New Zealand Journal of Geology and Geophysics abstracts
Performance of the program ASHFALL for forecasting ashfall during the 1995 and
1996 eruptions of Ruapehu volcano
A. W. HURST
Institute of Geological & Nuclear Sciences
P.O. Box 30 368
Lower Hutt, New Zealand
email: t.hurst@gns.cri.nz
RICHARD TURNER
National Institute of Water & Atmospheric Research
P.O. Box 14 901
Wellington, New Zealand
Abstract The program ASHFALL was developed to predict the
ashfall that would result from a volcanic eruption, primarily for civil defence
purposes, such as giving public warnings and planning mitigation measures. It
can also be used for volcanic hazard assessments and for studies of past
eruptions. During the 1995 and 1996 eruptions of Ruapehu, a volcano in the
North Island of New Zealand, the program was used to provide forecasts of where
ash would land if a significant eruption occurred, based on forecast wind
patterns. This paper compares the program predictions with the actual ash
distribution of three major ash-producing events from Ruapehu in 1995 and 1996,
and looks at the main sources of error.
The accuracy of the forecast wind direction is the main factor affecting the
ash distribution. If the forecast wind direction is correct, ASHFALL gives a
good indication of where ash will fall. The wind strength has less effect on
the ash distribution. The quantity of ash falling downwind depends on the
volume 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 of
the three events for which the total 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.
Keywords volcanic ash; Ruapehu; ASHFALL; prediction maps
New Zealand Journal of Geology & Geophysics, 1999, Vol. 42:
615-622
0028-8306/99/4204-0615 $7.00/0 (c) The Royal Society of New Zealand
1999
PDF file of entire paper: medium quality (1477K); (scanned from paper original: notes about this process)
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