FALL 3 D : A Computational Model for Trans - 1 port and Deposition of Volcanic Ash ⋆

6 FALL3D is a 3-D time-dependent Eulerian model for the transport and deposition of 7 volcanic ash. The model solves the advection-diffusion-sedimentation (ADS) equa8 tion on a structured terrain-following grid using a second-order Finite Differences 9 (FD) explicit scheme. Different parameterizations for the eddy diffusivity tensor 10 and for the particle terminal settling velocities can be used. The code, written 11 in FORTRAN 90, is available in both serial and parallel versions for Windows and 12 Unix/Linux/Mac X Operating Systems (OS). A series of preand post-process util13 ity programs and OS-dependent scripts to launch them are also included in the 14 FALL3D distribution package. Although the model has been designed to forecast 15 volcanic ash concentration in the atmosphere and ash loading at ground, it can also 16 be used to model the transport of any kind of airborne solid particles. The model 17 inputs are meteorological data, topography, grain-size distribution, shape and den18 sity of particles, and mass rate of particle injected into the atmosphere. Optionally, 19 FALL3D can be coupled with the output of the meteorological processor CALMET, a 20 diagnostic model which generates 3-D time-dependent zero-divergence wind fields 21 from mesoscale forecasts incorporating local terrain effects. The FALL3D model can 22 be a tool for short-term ash deposition forecasting and for volcanic fallout hazard 23 assessment. As an example, an application to the 22 July 1998 Etna eruption is also 24 presented. 25