The role of wave coupling from the mesoscale to the sub mesoscale in a sting jet windstorm

Windstorms associated with extratropical cyclones are destructive natural hazards. Processes governing the formation of near-surface winds are crucial for their societal impact but are not well understood and too small scale to be explicitly represented in numerical weather prediction models.

 

The explosive cyclone Alex made landfall in southern Britany in October 2020 causing extensive wind damage in Belle-Ile and further inland. We investigate the mechanisms allowing high momentum air to descend to the surface and the influence of wave driven air-sea interactions from the mesoscale O(100km) to the sub-mesoscale O(100m). For this purpose we run a series of stand-alone atmospheric simulations and coupled wave-atmosphere simulations using the Meso-NH and WAVEWATCH-III models incrementally decreasing the horizontal grid spacing by two from 1.6 km to 100 m. The high resolution allows explicit representation of shallow convection and of the most energetic turbulent eddies in the atmospheric boundary layer (Large Eddy Simulations). 

 

Online trajectory calculations allow for a Lagrangian air mass tracking in Meso-NH. In the 1.6 km simulations, these reveal the presence of 3 distinct airstreams responsible for the strongest winds. The evolution of state parameters along these trajectories helps match the airstreams to the classical conceptual model for extra tropical cyclones. Evidence hints to the presence of a rare sting jet associated with Alex’s extreme winds, along with the more common cold conveyor belt and dry intrusion. In the Large Eddy Simulations, the same Lagrangian approach shows how the high momentum air in the airstreams is brought down by coherent boundary-layer structures. The vertical momentum transport is further controlled by wave coupling, which influences the stability of the boundary layer and the surface drag. The impact of wave coupling and resolution on extreme winds is discussed for the different mesoscale airstreams.  The results show that the representation of both sub-mesoscale processes and air-sea interactions constrains the formation of near-surface winds under storm conditions.