The study investigated climatological characteristics of mid-latitude cyclones and windstorms (the mid-latitude cyclones with the strongest winds associated with them) in northern Europe using ERA5 reanalysis. The study shows that there are no significant trends in the number of mid-latitude cyclones nor windstorms in northern Europe during 1980-2019 and that the inter-annual variability in the number of windstorms was large. The well-known annual cycle – with more windstorms in winter than in summer – was also clearly identified in this study. What was most surprising is that the study found no annual cycle in the number of mid-latitude cyclones (i.e. the more “normal”, weaker weather systems).
The study also examined how the structure of windstorms changes during the life time of the storm. During the development of a windstorm, the minimum pressure decreases, the horizontal pressure and temperature gradients increase, and the winds get stronger. The location of the highest winds moves and extends during the development from the warm sector to behind the cold front following the location of the strongest pressure gradient.
The study also examined “precursors” which are features in the atmosphere that precede the windstorms and can be used by forecasters as early warning signs of potentially strong winds and high-impact weather. The precursors to windstorms were investigated using an ensemble sensitivity analysis where higher sensitivity means a stronger correlation between the precursur and the eventual intensity of the windstorm. The sensitivity was higher in cold-season windstorms than in warm-season windstorms which indicates that windstorms are potentially better predictable during the cold season than the warm season. The study showed that the highest sensitivities exist between the low-level horizontal temperature gradient and the eventual windstorm intensity. Therefore, the low-level temperature gradient is an important variable when predicting the windstorm intensity and forecasters should pay careful attention when stronger than average temperature gradients are present.
Reference:
Laurila, T. K., Gregow, H., Cornér, J., and Sinclair, V. A.: Characteristics of extratropical cyclones and precursors to windstorms in northern Europe, Weather and Climate Dynamics, 2, 1111-1130, https://doi.org/10.5194/wcd-2-1111-2021, 2021.