Extending the forecast range of the UK storm surge ensemble

Flooding due to coastal storm surges presents a significant threat to life and property. The UK has long had a storm surge forecasting system based on a single deterministic' simulation. This was augmented with an operational storm surge ensemble in December 2009. By producing several simulations sampling the forecast uncertainty, the ensemble estimates the probability of reaching critical water levels and thus supports a more risk-based approach to civil protection. The original storm surge ensemble provided forecasts out to T + 54 h, limited by the forecast range of the driving MOGREPS-R atmospheric ensemble. Longer-range forecasts could provide advance notice of the potential for a significant event, allowing suitable preparatory actions to be taken. This study investigates the possibility of extending the storm surge ensemble to between 5 and 7 days using atmospheric data from the lower-resolution Met Office 15-day ensemble (MOGREPS-15). Both case studies and statistical verification indicate the potential for useful forecasts out to the full 7.25 days tested. The best performance is obtained by extending the existing surge ensemble products with output from separate runs of the storm surge model, which have been driven by MOGREPS-15 meteorology from T + 0 h. An attempt to create a single surge history for each member by switching from MOGREPS-R to MOGREPS-15 input at T + 54 h led to spurious oscillations in some cases, and poorer performance on several statistical measures. These issues might be improved by smoothing the discontinuity in atmospheric forcing. Following this successful trial, the separate-runs extension to the surge ensemble was implemented operationally in summer 2011. The study also demonstrates the benefit of online bias correction and dressing' the forecast members to account for errors which the system does not otherwise sample. The operational implementation of these features is left for future work. Copyright (c) 2012 British Crown copyright, the Met Office Published by John Wiley & Sons Ltd.