NIWA's work seeks to improve our understanding of the key dynamic and physical processes which are responsible for heavy rainfall , and to improve the heavy rainfall forecast.
Current research – the context
Heavy rainfall can occur in most of New Zealand, and throughout the year. The record for the highest recorded daily rainfall is Colliers Creek, in Hokitika, which got 682mm (Jan 21–22 1994).
Very heavy rainfall in a short time period is likely to cause flooding and landslides.
In New Zealand, heavy rainfall is directly associated with specific weather systems such as ex-tropical cyclones that begin in the tropics, Tasman lows that are usually generated in the west Tasman Sea, and mid-latitude cyclones/lows that begin over the mid-latitude, cold sea.
In order to understand how heavy rainfalls occur, and accurately forecast them, we need to study the physics and factors behind how such weather systems develop, to develop the regional numerical model so that this information can be described /resolved by the model.
In the short term, studies look at the evolution of these weather systems over time scales ranging from a few hours to a few days. Long-term, the incidence of heavy rainfall over New Zealand can also be affected by climate variability associated with ENSO, and by global warming-related climate change.
Over the last two decades, 3D atmospheric data analysis has become increasingly available. In addition, some regional numeric weather experiment studies, as well as diagnostic and statistical analysis, have been conducted to understand heavy rainfall over New Zealand.
We have developed an overall understanding of how these systems occur, develop, and move. Our observations from field experiments over the South Island have played a particularly important role in understanding how mountains induce heavy rainfall over the Southern Alps.
On the other hand, the sparse observations we have of the southwest Pacific Ocean, which is where these systems usually start and develop, limits our complete understanding. There are still many questions which need to be answered about heavy rainfall and its associated weather patterns.
Important research questions
- How does land–air interaction and the orographic lifting and descent of airflow by New Zealand terrain affect the strength, development, and movement of weather systems which result in heavy rainfall?
- How does air-sea interaction - for example, the sea surface temperature and its distribution in the southwest Pacific Ocean - affect the development and movement of Tasman lows, winter storms, and the extra-tropical cyclones?
- How does interaction of the troposphere and stratosphere affect the cyclones? What is the role of the downward transportation of potential vorticity - a physical variable that describes the strength and development of a cyclone?
- How do these physical processes affect the location and amount of heavy rainfall over New Zealand?
With the development of remote sensing and satellite data retrieval techniques over the past decade, a large amount of meteorological data has been produced from the atmosphere over the sea. This data can be used to investigate these questions.
Understanding the processes means their representation in the model is improved, leading to better forecasts.
Numerical modelling and forecast of heavy rainfall
Because of the potentially severe effects of heavy rainfall, reliable forecasts for heavy rainfall are in great demand.
Meso-scale and cumulus-resolved numerical modelling and forecasting are considered to be the best ways to quantitatively forecast heavy rainfall over NZ. However, an accurate forecast of heavy rainfall in terms of both its location and timing over 1-3 days is still a major challenge.
One of the issues is that the many atmospheric and surface physics related to heavy rainfall are not fully understood, or well described by numeric models. In addition to the important research questions listed above (ANCHOR), NIWA scientists are also conducting research to address these issues, in order to improved heavy rainfall forecasts over NZ.
Research areas
- How to improve the description of these dynamic and physical processes in a regional weather or climate model, for example a New Zealand limited area model (NZLAM) and New Zealand convection resolved model (NZCONV).
- How to show that their inclusion in weather models improves heavy simulation capabilities.
- How to improve the description of the atmospheric status in the NZLAM and NZCONV models for weather systems which might result in heavy rainfall. This will be done through the assimilation of all available meteorological and satellite observation data.
Currently at NIWA, the NZLAM-12 (New Zealand Limit Area model) is running daily to provide 48 hourly forecasts twice daily as part of Eco-connect. The forecast of NZCONV will be available soon. These numerical forecasts are very useful for predicting the location and timing of heavy rainfall and other weather over New Zealand.
Other related NIWA research
Heavy rainfall often results in flooding over New Zealand, please see flood modelling.
Past research involving NIWA scientists
Past and related research has sought to improve the understanding and forecasting of the heavy rainfall over New Zealand.
The goal of the Southern Alps Experiment (SALPEX'96) was to determine and understand the processes through which the Southern Alps influence precipitation. Studies from this project improved our understanding of the role of the Southern Alps in affecting heavy rainfall (Bulletin of the American Meteorological Society, 77, 683-692 (1996)
The Southern Hemisphere THORPEX project. THORPEX, established in May 2003, is a ten year international research programme aimed at accelerating improvements in the accuracy of 1-day to 2-week high impact weather forecasts. Improving the accuracy of heavy rainfall forecasts is one of the project goals.