Volcanic plume simulation on large scales

The plume model ATHAM (Active Tracer High Resolution Atmospheric Model) is designed to simulate explosive volcanic eruptions for a given mass flux of pyroclastic material under realistic atmospheric background conditions. Bas ed on the assumption that all particles are small the model's equations are simplified such that, besides equations for gaseous, liquid and solid cons tituents of arbitrary concentrations, only the volume means of momentum and heat are predicted. The exchange of momentum and heat between the fluid's constituents are treated diagnostically. A prognostic turbulence closure sc heme describing the entrainment of ambient air into the plume takes into ac count the anisotropy of the horizontal and vertical components of turbulenc e. Its length scale is assumed to be isotropic. Microphysical processes suc h as the exchange of heat and momentum between dry air, water vapor, cloud water, precipitable water, ice crystals and graupel are parameterized. Ash and lapilli represent the spectrum of silicate particles. A diagnostic sedi mentation velocity allows for the separation of gas and particles. The mode l is formulated with an implicit time stepping scheme. The equations of mot ion and the transport equations for tracers are formulated in flux form in order to guarantee the conservation of momentum and all tracer masses. Thr heat transport equation is in advective form. The wave equation and the equ ations for the transport of momentum, heat and tracers are solved using a c ombined line-relaxation successive overrelaxation scheme. Two-dimensional e xperiments for symmetric cases with cylindrical coordinates yield qualitati vely similar results to other dynamic-thermodynamic models. However, entrai nment processes are now computed quantitatively through the turbulence clos ure and condensed matter has a sophisticated description. In order to study the transferability of results from computationally cheap two-dimensional experiments to costly three-dimensional simulations of a realistic plume, c omparisons between calculations with and without cylindrical coordinates ar e performed. Finally, experiments for different atmospheric background cond itions allow investigation of plume development on the influence of cross w ind effects, and temperature and humidity profiles. (C) 1998 Elsevier Scien ce B.V. All rights reserved.