Loss potentials based on an ensemble forecast: how likely are winter windstorm losses similar to 1990?

In this paper we investigate the feasibility and added value of using the seasonal hindcasts of the ECMWF System 4 as a hazard event set for European winter windstorms damage calculations. The windstorms are identified for every ensemble member and every year by an objective windstorm tracking algorithm. The damages are calculated directly from the obtained wind footprints via the open source natural catastrophe damage model CLIMADA for Germany, the UK, France and Spain and compared to the loss from ERA-Interim. The results show that the ensembles of losses in System 4 nicely capture the inter-annual loss variability of the reanalysis. Due to more than 1,500 years of “virtual reality” windstorm data from the hindcasts, the return levels of extreme losses can be estimated fairly accurately. Based on System 4, the losses in the scale of 1990 (January, February, March and December including the prominent windstorm Daria) represent a 20-year event in Germany whereas they represent a 100-year event for the UK. Thus, a considerably shorter return period compared to return periods calculated from ERA-Interim alone. Further we investigate the link between the annual losses and large-scale drivers derived from mean-sea-level-pressure (MSLP) data in System 4. We can show that within System 4 there is a significant link between increased loss potentials for strongly positive North Atlantic Oscillation (NAO) phases for Germany and the UK as well as a reduced loss potential for Spain. The link between the other analysed indices is weak bar the East Atlantic (EA) pattern index. Thus, if the NAO in System 4 is correct we can assume that the windstorms in System 4 are useable. If this premise is given our study shows that the loss estimates and ultimately the return levels of losses from System 4 can be used in an operational way.