resolution and combines a Smagorinsky subgridscale (SGS) model giving the diffusion by transport (k_{ht}) with a formula that counteracts numerical overdiffusion in coarse discretisations (k_{hn}) so that the smaller one between k_{ht} and k_{hn} dominates. In this way, the effect of the transportive diffusion is minimised for coarse grids, whereas for fine discretisations the numerical diffusion term is reduced.
The vertical diffusion coefficient k_{v} can also be either assumed constant or parameterised according to the similarity theory and distinguishing among surface layer, atmospheric boundary layer (ABL) and free atmosphere (e.g. Neale et al., 2010):
where κ is the von Karman constant (κ=0.4), z is the distance from the ground, u_{*} is wind friction velocity, ϕ_{h} is the atmospheric stability function for temperature, h_{p} is the ABL height, l_{c} is a characteristic length scale, u_{h} is the horizontal wind velocity modulus, and F_{c} is a stability function which depends on the Richardson number Ri. For l_{c} and F_{c}, FALL3D8.0 adopts the relationships used by the Community Atmosphere Model (CAM) 4.0 model (Neale et al., 2010):
where λ_{c} is the socalled asymptotic length scale (λ_{c}≈30 m). The atmospheric stability function for temperature ϕ_{h} is calculated as
where β_{h}=5, γ_{h}=15, and L is the Monin–Obukhov length, defined as
with g denoting gravity, ${\stackrel{\mathrm{\u203e}}{\mathit{\theta}}}_{\text{v}}$ the mean potential virtual temperature and θ_{*} the potential temperature scale. The parameters in Eq. (13) (i.e. L or ${u}_{*}/{\mathit{\theta}}_{*}$) are ideally furnished by the driving meteorological model. If not, and alternatively, FALL3D8.0 estimates the friction velocity u_{*} and the potential temperature scale θ_{*} from the potential virtual temperature θ_{v} and the Richardson bulk number Ri_{b} as (Louis, 1979; Jacobson, 1999)
where z_{r} and z_{o} denote the reference and the ground roughness heights, and ${\stackrel{\mathrm{\u203e}}{\mathit{\theta}}}_{\text{v}}$ denotes the average between the two vertical levels. Given Ri_{b}, one can estimate u_{*} and θ_{*} as
where Pr_{t} is the turbulent Prandtl number (Pr_{t}≈1) and the stability functions G_{m} and G_{h} are given by (Louis, 1979; Jacobson, 1999)
3.2.2 Sedimentation velocity
Particle bins in the model are assumed to settle down with a sedimentation velocity ${\mathit{u}}_{\text{s}}=(\mathrm{0},\mathrm{0},{w}_{\text{s}})$ equal to its terminal velocity:
where ρ_{a} and ρ_{p} denote air and particle density, d is the particle equivalent diameter, and C_{d} is the drag coefficient that depends on the Reynolds number, $\mathit{Re}=d{u}_{\text{s}}/{\mathit{\nu}}_{\text{a}}$ (${\mathit{\nu}}_{\text{a}}={\mathit{\mu}}_{\text{a}}/{\mathit{\rho}}_{\text{a}}$ being the kinematic viscosity of air and μ_{a} its dynamic viscosity). For irregular particles, the drag coefficient C_{d} has to be obtained from experimental measurements. FALL3D8.0 includes several parameterisations derived from laboratory results using natural and synthetic particles and that cover a wide range of particle sizes and shapes (characterised by sphericity, by circularity, or by some other model shape factor). Model options for the drag coefficient C_{d} include the following:

The GANSER model (Ganser, 1993) shows that
$$\begin{array}{ll}\text{(20)}& {\displaystyle}{C}_{\text{d}}=& {\displaystyle \frac{\mathrm{24}}{\mathit{Re}{K}_{\mathrm{1}}}}{\displaystyle}\left\{\mathrm{1}+\mathrm{0.1118}{\left(\mathit{Re}\phantom{\rule{0.125em}{0ex}}{K}_{\mathrm{1}}{K}_{\mathrm{2}}\right)}^{\mathrm{0.6567}}\right\}{\displaystyle}& {\displaystyle}+{\displaystyle \frac{\mathrm{0.4305}{K}_{\mathrm{2}}}{\mathrm{1}+\frac{\mathrm{3305}}{\mathit{Re}{K}_{\mathrm{1}}{K}_{\mathrm{2}}}}},\end{array}$$where ${K}_{\mathrm{1}}=\mathrm{3}/\left[\right({d}_{n}/d)+\mathrm{2}{\mathrm{\Psi}}^{\mathrm{0.5}}]$ and ${K}_{\mathrm{2}}={\mathrm{10}}^{\mathrm{1.8148}(\text{Log}\mathrm{\Psi}{)}^{\mathrm{0.5743}}}$ are two shape factors, d_{n} is the average between the minimum and the maximum axis, and Ψ is the particle sphericity (Ψ=1 for spheres). For calculating the sphericity, it is practical to use the concepts of “operational” and “working sphericity”, Ψ_{work} introduced by Wadell (1933) and Aschenbrenner (1956), which are based on the determination of the volume and of the three dimensions of a particle, respectively:
$$\begin{array}{}\text{(21)}& {\mathrm{\Psi}}_{\text{work}}=\mathrm{12.8}{\displaystyle \frac{({P}^{\mathrm{2}}Q{)}^{\mathrm{1}/\mathrm{3}}}{\mathrm{1}+P(\mathrm{1}+Q)+\mathrm{6}\sqrt{\mathrm{1}+{P}^{\mathrm{2}}(\mathrm{1}+{Q}^{\mathrm{2}})}}},\end{array}$$with $P={d}_{\mathrm{3}}/{d}_{\mathrm{2}}$, $Q={d}_{\mathrm{2}}/{d}_{\mathrm{1}}$, where d_{1} is the longest particle dimension, d_{2} is the longest dimension perpendicular to d_{1}, and d_{3} is the dimension perpendicular to both d_{1} and d_{2}.

The PFEIFFER model (Pfeiffer et al., 2005), based on the interpolation of previous relationships by Walker et al. (1971) and Wilson and Huang (1979), shows
$$\begin{array}{}\text{(22)}& {C}_{\text{d}}=\left\{\begin{array}{ll}\frac{\mathrm{24}}{\mathit{Re}}{\mathit{\phi}}^{\mathrm{0.828}}+\mathrm{2}\sqrt{\mathrm{1}\mathit{\phi}}& \mathit{Re}\le {\mathrm{10}}^{\mathrm{2}}\\ \mathrm{1}\frac{\mathrm{1}{C}_{\text{d}}{}_{\mathit{Re}={\mathrm{10}}^{\mathrm{2}}}}{\mathrm{900}}({\mathrm{10}}^{\mathrm{3}}\mathit{Re})& {\mathrm{10}}^{\mathrm{2}}\le \mathit{Re}\le {\mathrm{10}}^{\mathrm{3}}\\ \mathrm{1}& \mathit{Re}\ge {\mathrm{10}}^{\mathrm{3}},\end{array}\right.\end{array}$$where $\mathit{\phi}=(b+c)/\mathrm{2}a$ is the particle aspect ratio ($a\ge b\ge c$ denote the particle semiaxes).

