Modelling the ash concentration, transport, and dispersal of co-PDC ash clouds: implications for the aviation hazard

Pyroclastic density currents (PDCs) have the potential to generate co-PDC plumes, which segregate and buoyantly rise from the underlying gravity current. Co-PDCs are composed of hot gas and fine particles (e.g., < 90 μm) and typically have high-aspect ratio source geometries. Using the atmospheric-dispersion model, NAME, we perform a series of model runs that vary the particle release height and associated mass eruption rate for the eight different weather patterns that characterise the UK and the surrounding European area. We examine the ash cloud concentration as a function of vertical elevation (or flight level) within the atmosphere. We find that the ash clouds generated by PDCs have relatively small areas but are compact in shape and contain high ash concentrations, especially in early hours after particle release. The elevation of maximum mass resides in the vertical release region (within the first 36 h), and the maximum flight level achieved by the ash is 50 to 150 flight levels above the release region. Our results are discussed in terms of operational modelling by volcanic ash advisory centres for the aviation sector and the newly introduced concentration thresholds for quantitative volcanic ash forecasts (QVA). When applying these thresholds, most clouds are very high-concentrated, often above 10 mg m^-3 within the first hours of particle release and thus represent a hazard to aviation.