Incorporating eruption source parameter and meteorological variability in the generation of probabilistic volcanic ash hazard forecasts

Volcanic ash represents a hazard to aviation and a global network of Volcanic Ash Advisory Centres (VAACs) are tasked with providing advice and guidance to the aviation industry on its presence in the atmosphere. Forecasts of the expected location of the ash cloud are generated using atmospheric transport and dispersion models, initialised with a set of Eruption Source Parameters (ESPs) and driven by forecast meteorological data. In the future the VAACs will be required to adopt a Quantitative Volcanic Ash (QVA) approach of issuing probabilistic volcanic ash concentration information to the aviation industry, as such there is a need to develop a framework for producing a probabilistic forecast which incorporates both the uncertainty in forecast meteorology and in the eruption source parameters (ESPs). Variability in the meteorological forecasts is typically expressed as an ensemble of meteorological data, which can be provided to an atmospheric dispersion model to produce an ensemble of simulations in which ESPs are kept constant, and outputs are aggregated to remove the conditioning on the meteorology. Dispersion model forecasts are also sensitive to the ESPs used to initialise the simulations, notably the plume height and the mass eruption rate (MER) of volcanic ash injected into the atmosphere. The latter is difficult to measure in real-time, and for event response it is assumed to scale with the plume height. There remains a high degree of uncertainty in these linked ESPs, which can be modelled via a Bayesian linear modelling approach. We will present a method for incorporating the combined variability of ESPs and meteorological forcing into the ensemble to obtain values of exceedance probabilities for airborne volcanic ash concentrations of interest, using standard statistical techniques and numerical methods, whilst keeping computational and time costs down for efficient evaluation in an emergency.