Heavy rain fell over large areas of New Zealand during the weekend of June 12 and 13. The heaviest falls occurred over the hill country of Buller, Nelson and Western Marlborough, where between 100 and 200 mm of rain fell in 24 hours, causing rivers to flood.
A man was killed on Saturday evening at the height of the downpour when his car left the road in the Buller Gorge and plunged into Brown Creek. Poor visibility as well as bad road conditions are likely to have contributed to the accident. The flow of Brown Creek peaked at five metres above normal on Sunday morning, and the car could not be retrieved until Monday.
The Buller peaked at about 12 metres and was described by one eye witness as “filthy brown and full of logs travelling at 35 kilometres an hour.” A number of roads were closed by slips, and the Grey River took another two hectares of land at Kamaka, bringing the flooded river to within 50 metres of State Highway 7 and the railway line.
In Marlborough, the Wairau river reached its highest level in ten years, peaking at 6.7 metres above average. More than 100 bull calves were marooned in the upper reaches when the river flowed through a gap in the stopbank caused by a flood two years ago. A tin shed from a gravel crushing plant was washed down the river and was for a time mistaken for a house truck.
Mudslides in the Marlborough Sounds pushed two baches into the sea. Fears were expressed that they would become navigational hazards. These were put into perspective by the Marlborough District Council harbourmaster, who pointed out that the amount of timber in the baches was about one fifth that of the logs and stumps normally seen floating in the Sounds after this sort of rain.
Slips and surface flooding closed some roads in the Wellington area for a time. A canoeist in the flooded Hutt River was tipped out of his canoe and knocked unconscious. Although he was pulled from the river and resuscitated, he died in hospital several days later.
Heavy rain also fell over the centre and north of the North Island, causing rivers to rise.
What caused such heavy rain?
A trough of low pressure intensified as it moved across the Tasman Sea towards New Zealand during Saturday. As the pressure became lower in the trough, the number of isobars increased, and so the wind strength in the northwesterlies on the eastern side of the trough increased.
The northwest winds brought relatively warm, moist air towards New Zealand from further north. As the strong winds drove the air against the mountains it was forced to rise, and consequently was cooled by expansion, forcing much of its water vapour to condense and fall out as rain.
The rising of the air in the middle atmosphere was also helped by the presence of a strong maximum of over 330 kph in the high-altitude jet stream that was moving east across the Tasman Sea in the latitude of Cape Reinga. Where air exits from the jet maximum on its poleward side, divergence occurs, forcing air from lower levels to rise. (See Issue 14).
Although in many places the heavy rain occurred predominantly in the hills, on the west coast of the South Island the rain extended all the way down to the coast. Westport, for example, had 83 mm in the 24 hours to 9 a.m. Sunday. As the strong northwest winds blew towards the Southern Alps, the air near the Earth’s surface was too
stable to rise easily, and was mostly deflected southwards to run parallel to the coast as a northeast wind. This air current is known as a “barrier jet.”
The northwest wind further out to sea rises over the barrier jet, much as if it were a solid obstacle like the hills. Since the air in the northwesterlies is rising before it reaches the coast, it begins to drop its rain before it reaches the land.
On the western side of the trough, beyond the deep low of 962 Hpa, there was a broad belt of gale and storm force southerlies. These winds moved a large amount of cold air north into the ‘Tasman Sea, and this air subsequently crossed the North Island on Monday with squally showers, some thunderstorms and hail, and snow on Ruapehu.
A truck was blown over on the Auckland Harbour Bridge, and a yacht was driven on to the rocks at Orakei.
In the western Bay of Plenty, a man was drowned when a three-metre runabout capsized as it headed out to sea from the Kaituna river mouth.
The squalls were so strong because the cold air became so unstable as it moved over the relatively warmer ocean surface in New Zealand’s latitudes. Surface wind speeds are normally much less than the wind speed a thousand metres up, because of friction. However, when the air is very unstable there are vigorous upward and downward air movements, and these bring the stronger winds from aloft down to the surface for short periods of time.
The overall driving force for the weather is the fact that the tropics receive far more energy from the sun than the poles do. However, the temperature in the tropics does not keep inexorably rising, because the wind and the ocean currents transport heat away towards the poles. This ultimately gives rise to the anticyclones and depressions we see on the weather map.
So the next time a deep depression moves some particularly cold air over you, just remember, that is their job; that is what they have to do in the interests of a well-ventilated planet.