Ash predictions, how successful were they?

Abstract

The program ASHFALL was developed to enable the rapid calculation of the ashfall from a volcanic eruption, primarily for Civil Defence purposes when an eruption is either thought to be imminent, or is actually underway. It could also be used for the estimation of volcanic hazards and for studies of past eruptions, by calculating the ash distributions produced by a range of possible eruptions, under various likely wind conditions.

During the 1995 and 1996 eruptions of Ruapehu, the program was used to provide forecasts of where ash was likely to travel if a significant eruption occurred, based on forecast wind patterns. For the three significant eruptions that occurred during this period, this note looks at the performance of this program, compared to available information on the ash distribution, and looks at the main sources of error.

For the question of most interest to Civil Defence authorities, namely will a certain area receive any ash at all, the error in the forecast wind direction is by far the most important. If the forecast wind direction is correct, ASHFALL gives a good indication of where ash will fall. The quantity of ash falling downwind is a function more of the size of the eruption, and it is difficult to give more than an order of magnitude estimate of this at the time of the eruption. However, analysis following the 1995 and 1996 eruptions of Ruapehu, when the eruption volumes could be derived from measured ash thicknesses, showed that the ash thickness at any point was generally within a factor of two of that forecast by ASHFALL.