Abstract
Abstract. New insights into the synoptic-scale predictability of 25 severe European winter storms of the 1995–2015 period are obtained using the homogeneous ensemble reforecast dataset from the European Centre for Medium-Range Weather Forecasts. The predictability of the storms is assessed with different metrics including (a) the track and intensity to investigate the storms' dynamics and (b) the Storm Severity Index to estimate the impact of the associated wind gusts. The storms are well predicted by the whole ensemble up to 2–4 days ahead. At longer lead times, the number of members predicting the observed storms decreases and the ensemble average is not clearly defined for the track and intensity. The Extreme Forecast Index and Shift of Tails are therefore computed from the deviation of the ensemble from the model climate. Based on these indices, the model has some skill in forecasting the area covered by extreme wind gusts up to 10 days, which indicates a clear potential for early warnings. However, large variability is found between the individual storms. The poor predictability of outliers appears related to their physical characteristics such as explosive intensification or small size. Longer datasets with more cases would be needed to further substantiate these points.
Highlights
One of the most important natural hazards over Europe arises from winter storms associated with low-pressure systems from the North Atlantic, referred to as cyclonic windstorms (Lamb and Frydendahl, 1991)
Three metrics are combined to assess the predictability of the storms with regard to different properties: the dynamics are evaluated with the track and intensity of the storms (Fig. 1a; Sect. 2.3.1), the impact is estimated with the footprint of wind gusts (Fig. 1b; Sect. 2.3.2) and the potential for early warnings is computed from the area of predicted gusts that are well above the model climate (Fig. 1c; Sect. 2.3.3)
Either the ensemble average or the individual members are used for the verification of the reforecasts of the selected storms based on these metrics
Summary
One of the most important natural hazards over Europe arises from winter storms associated with low-pressure systems from the North Atlantic, referred to as cyclonic windstorms (Lamb and Frydendahl, 1991). Pirret et al (2017) recently applied this tracking approach to severe European storms in operational ECMWF ensemble forecasts and found a negative bias in intensity in addition to the slow bias in track They further investigated the relative contribution of diabatic and baroclinic processes to the intensification of the storms. Their results were, limited by the use of operational forecasts, whose skill improves with updates in the model version and with increases in the horizontal resolution in particular Building on these previous studies, the predictability of severe European winter storms is systematically investigated here for a 20-year period in an ensemble prediction system by taking advantage of the recently available ECMWF retrospective forecasts (reforecasts; Hagedorn et al, 2008, 2012).
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