Accounting for uncertainties in forecasting tropical-cyclone-induced compound flooding

Abstract

Abstract. Tropical-cyclone impacts can have devastating effects on the population, infrastructure, and natural habitats. However, predicting these impacts is difficult due to the inherent uncertainties in the storm track and intensity. In addition, due to computational constraints, both the relevant ocean physics and the uncertainties in meteorological forcing are only partly accounted for. This paper presents a new method, called the Tropical Cyclone Forecasting Framework (TC-FF), to probabilistically forecast compound flooding induced by tropical cyclones, considering uncertainties in track, forward speed, and wind speed and/or intensity. The open-source method accounts for all major relevant physical drivers, including tide, surge, and rainfall, and considers TC uncertainties through Gaussian error distributions and autoregressive techniques. The tool creates temporally and spatially varying wind fields to force a computationally efficient compound-flood model, allowing for the computation of probabilistic wind and flood hazard maps for any oceanic basin in the world as it does not require detailed information on the distribution of historical errors. A comparison of TC-FF and JTWC operational ensembles, both based on DeMaria et al. (2009), revealed minor differences of <10 %, suggesting that TC-FF can be employed as an alternative, for example, in data-scarce environments. The method was applied to Cyclone Idai in Mozambique. The underlying physical model showed reliable skill in terms of tidal propagation, reproducing the storm surge generation during landfall and flooding near the city of Beira (success index of 0.59). The method was successfully applied to forecasting the impact of Idai with different lead times. The case study analyzed needed at least 200 ensemble members to get reliable water levels and flood results 3 d before landfall (<1 % flood probability error and <20 cm sampling errors). Results showed the sensitivity of forecasting, especially with increasing lead times, highlighting the importance of accounting for cyclone variability in decision-making and risk management.

Exploring coastal climate adaptation through storylines: Insights from Cyclone Idai in Beira, Mozambique

“…SFINCS is a reduced-physics solver that accurately simulates compound coastal flooding by solving simplified two-dimensional overland flow equations. Its suitability for simulating compound flooding resulting from TCs has been demonstrated in previous studies (Leijnse et al, 2021;Sebastian et al, 2021;Eilander et al, 2023b;Goulart et al, 2024;Nederhoff et al, 2024). A full description of the model is available at Leijnse et al (2021).…”

“…The onshore simulation is then forced with the generated water levels and precipitation to produce inland flooding levels in Beira. The surface elevation is obtained from a merged dataset that combines several local and global datasets, achieving a 5 m resolution in Beira (Nederhoff et al, 2024). The roughness coefficients are sourced from the Copernicus Global Land Service (Buchhorn et al, 2020) and infiltration rates derived from the GCN250 dataset (Jaafar et al, 2019).…”