A Meteorological Approach to Calculate Volcanic Cloud Parameters

Remote sensing has shown that ice is an important component of volcanic clouds. The mass of ice in a given volcanic cloud is determined by total water content (TWC), the mass of water from erupted magma, entrained tropospheric air, and vaporized surface water, and by cloud temperature. This presentation provides a theoretical approach to calculate volcanic cloud TWC, temperature and density from satellite observations of ash and SO2 masses, motivated by a need to better model volcanic cloud destabilization and sedimentation mechanisms. Using parcel theory, a recently emplaced volcanic cloud can be modeled as a closed-system homogeneous fluid parcel with 3 components: Dry air, ash, and magmatic water. Cloud thickness is estimated from trajectory modeling and area is measured using remote sensing, which are combined to give parcel volume. Mass of water entrained during plume rise is ignored, providing a minimum value for TWC (however, measured parcel dimensions after emplacement account for the volume of entrained air). Magmatic water mass is calculated from knowledge of typical gas ratios in source volcano emissions and satellite-based measurement of SO2 in the volcanic cloud of interest. Other volcanic gases are ignored in the calculation because concentrations are low and densities are relatively similar to atmospheric gases. Cloud density and temperature can then be approximated using the ideal gas law under the following assumptions: (1) The volcanic cloud is in hydrostatic equilibrium; (2) fine ash (1-25 μm) and magmatic water are not removed by sedimentation; (3) the volumetric fraction of magmatic water and volcanic ash is so small that displaced air is negligible in the parcel density determination. Analysis of the 3.7 hr 17 September 1992 Spurr cloud (an ash-dominant cloud) shows that water was abundant with ~0.2 g/kg at -62°C and 191 hPa (compared to almost zero in ambient atmosphere). Cloud density and temperature was within 0.05% of the ambient atmosphere. Furthermore, almost all of the water existed as ice which then rapidly sublimated. The presence of abundant water in various phases must therefore, play an important role in volcanic cloud dynamics.