Estimation of total discharges of volcanic ash using atmospheric-transport models

摘要

We propose a method for calculating the total mass and grain-size composition of the ash that enters the atmosphere during explosive eruptions based on direct numerical modeling of transport and precipitation of ash particles and measurements of deposited mass in the ashfall area. The solution reduces to an overdetermined problem in a single parameter, viz., discharge rate as estimated by least squares from the minimal prior information on wind field structure and discharge heights. The example we consider in this study is the explosive event of January 13, 2011 on Kizimen Volcano accompanied by emission of pyroclastic products to heights of 6-9 km a.s.l. with subsequent propagation of the downwind plume to distances on the order of a few hundred kilometers. The ultimate estimate for the emitted volume (0.68-1.67 million tons) is in overall agreement with calculations made using the isopach method along with the Fedotov nomogram based on data on the eruptive cloud height and wind velocity. It is shown that meso-scale (2-200 km) atmospheric disturbances above mountains exert the controlling influence on the conditions of transfer/deposition of ash particles; this influence should be taken into account in order to derive correct estimates for emission mass and the role of wind-caused gravity differentiation of ash material during the formation of volcanogenic deposits.