The DIOGUARDI model (Dioguardi et al., 2018) demonstrates that
$$\begin{array}{}\text{(23)}& \begin{array}{rl}{C}_{\text{d}}& ={\displaystyle \frac{\mathrm{24}}{\mathit{Re}}}{\left({\displaystyle \frac{\mathrm{1}\mathit{\xi}}{\mathit{Re}}}+\mathrm{1}\right)}^{\mathrm{0.25}}\\ & +{\displaystyle \frac{\mathrm{24}}{\mathit{Re}}}\left(\mathrm{0.1806}{\mathit{Re}}^{\mathrm{0.6459}}\right){\mathit{\xi}}^{{\mathit{Re}}^{\mathrm{0.08}}}{\displaystyle \frac{\mathrm{0.4251}}{\mathrm{1}+\frac{\mathrm{6880.95}}{{\mathit{Re}}^{\mathrm{2}}}{\mathit{\xi}}^{\mathrm{5.05}}}},\end{array}\end{array}$$where ξ is a particle shape factor (sphericity to circularity ratio), for which Dioguardi et al. (2018) suggested an empirical correlation with sphericity Ψ as ξ=0.83Ψ.
Note that, in any case, the terminal velocity w_{s} is defined by a triplet $(d,{\mathit{\rho}}_{\text{p}},\mathrm{\Psi})$. As a result, particles with similar values of the three parameters can be grouped within the same model bin.
3.2.3 Emissions
The emission source term for the ith bin (${S}_{i}^{\text{e}}$ term in the bin equations; see Eq. 4) gives the mass per unit of time and volume (units of $\mathrm{kg}\phantom{\rule{0.125em}{0ex}}{\mathrm{m}}^{\mathrm{3}}\phantom{\rule{0.125em}{0ex}}{\mathrm{s}}^{\mathrm{1}}$) released at each point (cell) of the computational domain. FALL3D8.0 can generate and handle multiple types of emission sources, internally defined as a data structure made of n_{p} discrete points, each “tagged” with a timevarying position and bin emission rate (source strength) M_{ip} (in kg s^{−1}). As a result, ${S}_{i}^{\text{e}}$ in a model grid cell results from summing emissions from all point sources laying within the cell volume V:
The total source strength M_{o} results from summing over all source points and bins, i.e.
Table 5 summarises the different (exclusive) options available in FALL3D8.0 for the emission term and related source strength.

The POINT option assumes that all mass is emitted from a single point (n_{p}=1) located at height z_{t} above ground level:
$$\begin{array}{}\text{(26)}& {M}_{i\mathrm{1}}=\left\{\begin{array}{ll}{f}_{i}{M}_{\text{o}}& z={z}_{\text{t}}\\ \mathrm{0}& z\ne {z}_{\text{t}},\end{array}\right.\end{array}$$where f_{i} is the ith bin mass fraction.

The HAT option defines a uniform vertical line of n_{p} source points spanning in height from z_{b} (bottom) to z_{t} (top) above the ground (i.e. with thickness z_{t}−z_{b}):
$$\begin{array}{}\text{(27)}& {M}_{ip}=\left\{\begin{array}{ll}\frac{{f}_{i}\phantom{\rule{0.25em}{0ex}}{M}_{\text{o}}}{{n}_{\text{p}}}& {z}_{\text{b}}\le z\le {z}_{\text{t}}\\ \mathrm{0}& \text{otherwise}.\end{array}\right.\end{array}$$Note that this option includes as endmembers the POINT option (if z_{b}=z_{t}) and a vertically uniform emission from ground to top (if z_{b}=0).

The SUZUKI option (Suzuki, 1983; Pfeiffer et al., 2005) assumes a mushroomlike vertical distribution of n_{p} emission points depending on two dimensionless parameters (A_{s} and λ_{s}):
$$\begin{array}{}\text{(28)}& {\displaystyle}{M}_{ip}={\displaystyle \frac{{f}_{i}\phantom{\rule{0.25em}{0ex}}{M}_{\text{o}}}{{n}_{\text{p}}}}{\left[\left(\mathrm{1}{\displaystyle \frac{z}{{z}_{\text{t}}}}\right){e}^{{A}_{\text{s}}\left(\frac{z}{{z}_{\text{t}}}\mathrm{1}\right)}\right]}^{{\mathit{\lambda}}_{\text{s}}}{\displaystyle}\mathrm{0}\le z\le {z}_{\text{t}}.\end{array}$$The Suzuki parameter A_{s} controls the vertical location of the maximum of the emission profile, whereas the parameter λ_{s} controls the distribution of the emitted mass around the maximum. When any of the previous source options are defined for volcanic plumes, it is useful to prescribe the total source strength (eruption mass flow rate) M_{o} in terms of the eruption column height H because this parameter is easier to be obtained from direct observations. To this purpose, FALL3D8.0 includes two relationships that correlate M_{o} with H based on empirical observations and on 1D plume model simulations, respectively. The first and simplest case considers the fit proposed by Mastin et al. (2009):
$$\begin{array}{}\text{(29)}& {M}_{\text{o}}=a{H}^{\mathrm{4.15}},\end{array}$$where a=140.8 is a constant and H is the eruption column height expressed in kilometres above the eruptive vent. Alternatively, the 1D model fit by Woodhouse et al. (2016) can also be used to provide M_{o} depending on the surrounding atmospheric conditions:
$$\begin{array}{}\text{(30)}& {M}_{\text{o}}={c}_{\text{o}}{N}^{\mathrm{3}}{H}^{\mathrm{4}}f\left(\stackrel{\mathrm{\u203e}}{W}\right),\end{array}$$where N is the Brunt–Väisälä frequency, c_{o} is a constant, and f is a function of the parameter $\stackrel{\mathrm{\u203e}}{W}=\mathrm{1.44}\stackrel{\mathrm{\u203e}}{\mathit{\gamma}}/N$ given by
$$\begin{array}{}\text{(31)}& f\left(\stackrel{\mathrm{\u203e}}{W}\right)={\left({\displaystyle \frac{\mathrm{1}+b\stackrel{\mathrm{\u203e}}{W}+c{\stackrel{\mathrm{\u203e}}{W}}^{\mathrm{2}}}{\mathrm{1}+a\stackrel{\mathrm{\u203e}}{W}}}\right)}^{\mathrm{4}},\end{array}$$with the coefficients $a=\mathrm{0.87}+\mathrm{0.05}{\mathit{\beta}}_{\text{e}}/{\mathit{\alpha}}_{\text{e}}$, $b=\mathrm{1.09}+\mathrm{0.32}{\mathit{\beta}}_{\text{e}}/{\mathit{\alpha}}_{\text{e}}$, and $c=\mathrm{0.06}+\mathrm{0.03}{\mathit{\beta}}_{\text{e}}/{\mathit{\alpha}}_{\text{e}}$, with α_{e} and β_{e} the radial and crosswind plume entrainment coefficients (Costa et al., 2016b) and $\stackrel{\mathrm{\u203e}}{\mathit{\gamma}}$ the mean shear rate of wind. The constant c_{o} in Eq. (30) depends on the conditions at the vent:
$$\begin{array}{}\text{(32)}& {c}_{\text{o}}={\displaystyle \frac{\mathrm{0.35}\phantom{\rule{0.25em}{0ex}}{\mathit{\alpha}}_{\text{e}}^{\mathrm{2}}\phantom{\rule{0.25em}{0ex}}{\mathit{\rho}}_{a}{C}_{\text{a}}{T}_{\text{a}}}{g\left[({C}_{\text{v}}{n}_{\text{o}}+{C}_{\text{s}}(\mathrm{1}{n}_{\text{o}}\left)\right){T}_{\text{o}}{C}_{a}{T}_{\text{a}}\right]}},\end{array}$$with C_{s}, C_{v} and C_{a} the specific heat capacities at constant pressure of the solid pyroclasts, gas phase and air, respectively, T_{a} and T_{o} the air and vent magma mixture temperatures, and n_{0} the gas mass fraction.

