Comment on acp-2021-973

Abstract

Due to the remote location of many volcanoes, there is large uncertainty in the timing, amount and vertical distribution of volcanic ash released when they erupt. One approach to determine these properties is to combine prior estimates with satellite retrievals and simulations from atmospheric dispersion models to create posterior emissions estimates constrained by both the observations and the prior estimates using a technique known as source inversion. However, the results are dependent not only on the accuracy of the prior assumptions, the atmospheric dispersion model and the observations used but also the accuracy of the meteorological data used in the dispersion simulations. In this study we advance the source inversion approach by using an ensemble of meteorological data to represent the uncertainty in the meteorological data and apply it to the 2019 eruption of Raikoke. This provides confidence in the posterior emission estimates and associated dispersion simulations that are used to produce ash forecasts. Prior mean estimates of fine volcanic ash emissions for the Raikoke eruption based on plume height observations are more than 15 times higher than any of the mean posterior ensemble estimates. In addition, the posterior estimates have a different vertical distribution with 27–44 % of ash being emitted into the stratosphere compared to 8 % in the mean prior estimate. This has consequences for the long-range transport of ash as deposition to the surface from this region of the atmosphere happens over long time-scales. The posterior ensemble spread represents uncertainty in the inversion estimate of the ash emissions. For the first 48 hours following the eruption, the prior ash column loadings lie outside an estimate of the error associated with a set of independent satellite retrievals whereas the posterior ensemble column loadings do not. Applying a risk-based methodology to an ensemble of dispersion simulations using the posterior emissions shows that the area deemed to be highest risk to aviation, based on the fraction of ensemble members exceeding predefined ash concentration thresholds, is reduced by 51 % compared to estimates using an ensemble of dispersion simulations using the prior emissions with ensemble meteorology. If source inversion had been used following the eruption of Raikoke it would have had the potential to significantly reduce the disruption to aviation operations. The posterior inversion emission estimates are also sensitive to uncertainty in other eruption source parameters (e.g., the ash density and size distribution) and internal dispersion model parameters (e.g., parameters relating to the turbulence parameterisation). Extending the ensemble inversion methodology to account for uncerainty in these parameters would give a more complete picture of the emission uncertainty, further increasing confidence in these estimates.