Estimates of total ash content from 2006 and 2009 explosion events at Bezymianny volcano with use of a regional atmospheric modeling system

• Ash-falls at Bezymianny in a diverse meteorological conditions are studied.
• First-order effect of topography-induced atmospheric perturbations on deposit pattern
• Fallout rates of ash particles are well constrained with multiple regression approach.
• Fine ash composes ~ 40–60 wt.% of ash-sized ejecta.

The December 24, 2006, and December 16, 2009, strong explosion events at Bezymianny Volcano (Kamchatka Peninsula) were accompanied by extensive ash-falls in proximal and medium–distal area (< 100 km) downwind following the direction of prevailing upper tropospheric winds. In the present study, we apply a limited area atmospheric modeling system RAMS6.0 and a lagrangian stochastic model HYPACT1.5 to predict local airflows in a mountain area around the volcano during these events and quantify effects of atmospheric dispersal, gravitational settling, and particle aggregation on the observed ash-fall deposit patterns. It was found that the orography-induced atmospheric disturbances provided first-order influence on ash dispersal regime in the events owing to enhanced turbulence rates in a free troposphere above mountains and low-level airflows generated by mesoscale pressure perturbations. A total mass of ash from these eruptions is inverted based on grain-size sample data and model-calculated Green's function for atmospheric transport with use of a multiple regression approach. We demonstrate that in the absence of precise data on individual and collective settling rates the proposed inversion technique, which explicitly constrains fall velocity spectrum within individual sieve classes and aggregated modes, provides more reliable estimate for total erupted mass compared to procedures employing constant shape factor or prescribed settling rates within the framework of a simple linear regression model.