Modelling the Tracy storm surge – implications for storm structure and intensity estimation

Abstract

The results of hydrodynamic storm surge modelling are used to inform and construct a detailed quantitative description of the possible changing structure and intensity of severe tropical cyclone Tracy as it proceeded across Beagle Gulf to landfall at the City of Darwin in 1974. The measured storm surge is used as an independent wind calibration proxy to augment the measured eyewall winds and pressures from the airport, which is located close to the coast. The investigation resulted in a seemingly subtle change in the modelled wind speed for the airport site but enabled an accurate reproduction of the magnitude of the measured storm surge in the nearby harbour and wave impacts on the local beaches. It is therefore hypothesised that Tracy likely underwent an eyewall-replacement contraction when approaching near land west of Darwin and, locally at least, significantly increased its peak winds during the critical period when the storm tide was generated and enabled the propagation of the surge into the harbour. It is further hypothesised that the storm subsequently rapidly decayed or became unstable as it came ashore. As a result of these experiments, there is also evidence that winds at the nearby coastline may have been substantially higher than those recorded at the airport.