The PLUME option, valid only for volcanic plumes, uses the FPLUME1.0 model (Folch et al., 2016) embedded in FALL3D8.0. FPLUME1.0 is a steadystate 1D crosssectionaveraged eruption column model based on the buoyant plume theory (BPT). The model accounts for plume bending by wind, entrainment of ambient moisture, effects of water phase changes, particle fallout and reentrainment, and a model for wet aggregation of ash particles in the presence of liquid water or ice. As opposed to the previous cases, the PLUME source option automatically determines a bindependent vertical distribution of mass and computes height from M_{o}, or vice versa by solving an inverse problem.

The RESUSPENSION option considers the remobilisation and resuspension by wind of soil particles (e.g. mineral dust or volcanic ash previously deposited on the ground). Up to three different emission schemes are available in FALL3D8.0 to obtain the vertical flux of suspended particles, from which M_{o} is obtained by multiplying by the associated surface cell area (see Folch et al., 2014, for details). Typically, the emission schemes for mineral dust are formulated in terms of the friction velocity. For example, emission scheme 1 (Westphal et al., 1987) considers
$$\begin{array}{}\text{(33)}& {F}_{\text{V}}=\left\{\begin{array}{ll}\mathrm{0}& {u}_{*}<{u}_{*t}\\ {\mathrm{10}}^{\mathrm{5}}{u}_{*}^{\mathrm{4}}& {u}_{*}\ge {u}_{*t},\end{array}\right.\end{array}$$where F_{V} is the vertical flux (in $\mathrm{kg}\phantom{\rule{0.125em}{0ex}}{\mathrm{m}}^{\mathrm{2}}\phantom{\rule{0.125em}{0ex}}{\mathrm{s}}^{\mathrm{1}}$), occurring only above a (constant) threshold friction velocity u_{*t} (u_{*} given in m s^{−1}). An important limitation of Eq. (33) is that the vertical flux does not depend on either particle size or soil moisture. However, despite its simplicity, this parameterisation can be useful when information on soil characteristics (e.g. particle sizes and densities, moisture, roughness) is unavailable or poorly constrained. Emission scheme 2 (Marticorena and Bergametti, 1995; Marticorena et al., 1997) considers
$$\begin{array}{}\text{(34)}& {F}_{\text{V}}=\left\{\begin{array}{ll}\mathrm{0}& \phantom{\rule{0.33em}{0ex}}{u}_{*}<{u}_{*t}\\ {S}_{\text{c}}{\displaystyle \frac{{\mathit{\rho}}_{\text{a}}}{g}}{u}_{*}^{\mathrm{3}}\left(\mathrm{1}{\displaystyle \frac{{u}_{*t}^{\mathrm{2}}}{{u}_{*}^{\mathrm{2}}}}\right)\left(\mathrm{1}+{\displaystyle \frac{{u}_{*t}}{{u}_{*}}}\right)& \phantom{\rule{0.33em}{0ex}}{u}_{*}\ge {u}_{*t},\end{array}\right.\end{array}$$where the experimental coefficient S_{c} (in cm^{−1}) depends on the amount of available fine particles in the soil, and the threshold friction velocity is given by
$$\begin{array}{}\text{(35)}& \begin{array}{rl}& {u}_{*t}=\\ & \left\{\begin{array}{ll}\frac{\mathrm{0.129}\phantom{\rule{0.125em}{0ex}}K}{(\mathrm{1.928}{\mathit{Re}}^{\mathrm{0.092}}\mathrm{1}{)}^{\mathrm{0.5}}}& \mathrm{0.03}<\mathit{Re}\le \mathrm{10}\\ \mathrm{0.129}K(\mathrm{1}\mathrm{0.0858}{e}^{\mathrm{0.0617}(\mathit{Re}\mathrm{10})})& \mathit{Re}>\mathrm{10},\end{array}\right.\end{array}\end{array}$$with $K=\sqrt{\frac{{\mathit{\rho}}_{\text{p}}gd}{{\mathit{\rho}}_{\text{a}}}\left(\mathrm{1}+\frac{\mathrm{0.006}}{{\mathit{\rho}}_{\text{p}}g{d}^{\mathrm{2.5}}}\right)}$ and $\mathit{Re}=\mathrm{1331}\times {d}^{\mathrm{1.56}}$ (the lower bound of the fit corresponds to particles of ≈ 10 µm in size). Note that in Eq. (35), ρ_{p} and ρ_{a} are particle and air densities (expressed in g cm^{−3}), g is gravity (in cm s^{−2}), d is the particle size (in centimetres), Re is the Reynolds number parameterised as a function of the particle size, and u_{*t} is given in cm s^{−1}.
Finally, emission scheme 3 (Shao et al., 1993; Shao and Leslie, 1997; Shao and Lu, 2000) considers that the uplift from surface of the fine fraction of soil particles is controlled by the bombardment of saltating particles of larger sizes (≥ 63 µm), which breaks the cohesive forces of smaller particles.
