IAVCEI Commission on Tephra Hazard Modelling

TEPHRA DISPERSAL MODELING

Tephra is one of the main products of explosive eruptions and forms after material has been explosively ejected from a vent producing an eruption column, which is a buoyant plume of particles and gas rising high into the atmosphere. Tephra can be transported in the atmosphere for long time and distance after the eruptive event causing respiratory problems to human and animals, serious damage to buildings and also affecting several economical sectors such as agriculture and tourism. The study of tephra dispersal represents an important aspect of risk mitigation necessary in those populated area which have developed close to active volcanoes or characterized by significant aviation traffic. The understanding of dispersal and sedimentation from volcanic plumes has significantly progressed in the last few decades and new tephra models have been developed with different degrees of complexity.

Depending on the application, different simplifications and assumptions had to be introduced in order make the problem treatable. Approaches include i) empirical models dedicated to the estimation of eruptive parameters, ii) analytical solutions widely used for hazard assessments and iii) numerical models mainly used for real-time forecast of plume evolution and sedimentation. 1D analytical models are mainly focused on turbulent dispersion of particles during fallout, particle deposition in a wind field, large-eddy sedimentation from plume margins combined with gravity-driven intrusion of the volcanic current at the neutral buoyancy level, and sedimentation from plumes bent over by strong winds. Quasi 2D analytical models are based on the assumption of a constant and isotropic atmospheric diffusion, negligible vertical wind velocity and vertical diffusion. Finally, 3D time-dependent models are designed to locate the movement of volcanic clouds and to forecast real-time sedimentation, becoming an important tool in volcano observatories and aviation operations.

Models used for tephra hazard assessment (Hazard models) typically result from the combination and integration of different theories and approaches depending on the specific eruptive scenario and mitigation program required.