Forecasting Gita - extreme storm surge and wave heights

When tropical storm Gita passed over New Zealand on 20-21 February this year, high winds and low pressures combined with energetic ocean swells caused significant storm surges along the New Zealand West Coast.  

Storm surge is the abnormal rise of water generated by a storm, over and above the predicted astronomical tides. When accompanied by high waves, this surge can cause significant damage to coastal areas, including flooding and accelerated erosion.

“The storm offered an opportunity to validate our inhouse wave and surge models,” explains Dr Séverin Thiébaut, senior oceanographer at MetOcean Solutions. “Validation is when we compare the model results directly with in situ measurements, and provides us with a clear indication of how good our models are at predicting the timing and the magnitude of extreme storms. For Gita, we used data from a tide gauge at Charleston on the West Coast and a wave buoy in offshore south Taranaki.”

The forecast model predictions of storm surge and wave height during the storm are shown in Figures 1 and 2, respectively. Modelled wind and rainfall are also presented in these figures. These operational models are produced by MetOcean Solutions for a range of applications in NZ waters. The models are tuned to replicate the typical conditions, so verifying the predictions under an extreme storm is a powerful test.

 
 Figure 1: Predicted storm surge progress: wind and rain (left) and storm surge (right) coincide as tropical storm Gita passes over New Zealand.

Figure 1: Predicted storm surge progress: wind and rain (left) and storm surge (right) coincide as tropical storm Gita passes over New Zealand.

 
 
 Figure 2: Predicted wave progress: wind and rain (left) and wave height (right) forecast as tropical storm Gita passes over New Zealand.

Figure 2: Predicted wave progress: wind and rain (left) and wave height (right) forecast as tropical storm Gita passes over New Zealand.

 

In Figures 3 and 4 we present the time series comparison of measured and forecast storm surge and wave heights.

“The test shows our model slightly underestimated the storm surge as measured by the nearshore tide gauge,” continues Séverin. “This is likely due to the geometry of the small bay where the tide gauge is located plus the effects of wave setup on the measured water levels. Near the shore, waves will produce a localised increase in water level, which magnifies the observed storm surge. However, we are delighted the timing of the predictions was good and the magnitude acceptable for an open-coast extreme.”

“The Charleston tide gauge was located on the margin of the main storm surge effect; the model predicted coastal water level elevations up to 0.50 m in the northern parts of the South Island but unfortunately no open coast tide gauges are located in that area.”

 Figure 3: Time series of the storm surge as measured by the Charleston tide gauge data (Source:  LINZ ) and the MetOcean Solutions forecast values. The tide gauge reads higher values due to wave setup and the close proximity to shore.

Figure 3: Time series of the storm surge as measured by the Charleston tide gauge data (Source: LINZ) and the MetOcean Solutions forecast values. The tide gauge reads higher values due to wave setup and the close proximity to shore.

“The comparison for offshore wave height was excellent for a such rapidly moving system. At the offshore wave buoy, the forecast timing and height of the waves were very well correlated with the measured storm values. The highest measured significant wave height was 8.8 m, while the forecast value was 8.4 m. “It is very encouraging to see the models perform with such confidence under these extreme storm conditions,” notes Séverin. “For context, the largest significant wave height ever recorded on the West Coast of NZ was 10.4 m, measured in May 1977 near the Maui A platform.”

 Figure 4: Time series of the measured and modelled significant wave height at the offshore wave buoy (Source:  OMV ).

Figure 4: Time series of the measured and modelled significant wave height at the offshore wave buoy (Source: OMV).

Through our web portal MetOceanView, MetOcean Solutions provide automated storm watch services for coastal and offshore operators. These are automatically generated when preset conditions are identified within the forecast horizon. During the lead up to Gita’s landfall, significant wave heights of 8-9 m were predicted more than 3 days ahead (Figure 5). Warnings are sent via email alerts for operational decision-making - increasing safety and efficiency for people working at sea. These warnings are based on an ‘ensemble’ of  forecasts of the winds and waves, which captures the inherent uncertainty over a 7-day forecast horizon and the chaotic nature of a revolving tropical storm. Probabilistic guidance allows effective management decisions to be made within a robust, quantitative framework.

 
 Figure 5: Gita was forecast to be wild! Example of the ensemble forecast warning system that provided guidance on the range of possible outcomes for wind speed (top), significant wave height (middle) and wave direction (bottom) in the offshore Taranaki area.

Figure 5: Gita was forecast to be wild! Example of the ensemble forecast warning system that provided guidance on the range of possible outcomes for wind speed (top), significant wave height (middle) and wave direction (bottom) in the offshore Taranaki area.

 

MetOcean Solutions is a wholly owned subsidiary of state-owned enterprise, Meteorological Service of New Zealand (MetService). MetService is New Zealand’s national weather authority, providing comprehensive weather information services, to help protect the safety and well-being of New Zealanders and the economy.

For more information on our forecast models, contact us at enquiries@metocean.co.nz.