Abstract/Summary

Volcanic ash poses an ongoing risk to the safety of airspace worldwide. The accuracy to which we can forecast volcanic ash dispersion depends on the conditions of the atmosphere into which it is emitted. In this paper we use meteorological ensemble forecasts to drive a volcanic ash transport and dispersion model for the 2010 Eyjafjallajokull eruption. From analysis of these simulations we determine why the skill of deterministic-meteorological forecasts decrease with increasing ash residence time, and identify the atmospheric conditions in which this drop in skill occurs most rapidly. Large forecast errors are more likely when ash particles encounter regions of large horizontal flow separation in the atmosphere. Nearby ash particle trajectories can rapidly diverge leading to a reduction in the forecast accuracy of deterministic forecasts which do not represent variability in wind fields at the synoptic-scale. The flow separation diagnostic identifies where and why large ensemble spread may occur. This diagnostic can be used to alert forecasters to situations in which the ensemble mean is not representative of the individual ensemble member volcanic ash distributions. Knowledge of potential ensemble outliers can be used to assess confidence in the forecast and to avoid potentially dangerous situations in which forecasts fail to predict harmful levels of volcanic ash.