Based on theoretical and experimental results, Shao et al. (1993) found an expression for the vertical flux of dust particles of size d ejected by the impact of saltating particles of size d_{s}:
$$\begin{array}{}\text{(36)}& {F}_{\text{V}}(d,{d}_{\text{s}})={\displaystyle \frac{{\mathit{\alpha}}_{\text{s}}(d,{d}_{\text{s}})}{{u}_{*t}^{\mathrm{2}}\left(d\right)}}{F}_{\text{H}}\left({d}_{\text{s}}\right),\end{array}$$where α_{s} (in m s^{−2}) is the coefficient of sandblasting efficiency determined experimentally (Shao and Leslie, 1997) and F_{H} is the horizontal flux (in $\mathrm{kg}\phantom{\rule{0.125em}{0ex}}{\mathrm{m}}^{\mathrm{1}}\phantom{\rule{0.125em}{0ex}}{\mathrm{s}}^{\mathrm{1}}$) of saltating particles:
$$\begin{array}{}\text{(37)}& {F}_{\text{H}}\left({d}_{\text{s}}\right)=\left\{\begin{array}{ll}\mathrm{0}& {u}_{*}<{u}_{*t}\left({d}_{\text{s}}\right)\\ {c}_{\text{o}}\frac{{\mathit{\rho}}_{\text{a}}{u}_{*}^{\mathrm{3}}}{g}\left(\mathrm{1}\frac{{u}_{*t}^{\mathrm{2}}\left({d}_{\text{s}}\right)}{{u}_{*}^{\mathrm{2}}}\right)& {u}_{*}\ge {u}_{*t}\left({d}_{\text{s}}\right),\end{array}\right.\end{array}$$where c_{o} is an empirical dimensionless constant close to 1. In this scheme, the threshold friction velocity u_{*t}(d) is given by
$$\begin{array}{}\text{(38)}& {u}_{*ts}=\sqrt{\mathrm{0.0123}\left({\displaystyle \frac{{\mathit{\rho}}_{\text{p}}gd}{{\mathit{\rho}}_{\text{a}}}}+{\displaystyle \frac{\mathit{\gamma}}{{\mathit{\rho}}_{\text{a}}d}}\right)},\end{array}$$where γ is an experimental parameter ranging between $\mathrm{1.65}\times {\mathrm{10}}^{\mathrm{4}}$ and $\mathrm{5}\times {\mathrm{10}}^{\mathrm{4}}$ kg s^{2} (a value of $\mathrm{3}\times {\mathrm{10}}^{\mathrm{4}}$ kg s^{2} is assumed in FALL3D8.0).
3.2.4 Deposition mechanisms
In FALL3D8.0, dry and wet deposition mechanisms can be activated for any type of bin below a certain particle/aerosol size.
Dry deposition on the ground is imposed prescribing the deposition velocity through a Robin boundary condition in Eq. (2). FALL3D8.0 admits two dry deposition parameterisations which describe the vertical depositional fluxes by Brownian diffusion and inertial impaction, parameterised through the Schmidt and the Stokes numbers, respectively. The first option considers the massconsistent formulation proposed by Venkatram and Pleim (1999):
where r_{a} describes the effects of aerodynamic resistance and r_{b} the quasilaminar resistance (e.g. Brandt et al., 2002, and references therein). The aerodynamic resistance r_{a} can be calculated as
with z_{o} denoting the ground roughness height and ϕ_{h} the atmospheric stability function for temperature given by Eq. (12). The quasilaminar resistance r_{b} can be expressed in terms of the Schmidt number $Sc=\mathit{\nu}/D$ and Stokes number $\mathit{St}={w}_{s}{u}_{*}^{\mathrm{2}}/\left(g\mathit{\nu}\right)$ (with ν kinematic viscosity of air and D molecular diffusivity of particles) (e.g. Brandt et al., 2002):
The second option is that proposed by Feng (2008), which essentially differs from Eq. (39) in the estimation of r_{b}:
where c_{1}=0.0226, c_{2}=40300 and c_{3}=15330 are dimensionless parameterisation constants, Re^{*} is the Reynolds number (computed with the friction velocity u_{*}), and a and b are coefficients that depend on the particle size and surface characteristics. Note that Feng (2008) gives a and b bestfit values for seven land use categories and four aerosol size modes: nuclei (up to 0.1 µm), accumulation (up to 2.5 µm), coarse (up to 10 µm) and giant (up to 100 µm). A cutoff is assumed above this size because the sedimentation velocity term w_{s} dominates, and therefore the dry deposition contribution can be neglected.
Wet deposition mechanisms in FALL3D8.0 are assumed to occur only within the ABL, and the corresponding sink term in Eq. (3) is parameterised as
where Λ differs for incloud (ic) and belowcloud (bc) sinks. For belowcloud scavenging (precipitation), Λ_{bc} is estimated from the total precipitation rate as (e.g. Brandt et al., 2002; Jung and Shao, 2006)
where P is the precipitation rate (in mm h^{−1}), and $a=\mathrm{8.4}\times {\mathrm{10}}^{\mathrm{5}}$ and b=0.79 are two empirical constants. For incloud scavenging (rainout), the model considers a parameterisation based on the atmospheric relative humidity (RH; in %) as in Brandt et al. (2002):
with ${A}_{\text{RH}}=\mathrm{3.5}\times {\mathrm{10}}^{\mathrm{5}}$, RH_{t}=80 % (threshold value) and RH_{s}=100 % (saturation value). Two critical particle cutoff sizes of 100 and 1 µm are assumed for below and incloud scavenging, respectively.
3.2.5 Gravity spreading of the umbrella region
Large explosive volcanic eruptions can generate gravitydriven transport mechanisms that dominate over passive transport close to the vent and cause a radial spreading of the cloud (e.g. Woods and Kienle, 1994; Sparks et al., 1997). In order to simulate this mechanism, FALL3D8.0 includes a gravity current model (see Costa et al., 2013, and the erratum published in June 2019). This option consists on adding a radial velocity field to the background wind, so that contributions from both passive and densitydriven mechanisms are accounted for. The added radial wind is centred above the eruptive vent in the umbrella region and extended up to a radius R given by
where t is time since eruption onset, λ_{g} is an empirical constant constrained to ≈0.2 from direct numerical simulations (Suzuki and Koyaguchi, 2009), N is the Brunt–Väisälä frequency, and q is the volumetric flow rate into the umbrella region, estimated as (Morton et al., 1956; Suzuki and Koyaguchi, 2009; Costa et al., 2013, as correct in the erratum published in 2019)
where M_{o} is the total source strength (i.e. mass eruption rate), k_{e} is the air entrainment coefficient, and c is a constant that from varies from tropical to midlatitude/polar locations:
Given the radius R, the radial velocity field as a function of distance r is calculated as (Costa et al., 2013)
where u_{r}(R) is the front velocity:
In order to avoid sudden jumps at the gravity current front, FALL3D8.0 interpolates the front velocity u_{r}(R) with far field wind velocity using an exponential decay function of the cloud thickness h_{c} as
where r is the distance from the current front.
