Langragian analysis of the extreme-windstorm dynamics associated to post-tropical cyclone Leslie landfall in Portugal

Windstorms in Europe are responsible for more than half of the economic loss associated with natural disasters. In October 2018 a post-tropical cyclone, formerly Hurricane Leslie, made landfall in continental Portugal. This event was characterized by very intense winds, with a gust record-hitting value of 176 km/h registered near Figueira da Foz, a coastal city located in the center of the country. The main factors causing this event of extreme winds were likely a “cold-conveyor belt jet” or a “jet sting”, roughly 12 hours after losing its main tropical characteristics. Despite the strong impact associated with this windstorm there are still few studies modeling this kind of dynamics, and here we present a simulation and thorough analysis of the rare dynamics linked with this post-tropical cyclone affecting western Europe.

The WRF-ARW model, version 4.4.1, was used to numerically model Leslie as it transitioned from a hurricane to post-tropical cyclone. Three one-way nested domains were used with a large (5 km), medium (1 km), and lower (200m) resolution, with 68 hybrid levels (15 m - 20 hPa). The larger domain covers the Iberian Peninsula and a large portion of the Atlantic Ocean nearby, while the inner ones are focussed in the central and northern sectors of continental Portugal - the most affected areas. Initial and boundary conditions were retrieved from the GFS operational analysis at 0.25º spacing, in 6-hour intervals. Due to the difficulties modeling this cyclone, nudging was used in the outer domain to ensure that the cyclone would make landfall as close as possible to the real location.

Several state-of-the-art thermodynamics-based diagnostics were used to analyze in-depth the midlatitude cyclone dynamics observed in the recently transitioned cyclone Leslie. Midlatitude cyclone-related dynamics were identified in the simulation, leading to the extreme winds in the most impacted region. The set of final simulated data reveals a close resemblance to the real event, with parameterized wind gusts presenting a lower intensity around 140 km/h, but the largest values impacting approximately the same region of center Portugal. A Langragian approach was also used to study particle trajectories and evaluate the atmospheric circulation leading to the extreme winds showing vertical downdrafts up to 4 m/s. This study highlights the catastrophic potential a post-tropical cyclone such as Leslie has and, while at the end of their life-time with presumably less intensity, storms of this type should not be disregarded for warnings and need to be considered in general evaluations of midlatitude storm impacts.

Acknowledgements: This work was funded by the Portuguese Fundação para a Ciência e a Tecnologia (FCT) I.P./MCTES through national funds (PIDDAC) – UIDB/50019/2020. M. M. Lima was supported through the PhD MIT Portugal MPP2030-FCT programme grant PRT/BD/154680/2023. L. C. Santos is supported by the EarthSystems Doctoral School, at University of Lisbon, supported by Portuguese Fundação para a Ciência e a Tecnologia (FCT) project UIDP/50019/2020-2023, University of Lisbon.