Research calculates ash fall
Research led by Victoria University of Wellington researcher Dr Simon Barker calculates where ash from a volcano is most likely to fall and how thickly it will cover the ground, in different areas of New Zealand.
The method developed by Dr Barker and his team could play a major role in eruption forecasting and emergency management.
They used complex computer models to show how ash would be distributed after an eruption of Taupō volcano in the central North Island. They also added historic weather data from the area to track how that would affect the spread of ash.
“We created scenarios based on different eruption sizes as seen in the geological record, spanning several orders of magnitude in volume, added in the weather data, and ran each scenario 1000 times to see where the ash would fall and how thick the resulting deposits will be,” Dr Barker says.
Dr Barker says weather data, particularly wind, is a big factor in calculating the spread of volcanic ash, as different wind directions will send the ash across completely different areas. The height of the volcanic ash cloud also plays a key part, as a taller cloud will be affected by changing winds at different altitudes. “A westerly will blow the ash over Hawke’s Bay and out to sea, and a southerly will blow it up towards Auckland.”
Another factor is the shape of the volcanic ash cloud which ranges from a weaker ‘bent’ cloud like from Ruapehu in 1995 to the massive ‘umbrella’ cloud that can blast more than 20 km up into the stratosphere, pushing ash upwind. The distribution of ash is dictated not only by the wind conditions on the day, but also the power of the eruption.
“By adding in the interaction with the weather systems, we can work out how long it will take for the ash to start falling in different locations. This kind of information can help emergency managers work out which areas need immediate evacuation.
“For instance in a large Taupō explosion, in the most common westerly weather pattern, it would take about three hours for the ash to reach Napier,” Dr Barker says. In contrast, it would take approximately 12 hours to reach Wellington.
Dr Barker says another benefit of the research has been the close work they have done with the United States Geological Survey.
“We’ve been able to use a 3D ash fall model from the United States and add our knowledge about past eruptions from Taupō, which has been great for us and the United States-based researchers in testing ideas on large explosive eruptions.”
The research was funded by the Earthquake Commission.