New Zealand's tornado window
New Zealand appears to be in a period of slightly elevated incidence of tornadoes, according to NIWA scientist Dr Richard Turner.
Dr Turner says that more tornadoes were reported over the past decade than during the 1980s and 1990s.
“We're seeing more than we’ve seen for 40 years,” he said. “We’re experiencing about the same frequency as occurred during the fifties and sixties.”
On average, there are about seven to ten moderate to strong tornado events reported in New Zealand each year. In the past nine years, six people have been killed when tornadoes struck.
The Insurance Council estimated the cost of the fatal tornadoes that hit Auckland in December 2012 at $8.7 million. The tornado hit Hobsonville and Whenuapai, killing three builders and damaging almost 400 homes. The Defence Force insurance bill for damaged property was $1.4 million. It cost power companies about $3.5 million to fix lines. The Auckland Council paid about $1.5 million for staff to clean up the area.
Dr Turner says the history of tornadoes in New Zealand is not heavily studied and, because many go unobserved, it’s hard to discern historical frequency trends and relate them to climate variations such as La Niña or the Pacific Decadal Oscillation.
“It’s hard to ascertain, especially from old newspaper accounts, the actual wind phenomena that caused damage. For instance, damage caused by microburst winds is often wrongly attributed to a tornado.
“Taking this into account, there still does appear to be some trend on decadal timescales,” he says.
One strand of research is developing the NIWA and GNS Science ‘RiskScape’ tool, which estimates asset impacts and losses for various natural hazards. RiskScape has damage models that relate building and human impacts and losses to wind hazard exposure.
Turner says that, until recently, damage estimates used international formulae. But New Zealand’s housing stock has a prevalence of different construction materials and structural attributes, such as a timber frame with weatherboard cladding and sheet-metal roofs. Tornado damage here generally results from weaker events than overseas.
“So we’ve gone into the field to collect data about the sort of damage caused by tornadoes as well as different types of wind phenomena.
“We are collecting real New Zealand data for real New Zealand buildings, because relying on damage models developed overseas hasn’t always given us realistic results.
“We can relate the actual damage and loss incurred to the likely range of wind speeds caused by a tornado (or microburst) like those in Hobsonville in December 2012, Albany in 2011 and Taranaki in 2007.
“This will improve the ability of RiskScape to accurately assess the impact of hazardous wind events such as tornadoes.”
Dr Turner is optimistic about current research. “NIWA is designing very-high-resolution weather forecast models (which demand huge computational resources – NIWA’s supercomputer) to predict the occurrence of mesocyclones (see sidebar: Tornado watch). If successful, this could potentially help forecasters identify mesocyclone formation between six and 12 hours in advance, increasing warning times.”
In New Zealand, tornadoes are usually around 20 to 100 metres in diameter, last for a couple of minutes, and travel two to five kilometres during that time. Wind speeds are typically between 115 and 180 kilometres per hour. Local tornadoes are usually spawned from mesocyclones – spinning updrafts of air in thunderstorms.
Warning signs of tornadoes can include hail or heavy rain followed by dead calm or a fast, intense wind shift; and large, dark, low-lying clouds