The remnants of ex-Tropical Cyclone Debbie has been hammering the North Island this week. What's driving the system? And can we expect more like it in the future? Science reporter Jamie Morton talks to Niwa meteorologist Ben Noll.
We talk about experiencing "remnants" of ex-Tropical Cyclone Debbie. What physically do we mean here and how much of what was Debbie is still intact?
While the intense cyclone that battered Australia is long gone, Debbie left plenty of energy in the atmosphere as it moved from Queensland in the Tasman Sea.
This energy - also known as vorticity - was then strung out along a frontal boundary that stretched from the Tasman Sea, across New Zealand, and well to the east of the country this week.
In the Southern Hemisphere, an ex-tropical cyclone is a weather system that was once a tropical cyclone, but has now moved outside of the tropics - or south of 23.4 degrees south latitude.
As a tropical cyclone moves south away from the tropics, it often encounters progressively stronger upper level winds and moves over colder ocean waters, which change the characteristics and structure of the weather system and result in a loss of tropical characteristics.
A tropical cyclone is a "warm core" low pressure system that develops over water.
Warm core low pressure systems are most intense at the surface and decrease in intensity with atmospheric height.
Typical mid-latitude low pressure systems - the ones that affect New Zealand regularly during the winter - are "cold core" lows that develop because of thermal (temperature) gradients.
Cold core low pressure systems increase in intensity with atmospheric height.
Flooding in Hawke's Bay. Photo / Supplied
What are some of the interesting characteristics we've seen in this system?
The remnants of ex-Tropical Cyclone Debbie helped to siphon deep tropical moisture southward from the Pacific Islands, which was part of something called an atmospheric river.
Atmospheric rivers are relatively long, narrow regions in the atmosphere - like rivers in the sky - that transport most of the water vapour outside of the tropics.
In addition to the very heavy rainfall (in some cases more than the normal April monthly rainfall in less than 24 hours) perhaps those in the upper North Island can attest to the exceptional humidity the storm has brought - rivalling or exceeding some of the most humid air experienced this past summer.
Flooding in Ngaruawahia after heavy rainfall due to the remnants of Cyclone Debbie. Photo / Facebook
Is there anything about its nature that makes it more challenging to track and model? Is it much more dynamic than a normal storm? And how accurate has our forecasting been so far?
Not necessarily - weather models actually have an easier time tracking well-defined weather features, such as an ex-tropical cyclone.
The signal for a potent storm was there five to seven days in advance.
However, the challenge comes in predicting the placement of the most intense rain, which is a challenge outside of 48 to 72 hours before the storm's arrival.
How are meteorologists able to calculate precise rainfall totals and wind speeds for different areas?
Numerical weather prediction.
Numerical weather prediction takes current observations of weather and processes this data with computer models, loaded with the equations that govern atmospheric motions, to forecast the future state of weather.
In the end, the user sees an "easy on the eyes" output of forecast total rainfall or maximum wind gusts.
Nardine and Richard Theodore checking on flooding, caused by the remnants of Cyclone Debbie, close to their home on Broomfield Road near Woodville. Photo / Mark Mitchell
Can we expect to see a greater number of, or more intense, systems such as this under climate change projections? And why?
While no one weather event is caused by climate change, all events are influenced by climate change since the atmosphere is now warmer and moister than it was in the past.
Climate change increases the likelihood of extreme rainfall, given the appropriate weather setup.
Research suggests that there will be up to 8 per cent more intense rain for every 1C of warming.
