Ash-sulphuric interactions: Simulating major volcanic aerosol clouds as global dust veils

This presentation will describe the adaptation of the interactive stratospheric-tropospheric aerosol model UM-UKCA to represent major volcanic aerosol clouds as a mixed plume of ash-sulphuric and pure sulphuric aerosol particles.

Aligned with the recent re-classification of the stratospheric aerosol layer as comprising meteoric-sulphuric, pure sulphuric and organic-sulphuric particles, we propose a 4^th"ash-sulphuric" particle type in the years after eruptions powerful enough to inject volcanic material into the stratosphere.

We present results from initial UM-UKCA ash-sulphuric simulations of the Pinatubo aerosol cloud, where, in addition to the formation and growth of secondary pure-sulphuric particles nucleated homogeneously, the source of primary ultra-fine ash (sub-micron particles) in the plume is also resolved in the model. We assess the evolution of this mixed-phase volcanic aerosol cloud in the initial months after the eruption, both the main reservoir confined to the tropics, and initial dispersion to Southern Hemisphere and Northern Hemisphere mid-latitudes.

We test the sensitivity to varying grid-scale-detrainment-altitude and emission-particle-size of the ultra-fine ash and compare to standard non-ash-resolving model experiments. We plan to calibrate these model uncertainties, comparing simulated vertical extent of the ash to observed depolarisation within tropical airborne lidar measurements in July 1991, and within ground-based lidar measurements in the mid-latitudes of both hemispheres (Aberystwyth, U.K. : Aug 1991 to Mar 1992 and Aspendale, Australia: backscatter from Jul 1991, depolarization from 6^thDec 1991).

The measurements align with the data-rescue activity within the SPARC activity on stratospheric sulphur "SSiRC", which is recovering measurement datasets of in-situ and remote sensing stratospheric aerosol measurements, focussing initially on periods of major volcanic enhancement.