Seasonal forecast skill for extratropical cyclones and windstorms

Daniel J Befort,Julia F Lockwood,Antje Weisheimer,Jeff R Knight,Simon Wild,Leon Hermanson,Gregor C Leckebusch,Philip E Bett,Hazel E Thornton

doi:10.1002/qj.3406

Abstract

Extratropical cyclones and their associated extreme wind speeds are a major cause of vast damage and large insured losses in several European countries. Reliable seasonal predictions of severe extratropical winter cyclones and associated windstorms would thus have great social and economic benefits, especially in the insurance sector. We analyse the climatological representation and assess the seasonal prediction skill of wintertime extratropical cyclones and windstorms in three multi‐member seasonal prediction systems: ECMWF‐System3, ECMWF‐System4 and Met Office‐GloSea5, based on hindcasts over a 20‐year period (1992–2011).Small to moderate positive skill in forecasting the winter frequency of extratropical cyclones and windstorms is found over most of the Northern Hemisphere. The skill is highest for extratropical cyclones at the downstream end of the Pacific storm track and for windstorms at the downstream end of the Atlantic storm track. We also assess the forecast skill of windstorm frequency by using the North Atlantic Oscillation (NAO) as the predictor. Prediction skill improves when using this technique over parts of the British Isles and North Sea in GloSea5 and ECMWF‐System4, but reduces over central western Europe. This suggests that using the NAO is a simple and effective method for predicting windstorm frequency, but that increased forecast skill can be achieved in some regions by identifying windstorms directly using an objective tracking algorithm. Consequently, in addition to the large‐scale influence of the NAO, other factors may contribute to the predictability of windstorm frequency seen in existing forecast suites, across impact‐relevant regions of Europe.Overall, this study reveals for the first time significant skill in forecasting the winter frequency of high‐impact windstorms ahead of the season in regions that are vulnerable to such events.

Highlights

Extratropical cyclones can produce high wind speeds near the surface, damaging physical structures, causing fatalities and enormous financial losses
As expected, the results show lower skill along the nodal line of the North Atlantic Oscillation (NAO) including most central western European countries when compared to the direct approach
Our results appear promising overall and corroborate the emerging evidence of predictability on seasonal time-scales regarding extratropical cyclones and windstorms (Renggli et al, 2011; Riddle et al, 2013; Scaife et al, 2014; Smith et al, 2016). Our analyses extend these recent studies by considering different seasonal prediction systems

Summary

INTRODUCTION

Extratropical cyclones can produce high wind speeds near the surface, damaging physical structures, causing fatalities and enormous financial losses. Strong midlatitude cyclones affecting Europe form when baroclinic disturbances over the North Atlantic undergo rapid intensification, leading to a fall in surface pressure and steep pressure gradients In conjunction with their related frontal structures, these intense cyclones can produce extremely high surface wind speeds over a large footprint region. The earliest one known to the authors is a study of the ENSEMBLES and DEMETER seasonal hindcast experiments (Renggli et al, 2011) In these early seasonal forecast systems, they found small but significant skill for extratropical cyclone-related windstorm frequency over parts of central western Europe. More recent studies investigating the Met Office GloSea hindcast dataset, based on the HadGEM3-GA3 model, have revealed a step-change in midlatitude seasonal forecasting skill, demonstrating that this forecast system does show significant, usable skill for the major climate variability mode over Europe, the NAO (Scaife et al, 2014; 2016; Palin et al, 2016; Clark et al, 2017). The results obtained from the seasonal forecast models appear smoother than the single realization of the reanalysis

IDENTIFICATION AND TRACKING OF CYCLONES AND WINDSTORMS

RESULTS

SUMMARY AND DISCUSSION