Early warnings of extreme winds using the ECMWF Extreme Forecast Index

Abstract

ABSTRACT:The European FP7 SafeWind Project aims at developing research towards a European vision of wind power forecasting, which requires advanced meteorological support concerning extreme wind events. This study is focused mainly on early warnings of extreme winds in the early medium-range. Three synoptic stations (airports) of North Germany (Bremen, Hamburg and Hannover) were considered for the construction of time series of daily maximum wind speeds. All daily wind extremes were found to be linked to very intense surface cyclonic circulation systems being advected mainly by southwest and northwest flow regimes. Overall, it becomes clear that the first indications of an extreme wind event might come from the ECMWF deterministic and/or probabilistic components capturing very intense weather systems (possible windstorms) in the medium term. For early warnings, all available EPS Extreme Forecast Index (EFI) formulations were used, by linking daily maximum wind speeds to EFI values for different forecast horizons. From all possible EFI schemes deployed for issuing early warnings, the highest skill was found for the Gust Factor formulation (EFI-10FGI). Using EFI-10FGI, the corresponding 99% threshold could provide an early warning for a considerable portion of the >99% wind extremes, but not for all. By lowering this threshold the number of hits is increased until all extremes are captured (zero misses), although by doing so the number of false alarms increased significantly. Consequently, an optimal trade-off between hits and false alarms has to be made when setting different (critical) EFI thresholds.

Revisiting the synoptic-scale predictability of severe European winter storms using ECMWF ensemble reforecasts

“…Similarly, a better performance for Anatol than for the relatively smaller storms Lothar and Martin was previously noted by Buizza and Hollingsworth (2002) in the operational ECMWF ensemble forecast. However, neither this better performance nor differences between the predictability of specific storms found by other authors using the EFI (Lalaurette, 2003;Petroliagis and Pinson, 2014;Boisserie et al, 2016) are confirmed here. This suggests a sensitivity to the ensemble prediction system and to the type and region of the reference data used for their validation, which vary from study to study.…”

“…This suggests a potential for warnings but with possible false alarms, as already noted by Lalaurette (2003). The use of EFI and SOT thus requires an appropriate balance between hit and false alarm rates (Petroliagis and Pinson, 2014;Boisserie et al, 2016).…”

“…As noted by Lalaurette (2003), the EFI gives useful warnings of extreme events but also frequent false alarms. Petroliagis and Pinson (2014) therefore suggested the use of an optimal threshold to balance between hit rate (H ) and false alarm rate (F ), a lower or higher thresh- old increasing or decreasing both H and F . Boisserie et al (2016) further suggested to maximise the Heidke skill score (HSS; Heidke, 1926) to define the optimal threshold.…”

“…2). In contrast to the previous studies of Petroliagis and Pinson (2014) and Boisserie et al (2016), however, which focused on specific storms or storm categories, an optimal threshold is first computed for the whole dataset and only then applied to the selected storms. This ensures that the result is not biased by verifying the forecast with extreme events only.…”

“…Lalaurette (2003) further showed that the extremeness of the ensemble as a whole, measured by its deviation from the model climate, allows identifying areas of unusually strong winds up to 120 h lead time in the case of Lothar, although failing in the case of Martin. Petroliagis and Pinson (2014) and Boisserie et al (2016) recently extended this method to longer periods with, respectively, operational and retrospective forecasts. While they found contrasting results from case to case, the authors confirmed the potential of ensemble forecasts for the early warning of severe European storms.…”