Scientists ‘move’ New Zealand into historical cyclones to model impact 

Met Office scientist Dr Ian Boutle has been working with NIWA in New Zealand on modelling weather events for the country’s contingency planners. The below blog highlights how they took on the challenge in an unusual way. 

Tropical cyclones don’t tend to directly impact on New Zealand. So, when scientists from the Met Office and NIWA (New Zealand’s National Institute of Water and Atmospheric Research) were asked to model potential impacts from cyclones on the country, it was a problem that needed some creative thinking. Their unique answer? Simply ‘move’ the land into the path of historical storms – metaphorically speaking.  

By the time tropical cyclones reach New Zealand’s latitudes, they have already undergone a transition into extra-tropical cyclones. These tend to be a bit weaker than the tropical cyclone they derived from, but they can still pack a heavy punch and there is a history of ex-tropical cyclones impacting on the country, hence the need for this project to see potential impacts from a stronger storm getting to the country.  

The research is part of New Zealand’s mitigation and adaptation planning for the future and the data provided by the simulations is now being worked on by New Zealand contingency planners to see how infrastructure in Auckland, New Zealand’s most populous city but one that has historically escaped the worst impacts of previous ex-tropical cyclones, would cope with these impactful weather events. Modelling weather events in this way provides contingency planners with reliable data, even when there’s little history of such impactful weather hitting the region. 

Met Office Scientific Manager Dr Ian Boutle, who worked with NIWA as part of a secondment, said, “Historically, New Zealand doesn’t get affected by fully-fledged tropical cyclones but a changing climate means that it’s something that’s possible in the future, as warmer sea surface temperatures wouldn’t weaken these cyclones as much as they travel south.” 

Climate change is just one reason behind the research. City planners, flood experts and scientists are currently working through the data provided by NIWA to see how the country would cope in the face of direct impact from an ex-tropical cyclone, including investigating the potential for landslides, flooding and building damage.  

Shifting into real weather 

The requirement for modelling these different realistic scenarios created an opportunity for the scientists to consider how to model the most realistic weather events for the country. The answer, of course, was to use real weather events.  

“Historically for New Zealand, there’s not much to go on in terms of active cyclones making landfall,” explained Dr Boutle.  

“One idea is to create an atmospheric state inside the Met Office’s Unified Model to create a cyclone that follows the path you want and go from there. However, that’s very tricky to do within the model and raises questions as to what would be a realistic cyclone coming at New Zealand from the tropics.  

“What turned out to be much easier and more effective to do was to look at recent ex-tropical cyclones that have passed close to New Zealand, and simply pick up and shift the landmass into its path and let the simulations show how the cyclone would behave and develop if it did hit land.”  

This map, as captured from the research, shows the north of New Zealand being affected by Tropical Cyclone Cook

In order to lessen the impacts of sea temperatures being warmer further north, scientists only shifted New Zealand on the east-west axis. With New Zealand in its ‘new’ location, the Met Office Unified Model ran the simulations for the weather set up, but this time taking in to account the impact of the new landmass and developed the weather system differently.  

This meant NIWA could provide the New Zealand government with realistic scenarios, as it was a real weather event in the vicinity of the country. Model runs were done for five different cyclones, shifting New Zealand into the path of each one to provide a number of scenarios for the planners to analyse.  

“When we move the landmass in to the path of a cyclone we already know has happened, what it does is develop the system in a different way once it hits land instead of sea. In short, cyclones evolve differently over land and the model reflects that and provides realistic data for us on wind speed, rainfall amounts and trajectory,” said Dr Boutle.  

“What we found is that by introducing land to these historical events, wind speeds obviously drop when land is reached, but New Zealand’s northern city of Auckland is right on the coast and so impacts from wind were more severe here. In terms of rainfall amounts, this could change either way when we introduced the new landmass, with some model runs giving higher levels of rain and some producing less than if it didn’t hit land.” 

Proof of concept and climate change 

The research, published with NIWA colleagues Stuart Moore and Richard Turner, plays an important role for New Zealand’s planning going forward, but it also acts as a proof of concept for other scientists researching in similar areas.  

Dr Boutle concluded: “With the rise in global temperatures we’ve seen since the industrial revolution, it’s possible for cyclones to move differently in the future, using a warmer sea surface temperature to retain strength and this increases up the possibility of new routes. Modelling in the way we have opens new possible research into potential impacts of these long-term climate changes.” 

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