Parameterization of the microphysics of hydrometeors and ash

3The parameterization includes the condensation of water vapor in the rising eruption column. The formation of liquid and solid hydrometeors and the effect of latent heat release on the eruption column dynamics are considered. The interactions of hydrometeors and volcanic ash within the eruption column that lead to aggregate formation are simulated for the first time within a rising eruption column. The microphysical parameterization follows a modal approach. The hydrometeors are described by two size classes, each of which is divided into a liquid and a frozen category. By analogy with the hydrometeor classification, we specify four categories of volcanic ash particles. We imply that volcanic particles are active as condensation nuclei for water and ice formation. Ash can be contained in all categories of hydrometeors, thus forming mixed particles of any composition reaching from mud rain to accretionary lapilli. Collisions are caused by gravitational capture of particles with different fall velocity. Coalescence of hydrometeor–ash aggregates is assumed to be a function of the hydrometeor mass fraction within the mixed particles. Th