An approach to build an event set of European windstorms based on ECMWF EPS

Abstract

Abstract. The properties of European windstorms under present climate conditions are estimated on the basis of surface wind forecasts from the European Centre for MediumRange Weather Forecast (ECMWF) Ensemble Prediction System (EPS). While the EPS is designed to provide forecast information of the range of possible weather developments starting from the observed state of weather, we use its archive in a climatological context. It provides a large number of modifications of observed storm events and includes storms that did not occur in reality. Thus it is possible to create a large sample of storm events, which entirely originate from a physically consistent model, whose ensemble spread represents feasible alternative storm realizations of the covered period. This paper shows that the huge amount of identifiable events in the EPS is applicable to reduce uncertainties in a wide range of fields of research focusing on winter storms. Windstorms are identified and tracked in this study over their lifetime using an algorithm based on the local exceedance of the 98th percentile of instantaneous 10 m wind speed, which is associated with a storm severity measure. After removing inhomogeneities in the data set arising from major modifications of the operational system, the distributions of storm severity, storm size, and storm duration are computed. The overall principal properties of the homogenized EPS storm data set are in good agreement with storms from the ERAInterim data set, making it suitable for climatological investigations of these extreme events. A demonstrated benefit in the climatological context by the EPS is presented. It gives clear evidence of a linear increase of maximum storm intensity and wind field size with storm duration. This relation is not recognizable from a sparse ERA-Interim sample for long-lasting events, as the number of events in the reanalysis is not sufficient to represent these characteristics.

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

“…The fundamental idea is to identify windstorms in the 51 members of the ECMWF System 4 seasonal forecast system (Molteni et al , 2011) and treat each member as a physically consistent realisation of a potential reality. This approach is similar to Osinski et al () who used the ECMWF EPS model to build a windstorm “hazard set.” This will lead to a substantial increase in the available physically consistent sample of extreme events. The annual losses for four different European countries for every member of the ensemble are estimated from the tracked windstorm events with the help of the open source natural catastrophe damage model CLIMADA (Bresch, ).…”

“…The fundamental idea is to identify windstorms in the 51 members of the ECMWF System 4 seasonal forecast system (Molteni et al, 2011) and treat each member as a physically consistent realisation of a potential reality. This approach is similar to Osinski et al (2016) who used the ECMWF EPS model to build a windstorm "hazard set." This will lead to a substantial increase in the available physically consistent sample of extreme events.…”

Spatial variability and potential maximum intensity of winter storms over Europe

“…The 51 members are used in a climate archive context similar to Osinski et al (2016), thus every member is treated as an individual artificial reality of 31 years resulting in more than 1500 years of physically consistent data. As the forecast skill for very high local wind speeds is relatively small (Walz et al (2018b)), the members can be seen as statistically independent since statistical dependence and forecast skill can be seen as equivalent.…”

“…In order to estimate the uncertainty of high impact windstorms in terms of frequency and severity however, the amount of data is still too sparse to produce reasonable confidence intervals. Similar to Osinski et al (2016) in which the ECMWF Ensemble Prediction System (EPS) is used as a data archive for creating a windstorm catalogue, this study approaches the retrospective predictions of ECMWF Seasonal Forecast System 4 (Molteni et al, 2011) as an archive of potential windstorms. Clearly none of the windstorms found in these forecasts ever happened, however each of them represents one possible physical consistent realisation of a potential reality.…”

Ensemble hindcasting of wind and wave conditions with WRF and Wavewatch III<sup>®</sup> driven by ERA5

“…This difference can be due to a different dynamical evolution of the storm or due to different tracks in the atmospheric model members (compare Osinski et al, 2016), leading to differences in the position. Already a slight change in the track of the storm can provoke large differences in the maximum if looking at a specific location in such a high resolution.…”