3.2.6 Aggregation
Aggregation of tephra particles can occur inside the eruptive columns or even downwind in ash clouds during atmospheric dispersion, thereby affecting the sedimentation dynamics and deposition of volcanic ash. FALL3D8.0 includes some simple a priori aggregation options and a wet aggregation model (Costa et al., 2010; Folch et al., 2010) that can be activated for tephra bins. The a priori options consist of userdefined or empirically based predefined fractions of aggregating classes being transferred to one or more class of aggregates at the source points (i.e. aggregation is performed before transport). In contrast, in the wet aggregation model, ash particles aggregate on a single effective class of diameter d_{A}; i.e. aggregation only affects tephra bins with diameter smaller than d_{A}, typically in the range 100–300 µm. This option can run embedded in FPLUME1.0 or as a standalone module. Consider a tephra grain size distribution in which k particle bins can aggregate. Then, the aggregation model defines the source (S^{a}) and sink (I^{a}) bin terms for the corresponding k+1 bins as
where d_{j}(<d_{A}) and ρ_{j} are, respectively, the diameter and density of particles in bin j, and ${\dot{n}}_{j}$ is the number of particles per unit volume and time that aggregate. The model assumes that this is proportional to the total particle decay per unit volume and time ${\dot{n}}_{\text{tot}}$, i.e.
where
is the number of primary particles of diameter d_{j} in an aggregate of diameter d_{A}, k_{f} is a fractal prefactor (k_{f}≈1), and D_{f} is the fractal exponent (D_{f}≤3). The model estimates the total particle decay per unit time ${\dot{n}}_{\text{tot}}$ integrating the coagulation kernel over all particle sizes, depending on the sticking efficiency times a collision frequency function which accounts for Brownian motion, collision due to turbulence as a result of inertial effects, laminar and turbulent fluid shear, and differential sedimentation (see Costa et al., 2010; Folch et al., 2016, for details).
3.2.7 Radioactive decay
FALL3D8.0 can handle the fate of radioactive material dispersed from accidental releases (e.g. Brandt et al., 2002; Leelössya et al., 2018). Five common species of radionuclides have been implemented: cesium (^{134}Cs, ^{137}Cs) and iodine (^{131}I), which decay to stable isotopes, and ^{90}Sr, which decays to the unstable isotope ^{90}Y (see Table 6).
Radionuclide species need to specify the source (${S}_{n}^{\text{r}}$) and the sink (${I}_{n}^{\text{r}}$) terms in Eq. (3) associated with the radioactive production or decay of the isotope n, respectively. The radioactive decay term indicates the mass per unit volume of the isotopes of type n that decays per unit time:
where c_{n} is concentration (expressed in kg m^{−3}) and k_{r} a constant specific of each isotope (decay rate) that can be calculated from the radioactive element half life t_{1∕2} as ${k}_{\text{r}}=\mathrm{ln}\left(\mathrm{2}\right)/{t}_{\mathrm{1}/\mathrm{2}}$. Values of t_{1∕2} and k_{r} for common radionuclides are reported in Table 6. Note that the decay term is more relevant for isotopes with short half lives, e.g. for ^{131}I, which has ${t}_{\mathrm{1}/\mathrm{2}}\simeq \mathrm{8}$ d (Brandt et al., 2002; Leelössya et al., 2018).
The radioactive decay term of the isotope n constitutes a sink ${I}_{n}^{\text{r}}$ for the isotope itself. However, the decay of the isotope m, father of isotope n, constitutes a source ${S}_{n}^{\text{r}}$ for the isotope n:
where p_{mn} is the relative probability of decay of the isotope m to the isotope n. If the isotope m has only one child n, the relative probability of the branch m↦n is p_{mn}=1. Note from Table 6 that ^{134}Cs, ^{137}Cs and ^{131}I decay to stable isotopes, whereas ^{90}Sr decays to ^{90}Y which, in turn, is unstable and decays to the stable isotope ^{90}Zr. The production rate ${S}_{n}^{\text{r}}$ of ^{90}Y is therefore equivalent to the decay rate of ^{90}Sr.
In FALL3D8.0, the radioactive decay is implemented by firstly transporting the radionuclides for a time step Δt and then by evaluating the decay during the same time step. For radionuclides that decay to a stable isotope (^{134}Cs, ^{137}Cs and ^{131}I), it is considered that after a time step Δt the concentration decreases as
whereas for decay to an unstable isotope (yttrium), the concentration varies as
where c_{Y} and c_{Sr} are, respectively, the concentrations of ^{90}Y and ^{90}Sr, and k_{Y} and k_{Sr} are the corresponding decay rates.
3.3 Data insertion
Instrumentation aboard the new generation of geostationary satellites provides an unprecedented level of spatial resolution and temporal frequency (2 to 4 km pixel size and 10 to 15 min observation period; see Table 7), yielding a quasiglobal coverage considering the overlap of different existing platforms. This is very suitable for highresolution model data assimilation and related uncertainty quantification, as well as to implement ensemblebased dispersal forecast systems. These aspects are still under development and hopefully will be part of next FALL3D model distributions. However, FALL3D8.0 already includes the possibility of initialising a model run from satellite retrievals. This option, known as data insertion, is typically used in dispersal of volcanic ash and aerosols (mainly SO_{2}) in order to reduce model uncertainties coming from the eruption source term.
Satellite retrievals giving cloud column mass of fine volcanic ash and aerosols can be furnished to the model together with values of cloud thickness, with the latter needed in order to compute initial concentration (in kg m^{−3}) from column mass (in kg m^{−2}). In the model initialisation step, gridded satellite data are interpolated into the model grid imposing conservation of mass when concentration values are computed for each model grid cell, i.e. ensuring that the resulting column mass in the model (computed concentration multiplied by cloud thickness) equals that of satellite data over the same cell area. Examples showing how data insertion improves model accuracy are given in the companion paper (Prata et al., 2020).
4.1 Coordinate mappings and scaling
Consider the ADS equation (Eq. 1) written in a Cartesian system of coordinates $(x,y,z)$ assuming a diffusion tensor as in Eq. (5) and a sedimentation velocity ${\mathit{u}}_{\text{s}}=(\mathrm{0},\mathrm{0},{w}_{\text{s}})$ aligned with the vertical coordinate z:
It is straightforward to discretise the above equation in a “bricklike” computational domain Ω_{c} using a structured regular (i.e. equally spaced) mesh, although the regularity condition is typically relaxed across the vertical direction so that the vertical grid resolution increases close to ground, where higher gradients are expected. In order to use other coordinate systems, Eq. (59) can be written on a generalised orthogonal system of coordinates $({X}_{\mathrm{1}},{X}_{\mathrm{2}},{X}_{\mathrm{3}})$ (e.g. Toon et al., 1988; Byun and Schere, 2005):
where C is the scaled concentration, $(U,V,W)$ are the scaled wind components, $({K}_{\mathrm{1}},{K}_{\mathrm{2}},{K}_{\mathrm{3}})$ are the scaled diffusion coefficients, and S^{*} and I^{*} are the scaled source and sink terms. The implementation of a generalised equation like Eq. (60) presents two major advantages. On one hand, the generalised equation reads formally equal to that in Cartesian coordinates, so that little computational penalty exists to map physical domains (e.g. accounting for Earth's curvature and topography) to a “bricklike” computational domain (see Fig. 1) by using coordinatedependent horizontal and vertical mappings. On the other hand, a generalised form simplifies the structure and implementation of the code because the model can be solved on various horizontal (Cartesian, spherical, Mercator, polar stereographic, etc.) and vertical (terrainfollowing, σcoordinate, etc.) coordinate systems using only one solving routine. To this purpose, one needs first to scale the model coordinates and some terms in the equation using adequate mapping and scaling factors, then solve for the scaled concentration C in the regular computational domain Ω_{c} (as in Cartesian coordinates) and, finally, transform the scaled concentration back to the original one.
