Modelling wind behaviour over complex terrain
Understanding New Zealand’s wind climatology is very important in a number of areas, such as:
- Identifying areas of high daily wind runs for the siting of wind-farms.
- Identifying areas prone to extreme wind gusts. This is important for hazard mapping.
- Identifying local katabatic winds (drainage flows) on cold stable nights. Important in knowing the direction of movement of plumes of highly concentrated pollutants or viral airborne diseases.
New Zealand has very complex terrain, and terrain effects are very important in all three of the above applications.
Wind observations that have been archived in the past, while being of a high quality, may not always be representative of winds at locations near the observing site due to these complex terrain effects. One way to get around this problem is to use computer models, such as BLASIUS, CALMET, and the UK Unified Model.
These models are used by NIWA research meteorologists in the applications mentioned above. However, we still also need to compare their performance against each other and the observations for various meteorological conditions and locations in New Zealand.
This helps to identify the limitations and strengths of each of the models, and tells us when and where we can apply a model or the conditions under which models with known limitations should not be used.
The figures presented (left) are from numerical simulations of typical conditions over the Manawatu Gorge – that of a neutral to weakly stable atmosphere in a prevailing west-northwest airflow. The top graphic gives a high resolution flow simulated by BLASIUS; the next shows a lower resolution flow over Manawatu from CALMET. The bottom one compares the flows from both CALMET and BLASIUS. The red areas are regions of high wind speed and the blue are areas of low wind speeds.
Both models are providing similar results by showing higher speeds over the ranges for the same situation. Neither model is outperforming the other, increasing the confidence of the meteorologist in applying such models in a range of applications.