Global-to-local scale storm surge modelling

Abstract

Storm surge that results from tropical and extra-tropical cyclones is driven primarily by wind strength and inverse barometer effects which have global scale dynamics, but also local scale effects. In fact, storm surge is influenced by a multitude of local factors, including; shoreline geometry; the nearshore slope and bathymetry; cyclone wind obliquity to the coast and forward-moving speed; harmonic coupling with the astronomic tide; and cyclone intensity. The complexity of storm-surge dynamics means that both global-scale and local-scale processes need to both be resolved. In order to resolve the global scale processes, Deltares has developed the global numerical hydrodynamic Delft3D FM model “Global Tide and Storm surge Model” (GTSM). The Global Storm Surge Information System (GLOSSIS) runs the GTSM operationally, producing four forecast a day. To better understand the sensitivities of global to local scale storm surge modelling, the GTSM model has been applied in real-world operational forecasting systems (through GLOSSIS), as well as in a model sensitivity analysis study. Two operational systems are described in this paper: forecasting surface currents to gain an edge in the global sailing contest “Volvo Ocean Race” and storm surge and wave forecasting in TC prone Mozambique. The model sensitivity analysis described in this paper has been performed for a case study in North Queensland. In this study the global GTSM model provides forcing (boundary water level and atmospheric (velocity and pressure)) for a local Delft3D FM model operated by Risk Frontiers. This local model benefits from the availability of high-resolution bathymetry and coastal water level observations. The model sensitivity analysis looked at the impact of bathymetry and grid resolution. Separately, the GTSM v3 hydrodynamic model has been verified on a global scale through a validation study for a global observation dataset for both tide and surge, and compared to other state-of-the-art tidal models, as well as in a dedicated validation study for Europe. The validation shows good agreement of the model against both tide and surge observations, demonstrating the capability of the model to accurately represent water-levels at the coastal zone. The accuracy of the forecasting model is comparable to existing data-assimilated global tide models and outperforms modern non-assimilated global tide models. The surge results show very high correlation and low standard error (STDE) values for both datasets, and tropical-cyclone induced surges are also shown to be well captured. This paper describes the ongoing improvements of GTSM, as well as two forecasting applications of GLOSSIS in the real world as well as the use of GTSM in a model sensitivity analysis performed for a case study in North Queensland.