Microphysical Controls on Ascent of Water-Rich Ash Clouds from Supereruptions

Renewed interest in assessing the reliability of ash cloud forecasting has posed new questions regarding the methodologies used in and the limitations of available eruption cloud models. Supereruptions represent end-member processes in terms of eruptive scale, and their reconstruction pushes the standard modeling techniques to their limits. This study presents results from simulations of eruption clouds generated during selected phases of the 27 ka Oruanui supereruption (Taupo volcano, New Zealand), using the non-hydrostatic cloud resolving model ATHAM (Active Tracer High resolution Atmospheric Model). Working within a range of volcanic and meteorological scenarios, sensitivity studies have identified likely ash cloud heights and thicknesses that characterize the water-rich eruptive phases. In particular, the role of water phase changes is investigated by varying water contents entrained from the surface (the eruption occurred through a large lake) and from atmospheric moisture. These controls on plume ascent provide insight into how large-scale, water-rich ash clouds interact with and intrude into a stratified atmosphere, with allowance for climatic conditions during the Last Glacial Maximum. The combined use of ATHAM simulations and operational advection-diffusion-sedimentation models is explored and compared with field data from the Oruanui case study (e.g. distribution of thickness, grain size and the occurrence of ash aggregates). Further uncertainties arising from simplifying the interplay between plinian-style fall and co-ignimbrite input to hybrid ash clouds are addressed, and the broad scale limitations versus insights gained from simulating unobserved, multiphase events are discussed.