Abstract
Volcanic ash clouds are common, often unpredictable, phenomena generated during explosive eruptions. Mainly composed of very fine ash particles, they can be transported in the atmosphere at great distances from the source, having detrimental socio-economic impacts. However, proximal settling processes controlling the proportion (ε) of the very fine ash fraction distally transported in the atmosphere are still poorly understood. Yet, for the past two decades, some operational meteorological agencies have used a default value of ε = 5% as input for forecast models of atmospheric ash cloud concentration. Here we show from combined satellite and field data of sustained eruptions that ε actually varies by two orders of magnitude with respect to the mass eruption rate. Unexpectedly, we demonstrate that the most intense eruptions are in fact the least efficient (with ε = 0.1%) in transporting very fine ash through the atmosphere. This implies that the amount of very fine ash distally transported in the atmosphere is up to 50 times lower than previously anticipated. We explain this finding by the efficiency of collective particle settling in ash-rich clouds which enhance early and en masse fallout of very fine ash. This suggests that proximal sedimentation during powerful eruptions is more controlled by the concentration of ash than by the grain size. This has major consequences for decision-makers in charge of air traffic safety regulation, as well as for the understanding of proximal settling processes. Finally, we propose a new statistical model for predicting the source mass eruption rate with an unprecedentedly low level of uncertainty.
Highlights
Volcanic ash clouds are common, often unpredictable, phenomena generated during explosive eruptions
Volcanic ash clouds generated by explosive eruptions are distally transported in the atmosphere up to distances ranging from a few hundreds to thousands of kilometres from the vent
We show that ε of sustained eruptions spans a wide range of values, from 0.1% (e.g., Plinian Kelut 2014 eruption) to 6.9% (Small/Moderate Ruapehu 1996 eruption; Table 1)
Summary
Volcanic ash clouds are common, often unpredictable, phenomena generated during explosive eruptions. We examine this partitioning (ε; given in percentage) as the ratio between the very fine ash flux transported in distal clouds (Qa) estimated from satellite-based infrared measurements, and the total flux of tephra emitted at the source (Qs) inferred from ground studies of tephra deposits The latter is referred to as the Mass Eruption Rate (MER). The database comprises satellite-based infrared measurements of Qa inferred from the extinction properties of ash using the split-window method, except for the May, 2011 Grimsvötn and October 27, 2002 Etna eruptions, whose measurements were carried out using hyperspectral sounders This technique provides vertical column densities as a mass per unit area, and allows total mass of very fine ash to be retrieved from integration of ash-bearing pixels over the whole cloud surface. Overall uncertainty associated with satellite-based measurements was estimated to be in the range ±40–60%2
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