Extreme weather - heavy rainfall
In New Zealand, heavy rainfall is relatively common. Often, a significant amount of precipitation occurs in only a few hours, leading to severe flooding and landslide risk.
Causes of heavy rainfall
Heavy rainfall occurs over New Zealand mainly because of the following common weather systems:
- ex-tropical cyclones
- North Tasman Sea lows moving to NZ region
- depression/lows from the south
- cold fronts.
New Zealand's mountains tend to modify and amplify precipitation, and this often causes the frequent heavy rainfall we experience. Heavy rainfall tends be most common over the western coastal region of the South Island and the middle and upper North Island, and least common on the east side of the South Island (due to the prevailing westerlies).
Potential consequences of heavy rainfall
Heavy rainfall can lead to numerous hazards, for example:
- flooding, including risk to human life, damage to buildings and infrastructure, and loss of crops and livestock
- landslides, which can threaten human life, disrupt transport and communications, and cause damage to buildings and infrastructure.
Where heavy rainfall occurs with high winds, risk to forestry crops is high.
Incidences of very heavy rainfall in recent years
In early January 2012, heavy rain warnings were issued around the North Island. The southern Hawkes Bay area was the worst affected, with up to 150mm of rain falling on it over an 18-hour period. Additionally, houses in Wellington lost power as the region's electricity network was disrupted by 'wild weather', including heavy rain and strong winds (Mt Kaukau in Wellington was battered by winds up to 128 km/h: classified as 'hurricane force'). Heavy rainfall was also recoded in Nelson/the Tasman Region in December 2011, with Takaka experiencing the highest 1-day rainfall: 392mm on 14 December.
Heavy rain caused severe floods in Northland (March 2007), when over 40 mm/hr fell for several hours (Figures from Natural Hazards Update 2007)
Very heavy rainfall in mid February 2004 produced the most disastrous floods for many decades in the lower and middle North Island - 235mm fell in 19hrs at Pohangina River / Ruahine range (Fuller & Heerdegen 2005). The cost of damage and economic impact resulting from the floods (and associated wind damage) in the lower North Island was estimated to be close to $300 million (Fuller & Heerdegen 2005) .
Heavy rainfall research at NIWA
NIWA scientists are currently working within the Reducing the Impact of Weather-Related Hazards Programme to improve hazard mitigation and increase society's resilience to hazards such as heavy rainfall.
To achieve this, we are developing and applying improved weather prediction models capable of accurately forecasting severe events in New Zealand.
Current NIWA research projects
For heavy rainfall specifically, we are:
- Developing very high resolution numeric weather prediction (NWP) forecast models and investigating the effects of model resolution on heavy rainfall simulation. Models are run with a resolution of 12km and 1.5km on NIWA's supercomputer.
- Improving the model performance by incorporating all available meteorological observationsto generate the initial atmospheric conditions as accurately as possible.
- Improving the processes that control the dynamic characteristics and predictability of heavy rainfall in the New Zealand region into the NWP model formulations, including the land surface processes needed.
- Developing ensemble methods/forecasts by running the model several times with slightly different initial conditions to increase our confidence in forecasting heavy rainfall.
The output of this research is being incorporated into our environmental forecasting product, EcoConnect.
Additionally, NIWA has analysed measurements of extreme rainfall throughout New Zealand, and developed the free HIRDS software tool to provide public information on the risk of extreme rainfall. HIRDS is regularly used by engineers throughout New Zealand, to assist in designing bridges, culverts and waterways. HIRDS includes an option to make an assessment of the impact of climate change on extreme rainfall.