Differentiating average and extreme extra-tropical cyclones by their vertical tilt and potential vorticity structure

This study investigates the difference of the vertical tilt and potential vorticity (PV) structure between average and extreme extra-tropical cyclones (ETCs) in the Northern Hemisphere during the cold season (October–March) in ERA5 reanalysis. The vertical tilt is quantified as novel two-component (directional and lateral) horizontal distance between the near-surface and upper-level ETC. Compared to average ETCs, extreme ETCs exhibit an amplified tilt magnitude during the intensification phase and a change in the sign of the lateral tilt prior to maximum intensity. Accounting for the tilted vertical ETC structure, this study shows that average ETCs are associated with an upper-level PV anomaly, which is constant through the near-surface ETC lifecycle, while the PV intensifies from the bottom up. On contrast, extreme ETCs are characterised by a growing magnitude of upper-level PV anomaly, and a simultaneous intensification throughout the lower levels.