In general and given two orthogonal coordinate systems, $({x}_{\mathrm{1}},{x}_{\mathrm{2}},{x}_{\mathrm{3}})$ and $({X}_{\mathrm{1}},{X}_{\mathrm{2}},{X}_{\mathrm{3}})$, coordinate mapping factors are given by the terms m_{ij} of the Jacobian transformation matrix 𝕄:
but, for the transformations considered here, 𝕄 will always be diagonal with three nonzero components (m_{1}, m_{2} and m_{3}). For example, in the horizontal transformation to spherical Earth surface coordinates (λ,ϕ), one has
where R_{e} is the radius of the Earth, and λ, ϕ and γ are the longitude, latitude and colatitude, respectively (in Rad). Trivially, this transformation yields to m_{1}=sin γ and m_{2}=1. Table 8 gives horizontal mapping factors for different coordinate systems (Toon et al., 1988). In most practical cases, FALL3D8.0 simulations only consider spherical coordinates, but other options are in principle possible. For vertical transformations, FALL3D8.0 incorporates a new σcoordinate system with linear decay (GalChen and Somerville, 1975) in which
where h(x,y) is the terrain height and H_{T} the height of the top of the computational domain. In the σcoordinate system, the influence of the terrain decreases linearly with height, from terrainfollowing at the surface (X_{3}=0) to a rigid lid at the top of the computational domain (X_{3}=H_{T}). This option has been added to partially correct numerical oscillations in the previous terrainfollowing model mapping ($z={X}_{\mathrm{3}}+h$), which can appear near the surface in the case of flows over mountain ranges and propagate upwards (Schar et al., 2002).
Table 9 gives the vertical mapping factors for the different coordinate systems available in FALL3D8.0. Once defined, these coordinate mapping factors are used to scale the variables and parameters that appear in the generalised equation (Eq. 60). The scaling of scalar quantities is straightforward since it only involves the Jacobian determinant of the transformation, e.g. $C=\mid \mathbb{M}\mid c={m}_{\mathrm{1}}{m}_{\mathrm{2}}{m}_{\mathrm{3}}c$ for concentration and so on. Note that for a volume one has dV=m_{1}m_{2}m_{3}dv, so that the mass comprised in a cell dX_{1}dX_{2}dX_{3} of the computational domain is equal to that in the transformed cell of the physical domain. The horizontal velocity components are trivially scaled as
whereas for the vertical component one has to consider that, in terrainfollowing coordinate systems, the coordinate X_{3} depends also on (x,y) through the terrain elevation h(x,y):
Basic algebra manipulation yields to the scaling factors shown in Table 10 for the different vertical coordinate systems. Note that the expression above implicitly contains the correction for topography in the vertical velocity. Finally, scaling factors for diffusion coefficients can also be obtained after some manipulation (Toon et al., 1988; Byun and Schere, 2006).
4.2 Discretisation and solving algorithm
FALL3D solves for each model bin the 3D generalised equation (Eq. 60) using a fractional step method, which splits the equation along each spatial direction (Folch et al., 2009):
with the solving order of each resulting onedimensional ADS equation being permuted in each successive time step in order to avoid any privileged direction. One advantage of this splitting strategy is that, even if each onedimensional equation is solved explicitly in time, the result is a semiimplicit scheme that adds stability.
In FALL3D8.0, the “bricklike” computational domain is discretised using a variation of the staggered Arakawa D grid, in which the wind velocity components are evaluated at the respective cell faces and the rest of scalar quantities at the cell centres (Fig. 2). Note that this configuration is very convenient for solving the 3D equation in a fractional manner because, when solving each onedimensional case, wind velocities are already aligned with the boundaries of the corresponding onedimensional cells. The previous versions of FALL3D used the classical LW centraldifference scheme for solving the resulting onedimensional ADS equations in Eq. (67), combined with a slope limiter to reduce numerical over/undershooting near discontinuities. This resulted in a secondorder accuracy except near sharp concentration gradients where, nonetheless, accuracy remained higher than in a firstorder upwind method. The main advantage of using the LW scheme was its simplicity but, in contrast, it is well known that it introduces numerical dissipation that prevents accurate resolution of discontinuities, leading to overdiffusive results. In order to circumvent this drawback, FALL3D8.0 uses instead a highresolution Kurganov–Tadmor (KT) scheme that can be combined either with a fourthorder explicit Runge–Kutta or with a firstorder Euler timemarching method. The latter option, even if less accurate, is still supported for computational efficiency reasons because it implies 4 times less solver calculations.
Consider the general onedimensional advectiondiffusion equation for a scalar variable c(x,t) on its conservative form:
where, in our particular case, F=c u is the advective flux and $G=k\partial c/\partial x$ is the diffusive flux (as already introduced, u(x,t) and k(x,t) are the velocity and diffusivity, respectively). Consider also a 1D computational domain discretised as in Fig. 2, where c is computed at cell centres (“mass” points) and u is stored at staggered cell boundaries, which do not need to be equally spaced. The semidiscrete form of the KT scheme can be written at centre of each cell i in terms of fluxes at boundaries $i\pm \mathrm{1}/\mathrm{2}$ as (Kurganov and Tadmor, 2000):
where $\mathrm{\Delta}{\stackrel{\mathrm{\u203e}}{x}}_{i}$ is the ith cell width and
Note that, in the expression above, the last equality holds because our flux G depends only on the gradient of c, i.e. $G=G(\partial c/\partial x)$. In Eq. (70), ${a}_{i\pm \mathrm{1}/\mathrm{2}}$ is the maximum absolute value of the eigenvalue of the Jacobian matrix of F (in our particular case, it reduces to ${a}_{i\pm \mathrm{1}/\mathrm{2}}=\left{u}_{i\pm \mathrm{1}/\mathrm{2}}\right$), and c^{r} and c^{l} are, respectively, right and left values at a cell boundary, computed as
where
and ϕ(r) is a flux limiter function (e.g. Sweby, 1984). Options available in FALL3D8.0 are the wellknown superbee ϕ_{s}(r) and minmod ϕ_{m}(r) (Roe, 1986):
Note that c is needed at two extra mass points in order to evaluate ${c}_{i\mathrm{1}/\mathrm{2}}^{\text{l}}$ and ${c}_{i+\mathrm{1}/\mathrm{2}}^{\text{r}}$ at the left/right cell boundaries, respectively. In other words, the “stencil” of the KT scheme needs two ghost nodes at the boundaries of the computational domain or a twopoint halo for internal domains in the case of parallel domain decomposition. Note also that the solving strategy derives from a onedimensional finitevolume formulation that, in our onedimensional case, is also in practice equivalent to using linear finite elements.
