Nasa examines 'extreme' rainfall dumped on NZ by ex-cyclones Debbie and Cook
Cyclone Cook's onslaught of New Zealand can be seen in an "extreme rainfall" analysis by US space agency Nasa.
Nasa examined rainfall accumulations generated over eastern Australia, the Pacific, and New Zealand from April 4 to 15.
During a two-week period, Cyclone Debbie and Cyclone Cook slammed Queensland and New Caledonia and the remnants of the tropical storms deluged New Zealand, causing flooding, slips, and power outages.
One person was killed in New Caledonia.
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"Data indicates that rainfall totals of over 80mm (3.1 inches) were common in many areas of New Zealand. [Satellite data] shows that the two extra-tropical cyclones and other low pressure systems dropped heavy rainfall on both the North and South Islands of New Zealand."
Cyclone Cook swept over the north and east coast of New Zealand, tracking towards Auckland then along the coast of both islands.
A Nasa analysis says the Bay of Plenty, Hawke's Bay, the coastal region of the southwest North Island - near Wellington - and the western tip of the South Island had some of the heaviest rainfalls.
Total rainfall from the two storms was 80mm in many areas of New Zealand. To put this in context, however, required a consideration of regional variations around the country.
National Institute of Water and Atmospheric Research forecaster Seth Carrier said the NASA data was derived from satellite analyses, which use cloud cover characteristics and imagery of storms to estimate rainfall.
The agency's satellite data was useful for estimating rainfall over open ocean, he said.
"It looks like they're giving an estimate of rainfall total in a nine-day period using satellite derived rainfall amounts.
"A satellite derives rainfall amounts by analysing cloud cover and based on cloud cover they're estimating the rainfall out of a certain type of cloud.
"The most accurate way to determine where rain is falling is radar and that is land-based.
"Over the ocean, it's really just a black hole of data, so in recent years they've developed this technology where satellites can do a really rough estimate of rainfall over open ocean."
Over ocean, this was a useful way to estimate rainfall, but over land weather radar stations were more accurate.
In New Zealand, regional variations in rainfall must be taken into account. Rain falls mainly in the mountains and, for example, Milford Sound has annual rainfall of several metres.
Last year was the wettest recorded year at Milford since 1929.
The total amount of water that falls as rain and snow in New Zealand each year is estimated at 560,000 million cubic metres - or enough to cover the entire country to a depth of 2.1 metres, according to Niwa.
MetService meteorologist Angus Hines said many rain gauges had 50 - 70mm of rain during ex-Cyclone Cook but he would not call this "extreme" by New Zealand standards.
"There were a few other factors at play which made the impact of this one perhaps more than other events with a similar amount of rainfall.
"Debbie really saturated the soil.
"The couple of days before Cook there was a separate low off the west coast which brought more rain. Cook was moving quickly so a lot of rain fell really fast."
MetService criteria for "severe" rain used two benchmarks - at least 50mm in six hours or 100mm in 24 hours.
During Cook, many gauges recorded more than 50mm in six hours.
NASA used the global precipitation satellite, a joint mission between the US agency and the Japan space agency, to gather rainfall data from orbit.
Rainfall data was produced in half-hour intervals for the nine-day period when the cyclones tracked across the Pacific and towards New Zealand. Measurements were calibrated with rain gauge networks.