Kate Turnbull

Operational prediction of ash concentrations in the distal volcanic cloud from the 2010 Eyjafjallajökull eruption

[1] During the 2010 eruption of Eyjafjallajokull, improvements were made to the modeling procedure at the Met Office, UK, enabling peak ash concentrations within the volcanic cloud to be estimated. In this paper we describe the ash concentration forecasting method, its rationale and how it evolved over time in response to new information and user requirements. The change from solely forecasting regions of ash to also estimating peak ash concentrations required consideration of volcanic ash emission rates, the fraction of ash surviving near-source fall-out, and the relationship between predicted mean and local peak ash concentrations unresolved by the model. To validate the modeling procedure, predicted peak ash concentrations are compared against observations obtained by ground-based and research aircraft instrumentation. This comparison between modeled and observed peak concentrations highlights the many sources of error and the uncertainties involved. Despite the challenges of predicting ash concentrations, the ash forecasting method employed here is found to give useful guidance on likely ash concentrations. Predicted peak ash concentrations lie within about one and a half orders of magnitude of the observed peak concentrations. A significant improvement in the agreement between modeled and observed values is seen if a buffer zone, accounting for positional errors in the predicted ash cloud, is used. Sensitivity of the predicted ash concentrations to the source properties (e.g., the plume height and the vertical distribution of ash at the source) is assessed and in some cases, seemingly minor uncertainties in the source specification have a large effect on predicted ash concentrations.

A case study of observations of volcanic ash from the Eyjafjallajökull eruption: 1. In situ airborne observations

On 17 May 2010, the FAAM BAe-146 aircraft made remote and in situ measurements of the volcanic ash cloud from EyjafjallajA�¶kull over the southern North Sea. The Falcon 20E aircraft operated by Deutsches Zentrum fA�¼r Luft- und Raumfahrt (DLR) also sampled the ash cloud on the same day. While no A¢Â�Â�wingtip-to-wingtipA¢Â�Â� co-ordination was performed, the proximity of the two aircraft allows worthwhile comparisons. Despite the high degree of inhomogeneity (e.g., column ash loadings varied by a factor of three over �100 km) the range of ash mass concentrations and the ratios between volcanic ash mass and concentrations of SO2, O3 and CO were consistent between the two aircraft and within expected instrumental uncertainties. The data show strong correlations between ash mass, SO2 concentration and aerosol scattering with the FAAM BAe-146 data providing a specific extinction coefficient of 0.6A¢Â�Â�0.8 m2 g�1. There were significant differences in the observed ash size distribution with FAAM BAe-146 data showing a peak in the mass at �3.5 mm (volume-equivalent diameter) and DLR data peaking at �10 mm. Differences could not be accounted for by refractive index and shape assumptions alone. The aircraft in situ and lidar data suggest peak ash concentrations of 500A¢Â�Â�800 mg m�3 with a factor of two uncertainty. Comparing the location of ash observations with the ash dispersion model output highlights differences that demonstrate the difficulties in forecasting such events and the essential nature of validating models using high quality observational data from platforms such as the FAAM BAe-146 and the DLR Falcon.