Time marching from t^{n} to ${t}^{n+\mathrm{1}}={t}^{n}+\mathrm{\Delta}t$ in Eq. (69) can be performed with the explicit firstorderintime Euler method (EU1):
or, alternatively, using the classical fourthorder Runge–Kutta method (RK4), in which
with
where the function f(t,c) is given on the righthand side of Eq. (69). In any case, the Courant–Friedrichs–Lewy (CFL) condition can be imposed to guarantee convergence in time integration (e.g. Hindmarsh et al., 1984) along each onedimensional problem:
multiplied by a userdefined safety factor. This factor should theoretically be lower than 1 in fully explicit cases but, given the semiimplicit nature of the splitting algorithm, slightly larger values can also yield to stability.
4.3 Algorithm benchmarks
Three benchmark cases serve us to illustrate the gains of the KT + RK4 numerical scheme with respect the former LW + EU1 implemented in the previous versions of the code.
Example 1 considers the pure advection (k=0) of a steplike discontinuity. Consider a 1D domain $x\in [\mathrm{1},\mathrm{1}]$ with an initial concentration of
that is advected by a uniform velocity field u=1. Periodic conditions are imposed at the boundaries, so that “mass” leaving the computational domain at x=1 is reinjected at $x=\mathrm{1}$. As a result, the initial condition is periodically recovered after each cycle with a period t=2. Results are shown in Fig. 3. Note how, as opposed to LW, the KT scheme adds almost no numerical diffusion and preserves discontinuities. In addition, because of its higher order in time, the KT + RK4 scheme preserves the solution, whereas for LW + EU1 accuracy deteriorates with time (compare the accuracy of the LW + EU1 solutions after 1 and 10 cycles in Fig. 3).
Example 2 considers the classical 1D advection–diffusion problem for the onset of numerical instability in a domain $x\in [\mathrm{1},\mathrm{1}]$ subject to the boundary conditions c=0 at $x=\mathrm{1}$ and c=1 at x=1. The problem has a steadystate analytic solution given by
where $\mathit{Pe}=ul/\mathrm{2}k=u/k$ is the Péclet number. Figure 4 shows the steadystate solutions for different Péclet numbers, illustrating also how KT + RK4 outperforms LW + EU1 as the Péclet number increases.
Example 3 considers a case with pure advection (k=0) on a 2D domain $(x,y)\in [\mathrm{1},\mathrm{1}]\times [\mathrm{1},\mathrm{1}]$ with an initial condition at t=0 given by a conic concentration distribution with a unit (c=1) peak concentration centred at $({x}_{\text{c}},{y}_{\text{c}})=(\mathrm{0},\mathrm{0.695})$ and having a radius r=0.1 (Fig. 5a). The cone is advected by a rotating clockwise velocity field centred at (0,0):
with an angular velocity Ω=π (in Rad s^{−1}), so that each cycle is repeated with a period of t=2. Concentration profiles along two transects A (y=y_{c}) and B (x=0) after two cycles (i.e. at t=4) are shown in Fig. 5b and c, respectively. Note again the substantial improvement in the KT + RK4 scheme, which performs well even in this numerically challenging test.
In FALL3D8.0, the preprocess auxiliary programmes have been parallelised and embedded in the code, so that a single executable file exists for all the preprocess steps and execution workflow (see Fig. 6). These formerly independent programmes can still be run individually as model tasks (specified by a programme call argument) or, alternatively, concatenated with the model in a single execution. In the first case, preprocess tasks generate output files that are later given as inputs to the model task. This is similar to what occurred in the previous version (v7.x) but with the difference of a parallel preprocess. In contrast, the second option does not require intermediate file writing/reading and therefore saves disk space and overall computing time. In any case, all tasks share a unique model input file and generate their own log file to track execution and report eventual warnings and errors. Possible task options are summarised in Table 11 and include the following:

Task SetTgsd can generate Gaussian and biGaussian particle grain size distributions in Φ (lognormal in diameter) or, alternatively, Weibull and biWeibull distributions (Costa et al., 2016a), and assumes a linear variation of density and particle shape factor between two specified cutoffs. For other kinds of grain size distributions, the user must provide a total grain size distribution file (name.tgsd).

Task SetDbs interpolates all the required meteorological data from the original grid of the driving meteorological models to the computational domain. Table 12 summarises the different meteorological drivers available in FALL3D8.0. Global datasets include ERA5, the new reanalysis from the European Centre for MediumRange Weather Forecasts (ECMWF) (Hersbach and Dee, 2016), the National Centers for Environmental Prediction (NCEP) Global Forecast System (GFS) and Global Data Assimilation System (GDAS) final analysis. Regional models supported include the Advanced Research Weather Research and Forecasting (WRF) (ARW) core of the WRF model (Skamarock et al., 2008), the mesoscale models HARMONIEAROME (Bengtsson et al., 2017) and the COSMOLAMI, run by the Italian Regional Environmental Protection Agency (ARPA). Note that some former dataset options have been deprecated in v8.x. The FALL3D8.0 distribution package provides a set of utilities to download and preprocess meteorological data for the SetDbs model task. Python scripts are provided to download and crop the variables required by the model. ERA5 can be obtained on either model levels (137 vertical levels) or pressure levels (37 vertical levels) via the Climate Data Store (CDS) infrastructure. GFS datasets can be accessed using the online archive of realtime weather model output from the National Operational Model Archive and Distribution System (NOMADS) (Rutledge et al., 2006). The NCEP FNL (final) operational global analysis and forecast data from the Global Data Assimilation System (GDAS) can be accessed using the Opensource Project for a Network Data Access Protocol (OPeNDAP) through the Thematic Realtime Environmental Distributed Data Services (THREDDS) Data Server (TDS) offered by the National Center for Atmospheric Research (NCAR) Research Data Archive (RDA).

Task SetSrc generates different emission source terms (see Sect. 3.2.3 and Table 4), including the PLUME option based on the FPLUME1.0 model (Folch et al., 2016). If necessary, it also performs a priori tephra particle aggregation (Sect. 3.2.6) and a TGSD cutoff in order to select the effective bins considered in the atmospheric transport.

Task FALL3D runs the FALL3D8.0 model itself.

Task All runs all previous tasks consecutively as a single parallel execution.
Parallelisation in FALL3D8.0 considers a 3D domain decomposition, with freedom for user to choose the number of processors along each direction. In contrast, previous versions considered two different levels of parallelisation: one on particle bins and another on domain but only along the vertical dimension. Parallelisation on bins was convenient in v7.x because no interaction among bins existed, but such a form of trivial parallelism has been deprecated given that it would now yield to unnecessary communication penalties. Note also that the full 3D domain decomposition allows solving on much larger grid sizes before reaching hardware memory limits.
