Estimating return periods of extreme events from ECMWF seasonal forecast ensembles

Meteorological extremes have large impacts on society. The fact that approximately 40% of the Netherlands is below sea level makes this country especially vulnerable to flooding, both from the sea and from rivers. This has resulted in extensive research on the statistics of extremes. However, applications to meteorological and hydrological situations are always hampered by the brevity of the available datasets, as the required return levels exceed the record lengths by a factor of 10 to 100. In order to overcome this problem, we use archived data from all past seasonal forecast ensemble runs of the European Centre for Medium-Range Weather Forecasts (ECMWF) since 1987 as input for extreme-value statistics analysis. We make use of the fact that the seasonal forecast has little seasonal skill for the Netherlands, which implies that the ensembles can be regarded as independent sets that cumulate to over 1500 years.

We investigate the hydraulic response in the Netherlands to extreme synoptic-scale weather systems by studying the extreme-value distributions of sea storm surge levels, waves and river discharges. The application is detailed in four practical examples originating from coastal protection, river flooding protection, and water management problems. The long record length of the ECMWF data reduces the uncertainty in the 10^3-year and the 10^4-year return values considerably with respect to the results based on observational time series. The ECMWF dataset gives the opportunity to explore the distribution of events that depend on several kinds of extreme.