Forecasting tropical cyclone surge

Tropical cyclones, with strong winds and low central pressure, can produce very large coastal surges and have led to the most devastating flooding in history. For example, in Bangladesh, flooding from cyclones in 1970 and 1991 has been estimated to have cost the lives of over 300,000 and 150,000 people respectively. While forecasting of cyclones and emergency management have improved dramatically, flooding from cyclone surge still represents one of the most serious global natural hazards and a considerable challenge for humanitarian organisations. In 2020, Cyclone Amphan was forecast to make landfall on the border of Bangladesh and India and the UK Foreign, Commonwealth and Development Office (FCDO) approached HR Wallingford to provide a forecast of the potential cyclone surge. Following that event, the FCDO commissioned a pilot study to develop the forecasting of flooding globally, including surge modelling, response of rivers to rainfall, modelling of flooding from both fluvial and coastal sources and analysis of the impact on local population and infrastructure. The aim of the forecast is to provide advance warning of the location, extent and severity of flooding and impact on population in order to aid the coordination of humanitarian relief. The service is provided by a partnership of ECMWF, the Universities of Reading and Bristol, Fathom and HR Wallingford. As part of this service, a surge forecast system was developed using TELEMAC-2D forced by cyclonic wind and atmospheric pressure fields. The development of the system had to overcome a number of challenges: • The TELEMAC-2D models have to be large enough to cover all areas of interest to the FCDO and humanitarian agencies and at risk of flooding from cyclone surge; • The models must adequately resolve the wind and pressure fields of cyclones and coastal bathymetry and topography; • The models must include tide as it is the combination of both tide and surge that determines the elevation of the water and hence the extent and severity of flooding; • The models must run quickly. The goal is to provide a bulletin within one working day and the target for the surge model runtime is less than one hour. The surge modelling system includes a number of regional TELEMAC-2D models covering areas of the world vulnerable to cyclone surge flooding and of interest to the FCDO. The regional models include tide and atmospheric forcing. Cyclone tracks are downloaded from relevant meteorological agencies responsible for forecasting tropical cyclones. Within the modelling system, these are converted into wind and pressure fields to force the model. Model results are extracted along the coastline for input into an inundation model and population exposure analysis. The combined results showing areas forecast to be affected by flooding and the impact on the local population and infrastructure are summarised in a concise bulletin for the FCDO. The bulletin is then circulated to local and international aid organisations including the UN OCHA and International Federation of Red Cross and Red Crescent. Since the start of the pilot study in October 2020, the team have responded to tropical cyclones affecting Central America, Mozambique, Madagascar and the Philippines