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Richard Turner and Tony Hurst


The prediction of the dispersal of volcanic ash from events such as the Ruapehu eruptions of 1995 and 1996 is important, not only for civil-defense authorities who need to warn people in downwind areas, but for airline companies that have to reroute aircraft to avoid the encounters with volcanic ash clouds that can badly damage expensive jet engines and jeopardize passenger safety. The results of numerical simulations of volcanic ash dispersal using the Regional Atmospheric Modeling System (RAMS) and Hybrid Particle and Concentration Transport Model (HYPACT) for three periods (11–12 October 1995, 14 October 1995, and 17 June 1996) during the recent Ruapehu eruptive sequence are presented here. RAMS is a 3D atmospheric model that can be used to give detailed predictions of winds for regions such as the volcanic plateau. HYPACT is a particle dispersion model that uses the RAMS-generated wind fields to predict the movement and concentration of the volcanic ash cloud. Validation is achieved through comparison of predictions of airborne distributions and ashfall patterns with contour maps of ashfall depth and with satellite images of the ash cloud. Comparison of the performance of RAMS/HYPACT with that of the current Gaussian-plume “ASHFALL” Model currently used for ashfall advisory guidance in New Zealand indicates that the RAMS/HYPACT suite provides more accurate spatial and temporal forecasts than ASHFALL does, but that, like ASHFALL, its accuracy is limited by the accuracy of the initial and lateral boundary conditions provided and by the accuracy of the volcanological parameters that control the eruption-plume characteristics.

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