In terms of runtime performance, it is important to recall that the splitting algorithm combined with the RK4 time marching implies solving a series of 1D equations four times along each spatial dimension, contrasting with a single solution for the EU1 case. In other words, each time integration step of Eq. (67) using the RK4 scheme along X_{1} implies solving n_{y}×n_{z} onedimensional problems four times, the solution along X_{2} implies solving n_{x}×n_{z} problems four times and so on. In order to minimise the number of communications between neighbouring processors, it is more convenient to compute first each of the four partial increments in Eq. (78) for all the 1D problems on a given dimension. This optimises swapping communications among domain partitions because a single swapping request (i.e. one MPI send/receive call) communicates all the data necessary to compute the next RK4 increment for the whole mesh. In contrast, if all partial increments in Eq. (78) were computed for each 1D problem, it would require as many swapping requests as 1D problems involved. Clearly, this solving approach is convenient but it presents two drawbacks. Firstly, it obviously increases the amount of memory required and, secondly, it poses an issue on memory data access, with potential increase of cache memory misses given the more frequent access to data. Moreover, when one solves for the dimensions stored on array lowest axes, memory data access is scattered and therefore less efficient. This issue has been addressed in FALL3D8.0 by rearranging the components of the velocity and diffusion arrays, with the dimension to solve being stored always in the fastest axis, and also by transposing the concentration array that has to be updated at the first partial increment. Such a memory management strategy allows exploiting always contiguous cache memory positions on any spatial dimension.
Costa et al. (2016a)Byun and Schere (2006)All the aspects discussed above have improved substantially the performance and scalability of the code. As an example of strong scaling (i.e. time to solution for a fixed problem size), let us consider a realcase ash dispersal simulation from the 2011 Cordón Caulle eruption (e.g. Collini et al., 2013). The model was configured as shown in Table 13 and solved on a typical grid size of $\mathrm{500}\times \mathrm{500}\times \mathrm{100}$ computational cells (horizontal model resolution of $\approx \mathrm{0.03}{}^{\circ}$). For illustrative purposes, Fig. 7 shows model snapshots of ash cloud column mass at two different times. Figure 8a shows strong scaling results (speedup) up to 2048 processors obtained on the MareNostrum 4 supercomputer, composed by generalpurpose nodes with 48 Intel Xeon Platinum processors interconnected by a 100 GB Intel OmniPath fullfat tree. For comparison, this figure shows also the strong scaling curve obtained with latest code (v7.3.4), in this case limited to 64 processors (larger values were not possible on this grid using v7.3.4 given the former onedimensional domain decomposition along z). Figure 8b plots the corresponding parallel efficiencies, defined as
where t_{1} is the total computing time with one processing unit, and t_{N} is the time with N processing units. Note that ideal strong scaling implies a parallel efficiency of 100 %. Clearly, v8.x improves notably the scalability of the code, with a trend close to that of perfect scaling up to ≈ 100 processors (parallel efficiency ≥ 90 %). Depending on the time integration scheme, values of parallel efficiency above 50 % are obtained with up to 512 and 1024 processors for EU1 and RK4, respectively. This is in striking contrast with v7.3.4, for which parallel efficiency already drops below 50 % with only 16 processors (Fig. 8b).
Much better v8.x code performance is also observed not only in terms of code scalability but also in terms of total computing time. Figure 9 shows the computing time ratio (total elapsed time) between v7.3.4 and v8 depending on the number of processors and the time integration scheme. The EU1 case (i.e. equal order of accuracy in time to that in v7.3.4), shows a similar serial performance, only with an insignificant computation overhead probably due to algorithmic differences. However, this overhead is rapidly balanced with only four processors, and v8.x with EU1 is already up to 4 times faster than v7.3.4 with 64 processors. The RK4 case (i.e. fourth order of accuracy in time) is around 4 times slower than v7.3.4 for the serial execution. This is a logical result given that the RK4 scheme needs to iterate 4 times more to solve in time. However, this penalty is also rapidly balanced as the number of processors increases, and RK4 already outperforms v7.3.4 in absolute terms with only a few tens of processors. In summary, v8.x clearly shows a much better scalability and performance (absolute computing time to solution for a given problem and computational resources) than v7.3.4. The companion paper (Prata et al., 2020) contains a detailed model validation and shows that this is also true in terms of model accuracy.
After 15+ years, the atmospheric transport model FALL3D has been completely rewritten and modernised to overcome legacy constraints in the former release (v7.x) that precluded the introduction of new functionalities and seriously limited the scalability and performance of the code on hundreds or thousands of processors. With this, FALL3D8.0 can be considered as a baseline for the successive optimisations and preparation for exascale computing. However, as detailed in the paper, v8.x already contains remarkable improvements and updates on model physics, numerics and performance. In particular, the code has been prepared to deal also with particles different from tephra, aerosols and radionuclides, includes new coordinate mapping options, a more efficient and less diffusive solving algorithm (KT) that can be combined with a highorderintime solver (RK4) and a better memory management and parallelisation strategy based on a full 3D domain decomposition. Strong scaling results have shown perfect scaling with a few hundred processors and a parallel efficiency above 50 % with 1024 processors. This remarkable improvement is also true in terms of performance (total computing time), with v8.x outperforming the previous release by a factor of 4 with only 64 processors. Further expected improvements in the preparation towards exascale include memory optimisation, introduction of thread parallelism (OpenMP), code vectorisation, porting to accelerators (GPUs), performance portability, load balance, asynchronous I/O and preparation for emerging heterogeneous architectures (exascale hardware prototypes).
FALL3D is available under version 3 of the GNU General Public License (GPL) at https://gitlab.com/fall3ddistribution (last access: 17 March 2020, Folch, 2020).
AF and LM have written the bulk of FALL3D8.0, with contributions from NG and GM. AC revised and updated all physical parameterisations implemented in the code. NG and MH have performed optimisations and the performance and scalability analysis. AF and AC wrote the manuscript with the input of all coauthors.
The authors declare that they have no conflict of interest.
This work has been funded by the H2020 Centre of Excellence for Exascale in Solid Earth (ChEESE) under grant agreement no. 823844. AC and GM acknowledge the European project EUROVOLC (grant agreement no. 731070) and the Ministero dell'Istruzione, dell'Università e della ricerca (MIUR, Rome, Italy) AshRESILIENCE project (grant agreement no. 805 FOE 2015). The authors thank Andrew Prata (BSC) for providing satellite retrievals of ash column mass, and Fabio Dioguardi and an anonymous reviewer for their constructive comments.
This research has been supported by the Centre of Excellence for Exascale in Solid Earth (ChEESE) (grant no. 823844), the EUROVOLC (grant no. 731070) and the AshRESILIENCE (grant no. 805 FOE 2015).
This paper was edited by Augustin Colette and reviewed by Fabio Dioguardi and one anonymous referee.
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