Volcanic plume simulation on large scales

Abstract

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. Based on the assumption that all particles are small the model's equations are simplified such that, besides equations for gaseous, liquid and solid constituents 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 scheme describing the entrainment of ambient air into the plume takes into account the anisotropy of the horizontal and vertical components of turbulence. Its length scale is assumed to be isotropic. Microphysical processes such 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 sedimentation velocity allows for the separation of gas and particles. The model is formulated with an implicit time stepping scheme. The equations of motion and the transport equations for tracers are formulated in flux form in order to guarantee the conservation of momentum and all tracer masses. The heat transport equation is in advective form. The wave equation and the equations for the transport of momentum, heat and tracers are solved using a combined line-relaxation successive overrelaxation scheme. Two-dimensional experiments for symmetric cases with cylindrical coordinates yield qualitatively similar results to other dynamic–thermodynamic models. However, entrainment processes are now computed quantitatively through the turbulence closure 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, comparisons between calculations with and without cylindrical coordinates are performed. Finally, experiments for different atmospheric background conditions allow investigation of plume development on the influence of cross wind effects, and temperature and humidity profiles.