The use of Numerical Weather Prediction and a Lagrangian transport (NAME-III) and dispersion (ASHFALL) models to explain patterns of observed ash deposition and dispersion following the August 2012 Te Maari, New Zealand eruption

Abstract

The August 6, 2012 Te Maari, New Zealand eruption produced a very small ash-dominated plume (~230,000m3, 8–10km high) that was rapidly and widely dispersed, covering 1600km2 within an hour. This paper documents for the August 6, 2012 Te Maari eruption the upper level (troposphere) plume movement based on ash-detection algorithms applied to IR satellite imagery. It also presents the distribution of airborne ash and wind-influenced ashfall as determined by NAME-III aerial dispersion modelling using observed particle characteristics and grain size distribution measurements (that are also presented) and compares the ashfall with observations.The upper level (troposphere) ash movement was also evaluated from ash-detection algorithms, applied to infra-red satellite imagery and the resulting distributions were compared to those forecast by the numerical dispersion models. Forecasts of upper level ash-dispersion patterns explained the satellite imagery observations well, predicting the correct altitudes when using plausible ash size distributions and release levels. Patterns in proximal ashfall could only be partly explained by aerial dispersal of large particles released at low altitudes in the eruption column. The extreme distal (100–150km away) observed ashfall distributions also cannot be fully explained by NAME-III when using: reasonably prescribed initial particle size distributions, eruption column height, eruption timing, well forecast winds, and dry sedimentation processes. Aggregation and ice nucleation effects (observed in deposits) were not included in the ash dispersion model, but appear as a plausible mechanism to account for the observed fraction of wind dispersed ash particles <30μm deposited but not captured by the models.