Tropical cyclones are usually associated with destructive winds and heavy rain but they can also cause widespread damage and loss of life due to the forcing of water onshore as they cross the coast, known as a 'storm surge'.
Predicting the increase in sea level above the normal tidal range is a critical part of the Bureau of Meteorology's warning services. For example, during Tropical Cyclone Debbie in March 2017, the combination of high winds, tides and intense low pressure caused the water level on the coast to rise by over two metres above normal in some places. Warnings from the Bureau assisted emergency managers in deciding to advise coastal communities in north Queensland to evacuate well ahead of the storm onset, thereby avoiding being caught in a life-threatening event.
The Bureau of Meteorology has been significantly upgrading its modelling systems to improve the accuracy and timing of their storm surge forecasts. The accuracy of the new system comes from running a high resolution dynamical storm surge model for the actual forecast track, as opposed to previous techniques that used parameterised pre-computed scenarios using straight line tracks. By running the model many times to allow for the range of possible alternate tracks, an understanding of the uncertainty of the predicted storm surge can be obtained. The more forecast runs the system has to work with, the better the uncertainty in the forecast can be represented.
For the region of coastline that a cyclone will pass over, the height that the sea level will peak is the most critical bit of information. Due to large tidal ranges experienced over much of Australia, the timing of the surge is also very important. While the storm surge model takes into account many complex details about the atmosphere, wind speed, tides, cyclone movement, intensity and symmetry, the final product can be as simple as a single data point showing this peak sea height. Computational Scientist from the Bureau of Meteorology Dr Justin Freeman says "Managing a large number of simulations as fast as possible introduces some new challenges – especially as this is needed to help with timely emergency response."
As part of the development of the new system, the HPC Scaling and Optimisation team from NCI analysed the model and made improvements to ensure optimal use of the modern HPC infrastructure. As a result, the Bureau can now produce one 3-day forecast run in under 6 minutes.
Dr Freeman and his team tested the model improvements using historical storm data and comparing the model results with the location and sea height based on the real observations. This testing included 2017's Tropical Cyclone Debbie to ensure that the most up-to-date information was considered. As a result of this work, the Bureau introduced the new storm surge model in October 2017, ahead of the 2017-18 cyclone season.