Combined effects of wind and column mass distribution on tephra fall deposits

A 2D1/2 model of volcanic ashes (tephra) dispersion, based on an analytical solution of the advection-diffusion equation, is presented. By this model the correlation between a double maximum in thickness and a punctual bimodal size distribution in plinian deposit of volcanic eruptions is outlined for the first time. Both the features are explained by the combined effect of vertical wind profile and of mass distribution along the eruptive volcanic cloud. The model is applied to 1980 Mt.St.Helens plinian eruption. The good reproduction of field data suggests the effective utilization of this model for hazard evaluation from volcanic ash fall.